Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI

Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
energies
Article
Evaluation of a 1 MW, 250 kW-hr Battery Energy
Storage System for Grid Services for the Island
of Hawaii
Karl Stein 1 , Moe Tun 2 , Keith Musser 3 and Richard Rocheleau 4, *
 1   Center for Climate Physics, Institute for Basic Science (IBS), Busan 46241, Korea; kjstein@gmail.com
 2   HNU Photonics LLC, Kahului, HI 96732, USA; moetunhawaii@gmail.com
 3   Integrated Dynamics, Inc., Fishers, IN 46037, USA; kmusser@idi-software.com
 4   Hawai‘i Natural Energy Institute, SOEST, University of Hawaii at Mānoa, Honolulu, HI 96822, USA
 *   Correspondence: rochelea@hawaii.edu
                                                                                                       
 Received: 27 October 2018; Accepted: 22 November 2018; Published: 1 December 2018                     


 Abstract: Battery energy storage systems (BESSs) are being deployed on electrical grids in significant
 numbers to provide fast-response services. These systems are normally procured by the end user,
 such as a utility grid owner or independent power producer. This paper introduces a novel research
 project in which a research institution has purchased a 1 MW BESS and turned ownership over to a
 utility company under an agreement that allowed the institution to perform experimentation and
 data collection on the grid for a multi-year period. This arrangement, along with protocols governing
 experimentation, has created a unique research opportunity to actively and systematically test the
 impact of a BESS on a live island grid. The 2012 installation and commissioning of the BESS was
 facilitated by a partnership between the Hawaii Natural Energy Institute (HNEI) and the utility
 owner, the Hawaiian Electric and Light Company (HELCO). After the test period ended, HELCO
 continued to allow data collection (including health testing). In 2018, after 8500 equivalent cycles, the
 BESS continues to operate within specifications. HNEI continues to provide HELCO with expertise
 to aid with diagnostics as needed. Details about the BESS design, installation, experimental protocols,
 initial results, and lessons learned are presented in this paper.

 Keywords: battery energy storage system; field evaluation; wind smoothing; frequency regulation;
 grid-scale; lithium-titanate




1. Introduction
     At high penetration levels, the integration of intermittent renewable energy sources poses several
challenges for grid operations, due, in part, to the variability of renewable energy sources, but also
due to the reduction of system inertia via the displacement of traditional dispatchable generation.
These factors can result in a reduction in grid stability and reliability, manifesting in such ways as
increased frequency variability, voltage transients, and power quality reduction [1–3]. Such effects are
magnified on small island grids such as those on the Hawaiian islands [4]. The geographic isolation of
the islands’ electricity grids and the rapid growth of renewable generation make the Hawaiian grids
particularly susceptible to the adverse effects of variable renewable energy sources, but also an ideal
test-bed for various emerging grid solutions, including energy storage. With already high penetration
of wind and solar generation, recently passed legislation set a goal of 100% renewable energy by the
year 2045 for the state [5] and recent utility planning ensures that the islands’ grids will see increasing
renewable energy penetration in the coming years. Similar challenges and solutions are also being
addressed on other island grids [6–10].


Energies 2018, 11, 3367; doi:10.3390/en11123367                                 www.mdpi.com/journal/energies
Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
Energies 2018, 11, 3367                                                                             2 of 17



      Fast-acting battery energy storage systems (BESSs) show promise in mitigating many of the effects
of high renewable energy penetration levels [11–15]. Despite substantial numbers of deployments
worldwide, few studies have reported results of grid-connected systems [16–19]. This paper presents
a unique field test of a BESS operating on an island electric grid which was made possible by a
partnership between the Hawaii Natural Energy Institute (HNEI), at the University of Hawaii at
Mānoa, and the utility grid operator, the Hawaii Electric Light Company (HELCO, Hilo, HI, USA).
In accordance with the agreement, HNEI purchased the BESS, then relinquished ownership to HELCO,
but was allowed to perform experiments with the BESS while it was operating on the electric grid.
The experiments involved switching the BESS on and off every 20 min. This switching forms both an
independent and dependent variable over a relatively short time (minimizing changes in background
conditions). This allowed for much more direct measurements of the effect of the BESS on the island
grid than would be possible with data from long-term monitoring of the grid before and after BESS
installation. To the best of our knowledge, no other BESS research projects involve a multi-year
agreement that allows for robust scientific experimentation.
      The work presented here is part of a larger research program at HNEI, which includes assessment
of battery performance on the grid, optimization of control algorithms to maximize grid support while
minimizing battery cycling [20], and field and laboratory testing of cells to better understand cell
aging and degradation [21,22]. This paper focuses on the grid support performance aspect, along with
initial testing and methodology. The paper describes Hawaii Island BESS project including project
development and installation (Section 2), development of the control algorithms, acceptance test
results, and initial performance tests (Section 3) followed by concluding remarks and the plans for
future work (Section 4).

2. BESS Development and Installation
      The investigation into use of a BESS system for the island of Hawaii was initiated as a result
of a 2009 analysis of that electrical grid performed by General Electric (GE, Schenectady, NY, USA)
under contract to HNEI. The study utilized the Positive Sequence Load Flow (PSLF) power system
analysis software and historical data to model the frequency response of the Hawaii island grid to
rapid changes in wind generation and other contingency events. At the time of the study, the electric
grid, operated by the Hawaii Electric Light Company (HELCO), had a peak load of approximately
180 MW and a wind capacity of approximately 32 MW. While there has been rapid growth since, there
was negligible photovoltaics on the system at that time. The models indicated that as little as 1 MW
of short-duration fast-acting energy could significantly reduce the severity and duration of the grid
frequency events.
      These results prompted HNEI to develop a research project to procure and evaluate a 1 MW
grid-connected BESS. HNEI and the grid owner, HELCO, reached a mutually beneficial memorandum
of understanding that would facilitate the realization of this research project (and address any liability
issue). In accordance with the agreement, HNEI procured the BESS, and transferred ownership over
to HELCO immediately after commissioning. The agreement further stipulated that HELCO would
provide HNEI with the ability to perform experiments on the grid (provided adherence to protocol
that was agreed upon by both parties). A lithium ion titanate battery chemistry was chosen due to
the desire for an extended cycling lifetime and faster charge/discharge rates compared to the more
common carbon anode electrochemistry [23].
      HNEI assembled a public-private partnership (Table 1) for BESS development, installation,
and testing. The key tasks included: site preparation, material procurement, electrical design,
communications design, and algorithm development. Procurement of balance of plant (BOP) materials
(e.g., transformers, meters, and compatible communications devices) was undertaken by HELCO,
Altairnano (Anderson, IN, USA), and HNEI. The site preparation was conducted jointly by Altairnano
and the installation site land owner, Haw’i Renewable Development (HRD, Upolu Point, HI, USA),
which owns a collocated 10.6 MW wind farm. The electrical and communications systems were
Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
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Dynamics Inc. (IDI, Fishers, IN, USA) and SCADA Solutions Inc. (Irvine, CA, USA). HNEI provided
designed management
program    by HELCO and        Altairnano.
                             and              Algorithmindevelopment
                                  technical oversight                      wasdevelopment
                                                             all areas of the   performed byeffort.
                                                                                                Integrated Dynamics
Inc. (IDI,
     The BESS was installed at a transmission level site on the grid, at the point of provided
           Fishers,    IN,  USA)   and   SCADA     Solutions   Inc.  (Irvine, CA,  USA).   HNEI               program
                                                                                                   common coupling
management
(PCC)  between  and thetechnical
                         HRD wind oversight
                                       farm andin all
                                                   theareas of the
                                                        nearby      development
                                                                 Waimea            effort.
                                                                           substation.  Site construction started in
      The  BESS   was    installed  at a  transmission   level site
April of 2012 and the BESS was commissioned in December of 2012. Theon  the grid, at the point
                                                                                            BESSof common     coupling
                                                                                                    was designed    for
(PCC)   between      the  HRD    wind    farm  and   the nearby    Waimea    substation.   Site
interconnection to electrical power systems (as defined by ANSI C84.12006), and to be compliant construction    started
                                                                                                                  with
in April ofstandards
applicable   2012 and the      BESS
                           during      was commissioned
                                   discharge                  in December
                                                (IEEE 15472003)               of 2012. The BESS
                                                                    and standby/charging      modes was  designed
                                                                                                      (IEEE         for
                                                                                                             519) [24].
interconnection to electrical power systems (as defined by ANSI C84.12006), and to be compliant with
applicable
     Table 1.standards     during
               List of funding      discharge
                                sources         (IEEEfor
                                         and partners   15472003)   and standby/charging
                                                          Hawaii Battery                      modes
                                                                          Energy Storage Systems       (IEEE
                                                                                                  (BESS)      519) [24].
                                                                                                         project.

                                                          Project
     Table 1. List of funding sources and partners for Hawaii      Management
                                                              Battery Energy Storage Systems (BESS) project.
  Hawaii Natural Energy Institute (HNEI)                  Algorithm Development
                                                          Technical   Oversight
                                                            Project Management
      Hawaii Natural Energy Institute (HNEI)               Algorithm
                                                          Primary       Development
                                                                    Funding  Source
  Office of Naval Research (ONR)                           Technical   Oversight
                                                          Program    Oversight
                                                            Primary Funding Source
      Office ofof
  Department    Naval   Research
                   Energy  (DOE) (ONR)                    Partial Funding  for Algorithm Development
                                                            Program   Oversight
      Department of Energy (DOE)                          Infrastructure  Development
                                                            Partial Funding for Algorithm Development
  Hawaii Electric Light Company (HELCO)                   Planning
                                                            Infrastructure Development
      Hawaii Electric Light Company (HELCO)                 Planning
                                                          Grid  Management
                                                            Grid  Management
                                                          Host Site  Owner
  Haw’i Renewable Development (HRD)                         Host Site Owner
      Haw’i Renewable Development (HRD)                   Site Preparation
                                                            Site Preparation
                                                          Battery
                                                            BatterySystem
                                                                     SystemManufacturer
                                                                            Manufacturer
  Altairnano
      Altairnano                                          Systems   Integration
                                                            Systems Integration
                                                            FrequencyAlgorithm
                                                          Frequency     AlgorithmDevelopment
                                                                                  Development
      Integrated
  Integrated       Dynamics,
              Dynamics,        Inc. (IDI)
                          Inc. (IDI)                        SoftwareDevelopment
                                                          Software    Development
      SCADA Solutions                                       Wind Algorithm Development
  SCADA Solutions                                         Wind Algorithm Development
      Parker–Hannifin Company                               Power Conversion System Supplier
  Parker–Hannifin Company                                 Power Conversion System Supplier

     The system
     The system isis housed
                     housed in
                            in two
                               two containers,
                                    containers, one
                                                 one for
                                                       for the
                                                           the power
                                                                power module
                                                                        module (PM,
                                                                                 (PM, Figure
                                                                                       Figure 1a)
                                                                                               1a) and
                                                                                                    and aa second
                                                                                                           second
for the
for the power
        power conversion
                conversion system
                            system(PCS,
                                     (PCS,Figure
                                             Figure1b).
                                                      1b).The
                                                            ThePMPMcontains
                                                                      containsthe
                                                                                thebattery
                                                                                     batterystack,
                                                                                              stack,along
                                                                                                       along  with
                                                                                                            with  a
a ground  fault detection  system,  a fire suppression     system,  HVAC     temperature    control
ground fault detection system, a fire suppression system, HVAC temperature control system, the        system,  the
battery management
battery management system
                       system(BMS),
                               (BMS),andandthe
                                             thesite
                                                  sitedispatch
                                                       dispatchcontroller
                                                                  controller(SDC).
                                                                              (SDC).The  BMS
                                                                                       The  BMSis responsible   for
                                                                                                   is responsible
monitoring,
for          controlling,
    monitoring,           and and
                 controlling,  protecting  the battery
                                    protecting           cells, including
                                                 the battery                monitoring
                                                                cells, including         state-of-charge
                                                                                   monitoring               (SOC),
                                                                                                 state-of-charge
(SOC), preventing over-charge/under-charge, and protecting against thermal damage. The PCSa
preventing  over-charge/under-charge,       and protecting     against  thermal  damage.    The  PCS    contains
four-quadrant
contains       Parker-Hannifin
         a four-quadrant         inverter (+/−inverter
                            Parker-Hannifin       real and(+/−
                                                             reactive
                                                                 real power),  cooling
                                                                      and reactive      system,cooling
                                                                                      power),     metering   units,
                                                                                                          system,
processing units,  and associated  protection.
metering units, processing units, and associated protection.




                            (a)                                                        (b)
      Figure
      Figure 1.
             1. Containers
                Containers for
                           for the
                               the BESS
                                   BESS (a)
                                        (a) power
                                            power module
                                                  module (PM);
                                                         (PM); and
                                                               and (b)
                                                                   (b) power
                                                                       power conversion
                                                                             conversion system
                                                                                        system (PCS).
                                                                                               (PCS).
Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
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     The control center of the BESS is the site-dispatch controller housed in the power module, which
performs   the
     Energies    following
              2018,            tasks:
                    11, x FOR PEER REVIEW                                                  4 of 17

1.     Collects  and stores
            The control       allofdata
                         center          on the
                                    the BESS     cloud
                                             is the      server controller housed in the power module, which
                                                    site-dispatch
2.     Utilizes data
       performs       from thetasks:
                 the following     BMS (e.g., SOC and cell temperatures) and the inverter system in the PCS
      to determine
      1.             any
          Collects and   limits
                       stores allon available
                                 data          power
                                      on the cloud server
3.    Processes  the measured    data  to develop
      2. Utilizes data from the BMS (e.g., SOC and power    commands using
                                                     cell temperatures) and thereal-time  controlinalgorithms
                                                                                inverter system     the PCS
4.        to determine
      Executes  manualany   limits on available
                         commands      inputtedpower
                                                 by a user
      3. Processes the measured data to develop power commands using real-time control algorithms
      The SDC coordinates
      4. Executes             the PM and
                    manual commands          PCSby
                                        inputted  toaprovide
                                                      user     the desired amount of real and/or reactive
power at the point of common connection (PCC). It can accept automatic generation control (While the
             The SDC coordinates the PM and PCS to provide the desired amount of real and/or reactive
AGCpowercapability     exists, it was never connected or utilized.) (AGC) commands or operate in manual or
                 at the point of common connection (PCC). It can accept automatic generation control (While
automatic      modes.
      the AGC capability  In manual
                                 exists, itmode,   the connected
                                            was never   operator specifies        the(AGC)
                                                                      or utilized.)    amount      of real/reactive
                                                                                              commands       or operatepower     desired,
                                                                                                                         in manual
whileorinautomatic
            automaticmodes. mode In  themanual
                                           powermode,
                                                    command        is calculated
                                                            the operator             according
                                                                             specifies    the amountone ofof two   control algorithms
                                                                                                              real/reactive  power
(Section    3). Operators
      desired,                   interact mode
                   while in automatic        with the
                                                    the SDC
                                                         powervia     a PC-based
                                                                  command              Humanaccording
                                                                                is calculated      Machineone   Interface
                                                                                                                    of two (HMI)
                                                                                                                            control over
a secure     network
      algorithms          connection.
                      (Section               Theseinteract
                                  3). Operators      interactions
                                                              with the  typically
                                                                          SDC via include:
                                                                                     a PC-based   selection
                                                                                                      Human of    controlInterface
                                                                                                                Machine     algorithms,
      (HMI)parameter
algorithm        over a secure     network
                               settings,       connection.orThese
                                           diagnostics,                interactions
                                                               inputting     of manual typically   include: Manual
                                                                                             commands.        selection commands
                                                                                                                         of control are
oftenalgorithms,
        used to executealgorithm     parameter settings,
                                State-of-Health               diagnostics,
                                                     tests which              or inputting
                                                                      are performed            of manualscheduled
                                                                                           at regularly       commands.intervals.
                                                                                                                            Manual
      commands are often used to execute State-of-Health tests which are performed at regularly
      Figure 2 shows a simplified diagram of the system, indicating the location of the four meters
      scheduled intervals.
used for data collection. There are three Schweitzer SEL-735 m which sample current, voltage, and
             Figure 2 shows a simplified diagram of the system, indicating the location of the four meters
frequency;       one each for the BESS (“Battery”), the wind farm (“Wind Farm”), and combined signals
      used for data collection. There are three Schweitzer SEL-735 m which sample current, voltage, and
(“Totalizer”).
      frequency;    There    is also
                      one each    forathe
                                        Shark
                                            BESS200   m that can
                                                   (“Battery”),   thebe   usedfarm
                                                                        wind     to determine
                                                                                      (“Wind Farm”),the amount      of auxiliary
                                                                                                           and combined     signalspower
that the
      (“Totalizer”). There is also a Shark 200 m that can be used to determine the amount of auxiliary powerto the
            BESS     consumes.       The    meters    sample     at  10  Hz   for  control    purposes.      Data   is recorded
cloudthatserver    at a rate
             the BESS          of 5 HzThe
                          consumes.       for meters
                                              analysis.   Within
                                                       sample        theHz
                                                                 at 10    PM,forbattery
                                                                                  control cells   are organized
                                                                                            purposes.                as follows:
                                                                                                         Data is recorded     to the7 cells
      cloudgroup”,
to a “cell      server at   a rate
                         2 cell     of 5 Hz
                                 groups     to for analysis.and
                                               a module,       Within
                                                                   fourthe   PM, battery
                                                                          modules             cellsreplaceable
                                                                                       to a line     are organizedunitas(LRU,
                                                                                                                         follows:  7
                                                                                                                                Figure   2,
inset).cells  to a “cell
          Voltages     forgroup”,    2 cell groups
                            each group       (7 cells)toare
                                                         a module,
                                                             sampled   andatfour
                                                                             10 s modules
                                                                                   intervals,  to and
                                                                                                  a linetemperatures
                                                                                                          replaceable unitfor(LRU,
                                                                                                                               groups of
      Figure
28 cells         2, inset). consisting
           (2 modules       Voltages forofeach     group
                                              2 cell groups(7 cells)
                                                                each)are   sampled
                                                                         are recorded  at 10 s intervals,
                                                                                           every   minute.  and  temperatures
                                                                                                               These             for with
                                                                                                                       data, along
      groups of 28 cells (2 modules consisting of 2 cell groups each) are recorded every minute. These data,
diagnostics and environmental data within the container, are stored in a cloud server for later analysis.
      along with diagnostics and environmental data within the container, are stored in a cloud server for
7 cells are connected in parallel. 384 of these parallel groups are connected in series.
       later analysis. 7 cells are connected in parallel. 384 of these parallel groups are connected in series.




           Figure
      Figure       2. Schematic
              2. Schematic       of the
                             of the     Hawaiiisland
                                     Hawaii     islandBESS
                                                        BESS highlighting
                                                              highlighting metering
                                                                             meteringunits. TheThe
                                                                                       units.   pictures insets
                                                                                                    pictures    showshow
                                                                                                             insets
           the installed PM  and  PCS   units on site (top), and the line replaceable units (LRUs)  which
      the installed PM and PCS units on site (top), and the line replaceable units (LRUs) which house      house  the the
           battery cells within the  PM  (below).
      battery cells within the PM (below).
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3. Control Algorithms
3. Control Algorithms
     Most of the time, the Hawaii Island BESS operates in the automatic mode. In this mode, one
     Most of the time, the Hawaii Island BESS operates in the automatic mode. In this mode, one of
of two algorithms can be selected, a wind smoothing algorithm or a frequency response algorithm.
two algorithms can be selected, a wind smoothing algorithm or a frequency response algorithm. The
The algorithms were developed by SCADA Solutions, LLC and Integrated Dynamics, Inc. (IDI) on
algorithms were developed by SCADA Solutions, LLC and Integrated Dynamics, Inc. (IDI) on behalf
behalf of the battery manufacturer, Altairnano. These algorithms were developed in close consultation
of the battery manufacturer, Altairnano. These algorithms were developed in close consultation with
with the utility (HELCO) and the HRD wind farm. Both algorithms were developed through a similar
the utility (HELCO) and the HRD wind farm. Both algorithms were developed through a similar
process required by HNEI (Figure 3) which is described below. Process ensures that the algorithms
process required by HNEI (Figure 3) which is described below. Process ensures that the algorithms
were meeting the design and performance objectives.
were meeting the design and performance objectives.
     The development process began by defining a set of requirements and specifications for each
     The development process began by defining a set of requirements and specifications for each
algorithm. Once these were agreed upon by all stakeholders, Altairnano and IDI designed the
algorithm. Once these were agreed upon by all stakeholders, Altairnano and IDI designed the
algorithms and HMI user interface to meet the specifications. Next, simple modeling studies were
algorithms and HMI user interface to meet the specifications. Next, simple modeling studies were
performed to optimize control algorithm parameters and to ensure the predicted behavior of the
performed to optimize control algorithm parameters and to ensure the predicted behavior of the BESS
BESS under each algorithm met the design objectives. After the algorithms passed an acceptance
under each algorithm met the design objectives. After the algorithms passed an acceptance test based
test based on the modeling studies, laboratory testing with computational hardware in-the-loop
on the modeling studies, laboratory testing with computational hardware in-the-loop was executed.
was executed. This step determined whether the algorithms, running on the embedded computers
This step determined whether the algorithms, running on the embedded computers performed as
performed as intended. Lastly, a site acceptance test (SAT) plan was developed and executed during
intended. Lastly, a site acceptance test (SAT) plan was developed and executed during BESS
BESS commissioning
commissioning            to ensure
                  to ensure         thatalgorithms
                              that the   the algorithms    performed
                                                      performed         as intended
                                                                  as intended          in field.
                                                                                  in the   the field. Descriptions
                                                                                                 Descriptions      of
                                                                                                               of the
the two
two      algorithms
     algorithms   andandthethe  results
                             results of of
                                        sitesite acceptance
                                              acceptance    andinitial
                                                          and    initialtesting
                                                                          testingofofthe
                                                                                      thealgorithms
                                                                                          algorithmsareare presented
                                                                                                           presented
in the following   sections.
in the following sections.




          Figure
          Figure 3.
                 3. Flowchart
                    Flowchart of
                              of the
                                 the development
                                     development process
                                                 process for
                                                         for the
                                                             the frequency
                                                                 frequency response control algorithm.
                                                                           response control algorithm.

3.1. Frequency
     FrequencyResponse:
               Response:Algorithm
                        AlgorithmDevelopment
                                  Developmentand
                                              andTesting
                                                  Testing
     Some of the most important results in this section were made possible by the
                                                                              the agreement
                                                                                  agreement between
HNEI and HELCO. As the section progresses, it will become apparent that quantifying the impact of
a frequency response algorithm would be limited without the unique ability of experimentalists to
actively engage in control of the BESS, and hence, the response of the grid. Unlike power smoothing
(including wind smoothing), it is not possible to directly determine how the grid frequency ‘would
have behaved’ if the BESS (running a frequency
                                        frequency response
                                                   response algorithm)
                                                             algorithm) was not present.
                                                                                 present. This would
otherwise complicate the assessment of performance.
     The electric grid on the island of Hawaii operates with a relatively high penetration of non-
dispatchable renewable resources (70 MW of wind and solar on a 180 MW grid in 2013), which can
result in increased frequency fluctuations due to mismatch of load and generation and reduced
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      The electric grid on the island of Hawaii operates with a relatively high penetration of
non-dispatchable renewable resources (70 MW of wind and solar on a 180 MW grid in 2013), which can
Energies 2018, 11, x FOR PEER REVIEW                                                                                6 of 17
result in increased frequency fluctuations due to mismatch of load and generation and reduced
rotational inertia.
rotational    inertia. The The frequency
                                frequency response
                                              response algorithm
                                                          algorithm waswas designed
                                                                             designed toto mitigate
                                                                                             mitigate such
                                                                                                       such frequency
                                                                                                              frequency
deviations     by  sourcing    and sinking   real power    to/from    the
deviations by sourcing and sinking real power to/from the grid as needed. grid as needed.
      The grid
      The    grid frequency
                   frequency isismeasured
                                   measuredby   bytwo
                                                    twometers:
                                                         meters:(a)(a)Shark
                                                                       Shark200 m,m,
                                                                              200   and
                                                                                      and(b)(b)
                                                                                              SEL-735  m (Figure
                                                                                                 SEL-735   m (Figure 2, top
                                                                                                                         2,
left of  Figure    4). The   Shark  meter   proved   to  have  a  significant lag due  to  filtering, so
top left of Figure 4). The Shark meter proved to have a significant lag due to filtering, so the frequencythe frequency
signal is
signal   is obtained
            obtained from
                        from the
                              the SEL-735
                                  SEL-735 m  m in
                                                in the
                                                   the field
                                                       field for
                                                             for control
                                                                  control purposes.
                                                                          purposes. The
                                                                                      The frequency
                                                                                            frequency signal
                                                                                                        signal is
                                                                                                                is passed
                                                                                                                   passed
through     an  outlier   rejection filter to prevent   overly   large  ramp  rates and   a  low  pass
through an outlier rejection filter to prevent overly large ramp rates and a low pass filter to preventfilter to prevent
potential oscillations.
potential    oscillations.




                            Figure 4.
                            Figure    Schematic of
                                   4. Schematic of the
                                                    the frequency
                                                        frequency response
                                                                  response control
                                                                           control algorithm.
                                                                                   algorithm.

        The primary
        The   primary component
                          component for   for adjusting
                                               adjusting thethe frequency
                                                                 frequency response
                                                                                  response algorithm
                                                                                               algorithm is  is the
                                                                                                                the frequency-Watt
                                                                                                                     frequency-Watt
(f-W)     curve.    The   curve     can  be   configured     with    a    deadband
(f-W) curve. The curve can be configured with a deadband of various widths and with      of  various    widths    and    with various
                                                                                                                               various
proportional       gains,   i.e., the slope    of the  f-W   curve    in   MW/Hz        (Figure
proportional gains, i.e., the slope of the f-W curve in MW/Hz (Figure 4, inset). The result of the 4, inset). The   result  of  the f-W
                                                                                                                                    f-W
curve is
curve     is summed
             summed with with aa real
                                   real power
                                        powercomponent
                                                  componenttotomaintain
                                                                      maintainaatarget targetstate-of-charge
                                                                                                state-of-charge(SOC).
                                                                                                                    (SOC).This
                                                                                                                            This sum
                                                                                                                                   sum is
then    limited   by  total  available    power,    with  a further      limitation    based    on
is then limited by total available power, with a further limitation based on BMS and inverter data  BMS    and  inverter  data   before
sending
before       a real power
           sending            command
                      a real power           to the inverter.
                                       command       to the inverter.
        The   frequency      response     algorithm
        The frequency response algorithm was            wasfirst
                                                              firsttested
                                                                      tested    within
                                                                              within       a simple
                                                                                        a simple    gridgrid  model
                                                                                                          model        developed
                                                                                                                   developed          on
                                                                                                                                 on the
the   MATLAB/Simulink               platform.     A  model    of  the     BESS   was     incorporated
MATLAB/Simulink platform. A model of the BESS was incorporated into a simple model of the grid             into  a  simple   model    of
the grid dynamics
dynamics                   [25], inawhich
                [25], in which          singlea generator
                                                  single generator
                                                               model model           represented
                                                                            represented               the cumulative
                                                                                              the cumulative        effecteffect
                                                                                                                            of allofthe
                                                                                                                                      all
the   generators      on  the  grid,   and   which    included      a  wind     generation
generators on the grid, and which included a wind generation input (Figure 5). The unknown      input   (Figure   5).  The  unknown
parameters of
parameters       of the
                     thegrid
                          gridmodel
                                 modelwerewereinertia
                                                  inertiaconstant
                                                           constant  (M),    damping
                                                                          (M),  damping   constant
                                                                                              constant(D),(D),
                                                                                                            governor   gains
                                                                                                                governor       (Kp (Kp
                                                                                                                            gains   and
Ki), Ki),
and    and andgenerator    response
                   generator            time.
                                 response        These
                                              time.      were
                                                     These      estimated
                                                              were    estimated  based    on historical
                                                                                      based    on historicaltelemetry
                                                                                                                telemetryprovided
                                                                                                                            provided  by
HELCO.         The  consequence        of  a  large  number      of   unknowns         is  the
by HELCO. The consequence of a large number of unknowns is the possibility of parameter trades  possibility    of parameter      trades
(i.e., aa particular
(i.e.,    particular setset of
                            of parameters
                                parameters causes
                                                causes the
                                                         the model
                                                              model to    to “fit”
                                                                             “fit” the
                                                                                    the grid
                                                                                         grid frequency
                                                                                               frequency data,
                                                                                                             data, but
                                                                                                                    but this
                                                                                                                         this may
                                                                                                                              may notnot
be  the   physically    correct   set of parameters).      For this    reason,    a  variety  of  reasonable
be the physically correct set of parameters). For this reason, a variety of reasonable parameter sets           parameter    sets  were
used during
were              testing.
         used during         This, in
                          testing.     turn,inprovided
                                     This,                 a variety
                                                turn, provided            of possible
                                                                     a variety           predicted
                                                                                    of possible        behaviors.
                                                                                                    predicted        The full range
                                                                                                                 behaviors.   The full of
resultsofindicated
range                    that the that
             results indicated       BESSthewould
                                                BESSnot    worsen
                                                       would            the gridthe
                                                                not worsen           frequency.     Model runs
                                                                                        grid frequency.      Model  using
                                                                                                                      runsthe
                                                                                                                            usingsetsthe
                                                                                                                                      of
telemetry-estimated          parameters      showed     that  the   frequency       response
sets of telemetry-estimated parameters showed that the frequency response algorithm would        algorithm    would     improve    both
the transient
improve       bothresponse      of theresponse
                     the transient      grid to disturbances
                                                    of the grid and         the steady-state
                                                                    to disturbances         and variability     in gridvariability
                                                                                                   the steady-state      frequency.in
grid frequency.
     During the site acceptance test, the grid frequency was observed for several 6–8 Hr periods with
the BESS off, and several 6–8 Hr periods with the BESS on. The results of the test showed that the
BESS did not degrade frequency variability, but there was also no statistically significant evidence
that the BESS reduced frequency variability with the proportional gain at 10 MW/Hz. Subsequent
Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
testing by HNEI revealed that a statistically significant reduction in grid frequency variability could
be achieved by increasing the proportional gain (slope of the f-W curve) to 30 MW/Hz or more. Also,
because grid conditions and generator dispatch change over a shorter time frame, it was preferable
to make observations over shorter time periods rather than 6–8 Hr windows. A new protocol was
therefore    developed for testing the BESS under the frequency response control algorithm,
Energies 2018, 11, 3367
                                                                                                       as
                                                                                                   7 of 17
described in the following section.




     Figure 5.
     Figure    Schematic of
            5. Schematic  of the
                             the simple
                                 simple model
                                        model of
                                              of the
                                                 the Hawaii
                                                     Hawaii island
                                                            island grid,
                                                                   grid, which
                                                                         which was
                                                                               was used
                                                                                   used to
                                                                                        to develop
                                                                                           develop the
                                                                                                   the
     frequency  response algorithm.
     frequency response algorithm.

     During the site acceptance test, the grid frequency was observed for several 6–8 Hr periods with
     In order to test the effect of the BESS on grid frequency during field operations, HNEI designed,
the BESS off, and several 6–8 Hr periods with the BESS on. The results of the test showed that the BESS
and HELCO approved a test protocol to mitigate the effects of time varying background grid
did not degrade frequency variability, but there was also no statistically significant evidence that the
conditions. The protocol called for switching the BESS ON and OFF every 20 min allowing the
BESS reduced frequency variability with the proportional gain at 10 MW/Hz. Subsequent testing by
frequency variability of consecutive OFF/ON periods to be compared to assess the impact of the BESS.
HNEI revealed that a statistically significant reduction in grid frequency variability could be achieved
A time period of 20 min was chosen for each OFF and ON period in order to balance sufficient data
by increasing the proportional gain (slope of the f-W curve) to 30 MW/Hz or more. Also, because
collection against the changing background grid conditions.
grid conditions and generator dispatch change over a shorter time frame, it was preferable to make
     The “switching experiments”, typically 200 min in duration, were initially performed only in the
observations over shorter time periods rather than 6–8 Hr windows. A new protocol was therefore
evening hours to isolate the frequency variability due to wind fluctuations. Examples of two evening
developed for testing the BESS under the frequency response control algorithm, as described in the
experiments are shown in Figure 6. One experiment was conducted during an evening with light
following section.
winds (Figure 6a) and another was conducted during an evening with medium winds (Figure 6b). In
     In order to test the effect of the BESS on grid frequency during field operations, HNEI designed,
both, the red oscillating lines in Figure 6 (top) indicate when the battery was on showing real power
and HELCO approved a test protocol to mitigate the effects of time varying background grid conditions.
output of the BESS. The center plots show wind power output at the 10.6 MW Haw’i wind farm. The
The protocol called for switching the BESS ON and OFF every 20 min allowing the frequency variability
bottom plots display the grid frequency during the experiments. Comparing contiguous red and
of consecutive OFF/ON periods to be compared to assess the impact of the BESS. A time period of
black sections in the bottom plots, it is apparent that there was a reduction in grid frequency
20 min was chosen for each OFF and ON period in order to balance sufficient data collection against
variability when the BESS was ON (red) compared to when the BESS was OFF (black).
the changing background grid conditions.
     The “switching experiments”, typically 200 min in duration, were initially performed only in the
evening hours to isolate the frequency variability due to wind fluctuations. Examples of two evening
experiments are shown in Figure 6. One experiment was conducted during an evening with light
winds (Figure 6a) and another was conducted during an evening with medium winds (Figure 6b).
In both, the red oscillating lines in Figure 6 (top) indicate when the battery was on showing real power
output of the BESS. The center plots show wind power output at the 10.6 MW Haw’i wind farm. The
bottom plots display the grid frequency during the experiments. Comparing contiguous red and black
sections in the bottom plots, it is apparent that there was a reduction in grid frequency variability
when the BESS was ON (red) compared to when the BESS was OFF (black).
Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
Energies 2018,
  Energies     11,11,
           2018,   3367
                      x FOR PEER REVIEW                                                                                  8 8ofof
                                                                                                                              17 17
 Energies 2018, 11, x FOR PEER REVIEW                                                                                  8 of 17




                                 (a)                                                           (b)
                               (a)                                                           (b)
         Figure 6. Top: BESS power output with a gain of 30MW/Hz; Middle: wind power generation; Bottom:
       Figure6.6.Top:
                  Top:BESS
                        BESS power
                             power output
                                       output with
                                               with aa gain
                                                        gain of 30MW/Hz;      Middle: wind  power  generation; Bottom:
      Figure
         and grid frequency   on an evening     with lightof    30MW/Hz;
                                                              winds   (a) andMiddle:    wind
                                                                                an evening    power
                                                                                           with     generation;
                                                                                                medium   winds Bottom:
                                                                                                                (b). Black
       and
      and    grid
            grid  frequency
                 frequency   on
                            on anan   evening
                                     evening   with
                                              with   light
                                                    light   winds
                                                            winds  (a)
                                                                   (a) and
                                                                       and   an
                                                                             an  evening
                                                                                 evening with
                                                                                         with medium
                                                                                               medium winds
                                                                                                       winds (b).
                                                                                                              (b).Black
                                                                                                                   Black
         lines indicate the BESS is OFF and red lines indicate BESS is ON. The BESS was switched between
       lines
      lines   indicate
             indicate  the
                      the  BESS
                          BESS  is is OFF
                                    OFF    and
                                         and    red
                                              red     lines
                                                  lines      indicate
                                                          indicate BESSBESS
                                                                          is  is
                                                                             ON.  ON.
                                                                                   TheThe BESS
                                                                                       BESS was was switched
                                                                                                 switched     between
                                                                                                          between   OFF
         OFF and ON every 20 min.
       OFFON
      and     andevery
                  ON every   20 min.
                       20 min.
        Inorder
           ordertotoquantify
      Inorder
                       quantifythe thereduction
                                        reductioninin     grid   frequency variability,
                                                       grid frequency
                                                              frequency variability,
                                                                                            the standard   deviation of grid
     In           to quantify the     reduction in    grid                  variability, the
                                                                                          the standard
                                                                                              standarddeviation
                                                                                                         deviationofofgridgrid
  frequencywas
 frequency       wascalculated
                      calculatedfor foreach
                                        each20-min
                                               20-min     period.
                                                      period.     ToTo verify    that this  metric captured   grid frequency
frequency    was calculated     for each     20-min period.       To verify
                                                                      verify that
                                                                              that this
                                                                                    this metric
                                                                                         metriccaptured
                                                                                                 capturedgrid
                                                                                                            gridfrequency
                                                                                                                  frequency
  variabilityinina abelievable
 variability          believableway, way,5 5sample
                                              sampletime time   series
                                                             series   of of grid  frequency were     ordered by     eye (left    to
variability   in a believable way,       5 sample time       series   of grid
                                                                          grid frequency
                                                                                frequency were
                                                                                             were ordered
                                                                                                   orderedby byeye
                                                                                                                 eye(left
                                                                                                                       (lefttoto
  right)
 right)by by
        byaafewa few  researchers
                fewresearchers        (Figure
                     researchers (Figure
                                    (Figure 7). 7). The
                                              7). The      ordering    was   then   compared    to the  values  of  the  metric.
right)                                            The ordering
                                                        ordering was was then
                                                                           then compared
                                                                                  comparedtotothethevalues
                                                                                                     valuesofofthe
                                                                                                                 themetric.
                                                                                                                       metric.
  The  standard     deviations    (in  mHz)    were   7.1,  11.2,  14.2,  17.6, and  24.3  respectively  from  left  to right   in
The standard deviations (in mHz) were 7.1, 11.2, 14.2, 17.6, and 24.3 respectively from left to rightinin
 The  standard     deviations  (in   mHz)    were   7.1,  11.2, 14.2,  17.6, and   24.3 respectively  from  left to  right
  Figure   7, corresponding
 Figure7,7,corresponding
            correspondingto      to
                              tothe  the  visual
                                   thevisual      ordering
                                        visual ordering
                                                ordering of     of
                                                             of thethe  frequency
                                                                the frequency        variability.
                                                                     frequency variability.
                                                                                   variability.
Figure




            Figure
            Figure 7.
              Figure  Five
                   7.7.
                      Five 20-min
                        Five      frequency
                             20-min         time
                                    frequency    series
                                                 series
                                              time      shown
                                                        shown
                                                   series      inin
                                                              in
                                                          shown   order
                                                                 order  of of
                                                                        of
                                                                    order   increasing
                                                                           increasing  by-eye
                                                                                      by-eye
                                                                              increasing      variability.
                                                                                             variability.
                                                                                         by-eye variability.

     Table
      Table22shows
       Table 2shows
                shows the
                       the
                         thestandard
                             standard
                               standard deviations
                                         deviations
                                           deviations and  percent
                                                          percent
                                                        and  percent  changes
                                                                      changes
                                                                        changes  for
                                                                                for  the
                                                                                    the
                                                                                   for     data
                                                                                         data
                                                                                        the  data presented
                                                                                                 presented
                                                                                                    presented  ininFigure
                                                                                                                   inFigure 6.6.6.
                                                                                                                       Figure
Each
 Each OFF/ON
       OFF/ON    pair is  referred   to as  an  “interval”.
                                                “interval”.  The
                                                            The    standard
                                                                   standard    deviations
                                                                             deviations       for
                                                                                            for    the
                                                                                                  the
  Each OFF/ON pair is referred to as an “interval”. The standard deviations for the 5 intervals for the55 intervals
                                                                                                         intervals     for
                                                                                                                      for  the
                                                                                                                          the
evening
 evening
  eveningwith
         with
           withlow
                low
                  lowwind
                     wind
                       wind  is  shown
                              isisshown
                                   shown  in the
                                          inin
                                             thetheupper
                                                    upperpart
                                                            partofofthe table,
                                                                        table,
                                                                      the       and
                                                                               and
                                                                          table,  and the
                                                                                     the  55 intervals
                                                                                        the   intervals
                                                                                              5 intervals  forfor
                                                                                                          for   thethe
                                                                                                               the   evening
                                                                                                                    evening
                                                                                                                        evening
with
 with medium
      medium    winds
                 winds   is
                          is shown
                              shown   in
                                       in the
                                           the bottom.
                                                bottom.  The
                                                         The   mean
                                                               mean     percent
                                                                       percent    change
                                                                                 change
  with medium winds is shown in the bottom. The mean percent change for the light wind case for
                                                                                           for    the
                                                                                                 the   light
                                                                                                      light    wind
                                                                                                              wind     case
                                                                                                                      case  isis is
−−15.4%,
  15.4%, with  the  negative     indicating
                                indicating   aa  reduction
                                                reduction   of
                                                           of   frequency
                                                               frequency    variability
                                                                           variability     by
                                                                                          by    15.4%
                                                                                               15.4%
  −15.4%, with the negative indicating a reduction of frequency variability by 15.4% when the BESS       when
                                                                                                       when      the
                                                                                                                the   BESS
                                                                                                                     BESS   isis is
Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
Energies 2018, 11, x FOR PEER REVIEW                                                                                      9 of 17


ON. The mean percent change for the medium wind case is −37.1%, indicating that with increasing
grid frequency variability, the BESS has a stronger proportional effect.
      Although
 Energies            the above approach is a useful metric for assessing how the overall frequency
          2018, 11, 3367                                                                                                     9 of 17
variability is reduced, it does not provide any information on how well the frequency variability is
reduced on short timescales, and therefore does not optimally measure the effect of using a fast-acting
 ON. TheIf mean
battery.              percent of
              a timescales      change
                                    severalforminutes
                                               the medium       windwere
                                                          or longer      case is −37.1%,
                                                                               chosen,       indicating
                                                                                          then   the gridthat    with increasing
                                                                                                            frequency     control
 grid  frequency      variability,   the BESS    has  a stronger    proportional      effect.
would be dominated by generator governors, and one is not measuring the effectiveness of the BESS.
       Although
For this   reason,the     above
                      it was      approach
                              decided     thatisthe
                                                  a useful
                                                      20-minmetric    for assessing
                                                                periods    would behow      the overall
                                                                                        divided           frequency
                                                                                                   into several         variability
                                                                                                                   smaller   non-
 is reduced,    it does   not  provide     any   information     on   how   well  the  frequency
overlapping segments, and the standard deviation of each of the smaller segments taken. The mean     variability   is reduced   on
 short   timescales,     and  therefore     does   not  optimally      measure    the  effect  of
of those standard deviations is then determined and used to provide a single metric characterizing using  a  fast-acting   battery.
 If awhole
the   timescales
              20-min of several
                         period. minutes
                                    The length or longer
                                                    of the were
                                                            smaller chosen,   then is
                                                                        segments     thereferred
                                                                                          grid frequency
                                                                                                    to as thecontrol    would
                                                                                                                timescale.   Thisbe
 dominated      by   generator     governors,     and  one   is not   measuring     the  effectiveness
process is illustrated by example in Figure 8. The grid frequency data is separated into smaller 60 s     of  the BESS.   For  this
 reason, it was
segments.    The decided
                    standardthat     the 20-min
                                 deviations         periods
                                                of those   60would      be divided
                                                               s segments             into several
                                                                              are shown               smaller
                                                                                             just above   the non-overlapping
                                                                                                                plot. The mean
 segments,     and    the  standard     deviation     of  each   of  the  smaller    segments
of those values was calculated to be 7.5 mHz and represents the variability for the entire 20-min taken.   The    meanwindow.
                                                                                                                         of those
 standard deviations is then determined and used to provide a single metric characterizing the whole
 20-min
      Tableperiod.     The length
             2. The standard           of theofsmaller
                                  deviation               segments
                                                  the 20-min   intervalsis with
                                                                           referred
                                                                                BESS toOFFas versus
                                                                                              the timescale.     This process
                                                                                                      20-min periods    with      is
 illustrated
      BESS ON,  byalong
                    examplewith in
                                 theFigure
                                      percent8.change
                                                 The grid   frequency
                                                        between            data
                                                                   intervals, foristhe
                                                                                    separated
                                                                                       switchinginto    smaller 60
                                                                                                   experiments        s segments.
                                                                                                                  shown   in
 The Figure
       standard
              6. A:deviations      of those
                     Results for the   evening 60with
                                                   s segments     areB:shown
                                                       light winds;       Resultsjust  above
                                                                                  for an        the with
                                                                                          evening    plot.medium
                                                                                                            The mean      of those
                                                                                                                     winds.
 values was calculated to be 7.5 mHz and represents the variability for the entire 20-min window.
                                  A: Frequency Variability: Evening with Light Wind
          Interval             Standard Deviation of Standard Deviation of                     Percent Change
       Table 2. The standard deviation of the 20-min intervals with BESS OFF versus 20-min periods with
                               Frequency     w/  BESS  OFF    Frequency   w/  BESS  ON
       BESS ON, along with the percent change between intervals, for the switching experiments shown in
                                          [mHz]
       Figure 6. A: Results for the evening                            [mHz]
                                              with light winds; B: Results  for an evening with medium winds.
              1                            13.4                          8.3                         −38.1
              2                      A: Frequency
                                           10.3 Variability: Evening with6.0
                                                                          Light Wind                 −41.7
              3 Interval          Standard 9.0
                                             Deviation of          Standard
                                                                         7.7Deviation of      Percent−14.4
                                                                                                      Change
                              Frequency w/BESS OFF [mHz]       Frequency w/BESS ON [mHz]
              4                             7.1                          6.7                         −5.6
                    1                       13.4                            8.3                    −38.1
              5 2                           6.6
                                            10.3                         8.16.0                      22.7
                                                                                                   −41.7
                   3            B: Frequency9.0
                                             Variability: Evening with Medium
                                                                        7.7   Wind                         −14.4
                   4                           7.1                              6.7                         −5.6
           Interval
                   5
                                  Standard     Deviation
                                               6.6
                                                            of    Standard     Deviation
                                                                                8.1
                                                                                             of         Percent Change
                                                                                                            22.7
                                  Frequency w/ BESS OFF           Frequency w/ BESS ON
                                      B: Frequency Variability: Evening with Medium Wind
                                            [mHz]                          [mHz]
                                     Standard Deviation of             Standard Deviation of
               1 Interval                    15.9 OFF [mHz]
                                 Frequency w/BESS                            8.6
                                                                   Frequency w/BESS  ON [mHz]
                                                                                                       Percent−45.9
                                                                                                               Change

               2 1                           13.5
                                              15.9                           7.98.6                          −41.5
                                                                                                            −45.9
               3 2                            13.5
                                             14.3                            8.87.9                         −41.5
                                                                                                             −38.5
                     3                        14.3                              8.8                         −38.5
               4 4                           12.6
                                              12.6                           7.27.2                          −42.9
                                                                                                            −42.9
               5 5                           11.4
                                              11.4                           9.59.5                          −16.7
                                                                                                            −16.7




       Figure 8. A frequency time-series is divided into several smaller segments, in this case 60-seconds in
       length. The standard deviation of each smaller time series is shown as just above the plot. The mean of
       those values is taken to be the “frequency variability” of the 20-min period on a 60-seconds timescale.
Evaluation of a 1 MW, 250 kW-hr Battery Energy Storage - MDPI
Energies 2018, 11, x FOR PEER REVIEW                                                                                  10 of 17


     Figure 8. A frequency time-series is divided into several smaller segments, in this case 60-seconds in
     length. The standard deviation of each smaller time series is shown as just above the plot. The mean
     of those values is taken to be the “frequency variability” of the 20-min period on a 60-seconds
Energies 2018, 11, 3367                                                                                   10 of 17
     timescale.

      Timescales
      Timescales thatthatare
                           aretoo
                                tooshort
                                      shortpresent
                                             presentproblems
                                                       problems as as well.
                                                                      well.Figure
                                                                             Figure9,9,shows
                                                                                        showsthethemean
                                                                                                    meanof   ofthe
                                                                                                                thestandard
                                                                                                                    standard
deviations
 deviations over a number of timescales and for the same samples as shown in Figure 7,the
             over   a number      of timescales     and  for the same  samples    as shown    in Figure   7, with      same
                                                                                                                    with  the
colors  (e.g., Sample      1 is  blue   in  both   figures).  At  timescales   from   seconds    to
 same colors (e.g., Sample 1 is blue in both figures). At timescales from seconds to tens of seconds,tens  of  seconds,  the
measured    standard
 the measured            deviation
                   standard            depends
                                deviation          on theon
                                              depends      chosen   time scale,
                                                              the chosen    timeso  one so
                                                                                  scale,  does
                                                                                             onenot  have
                                                                                                  does     a stable
                                                                                                         not   have ametric.
                                                                                                                       stable
Moreover,     the shortest
 metric. Moreover,             timescales
                        the shortest          do notdoshow
                                        timescales              goodgood
                                                          not show     separation    between
                                                                            separation    betweenthethe
                                                                                                      high
                                                                                                        high andandlow
                                                                                                                    lowgrid
                                                                                                                         grid
variability  periods.    At  timescales      of  approximately     one  minute    and   longer,
 variability periods. At timescales of approximately one minute and longer, the measured standardthe  measured     standard
deviation
 deviationvalues
             valueslevel
                       leveloff
                              offtowards
                                   towardsa near-constant
                                                a near-constant  value; in in
                                                                    value;  other  words,
                                                                               other  words,thethe
                                                                                                measured
                                                                                                    measured  values  are no
                                                                                                                  values  are
longer   timescale     dependent.       Moreover,      the  separation   between     more    and
 no longer timescale dependent. Moreover, the separation between more and less variable periods    less variable     periods
increases
 increasesupupto tothe
                    theone-minute
                          one-minutetimescale,
                                          timescale,and andthen
                                                             thenlevels
                                                                   levelsoff
                                                                          offasas well.
                                                                                  well. The
                                                                                         Theone-minute
                                                                                              one-minutetimescale
                                                                                                              timescalewas
                                                                                                                         was
therefore
 therefore chosen to balance the need to capture the quick BESS response with stable metricthat
           chosen     to balance     the  need   to  capture  the  quick  BESS   response    with  stable  metric    thatcan
                                                                                                                          can
distinguish    high  a  low  variability
 distinguish high a low variability periods.periods.




      Figure
       Figure 9.9. Frequency
                    Frequencyvariability
                              variabilitymetrics
                                         metricsfor
                                                 fordata
                                                     datashown
                                                          shownin
                                                                in Figure
                                                                   Figure 7,
                                                                          7, using
                                                                             using different
                                                                                   different timescales.
                                                                                             timescales. The
                                                                                                         The
      colors
       colorscorrespond
              correspondto tothe
                              thesamples
                                  samplesshown
                                          shownininFigure
                                                    Figure 7.
                                                           7.

      With
       Withthese
              theseresults,
                      results,the
                                thechosen
                                     chosen metric  to to
                                               metric  characterize
                                                           characterize gridgrid
                                                                               frequency
                                                                                   frequency (whether    the BESS
                                                                                                  (whether            was ON
                                                                                                               the BESS     was
or  OFF)  was   the  mean   of  consecutive    60-seconds    standard     deviations.    With
 ON or OFF) was the mean of consecutive 60-seconds standard deviations. With this metric, it is  this metric,   it is possible
to  study the
 possible       BESS the
            to study   performance      under various
                           BESS performance       under grid    conditions
                                                          various               and control
                                                                    grid conditions      and algorithm       settings.settings.
                                                                                                control algorithm        Initial
results
 Initial of  switching
         results         experiments
                  of switching            are shown
                                   experiments     arein  Figurein10,
                                                       shown            which
                                                                    Figure    10,shows
                                                                                   which40-min
                                                                                           showstime40-minintervals    of BESS
                                                                                                               time intervals
real  power
 of BESS      output
           real  power (top),
                         outputHRD    wind
                                   (top),    power
                                           HRD   windoutput
                                                        power (2nd   from(2nd
                                                                 output      top),from
                                                                                   grid top),
                                                                                         frequency     (2nd from(2nd
                                                                                                grid frequency        bottom),
                                                                                                                           from
and   the running
 bottom),              standard
            and the running         deviation
                                standard        (60-seconds
                                            deviation           timescale)
                                                       (60-seconds              of grid
                                                                       timescale)         frequency
                                                                                     of grid  frequency (bottom),
                                                                                                           (bottom),for fortwo
                                                                                                                            two
switching     experiment performed
 switching experiment        performedunder  under   similar
                                                  similar  gridgrid     conditions,
                                                                 conditions,            butdifferent
                                                                                 but with      with different
                                                                                                        frequency   frequency
                                                                                                                      response
response     control algorithm
 control algorithm                  settings. Specifically,
                       settings. Specifically,   the left side the  leftfigure
                                                                of the     side shows
                                                                                 of the anfigure   shows an
                                                                                              experiment         experiment
                                                                                                             conducted     with
conducted
 the full 1 MWwith BESS
                    the full 1 MWoutput.
                           power      BESS power     output. Theon
                                               The experiment       experiment
                                                                         the right on   theofright
                                                                                     side           side ofwas
                                                                                               the figure     the figure   was
                                                                                                                    conducted
conducted     with the
 with the power          power
                     output       output
                              limited      limited
                                        to 300  kW.toThe300winds
                                                            kW. The were winds    werehigh
                                                                            similarly    similarly    high and
                                                                                               and variable     invariable    in
                                                                                                                    both cases.
both   cases. The bottom-left
 The bottom-left     plot clearlyplot   clearly
                                    shows   thatshows
                                                 the BESSthatOFF
                                                              the BESS
                                                                   periods OFF   periods
                                                                              (black)   have(black)
                                                                                               largerhave   largerdeviations
                                                                                                       standard      standard
deviations
 that the BESSthatONtheperiods
                         BESS ON (red).periods
                                         This is (red).  This is in
                                                 less apparent    less
                                                                     theapparent
                                                                           plot on thein bottom-right
                                                                                         the plot on the        bottom-right
                                                                                                           because    the BESS
because
 is beingthe   BESS istobeing
           restricted     300 kW.restricted to 300 kW.
Energies 2018, 11, 3367                                                                                          11 of 17
   Energies 2018, 11, x FOR PEER REVIEW                                                                       11 of 17




                              (a)                                                      (b)
         Figure
      Figure  10.10.
                  AA   comparisonofoftwo
                     comparison       twodifferent
                                           differentdays
                                                    days when
                                                         when total
                                                              total wind
                                                                    wind power
                                                                          poweroutput
                                                                                  outputatatHaw’i
                                                                                             Haw’iRenewable
                                                                                                    Renewable
         Development (HRD) and variability were similar. The gain setting used for the f-W curve of the BESS
      Development (HRD) and variability were similar. The gain setting used for the f-W curve of the BESS
         control algorithm was 30MW/Hz in both cases. The only difference is the limiting of the BESS output
      control algorithm was 30MW/Hz in both cases. The only difference is the limiting of the BESS output
         power, set to 1 MW for the left plots and 300 kW for the right plots. (Top) Time series of BESS real
      power, set to 1 MW for the left plots and 300 kW for the right plots. (Top) Time series of BESS real
         power output. (2nd from top) Time series of wind power at the HRD wind farm. (2nd from bottom)
      power output. (2nd from top) Time series of wind power at the HRD wind farm. (2nd from bottom)
         Time series of grid frequency. (bottom) The 60-seconds standard deviation of grid frequency.
      Time series of grid frequency. (bottom) The 60-seconds standard deviation of grid frequency.
        Table3 3lists
      Table         liststhe
                           the60-seconds
                                60-seconds frequency
                                             frequency variability
                                                        variability metrics
                                                                    metrics associated
                                                                              associatedwith
                                                                                           withthe
                                                                                                 theswitching
                                                                                                      switching
   experiments    shown     in Figure 10. The average percent variability reduction for
experiments shown in Figure 10. The average percent variability reduction for the 1 MW  the 1 MW  case (left)
                                                                                                    case      is is
                                                                                                          (left)
   58.4 ± 15% with a 95% confidence interval, and the average percent reduction for the 300-kW case
58.4 ± 15% with a 95% confidence interval, and the average percent reduction for the 300-kW case
   (right) is 20.9 ± 11% with a 95% confidence interval. The indication is that limiting the BESS output to
(right) is 20.9 ± 11% with a 95% confidence interval. The indication is that limiting the BESS output
   30% of rated power yields about a linear 30% reduction in frequency variability for similar wind
to 30% of rated power yields about a linear 30% reduction in frequency variability for similar wind
   conditions. The linear relationship was later confirmed through experiments designed to examine of
conditions. The linear relationship was later confirmed through experiments designed to examine
   the effect of various frequency response control algorithm parameter settings on BESS frequency
of the effect of various frequency response control algorithm parameter settings on BESS frequency
   regulation and usage [20].
regulation and usage [20].
        Table 3. The 60-seconds frequency variability metric for the 20-min periods with BESS OFF against
      Table 3. The 60-seconds frequency variability metric for the 20-min periods with BESS OFF against
        adjacent 20-min periods with BESS ON show in Figure 10. A: Results for the evening with high winds
      adjacent 20-min periods with BESS ON show in Figure 10. A: Results for the evening with high winds
        and the BESS allowed to use full rated power of 1 MW; B: Results for the evening with similar high
      and the BESS
        winds       allowed
               and the         to use full
                         BESS allowed       rated
                                        to use 300power
                                                     kW of of 1 MW; B: Results for the evening with similar high
                                                            power.
      winds and the BESS allowed to use 300 kW of power.
                                  A: 60-Second Frequency Variability: 1000 kW Limit
             Interval                A: 60-Second
                                    Frequency       Frequency
                                                 Metric  w/ Variability:
                                                                   Frequency1000 kW    Limit
                                                                                   Metric w/         Percent Change
                                         Frequency
                                      BESS   OFF     Metric
                                                   [mHz]            Frequency
                                                                     BESS     ON  Metric
                                                                                    [mHz]
                      Interval                                                               Percent Change
                                       w/BESS OFF [mHz]            w/BESS ON [mHz]
                 1                            12.9                              7.4                       −74.3
                          1                    12.9                          7.4                  −74.3
                 2                            10.7                              7.2                       −48.6
                          2                    10.7                          7.2                  −48.6
                 3        3                   11.2
                                               11.2                             6.7
                                                                              6.7                 −67.2 −67.2
                 4        4                    10.9
                                              10.9                            7.0
                                                                                7.0               −55.7 −55.7
                          5                    10.8                           7.4                 −45.9
                 5                            10.8                              7.4                       −45.9
                                     B: 60-Second   Frequency  Variability: 300
                                  B: 60-Second Frequency Variability: 300 kW LimitkW  Limit

             IntervalInterval            Frequency
                                    Frequency        Metric
                                                 Metric  w/         Frequency Metric
                                                                   Frequency       Metric w/ Percent Percent
                                                                                                     Change Change
                                       w/BESS OFF [mHz]            w/BESS ON [mHz]
                                      BESS OFF [mHz]                 BESS ON [mHz]
                          1                     9.2                          7.0                  −23.9
                 1        2
                                              9.2
                                               11.5                          8.5
                                                                                7.0               −26.1
                                                                                                          −23.9
                 2        3                   11.5
                                               12.7                             8.5
                                                                              8.9                 −29.9 −26.1
                 3        4                    11.2
                                              12.7                           10.5
                                                                                8.9               −6.3 −29.9
                          5                  12.1                       9.9                  −18.2
Energies 2018, 11, x FOR PEER REVIEW                                                                                12 of 17

Energies 2018, 11, 3367                                                                                              12 of 17
               4                             11.2                           10.5                          −6.3
               5                             12.1                            9.9                          −18.2
3.2. Wind Smoothing: Algorithm Development and Testing
3.2. Wind Smoothing: Algorithm Development and Testing
      In contrast with the frequency response algorithm, where switching experiments were imperative,
only In    contrastpassive
       standard       with the    frequencywere
                             observations       response
                                                    required algorithm,    where the
                                                                 to characterize      switching    experiments
                                                                                          performance              were
                                                                                                          of the wind
imperative,     only standard   passive   observations   were   required   to  characterize   the
smoothing algorithm. This section is not intended to show novelty but was included in this report  performance    of the
wind    smoothing algorithm. This section is not intended to show novelty but was included in this
for completeness.
reportTheforHRD
             completeness.
                   wind farm that came online in 2006, consists of 16 Vestas American Wind Technology
      The   HRD    wind
660 kW wind turbines,    farm  that
                             for     camerated
                                 a total    online in 2006,ofconsists
                                                 capacity       10.56 MWof 16[26].
                                                                              Vestas   American
                                                                                     This wind plantWind   Technology
                                                                                                         and  purchase
660  kW    wind   turbines,  for a  total  rated capacity    of 10.56  MW    [26].   This wind
power agreement pre-dated the suite of modern wind plant capabilities available today, which      plant  and  purchase
                                                                                                                include
power     agreement     pre-dated   the  suite  of modern      wind   plant   capabilities   available
frequency response; primary frequency response would leverage the capabilities of the wind turbine       today,  which
include
controls frequency
           to respondresponse;    primary
                        naturally to         frequency
                                      negative           response impacts
                                                system frequency     would leverage      the capabilities
                                                                               from its volatility.        of the wind
                                                                                                     The requirements
turbine   controls   to respond   naturally   to negative    system   frequency     impacts
for the wind power smoothing algorithm were developed based on the Power Purchasing Agreement from  its volatility. The
requirements      for  the  wind    power    smoothing      algorithm    were     developed    based
(PPA) between the utility (HELCO) and the wind farm owner, which includes three limitations on the      on  the  Power
Purchasing
fluctuationsAgreement
                of the wind(PPA)    between
                             farm power        the utility
                                             output.       (HELCO)
                                                      Specifically,    and
                                                                     the PPAthestipulates
                                                                                  wind farm   owner, which includes
                                                                                            that:
three limitations on the fluctuations of the wind farm power output. Specifically, the PPA stipulates that:
•    the wind farm’s power output may not change more than 1 MW over any two seconds interval,
•     the wind farm’s power output may not change more than 1 MW over any two seconds interval,
•    the wind farm’s power output may not change more than 2 MW over any one-minute interval,
•     the wind farm’s power output may not change more than 2 MW over any one-minute interval,
•
•    the
      the one-minute
           one-minute average
                         average of   power output
                                   of power    output changes
                                                       changes over
                                                                  over any
                                                                        any 22 ss intervals
                                                                                  intervals may
                                                                                             may not
                                                                                                   not exceed
                                                                                                       exceed 300
                                                                                                               300 kW.
                                                                                                                    kW.

      A simplified
      A  simplified block
                       blockdiagram
                                 diagramofofthethe  wind
                                                  wind     power
                                                        power      smoothing
                                                                smoothing         algorithm
                                                                               algorithm         is shown
                                                                                            is shown          in Figure
                                                                                                         in Figure  11. The11.
The  algorithm    was   designed     to minimize    PPA  agreement    violations,   as well
algorithm was designed to minimize PPA agreement violations, as well as minimize the standard as  minimize    the standard
deviation of
deviation  of power
                power output
                         output over
                                   over the
                                         the 2-seconds
                                              2-seconds and
                                                          and one-minute
                                                               one-minute intervals.
                                                                              intervals. InIn the
                                                                                              the most
                                                                                                   most basic
                                                                                                           basic terms,
                                                                                                                 terms, the
                                                                                                                          the
algorithm   calculates     a  “target”  power    output  from  the wind   farm    based    on  the
algorithm calculates a “target” power output from the wind farm based on the filtered values of the filtered  values   of the
“raw”
“raw”windwindfarmfarmoutput
                         output overover
                                      an adjustable   periodperiod
                                          an adjustable      of time.ofAtime.
                                                                         standard     proportional-integral-derivative
                                                                                 A standard       proportional-integral-
(PID)  controller   then    calculates   the  desired  BESS   power    command
derivative (PID) controller then calculates the desired BESS power command to offsetto  offset  the   difference   between
                                                                                                             the difference
the actual   wind   farm     output    and  that target  value.  All of the   parameters      were
between the actual wind farm output and that target value. All of the parameters were first tuned    first tuned   to avoidto
oscillations
avoid          and then
       oscillations          optimized
                      and then             using
                                   optimized       previous
                                                using         SCADA
                                                       previous   SCADA  data
                                                                            dataof of
                                                                                    HRD
                                                                                      HRDwind windfarmfarm power    output.
                                                                                                            power output.
Additionally,
Additionally, thethe algorithm
                      algorithm was  was designed
                                          designed to to return
                                                         return the
                                                                 the BESS
                                                                     BESS toto the
                                                                                the nominal
                                                                                     nominal 50% 50% state
                                                                                                        state of
                                                                                                              of charge
                                                                                                                 charge by by
calculating   the  battery     “bias”,  which   is defined  as the amount      of power     needed
calculating the battery “bias”, which is defined as the amount of power needed to return the BESS to   to return  the  BESS
to 50%
50%   SOCSOC   from
            from   itsits currentSOC
                        current      SOCover
                                           over1515min.
                                                     min.Because
                                                           Becausethe
                                                                    thebattery
                                                                         batterybias
                                                                                   bias is
                                                                                         is continuously
                                                                                            continuously updated,
                                                                                                              updated, thethe
associated   bias  power     commands      exponentially   approach    zero as  the  battery
associated bias power commands exponentially approach zero as the battery nears 50% SOC, with  nears   50%   SOC,  with   the
result
the    being
    result     thatthat
           being    the SOC
                          the SOCmanagement
                                      management could  taketake
                                                      could  the battery   fromfrom
                                                                  the battery     a fulla charge   or zero
                                                                                          full charge        charge
                                                                                                         or zero      to 50%
                                                                                                                  charge   to
SOC   in approximately        one  hour.  The  final algorithm   was  validated   against    the
50% SOC in approximately one hour. The final algorithm was validated against the subset of SCADA  subset   of SCADA     data
that was
data  thatnot
           wasusednot for
                       used algorithm    tuning.tuning.
                                for algorithm      The algorithm   passedpassed
                                                          The algorithm      laboratory     and siteand
                                                                                      laboratory       acceptance    tests to
                                                                                                           site acceptance
the satisfaction
tests              of HNEIoftechnical
      to the satisfaction                  reviewers.
                                  HNEI technical     reviewers.




                           Figure 11.
                           Figure     Schematic of
                                  11. Schematic of the
                                                   the wind
                                                       wind smoothing
                                                            smoothing control
                                                                      control algorithm.
                                                                              algorithm.

     Figure 12
     Figure  12 shows
                 showsa atime
                            timeseries
                                  series
                                       of of  power
                                          power       output
                                                   output  from from   the HRD
                                                                  the HRD    windwind
                                                                                    farm farm
                                                                                         (black)(black)
                                                                                                  and theand  the
                                                                                                           BESS-
BESS-smoothed
smoothed          time (red),
           time series   seriesfor
                                (red),
                                   the for  the morning
                                       morning    of Aprilof4th,April
                                                                  2013.4th,
                                                                         The2013.
                                                                             bottom The bottom
                                                                                      three plotsthree  plots are
                                                                                                   are expanded
expanded   sections  of the time  series to  show  detail. Qualitatively,   it is apparent from   comparison
sections of the time series to show detail. Qualitatively, it is apparent from comparison of the two           of
the two time  series (black  and  red) that  the algorithm   is able to  smooth   output from
time series (black and red) that the algorithm is able to smooth output from the wind farm.    the  wind  farm.
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