MODULE-LEVEL CONVERTERS FOR BIPV APPLICATIONS - S. RAVYTS, M. DALLA VECCHIA, G. VAN DEN BROECK AND J. DRIESEN - PV OPMAAT
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Module-level Converters for BIPV Applications
S. Ravyts, M. Dalla Vecchia, G. Van den Broeck and J. Driesen
KU Leuven – ESAT – ELECTA
Simon.ravyts@kuleuven.be
(paper under revision @ Elsevier)
Example of a BIPV
Context - What is BIPV? module, developed by
KUL and imec
• Building Integrated PhotoVoltaics
• PV panels used as a building element
• Combine multiple functions
• Here, focus on façade BIPV, using ‘standard’
modules
2018-09-21 2
Research questions
• What are the requirements of a BIPV installation from the electrical system point of
view?
• Translation of general ideas to concrete evaluation points
• Is it beneficial to use LVDC grids for BIPV installations?
• More than the advantages of LVDC in general, how does it combine with the
requirements of a BIPV installation
• Can we translate these requirements to practical converter design recommendations?
Is there an impact from the grid configuration on the requirements?
• Do we really need a transformer?
2018-09-21 3
Outline • Introduction • System-level criteria for the electrical installation of BIPV • What is required? • Electrical installation • Overview of possibilities • Why would we use LVDC? • Converter requirements and challenges • Conclusions 2018-09-21 4
System-level criteria for the electrical installation of BIPV
• Important parameters from a system perspective
• KPI’s
• Energy yield
• Relates to the payback time ($$$)
• Partial shading -> Mismatch losses
• Installation dependent
• Ranges from 5-25%
• Limit this impact by doing a distributed MPPT
• Per module and not per string
2018-09-21 5
• Compatibility
• Architectural freedom
• Multiple types and sizes of PV
• Electrical parameters can (strongly) differ
• Electrical system needs to cope with these differences on the input of the
converter
• The ability to cope with different input voltages and currents
2018-09-21 6
• Engineering effort
• A lot of engineering for designing the system
requires man hours
• High cost, should thus be minimal
• Limit hours on the drawing table!
• Modularity
• An easy practial installation method (plug & play)
• Mechanical and electrical installation
simultaneously
2018-09-21 7
Source: bkprecision.desk.com • Reliability/ lifetime • Lifetime of min. 30 years • AC grid disturbances • Surge voltage : One of the main causes of PV inverter failure • If converter in frame -> not reachable after installation! • High reliability and fault-tolerance required to avoid system shutdown • Automotive: 300000km @ average speed of 50km/h -> 6000h • PV: 25 years @ 8h/day -> 73000h • BIPV: (at least) 30 years @ 8h/day -> 87600h AND not-repairable 2018-09-21 8
• Monitoring
• For analysis and performance assessment
• For online reliability monitoring
• Both PV and converter can be modeled
• Technical room space
• Should be minimized in densely populated areas (high €/m²)
• Depending on the chosen electrical system, also a strong increase in
cables and technical shafts
2018-09-21 9
Electrical installation possibilities • What systems are available? • String inverter (a) • Micro inverter (b) • Series PO (c) • Parallel PO (d) 2018-09-21 10
So is it beneficial to use LVDC in BIPV systems?
• LVDC general advantages
• Fewer conversion steps lead to higher overall efficiency
• More power can be transmitted over the same conductor cross-section
• Power flows are actively controllable
• Specific advantage of LVDC in BIPV systems?
• Fewer conversion steps (only DC/DC) also require fewer components
• Inherently higher reliability
• Increased compactness
2018-09-21 11• Specific advantage of LVDC in BIPV systems?
• No energy buffering required!
• Both in- and output are DC powers
Inside of an Enphase µinverter
The big electrolytic
capacitors can be left out!
2018-09-21 Source picture: Evaluation of Electrolytic Capacitor Application in Enphase Microinverters, 2009 12• Specific advantage of LVDC in BIPV
systems?
• No damage due to 50 Hz thermal cycles
at junction
• Less thermal fatigue
• Again increased lifetime
2018-09-21 13Converter and grid requirements • Parallel power optimizer can tackle most requirements from a system point of view • Where are now the challenges in the design of this Module-Level Converter (MLC)? • Important assumption: Converter is not reachable after installation • Non-repairable system 2018-09-21 14
• Compactness
• Flat, long design is preferred to fit in the frame
• High switching frequencies help to make the passives smaller
• Wide-bandgap (GaN/SiC)
• Transformers/inductors vs capacitors
• Reliability?
EPC commercial
2018-09-21 15• Wide power and input voltage range
• Compatibility with mono-, poly-, film PV
• For a ‘standard’ surface or for all surfaces?
• Impacts I-V curve!
• ‘One-converter-fits-it-all’
• Impacts efficiency and compactness
Source: greensarawak.com
• Overdimensioning
• Even higher gains required for very low voltages
• Interleaved converters
• Maintain topology
• Change the amount of phases, based on power level (scalability)
• Modularity on converter level
2018-09-21 16• Temperature range and cooling
• High thermal stresses
• Peaks around 80°C (ambient!)
• Reduces efficiency and reliability
• Heat sink
• Bulky, coincides with compactness criterion
• Attach components to module itself?
• Difficult to implement in practice
• Commercial converters auto-shutdown
2018-09-21 17• Fault-tolerance
• System-level
• One converter failure does not lead to system failure
• Requires adequate design on the input/output to fail safe
• Converter-level
• Component failure does not lead to converter failure
• Requires redundancy and thus increases costs
• Communication can help in achieving both targets
• But again increases costs
2018-09-21 18Fault types • Two main types: short-circuit and earth faults • Focus on earth faults as they are related to the LVDC grid confguration 2018-09-21 19
Galvanic isolation?
• LF transformer
• Change voltage level
• Change grid type
• HF transformer
• Increases cost, lowers compactness, reduces overall efficiency, leads to
parasitic oscillations
• Easier to get a high step-up
• But is it necessary or advisable for some reason?
2018-09-21 20• From a fault perspective, the only impact is when we have an earth fault on the PV side 2018-09-21 21
• Transformer thus improves reliability of the converter by adding fault-tolerance 2018-09-21 22
Conclusions • Electrical system • Overview of specific requirements • Overview of current systems • Parallel Power Optimizers give the best match • Specific advantages of LVDC for BIPV? • Higher compactness • Higher reliability 2018-09-21 23
Conclusions
• Converter
• A passive cooling method is strongly recommended
• Transformer further increases reliability
• Interleaved converters allow to span a wider BIPV application field in terms
of power and current
2018-09-21 24Thank you for your attention! • Questions? • Comments? • Remarks? 2018-09-21 25
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