Frequency - the third dimension - January 13, 2004

www.leonardo-energy.org                                                      Power Quality

Frequency – the third dimension
January 13, 2004
by Ronnie Belmans, KULeuven – ESAT/ELECTOA
email ronnie.belmans@esat.kuleuven.ac.be

Power electronics have liberated electrical engineering from 50Hz, allowing to generate
and use power at a wide range of frequencies, increasing efficiency and enabling new
applications. But there may be a price to pay: reduced power quality. However, power
electronics is both cause and solution to these problems.

1    Introduction

At the generation side, the introduction of distributed generation (both from classical
and renewable sources) leads to the use of non-grid frequencies in the generation process.
For instance, photovoltaics deliver dc power, wind energy can benefit from the use of
variable speed generators and micro gasturbines require high speed generators for direct
drives. The extra cost may seem high, but the power electronic converters can add other
functionalities as active filtering and reactive power supply.
At the user side, the increasing use of active rectifiers can reduce the power quality
problems, that are seen now.
Electric power is delivered to applications of various size and in a wide power range,
going from Watt’s to several MW. When electric energy was introduced at the turn of
the nineteenth century, in most systems direct current (dc) was used. At that time, it
was almost impossible to increase a dc voltage: when the power demand increased, the
current grew, thus the demand for conductor material in order to limit voltage drop and
losses, way beyond reasonable investment possibilities.
Soon it became clear that using alternating current, the voltage could be changed in an
easy way using a transformer. If a large amount of powers has to be transmitted, a high
voltage is used, reducing the current and the losses and the conductor material required. It
is evident that while an increase in the voltage, reduces the conductor cost, the insulation
cost increases. Therefore, for a given power level a minimum investment cost is found at
a certain voltage level and this level increases as more power is transmitted.
Therefore, over the years, the transformer was used to move power in two dimensions:
from a low voltage and high current to a high voltage and a low current, and vice versa
with a minimum of losses, thus keeping the product U ∗ I constant.

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www.leonardo-energy.org                                                         Power Quality

This situation has been maintained for most of the last century. In fact, as long as power
electronics was not available at a sufficiently low cost and an acceptable level of reliability,
power was delivered at a constant frequency being 50Hz in Europe and 60Hz in North-
America. Either of these frequencies is used in other parts of the world. All consumers
had to live with this frequency, limiting the flexibility of the applications. Control of
the process was done further on in the system. One of the best known examples is flow
control: constant speed drives were used, and the flow was controlled by mechanical
systems involving massive energy losses. Now flow control is obtained by introducing
a power electronic converter to change the speed of the motor driving the pump, thus
controlling the flow in an energy efficient way.
In general, the rational use of electrical energy assumes the power to be delivered to an
application at a specific frequency. This is now possible using advanced power electronics,
introduced to change the fixed supply frequency into almost any frequency from dc to
several tens of kHz. In the remainder of this paper, the internal of the power electronic
converters is not considered. They are treated as black boxes with input the 50Hz side and
output a controlled (in magnitude and frequency) voltage. Some of them can reverse the
energy flow direction. In order to illustrate the flexibility, a number of generic examples
are discussed.

2     Variable speed drives

Up to the beginning of the nineties, variable speed drives invariably used dc motors, with
the known drawback of maintenance due to the collector/brush combination.
Frequency convertors were introduced to control the speed of squirrel cage induction
motors.
The first generation was characterised by a low switching frequency, leading to acoustic
noise, vibrations and additional losses. Only the introduction of fast switching IGBT’s
(Insulated Gate Bipolar Transistors) allowed to use advanced PWM (Pulse Width Mod-
ulation) schemes avoiding the above mentioned drawbacks. Variable speed drives were
introduced to vary the speed of pumps, compressors and fans in order to control the flow.
The energy savings when compared to mechanical controls are large, leading to pay back
periods of less than 1 year. The reliability of such drives is very good, and may even be
better than classical systems, due to the avoidance of mechanical components.
Gradually, more advanced control strategies became available, so-called field-oriented con-
trol and direct torque control, leading to new applications of induction motors replacing
dc drives:
    • traction drives (trams, traditional trains, high speed trains, diesel-electric systems)

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www.leonardo-energy.org                                                        Power Quality

    • cranes and elevators
    • paper and metal processing systems.
Even highly dynamic servodrives now can use frequency converter induction motors. An
alternative for the latter are synchronous motors with permanent magnets.
Power electronics even allow to use motors that can not be supplied directly from the grid.
The best known is the so-called switched reluctance motor: it is a motor with a different
number of stator and rotor poles, and the rotor is very simple as it does not contain any
conductor material. The stator windings are connected stepwise to a dc supply (diode
rectifier). Other motor types emerge too, as pancake and transverse flux motors. The
latter motors seem to be very promising for electric and hybrid vehicles.
Several types of drives require the possibility to feed back energy to the grid. This assumes
an active front of the inverter. A similar kind of arrangement is found in wind energy sets,
where variable turbine speed can be used in order to improve the energy captured. Several
types of generators can be used, one of the most interesting being the so-called doubly fed
induction generators, where only the rotor power is handled by power electronics. Other
generator types are based on high performance permanent magnets.
Overall, it is found that in new installations the majority of large drives is speed controlled
nowadays.

3     Lighting

When looking at light sources, the impact of power electronics becomes even more pro-
nounced. Electronic ballasts can replace the electromagnetic counterparts for supplying
low-pressure mercury tube luminescense lamps. The high frequency supply leads to an
increased efficiency, longer lamp lifetime, avoiding flicker and improving lamp ignition.
Furthermore, dimming is readily possible, not being the case with electromagnetic ballasts.
The introduction of such devices has made smaller lamps possible, leading to so-called
Compact Fluorescent Lamps. Here the power electronics are built into the lamp socket.
New lamp types become available as for instance the QL lamp; an electrode free discharge
lamp with an extremely high lifetime (≥ 70.000h), and supplied by a very high frequency
(> 1M Hz). High Intensity Discharge lamps are very much suited for specific applications
where colour rendering is crucial, requiring electronic ballasts for a safe operation.
A last example are LED’s, where the (power) electronics and the light source are almost
integrated.

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www.leonardo-energy.org                                                      Power Quality

4    Electroheat

In industrial as well as domestic applications, high frequency supplies are required to
improve the efficient heating of materials. The frequency variation is required both due
to the nature of the material (electrically conducting or not) and due to the penetration
depth needed in the application. Electricity as a source of heat can reduce the primary
energy consumption as the efficiency of the heating process is extremely high: for example
when heating plastics with hot air, most of the energy goes to the heating of the air and
not to the material. Probably the best known example is the microwave oven in which
direct heating is generated in the load using very high frequencies. Similar equipment
is used in industry. A somewhat lower frequency is used in dielectric heating, via a
capacitor like arrangement. Frequencies in the range of several MHz are used for treating
non-conducting materials. For treating metals, induction heating is an important issue,
with frequencies ranging from 50 Hz to several tens of kHz.

5    Power electronics in the grid

In the grid, several developments are seen. The use of HVDC (high voltage direct cur-
rent) for transporting energy over long distances (especially from large hydro power) is
probably the best known. Other applications are coupling of grids by longer subsea cables
(for instance United Kingdom-France). These classical HVDC systems, based on thyris-
tor valve technology, suffer from different problems, as harmonic disturbances and high
reactive power requirements.
Recent developments have led to voltage source inverter based converters, using IGBT’s
as power electronic switches. As rectifiers and inverters, they are the key part of so-called
HVDC light systems. Such devices can be used for controlling active power flow, but can
also deliver reactive power (both capacitive and inductive). In this way, they contribute
to the stability of the grid.
Not only fixed frequency (50 or 60Hz) to dc conversion systems are used, but also links
between 50 and 60Hz are developed (e.g. Japan) or 50Hz and 16 2/3Hz (Germany).
Other developments based on similar technology are seen for controlling the active and
reactive power. They are known by the general term FACTS: Flexible Alternating Current
Transmission Systems.

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www.leonardo-energy.org                                                       Power Quality

6     Power quality and power electronics

Due to the increasing use of power electronics, the grid voltage becomes more and more
disturbed (harmonics, spikes, etc.). On the other hand, such power electronic circuits are
very vulnerable to voltage disturbances (e.g. dips, sags, etc.). Therefore, a lot of attention
will go in future to increasing the power quality level, and again, power electronics is part
of the solution.
Different voltage correcting devices are on and will come to the market, all of them based
on high frequency switching devices.
Some of them can be used to improve the power quality, as active filters, dynamic voltage
restorers (DVR) and Static compensators (StatCom).
They have to contain an energy storage device with a relatively small content. For power
quality problems with a longer time span more energy storage is required. Several existing
(battery, flywheels) and new (SMES, Supercapacitors) devices are introduced in new
systems. All of them require power electronic grid interfaces.
Power electronic interfaces are also used in renewable energy systems as variable speed
wind turbines and photovoltaic devices. Also micro-CHP’s that become available in the
near future (microgasturbine, Sterling motors, fuel cells), will require power electronic
converters.

7     Conclusions

Power electronics are, and increasingly will be a key element in the electrical energy flow,
in generation, transmission, distribution and supply to the end users, where the electrical
energy is transferred to an other energy. The frequency is changed to a value that is
appropriate for the given application and ranges from dc to several MHz.

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