POWER SYSTEMS VAR CONTROL & HARMONIC DISTORTION
CORRECTION:  AN INTRODUCTORY NOTE

Parviz Doulai MIEAust (CPEng)
Department of Electrical and Computer Engineering
University of Wollongong
September 7, 1992

Modern semiconductor switching devices are currently employed in a wide
variety of domestic and industrial loads. These loads are often referred to as
"power electronics loads". They offer reliable and economical solutions to con-
trol of electric power, from a few watts to many megawatts. The nonlinear
characteristic of semiconductor devices as well as the operational function of
most power electronics circuits cause distorted current and voltage waveforms
on the supply system. In contrast with the conventional linear loads, the power
electronics loads are categorized as nonlinear loads. Another example of a non-
linear load is an electric arc furnace [3]. These loads also are commonly referred
to as "power system polluters" or "distorting sources" in relevant literature.
The presence of power electronics related distorting elements in virtually all
major industrial loads is viewed by the power distribution authorities as the
major cause of an alarming amount of harmonic distortion in electric power
systems.

The development of the energy improving technologies, widely used for in-
dustrial loads, has already been expanded to domestic electrical appliances.
This has resulted a further significant increase in the background distortion lev-
els of harmonic frequencies within electric power systems. Reducing the adverse
effect of the cumulative distortion, caused by aggregated small industrial and
domestic loads, requires complicated and innovative power filtering techniques.
The problems associated with harmonically polluted power systems are well
known. Among often-cited problems caused by harmonic distortion in supply
systems the poor use of the AC source and distribution wiring volt-ampere ca-
pacity as well as distortion of line voltage waveform caused by harmonic currents
particularly in "weak" system buses are considered highly important. It is in-
teresting to note that the power semiconductor-based loads which are the major
contributors to power systems pollution tend to be sensitive to pollution caused
by other nonlinear loads [4].

Synchronous condensers can be very effective for system var flow/voltage
control. However, because of their relatively slow response time, they are un-
able to compensate fully for the undesirable effects of rapidly changing loads.
An alternative approach to the use of controllable reactive power devices utilizes
the "static var compensator (SVC)" systems, which have faster response and
a good potential for lower initial and operating costs [6]. The contribution to
suppression of harmonics and transient distortion correction is non existent for
the SVC's. The term SVC covers the inherently controlled reactive power de-
vices, such as the saturated reactor, and all different configurations of thyristor
controlled reactive components in which the controllable compensator output
results from an active feedback control. The basic principle of all SVC schemes
is the cyclic process of storing energy in passive reactive elements and releasing
it to the system. This implies that the maximum reactive output is directly
proportional to the size of the inductor or capacitor which has been used within
the SVC. This, of course, can compensate only the fundamental displacement
of current required by the load [3].

The problems associated with performing switching operations on large scale
capacitors or inductors within the static var compensators, simultaneously with
using harmonic frequency filters to absorb the harmonic distortion generated by
nonlinear loads and the SVC itself, motivated the investigation of utilizing fast
switching technology to generate the required "corrective" or "compensating"
waveform. This waveform has to be injected into a carefully selected point in
a power system to correct the voltage waveform on a distorted bus-bar. The
concept of injecting the compensating waveform into a power system bus are
commonly referred to as "active power filtering". The switching compensator
itself has known by different names such as : active power filter [16], active
power line conditioner [12], and static var and distortion compensator [6]. At the
present time, active power filtering systems are mainly concepts `on paper' [12]
with a dozen simulation and prototype laboratory implementation and a few
experimental versions.

The advantages associated with fast switching technology in power condi-
tioning techniques are apparent from a simple comparison of existing DC and
AC power processors [9]. These include: smaller filter size (as the switching fre-
quency is increased) and the possibility of implementing high frequency energy

processing techniques such as pulse width modulation (PWM) [5]. In spite of
significant developments in control techniques, the application of fast switching
technology has been so far confined to a few disciplines such as adjustable-
speed motor-drives and uninterruptible power supplies. There are two reasons
for this: firstly, the speed-power limitation of power semiconductor switching
devices, and secondly the lack of a fast digital controller capable of handling
both intensive calculations and providing the required interfacing facilities.
The fundamental approach to compensating instantaneous reactive power
was first suggested by Erlicki in 1968 [10]. The application of semiconductor
switches for instantaneous reactive power compensation, under constant loading
conditions, was reported in the early 1970's [11]. Closed loop control operation
implemented on a forced commutated inverter has been reported by Harashima
in 1976 [13]. Several circuits of this class have been proposed and a few of
them have been implemented in electric utility system [17] and research labora-
tories [3]. One interesting feature of this class of compensator is that the source
not only provides fundamental frequency reactive power for voltage control and
phase balancing but also has a controlled waveform to cancel undesirable har-
monics.
The recent attention to exploring the fast switching compensator for mains
voltage support and distortion correction has occurred for the following major
reasons :
a: A focus on "power quality": there is a growing concern to try to reduce the
power system pollution which has been caused by an ever-increasing number of
power electronics loads.
b: Recent advances in power semiconductor technology which have significantly
improved the speed-power characteristics of switches in the medium and high
power-level ranges.
c: The availability of powerful single-chip microcontrollers.
d: The expected further price reduction in power semiconductor devices and
increases in prices of reactive components.
From the distribution authorities point of view, an important factor, which
has a significant impact on power quality, is the higher background harmonic
levels in the power systems caused by small and medium power-level nonlinear
loads. Where the users can be identified as being the major source of har-
monics, they can be required on a case-by-case basis, to install an appropriate
SVC system accompanied with tuned passive filters. However, the presence of
a large number of small harmonic sources, none of which exceeds the limits
of the nationally recognized standard, can still lead to a situation where the
overall harmonic level rises. In areas such as a central business district, the
necessary function of supporting voltage, and reducing the effects of the sys-
tem's background voltage and current harmonic distortion, could be carried out
effectively by carefully designed active power filters, placed at selected points in
the distribution system.

Some interesting features normally associated with the active power filtering
approach can be outlined as follows :
a: While passive filters have to be designed with a kVA rating based on the
worst case total distortion at each frequency, the active power filters can be
designed for the lower kVA rating than the worst case total distortion.
b: They are capable of reducing the effect of distorted current/voltage wave-
forms as well as compensating the fundamental displacement component of cur-
rent drawn by nonlinear loads.
c: Because of high controllability and quick response of semiconductor devices,
they have faster response time than the conventional SVC's.
d: They primarily utilize power semiconductor devices rather than conventional
reactive components. This results in reduced overall size of a compensator and
expected lower capital cost in future due to the continuously downward trend
in price of solid state switches.
APF's: Methodologies
Active power filters are basically designed to implement the well known
concept of injecting the "equal-but-opposite" distortion waveform to a weak
bus-bar system in an attempt to suppress harmonic distortion and improve
the power quality on the distorted bus-bar. This is equivalent to synthesizing a
sinusoidal voltage waveform [5] on a system bus while the connected distribution
feeder is subjected to transient disturbances, or is supplying nonlinear loads.
Note that the expression of "equal-but-opposite distortion waveform injection"
was first used for off-line correction of bus-bar waveform. This expression still
is widely employed in the literature addressing the technical features of the
newly developed active power filters, even for on-line implementation of active
filtering.
From a control system point of view, waveform correction on the system bus
can be implemented either in time-domain or frequency-domain basis. More-
over, the required switching control strategy for each case (which ultimately
determines the desired compensating signal), could be based on an open loop or
a closed loop control concept [5]. The first task in designing an active filter is
to define the instantaneous error function, the difference between the measured
bus-bar voltage and the corresponding points on the desired sinusoidal refer-
ence waveform. The bus-bar waveform correction based on the time-domain
approach attempts to bias this error toward zero by implementing an appro-
priate switching decision within the power switching circuit. For this case, in
order to generate the required switching edges, based on the availability of the
instantaneous error function, two options are available.
(1) an error sawtooth comparison technique in which the error function is
compared with a high frequency triangular carrier signal. The intersection of
these two waveforms determines the switching instants for the switching cir-
cuit [7].
(2) Hysteretic control technique in which the error function is centered in
a preset hysteresis band. When the error exceeds the upper or lower hystere-
sis limit, the hysteretic controller makes an appropriate switching decision to
control the error within the preset band [8].
A frequency domain approach, in contrast, involves either injection of some
predetermined harmonic distortion to the distorted bus [1], or implementing a
"Fast Fourier Transform (FFT) on a measured waveform to determine the order
of predominant harmonics which have to be injected [2] [14]. The first option is
an off-line technique, and is only suitable for cases in which the nature of distur-
bances are known. The closed loop implementation of the FFT option, at the
present time, is a concept which its implementation requires a fast digital pro-
cessor accompanied by extensive memory space and input/output facility. The
involvement of the FFT implementation, however, delays the system response
by one mains cycle.
Based on state-of-the-art technology, on-line implementation of time-domain
compensation solution to harmonically polluted power system is shown to be
a viable option. This also has the advantage of controlling reactive power at
mains frequency. The experimental result obtained from on-line implementa-
tion of an error sawtooth comparison algorithm and a hysteretic-based control
strategy demonstrated the effectiveness of the time-domain control algorithm in
distortion correction as well as in providing lagging or leading vars for funda-
mental voltage control [7] [8]. This showed the filter can be viewed as a potential
replacement for passive filters and reactive power control systems.
The application switching compensators will also cover the progress on uti-
lizing multiple stage series or parallel connection of low-power low-switching
rate power switching circuit modules to meet the requirements of large scale
power systems harmonic distortion problems [15].
Application of resonant link circuit topology employing the gate turn-off
thyristor as switching elements is also considered as an alternative approach
that offers the possibility of single stage circuit configuration with high power
level combined with high switching frequency operation


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