1. MODERN ELECTRIC POWER SYSTEMS
Shunt compensation for power quality improvement
using a STATCOM controller: modelling and
simulation
R. Mienski, R. Pawelek and I. Wasiak
Abstract: The paper deals with compensation of frequently time-variable loads by means of
STATCOM controllers. An arc furnace is considered as a heavily distributing load. The
STATCOM system was used to ensure good power quality at the point of common coupling. For
analysis of the system performance, the PSCAD/EMTDC programme was applied. Simulation
models of the load and two types of STATCOM controllers, 12-pulse and 24-pulse, are discussed in
the paper. A PSCAD model of a measurement block is also proposed for power quality
assessment. Some results of simulation are presented, which show the compensation effectiveness.
1 Introduction
In recent years power quality issues have become more and
more important both in practice and in research. Power
quality can be considered to be the proper characteristics of
supply voltage and also a reliable and effective process for
delivering electrical energy to consumers. Binding standards
and regulations impose on suppliers and consumers the
obligation to keep required power quality parameters at the
point of common coupling (PCC).
Interest in power quality issues results not only from the
legal regulations but also from growing consumer demands.
Owing to increased sensitivity of applied receivers and
process controls, many customers may experience severe
technical and economical consequences of poor power
quality. Disturbances such as voltage fluctuations, flicker,
harmonics or imbalance can prevent appliances from
operating properly and make some industrial processes
shut down. On the other hand, such phenomena now
appear more frequently in the power system because of
systematic growth in the number and power of nonlinear
and frequently time-variable loads.
When good power quality is necessary for technical and
economical reasons, some kind of disturbance compensa-
tion is needed and that is why applications of power quality
equipment have been increasing.
For many years conventional static VAr compensators
(SVC) have been widely used in distribution power
networks to improve power quality. Providing fast reactive
power compensation, they prevent fluctuations in supply
voltage, which can be detrimental to consumers. They thus
maintain a constant voltage on the load buses and reduce
voltage flicker, keeping the power factor steady and
balancing the reactive power consumption. Different
conventional SVC configurations have been applied: a fixed
capacitor with thyristor controlled reactor (FC/TCR), a
thyristor switched capacitor (TSC) and a combined
thyristor switched capacitor with thyristor controlled
reactor (TSC/TCR). Such compensator performance has
been described and analysed in many publications [1–3].
The most recent approach for solid-state power com-
pensators is based on self-commutated converters using
components with a current blocking capability. Such a
compensation system is the static equivalent of the
synchronous compensator, hence the term STATCOM
(static synchronous compensator).
A STATCOM can provide fast capacitive and inductive
compensation and is able to control its output current
independently of the AC system voltage (in contrast to the
SVC, which can supply only diminishing output current
with decreasing system voltage). This feature of the
compensator makes it highly effective in improving the
transient stability. Therefore, STATCOM systems with
GTO thyristors have been initially used for improving
flexibility and reliability of energy transmission in FACTS
(flexible AC transmission system) applications [4–7]. As the
switching frequency of GTOs must be kept low, the control
with fundamental frequency switching has been used and
multi-phase configurations have been formed to reduce
harmonics production. The newest applications of STAT-
COMs concern power quality improvement at distribution
network level. Some examples given in the literature are the
reduction of flicker, voltage control and balancing single
phase load [6, 8]. These are systems of a smaller power
where IGBT or IGCT technology can be applied, allowing
fast switching with PWM control.
Although the possibility of using STATCOMs for the
reduction of influence of disturbing loads on the supply
network has already been proved in practice, there is still a
lack of information about the complex assessment of
compensation effectiveness and a method of system
selection and its control for a given network. Thus, the
purpose of the authors’ work was to develop a model of the
system consisting of supply network, disturbing load and
STATCOM compensator and the simulation tool enabling
selection of the compensator settings and examination of
the compensation effectiveness. An arc furnace was selected
as a representative example of a heavily disturbing load,
which can deteriorate power quality in the grid, particularly
The authors are with the Institute of Electrical Power Engineering, Technical
University of Lodz, 18/22 Stefanowskiego Str. 90-924, Lodz, Poland
r IEE, 2004
IEE Proceedings online no. 20040053
doi:10.1049/ip-gtd:20040053
Paper first received 30th January 2003 and in revised form 19th November 2003
274 IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 2, March 2004

2. because the power of such devices may be significant. It was
assumed that the STATCOM task was to maintain good
power quality at the PCC and provide quality indexes
according to the binding standards. The PSCAD/EMTDC
programme was selected for simulation, which is particu-
larly useful in the case of networks with power electronics
elements and systems [9, 10].
In this paper a PSCAD model is discussed for the system,
including supply network, arc furnace as a heavily
disturbing load, STATCOM controller, and a special
measurement system for power quality assessment. Two
STATCOM systems, 12-pulse and 24-pulse, have been
compared. The first system has been partly adopted from
the PSCAD library; the second one is the original
contribution of the authors. As there are many configura-
tions of the STATCOM systems, there are also different
control strategies. In multi-phase systems the method of
switching with line frequency is usually used. The authors
applied this type of control to the examined network,
adopting first the solution given in [10] which realises
reactive power control with a PI controller. A number of
simulations proved that it can ensure sufficiently good
STATCOM operation only for symmetrical, and not for
very fast, voltage disturbances. Therefore, a new control
circuit has been proposed, which appears to be more
suitable for distribution networks supplying unbalance and
frequently time variable loads. It combines two control
types: the control of AC voltage and of inverter capacitor
voltage.
Description of the models developed and some results of
simulation are presented in this paper.
2 System modelling
2.1 Electrical network
The examined network is presented, in general, in Fig. 1. A
HV/MV substation is supplied from the transmission
network through a transformer. The network is represented
by an equivalent voltage source, the reactance of which
results from a short-circuit power on the high voltage side of
the transformer. It has been assumed that a disturbing load,
which is the three-phase arc furnace, is connected to the
substation busbars which, in general, may draw asymme-
trical and strongly fluctuating active and reactive powers.
The load is thus a source of disturbances such as voltage
fluctuations, flicker, and unbalance, which may cause
problems for other customers connected to the grid. The
natural power factor is low and varies irregularly, and
reactive power compensation is not possible in a conven-
tional way. Moreover, load imbalance causes imbalance of
the supply voltage and additional network losses. To
compensate for these effects the substation is also equipped
with a STATCOM controller.
All elements were modelled in PSCAD/EMTDC. The
method of modelling is described below.
2.2 Arc furnace
The physical process inside the arc furnace is erratic in
nature, with one or several electrodes sticking electric arcs
into it. As a consequence, the consumption, especially of
reactive power, fluctuates in a stochastic manner.
A nonlinear model of the arc furnace was developed as a
rectangular voltage source whose amplitude is a function of
an arc length and phase shift corresponding to the arc
current phase shift. Such a model allows analysing of
harmonics in the supply network. The stochastic nature of
the furnace operation was obtained by means of three
random generators, which changed the arc length in
individual phases. The generators were constructed experi-
mentally with the criterion of obtaining typical active and
reactive power flows, of the real arc furnace, described in
[11]. The control mechanism for stabilisation of the arc
length was taken into consideration. The model also
represents a process of the arc distinguishing when the
current value decreases below the minimum value. A
renewed arc ignition is possible when the electrode
encounters the scrap. Then the control mechanism causes
the stretch arc to the stabilised value. Through an
appropriate selection of the random generator coefficients,
the model can represent both phases of the furnace
operation: melting and refining.
The furnace model was developed using typical PSCAD/
EMTDC modules. Its graphical form obtained from
PSCAD is shown in Fig. 2.
2.3 STATCOM system
The main component of the STATCOM is a VSI (voltage
source inverter), which is connected to the network through
an inductance, usually of the coupling transformer. The
most basic is a six-pulse system configuration, giving the
rectangular output voltage. As such a system produces
harmonics, its practical application is limited. A multi-pulse
scheme is one of the solutions used for harmonic reduction,
in which several identical six-pulse bridges are connected to
transformers having outputs that are phase displaced with
respect to one another. Star- and delta-connected windings
give a relative 301 phase shift and, with two six-pulse
converter bridges connected, allow 12-pulse STATCOM
operation to be obtained.
The 24-pulse configuration is an extension of the previous
one and consists of four six-pulse GTO bridges, connected
in parallel and supplied in pairs from the two three-
windings converter transformers with primary windings
connected in zig-zag and secondary windings connected in
star and delta. In this way one can obtain the 151 phase shift
between secondary voltages.
The authors used the model of 12-pulse STATCOM
system available in the PSCAD program and developed a
model for the 24-pulse system. In the first model the
converter transformer was represented by means of six
single-phase transformers. For the 24-pulse unit the
component single phase 4 winding UMEC transformer
was used, which is available in the PSCAD library.
Converter bridges were represented using switching device
components power electronic switch, which enable both
diode and the GTO thyristor to be obtainied together with
its snubber circuit.
A diagram of the 12-pulse STATCOM and its control
circuit in a graphical form obtained in the PSCAD
environment is presented in Fig. 3.
Components of the CSMF module (control system
modelling function) were applied in the special block
designed for generating firing pulses. These components
return the firing pulse and the interpolation time required
load
compensator
Fig. 1 Network under study
IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 2, March 2004 275

3. for switching on and off. Synchronising the valve switching
to the AC system voltage was done using the PLL (three-
phase PI-controlled phase-locked loop) element.
The control circuit was designed to control the magnitude
of the inverter output voltages using the fundamental
frequency switching method. Two signals are formed, which
uarc
DD
DD
+
_
V
V
C
iCarc
−0.0004
−0.0004
_ + F
D
B
G
1 + sT
+
+
+
+
+
+
F
D
B
+
+
+
F
D
B
B
D
D
A
iAarc
iBarc
_
+
_
+
_
+
_
+
V
V
−0.0004
−0.0004
V
V
−0.0004
−0.0004
G
iAarc
iBarc
G
1 + sT
G
1 + sT
* *
*
* *
*
* *
*
contr
−
+
−
+
−
+
D
D
D
0.395
RMS
RMS
RMS
2
*
2
*
2
*
contr
contr
1
sT
1
sT
1
sT
1 3 0
4
1.018
7.7
Random
1 3 0
4
1.018
7.7
Random
1 3 0
4
1.018
7.7
Random
iCarc
F
F
F
Fig. 2 Diagram of arc furnace model constructed using PSCAD components
#1 #2
#1 #2
#1 #2
A
A
B
B
C
C
C
RMS
3 Place
RMS
1 3 5
4 5 2
dcV hg
gd gy
101
dcV hg
RMSD
D
F
F
p
I
+_
I
p
A
A
B
B
C
C
1
2
1
2
1
2
1
2
1
2
1
2
12
11
10
9
7
8
6
5
4
3
12
11
10
9
7
8
6
5
4
3
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
gy6gy5gy4gy3gy2gy1
gd1gd2gd3gd4gd5gd6
+
−
0.025
*
gy_6
gd_6
gy
gd
gy1
2
gy3
2
gy5
2
gy4
2
gy6
2
gy2
2
gd1
2
gd3
2
gd5
2
gd4
2
gd5
2
gd2
2
1
3 5
4 6 2
0.001
#1
#1
#1 #2
#2
#2
1000000
3000
generator
offiringpulses
A
B
Fig. 3 12-pulse STATCOM diagram
276 IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 2, March 2004

4. are then passed to the PI regulator: the first one comes from
the capacitor voltage and the second one comes from the
AC voltage measured at the PCC. With suitable parameters
of PI regulators, this control enables both stabilisation of
voltage at the PCC and compensation of the negative
sequence component of the voltage vector. The control
circuit output is passed on to the firing block. The methods
of control and firing pulses generation are the same in both
types of STATCOM units, adjusted only to the appropriate
number of thyristors.
The proposed structure of the control circuit is original
and can be applied for compensation of any unbalanced
and variable load.
2.4 Power quality assessment
As has already been mentioned, the STATCOM controller
is designed to ensure the adequate power quality at the
PCC. Both supply voltage level and waveform shape
characterise the quality of supply. The quality assessment
should be made according to international and native
standards and regulations, which define voltage character-
istics at the supply terminals, give their admissible values
and specify a method for their measurement and evaluation.
The authors elaborated a special module for power quality
assessment, based on the rules described in the standards
[12, 13]. From the measured instantaneous voltage signals,
the following power quality indexes are determined in it:
voltage RMS,
unbalance factor,
THD factor and harmonic spectrum,
IEC flickermeter signal.
The module consists of two blocks, including components
from PSCAD library, which are shown in Figs. 4 and 5. In
the first block, which is the voltage parameter meter, the on-
line frequency scanner and harmonic calculator were used,
both based on on-line fast fourier transformation. The
second block, called the IEC flickermeter, is the digital
realisation of the system described in details in the Standard
[13]. Its first part is a voltage adaptor, which allows
reduction of the influence of the very slow voltage variations
on measurement results. The next part represents a
combined reaction of the bulb–eye–brain channel to supply
voltage fluctuations. The output signal of the flickermeter
nom
u1
u2
u3
N/D
N/D
N/DN/D
D
D
DD
U2
U3
U1
_
_
_
N
1
1
mag+
(7)
mag−
(7)
mag0
(7)
Ph+
(7)
Ph−
(7)
Ph0
(7)
XA
XB
XC
u1
u2
u3
dcA dcB dcC
F = 50 (Hz)
F=50(Hz)
F F T
mag1
(7)
mag2
(7)
mag3
(7)
Ph1
(7)
Ph2
(7)
Ph3
(7)
X1
X2
X3
u1u2u3
dc1dc2dc3
FFT
Asy
u1
u2
u3
Asy
Thd1
Thd2
Thd3
1
2
3
4
5
6
7
7 7
4
3
2
1
3
2
1
123567
7 7 7
7
7 7 7
7
7 7 7
7
T
T
T
T
T
As
RMS
RMS
RMS
RMS
Thd3
Thd2
Thd1
THD
mag
harm 1
harm 2
harm 3
7
7
7
7
7
7
2 3 4 5 6 7
1
2
3
4
5
6
7
1.0
2 3 4 5 6 7
1
2
3
4
5
6
7
1.0
2 3 4 5 6 7
1
2
3
4
5
6
7
1.0
total
individual
total
individual
total
individual
total
individual
total
individual
total
individual
harmonic
distortion
harmonic
distortion
harmonic
distortion
harmonic
distortion
harmonic
distortion
harmonic
distortion
Fig. 4 Voltage parameter measurement module constructed using PSCAD components
IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 2, March 2004 277

5. blok is the basis for assessment of the flicker phenomena, as
described in [13].
3 Simulation
The effectiveness of compensation was investigated for an
arc furnace of 80MW connected to the 30kV substation
through the transformer of 120MVA. The substation was
supplied from the 220kV grid through the distribution
transformer of the following data: rated power of 120MVA
and positive sequence leakage reactance of 0.105p.u. The
short-circuit power at HV side of the transformer was
3800MVA. Both phases of the arc operation: melting and
refining, were taken into consideration. Selection of
parameters of the STATCOM units and settings for their
control circuit was done by simulation with the target
function to obtain all power quality indexes in the
permissible range at minimum compensator power for
every load operation condition. This approach seems to be
the most effective for systems with dominating nonlinear
elements.
In each simulation case the power quality indexes were
assessed. The assessment method was analogous to the one
given in [12], but applied for the other timescale. Averaging
time was arbitrarily decreased from 10min to 0.2s and the
measurement period from 1 week to 10s.
The following quantities were determined, averaged, and
recorded:
voltage changes, according to the formula:
DU ¼ jU À 100%j ð1Þ
where U is a percentage value of the voltage at the PCC,
total harmonic distortion factor THD,
imbalance factor K2U.
Moreover, the IEC flickermeter signal was recorded on-line
during simulation to assess flicker phenomena.
The melting operation of the arc furnace was selected for
presentation, because this phase of work offers the worse
working conditions for the STATCOM units.
Figures 6 and 7 show total active and reactive powers for
both the arc furnace and compensator and the resultant
powers of the supplying network. Voltage changes at the
PCC when the arc furnace operates uncompensated and for
the case of furnace operation compensated with the
STATCOM system are presented in Fig. 8. Comparison
of operation of both 12-pulse and 24-pulse STATCOM
units is shown in Fig. 9 in the form of systematic graphs.
Similarly, Figs. 10 and 11, and also Figs. 12 and 13 allow
comparison of disturbances produced by the load with and
without the STATCOM compensation in terms of
imbalance and flicker. The figures enumerated demonstrate
+
_
+
_
+
_
U3
U2
U1
**
*
*
NNN
DDD
F
F
F
X
2
X
2
X
2
RMSRMSRMS
N/DN/DN/D
1
sT
1
sT
1
sT
1/sqrt(2)1/sqrt(2)1/sqrt(2)
sqrt(2)
nom
D
D
D
high pass
butterwth
order = 1
low pass
butterwth
order = 6
low pass
butterwth
order = 1
N(s)
D(s)
order = 2
X
2
X
2
X
2
Pf1
Pf2
Pf3
G
s
Wo
s
Wo
s2
Wo
21 + 2z +
high pass
butterwth
order = 1
low pass
butterwth
order = 6
high pass
butterwth
order = 1
low pass
butterwth
order = 6
low pass
butterwth
order = 1
low pass
butterwth
order = 1
N(s)
D(s)
order = 2
N(s)
D(s)
order = 2
G
s
Wo
s
Wo
s2
Wo
21 + 2z +
G
s
Wo
s
Wo
s2
Wo
21 + 2z +
Fig. 5 Flickermeter block constructed using PSCAD components
200
150
100
50
−50
0
0 2 4 6 8 10
time, s
Ps
Po
Pk
P,MW
Fig. 6 Total active powers for arc furnace (Po), STATCOM
controller Pk and network Ps
278 IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 2, March 2004

6. the effectiveness of the proposed STATCOM control circuit
in terms of compensation of voltage fluctuation and
imbalance in the PCC. Figures 14 and 15 prove that the
STATCOM units amplify the voltage distortion and
therefore may require a special filtering on the AC side of
200
150
100
−50
−100
−150
−200
0
0 2 4 6 8 10
time, s
Qs
Qo
Qk
Q,Mvar
50
Fig. 7 Total reactive powers for arc furnace (Qo), STATCOM
controller Qk and network Qs
115
110
105
100
95
90
85
0 2 4 6 8 10
time, s
U,%
arc furnace
without STATCOM
arc furnace
with STATCOM 12
Fig. 8 Voltage changes at PCC
15
13
10
8
5
3
0
0 10 20 30 40 50 60 70 80 90 100
time, %
U,%
arc furnace
STATCOM-12
STATCOM-24
Fig. 9 Systematic graphs of voltage changes produced by arc
furnace and compensated by STATCOM units
arc furnace
without STATCOM
arc furnace
with STATCOM 12
0 2 4 6 8 10
time, s
10
8
6
4
2
0
k2u,%
Fig. 10 Imbalance factor at PCC
10
8
6
4
2
0
0
k2u,%
arc furnace
STATCOM-12
STATCOM-24
10 20 30 40 50 60 70 80 90 100
time, %
Fig. 11 Systematic graphs of unbalance factor during uncompen-
sated furnace operation and operation compensated by STATCOM
units
arc furnace
without STATCOM
arc furnace
with STATCOM 12
0 2 4 6 8 10
time, s
2500
2000
1500
1000
500
0PIEC,p.u.
Fig. 12 Flickermeter signal at PCC
2500
2000
1500
1000
500
0
PIEC,p.u.
0 10 20 30 40 50 60 70 80 90 100
time, %
arc furnace
STATCOM-12
STATCOM-24
Fig. 13 Systematic graphs of flickermeter signal for case of
uncompensated furnace operation and operation compensated by
STATCOM units
arc furnace
without STATCOM
arc furnace
with STATCOM 12
0 2 4 6 8 10
12
10
8
6
4
2
0
THD,%
time, s
Fig. 14 THD factor at PCC
IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 2, March 2004 279

7. the converter transformer. Comparison of both STAT-
COM unit controllers in terms of harmonics production is
shown in Fig. 16. Another way of reducing harmonics may
be by changing the control method, (e.g. using the PWM,
pulse width modulation technique). These possibilities will
be investigated in future.
4 Conclusions
The results presented in the paper prove that the
STATCOM is an effective solution for reducing voltage
fluctuations caused by disturbing loads and may be applied
for power quality improvement. In particular, flicker
reduction in this case is extremely good. However, the level
of harmonics produced by the controllers is relatively high.
Increasing the number of pulses in a multi-pulse configura-
tion reduces the harmonics contents, as was proved in this
paper for the case of 12- and 24-pulse STATCOM units
considered.
For examination of electrical power networks the method
of simulation can be applied effectively and PSCAD/
EMTDC has been recognised as a good tool. A model of
the considered network was constructed using components
that are available in the program library. Some modules,
especially the arc furnace, converter transformers for the 24-
pulse unit, and the PQ assessment module, are original and
were worked out by the authors. The simulator allows
evaluation of the power quality at the PCC according to the
EN 50160 Standard. It may also be useful for studies and
transient analysis of power networks with nonlinear and
unbalanced loads.
The authors applied the simulation method to selecting
parameters and settings of the STATCOM controllers.
With this approach it is possible to select the controller for a
given network environment, which will ensure power
quality indexes at the PCC at the required level for
minimum compensator power. The simulation method is
especially useful and convenient for nonlinear systems.
The STATCOM system requires self-commutated con-
verters to be used. Up to the present day, they have been the
GTO thyristors, rather expensive and inconvenient in
practical applications. These disadvantages have been
overcome in IGCT technology. IGCT are characterised
by short switching time, small losses, and high reliability.
They do not need snubbers. Their ratings are in the range of
4.5kV peak and 4.0kA turn-off [14], which make it possible
to construct systems of up to a few hundred MVA over the
full range of MV. It should be mentioned that the design of
STATCOM systems is more compact, which eventually
leads to a significant reduction in equipment size and
installation costs. It is assessed that the costs of power
electronic equipment using the IGCT technology may be
reduced by approximately 35% [15]. Good switching
abilities makes it useful for applying the PWM control.
These favourable features of IGCT thyristors allow the
assumption that the STATCOM compensator will become
a good solution for power quality improvement, commonly
used in practice.
5 Acknowledgment
This work has been supported by the Polish State
Committee for Scientific Research under Contracts 1459/
T10/2000/18 and 1659/T10/2001/20.
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13 European Standard EN 60868:‘Flickermeter: functional and design
specifications’, 1993
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2001
12
10
8
6
4
2
0
THD,%
0 10 20 30 40 50 60 70 80 90 100
time, %
arc furnace
STATCOM-12
STATCOM-24
Fig. 15 Systematic graphs of THD factor for case of uncompen-
sated furnace operation and operation compensated by STATCOM
units
5 7 11 13 23 25
harmonic number
STATCOM-12
STATCOM-24
10
8
6
4
2
0
Un,%
Fig. 16 Harmonics spectrum produced by STATCOM controllers
280 IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 2, March 2004