National Power & Energy Conference (PECon) 2004 Proceedings, Kuala Lumpur, Malaysia
A STATCOM Simulation Model to Improve
Voltage Sag Due to Starting of High Power
A. F. Huweg, S. M. Bashi, N. Mariun and N. F. Mailah
Matlab/simulink software to examine its capability for voltage
sag mitigation due to starting high power induction motor. In
this paper, the main structure of simulink (STATCOM) model is
described briefly. Its capability to compensate reactive power to
the system when the voltage sag occurs was described. A phase
control thyristor (SCR) based voltage source inverter (VSI) is
employed for this application. The influences of the initial
operation point and DC capacitance are considered. The
behavior of this system during voltage sag caused by starting of
motor load has been examined.
Simulation result shows the fast response and the STATCOM
capability for mitigate voltage sag.
Index Terms--voltage sag, voltage sag mitigation, static
synchronous compensator, voltage source inverter.
n the past, equipment used to control industrial process was
mechanical in nature, which was rather tolerant of voltage
disturbances. Nowadays, modern industrial equipment
typically uses a large amount of electronic components, such
as program logic control (PLCs), adjustable speed drives and
optical devices, which can be very sensitive to such voltage
The must majority disturbance that causes problems for
electronic equipments is voltage sags.
Voltage sag is defined as a decrease between 0.1 and 0.9 p.u.
in the rms voltage at the network fundamental frequency with
duration from 0.5 cycles to few seconds. This definition
assumes that the sag can be defined by a magnitude and
duration. The duration is usually associated with the time
taken by the protection system to clear the fault.
Voltage sags are huge problems for many industries [1,2]
and they have been found especially troublesome because they
are random events lasting only a few cycles.
However, they are probably the most pressing power quality
problem facing many industrial customers today . Voltage
sags as shown in Fig .1 may cause tripping, production
disturbances and equipment damages. The concern for
mitigation voltage sag has been gradually increasing due to
the huge usage of sensitive electronic equipment in modern
Fig.1. Typical waveform of voltage sag
This paper engaged with voltage sag caused by induction
motor. The induction motors are subjected to the voltage sag
slow down, but usually do not stop operating, if not tripped by
contactors. Problems can occur due to torque oscillations that
can be associated with very deep sags or to tripping of over
current relays, due to the high currents drawn by the motor.
During the voltage sag, an induction motor slows down and
requests higher current. If the sag is unbalanced, it is also
subjected to a negative sequence voltage and therefore it
absorbs a large negative-sequence current component, because
the negative-sequence impedance of the motor is usually low.
High is also the current drawn by the motor after the voltage
has recovered, necessary to rebuild the air-gap flux and
reaccelerate the machine . This phenomenon causes
extended post-fault sag with a long duration (one second or
more) if the motor load is large with respect to the system
impedance. In  it has been shown that the induction motors
influence to the voltage sags during faults.
Some solution approached for compensation of voltage sag a
shunt injection of reactive current and a series injection of
Ambra Sannino, et al , in their paper has carried out
research to examine of the operation of a series connected
VSC for voltage sag mitigation. Static series compensator
A. F . Huweg, S. M. Bashi and N. Mariun are with Department of
Electrical and Electronic Engineering, Faculty of Engineering, Universiti
Putra Malaysia, 43400 Serdang, Selangor, Malaysia (e-mail: senan@
0-7803-8724-4/04/$20.00 2004 IEEE.
SSC is depending on a quit large dc source.
STATCOM is one of the available shunt compensation
devices. The STATCOM obtained by a voltage source
converter (VSC), DC bank charged via bridge rectifier and
proper control, as shown in Figure 2. The proposal of
STATCOM is to injection reactive power to the system when
voltage sag occurs. The amount of reactive power could be
control by change firing angle of the thyristor or the DC
This paper investigates in, the performance of the
STATCOM verified when supplying an induction motor. The
aim of this work is to develop STATCOM module based on
thyristor (SCR) and studies the compensation capability of
this module due to starting an induction motor.
II. STATCOM MODEL
In general, STATCOM use to generate or absorb reactive
power. The active power generation or absorption capability
of the STATCOM is normally used under special
circumstances such as to enhance the steady state and transient
voltage control, to improve the sag elimination capability.
A. Basic operation
The basic electronic block of the STATCOM is the voltage-
sourced inverter that converts an input dc voltage into a three-
phase output voltage at fundamental frequency.
In its simplest form, the STATCOM is made up of a
coupling transformer, a voltage-sourced inverter and a dc
capacitor. In this arrangement, the steady-state power
exchange between the device and the ac system is mainly
reactive. A functional model of the STATCOM is shown in
Fig. 2. STATCOM functional model
Regulating the amplitude of the STATCOM output voltage
controls the reactive power exchange of the STATCOM with
the ac system. If the amplitudes of the STATCOM output
voltage and the ac system voltage are equal, the reactive
current is zero and the STATCOM does not generate/ absorb
reactive power. If the amplitude of the STATCOM output
voltage is increased above the amplitude of the ac system
voltage, the current flows through the transformer reactance
from the STATCOM to the ac system, and the device
generates reactive power (capacitive). If the amplitude of the
STATCOM output voltage is decreased to a level below that
of the ac system, then the current flows from the ac system to
the STATCOM, resulting in the device absorbing reactive
power (inductive). Since the STATCOM is generating/
absorbing only reactive power, the output voltage and the ac
system voltage are in phase, when neglecting circuit losses.
The current drawn from the STATCOM is 90o_ shifted with
respect to the ac system voltage, and it can be leading
(generates reactive power) or lagging (absorbs reactive
A capacitor is used to maintain dc voltage to the inverter.
An uncontrolled rectifier based six diodes used to keep the
capacitor charged to the required levels.
B. Principle of reactive power control
The principle of control reactive power via STATCOM is
well known that the amount of type (capacitive or inductive)
of reactive power exchange between the STATCOM and the
system can be adjusted by controlling the magnitude of
STATCOM output voltage with respect to that of system
voltage. The reactive power supplied by the STATCOM is
Q is the reactive power.
VSTATCOM is the magnitude of STATCOM output voltage.
Vs is the magnitude of system voltage.
X is the equivalent impedance between STATCOM and
When Q is positive the STATCOM supplies reactive power
to the system. Otherwise, the STATCOM absorbs reactive
power from the system.
C. Sag mitigation
The application of a shunt device such as a STATCOM for
mitigation of voltage sag has some advantages when
compared with a series device, as a shunt devices can
simultaneously be used for steady-state voltage control, load
power oscillation damping and as a back up power source .
Some applications of STATCOM for mitigation voltage
sag are presented in [6 – 8].
D. Simulation Modeling
A three phase voltage-sourced inverter is typically made of
six thyristor switches (SRC) to shape the output waveform
and it is the heart of the STATCOM compensator. There are
also six uncontrolled switches (diodes) to maintain dc source
energy charged. The inverter bridge and diodes bridge are
connected together and connected to the grid via transformer
as show in Figure 4.
The inverter bridge, which is the heart of the STATCOM, is
typically injection current to the system. When the
STATCOM voltage is greater than that of the system voltage
the STATCOM will supply VARs to the system. Otherwise,
the STATCOM will absorb VARs from the system as show in
III. SIMULATION RESULTS AND DISCUSSION
The circuit shown in fig .4 was implemented in the
Matlab/simulink software. The system was operated twice.
Once without STATCOM and the second time with the
STATCOM. In both condition the motor started working after
Fig .3. Generation and absorption reactive power
Where V is the STATCOM voltage.
VT is the terminal voltage.
Fig.4. STATCOM simulation circuit
0.25 second of switching on the system. Fig. 5 shows the
system RMS voltage with and without STATCOM. Fig. 5
shows the result obtained during voltage sag of 76% in
magnitude and 0.53 sec duration. Fig 5a shows the RMS
voltage at load terminal during voltage sag before the
STATCOM connected to the system. Fig 5b shows the RMS
voltage when the STATCOM was operation with the system.
The voltage sag improvement clearly shown in fig 5b it is
91% in magnitude and 0.13 sec. figure 5a RMS voltage
Figure 6 summarized the active and the reactive power
injection by the STATCOM to the system when the voltage
sag event. The figure illustrates the active and reactive powers
are positive. That means the load absorbs both active and
reactive power during voltage sag.
Fig 5a Line voltage without STATCOM
Fig 5b Line voltage with STATCOM
Looking at figure 7b, from o.25 sec up to 0.65 sec during this
interval the voltage sag is occurring and also it is at this
interval that the STATCOM in making current injection into
the system as shown in figure 8. Comparing figure 7b with 7a
the interval of voltage sag occurs between 0.25 up to 0.78.
Fig 6 the active and reactive power generated by the STATCOM
The Figures 7a, 7b and 8 show the effect of connecting
STATCOM to the system, the first figure shows the system
before connecting STATCOM .the second figure shows the
system immediately the STATCOM was connected, and third
figure shows the current which drawn by the STATCOM. At
this time the STATCOM voltage is higher than system
voltage. It can also be seen there is a small current drawn by
the STATCOM that it has no effect on the system.
Fig 8 STATCOM current
Hence, it can be seen that the addition of STATCOM
increases the response of the system by 0.13 second. The
addition of STATCOM can thus improve the transient
stability of the system
The dc voltage during sag events is shown in Fig 9. It is show
that the capacitor discharged during voltage sag.
Fig 7a load current without STATCOM
Fig 7b load current with STATCOM
Fig 9 dc voltage
In this paper, the simulation model of static synchronous
compensator STATCOM based thyristor has been constructed
on Matlab/simulink software. Reactive power generation was
achieved by charging and discharging the energy storage
capacitor. The amount of reactive power is depending upon
the thyristor-firing angle as shown in tables.1
CHANGES OF REACTIVE POWER WITH RESPECT TO FIRING
Firing angle (degrees) Reactive power (VAR)
The magnitude of the STATCOM terminal voltage was
controlled with respect to the system voltage. STATCOM
model Tested on Matlab/simulink has shown that it can
improve the voltage sag vector (magnitude and duration).
Furthermore, it has shown the fast response of the STATCOM
to voltage sag phenomena. Simulation results shown that the
voltage sag improvement offered by a STATCOM may
significantly reduce the number of trips in the sensitive
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