International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 5, May 2016)
Designing and Modelling of the STATCOM for Voltage
Improvement in the Transmission Line.
Rubi Kumari1, Chitrangada Roy2
1,2
Sikkim Manipal Institute of Technology, Electrical and Electronics Engineering Department, Majhitar, Sikkim-737136
The power system supplies power to a large number of
loads and is feeding from many generating units, which
leads to a problem of maintaining voltages within the
required limits. As load varies, the requirements of reactive
power in the transmission system also vary. As it is known
that the reactive power cannot be transferred or transported
over long distances, voltage control can be controlled by
using special devices located through the system which
possess difficulties in keeping sufficient levels of voltage in
the power system network.
The proper selection and coordination of equipment for
controlling reactive power and voltage stability is very
important for upgrading the voltage level in the power
system. Due to these challenges some FACTS devices for
controlling and compensating reactive power in the power
system. To overcome the additional demand for reactive
power and to maintain the voltage stability in the power
system devices such as SVC (Static Var Compensator) ,
STATCOM (Static Synchronous Compensator)were
introduced. The main aim of FACTS device is only to
increase of power flows in the high voltage side of network
during both steady state and transient conditions. In recent
decades, there has been significant progress in terms of
equipment designed to improve the stability of voltage in
power systems. This is mainly due to the development of
power supply systems in the world, which requires seeking
better ways of adjusting and controlling power flows and
voltage levels.
In this paper detailed explanation about the effects of the
STATCOM at the time of voltage dip at 0.6sec has been
mentioned. The MATLAB Simulation results obtained
explains how effectively STATCOM injects the reactive
power in the transmission system in order to improve the
voltage dip in the power system.
Abstract— This paper presents the use of the Static
Synchronous Compensator (STATCOM) for the voltage
regulation in the power system. A STATCOM is a Power
Electronics based Voltage Source Converter (VSC). The
objective of this study was to decrease the voltage fluctuation
or dip and to increase the power flow in the power system.
The MATLAB Simulation was carried out which allowed to
analyse the response of the STATCOM at the time of change
in load.
Keywords—Static
Synchronous
Compensator
(STATCOM), State Space Vector PWM (SVPWM), Voltage
Source Converter (VSC), Flexible AC Transmission Systems
(FACTS)
I. INTRODUCTION
Due to continuous increase in the demand of the
electricity it has affected the stability of the power system.
The STATCOM is one of the FACTS devices which can
improve the voltages profile in the transient state and can
improve power quality of the transmission system. The
voltage stability, steady state and transient stabilities of a
power system can be improved by the use of FACTS
devices [1]. The STATCOM falls into Shunt Controllers
category which provides high efficiency, continuous and
fast response time, continuous and dynamic voltage
control.
The voltage control and reactive power control is an
important issue in power system operation. This is because
of the differences between generation and transmission
systems. This paper comprises of the techniques which will
avoid the voltage collapse in the power system. In order to
achieve efficient and reliable operation of power system,
the control of voltage and reactive power should satisfy the
following objectives:1. Voltages at all terminals of all equipment in the
system
1. are within acceptable limits
2. System stability is enhanced to maximize utilization
of the transmission system
3. The reactive power flow is minimized so as to reduce
I²R and I²X losses.
II. SYSTEM MODELLING
The MATLAB blocks represent a test system model.
The test system model consists of a three phase source
which generates 11 kV at 50Hz. The voltage has been
stepped up to 33 kV using a 11kV/33kV, 5MVA three
phase transformer.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 5, May 2016)
The length of three phase 33 kV pi-section line is 50
Km. A 1 MW load is connected to the 33 kV transmission
line. At the receiving end side voltage has been stepped
down to 11 kV using a 33kV/11kV, 5MVA three phase
transformer receiving end comprises of two RL loads of 1
MW and 4.25 MW at 0.8 pf lagging each . The transient
time given to the three phase breaker is 0.6 sec and at that
instant the 4.25 MW load is introduced to the system.
Fig. 2. Functional Block Diagram of the STATCOM
B. V-I Characteristics of the STATCOM
The Static synchronous compensator can be operated in
two different modes:
1. In voltage regulation mode (the voltage is regulated
inside limits as explained below).
2. In VAR control mode (the STATCOM reactive power
output is kept stable) when the STATCOM is
operated in voltage regulation mode.
As long as the reactive current stays inside the converter
rating, the voltage is regulated at the reference voltage Vref.
However, a voltage droop is normally used, and the V-I
characteristic has the slope indicated in the figure. The V-I
characteristic is described by the following equation in the
voltage regulation mode:
Fig. 1. SIMULINK MODEL of the Test System
III. STATCOM DESIGN
A. Operating Principle of the STATCOM
Let V be the voltage of power system and V s be the
voltageproduced by the voltage source converter (VSC).
During steady state working condition, the voltage Vs
produced by VSC is in phase with V (i.e.=0) in this case
only reactive power is flowing. If the magnitude of the
voltage (Vs) produced by the VSC is less than the
magnitude of V, the reactive power is flowing from power
system to VSC(the STATCOM is absorbing the reactive
power). If Vs is greater than V the reactive power is
flowing from VSC to power system (the STATCOM is
producing reactive power) and if the Vs is equal to V the
reactive power exchange is zero. The amount of reactive
can be given as:Q=
V = Vref + IXs
(2)
where,
V = Voltage at Positive sequence (pu)
I= Reactive Current
Xs = Slope or droop reactance
The following V-I characteristic of STATCOM is shown
below:-
(1)
217
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 5, May 2016)
Neglecting the power losses, the instantaneous power
balance between the AC and DC sides will result in:(5)
=
+
+
=
(6)
+
)
(7)
Where, vdc = DC link voltage and idc = DC link current.
Thus equation (8) can be written as :-
=
(8)
Equation (ix) shows that the DC bus voltage has been
maintained constant by controlling the d−component of
STATCOM current
. The instantaneous STATCOM
generated reactive power which has been calculated by
equation (ix):-
Fig. 3. V-I Characteristics of the STATCOM
C. STATCOM Controller Design
The STATCOM is a three phase VSC with a DC bus
capacitor [3]. The simplified equivalent circuit of the
STATCOM has been shown in fig. 2, which comprises of a
DC link capacitor, a VSC, three resistors (Rstat), and three
inductances (Lstat). The resistances Rstat has been connected
in series with the AC lines. The Rstat resistance represents
conduction losses of the transformer, whereas the Lstat
inductance represents the filter leakage inductances [3].
=
(9)
Considering equations (3) and (4), the decoupling of the
cross coupling between the d and q components of
STATCOM current has been done,
in
and
in
in the control law used. The two PI
controllers has been used for the implementation of the
voltage control schemes. From equations (3) and (4), the
dynamics of the current control loop are given as follows:-
=
G(s) =
=
(10)
Where ,
Fig. 4. STATCOM equivalent model
The equations of the STATCOM can be derived from
the fig.4 can be written as:=
+
-
+
(3)
=
+
+
+
(4)
=
+
(11)
=
+
(12)
=
=
+
(13)
(14)
Where,
Thus the modelling of Statcom comprises of the
following steps :1. Firstly, the voltage components has been transformed
from three phase to two phase d−q using Clark’s
transformation.
(vd ,vq), (id, iq) and (vsd, vsq) are the d-qq components of
the transmission line voltage (va, vb, vc) , transmission line
current (ia, ib, ic ) and voltage source converter output
voltage (vsa, vsb, vsc ) respectively.
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Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 5, May 2016)
2. Then from d−q frame to the stationary frame using
Clark’s transformation.
3. Then using Clark’s reverse transformation the voltage
components has been transformed from two-to-three
phase generate the three-phase reference voltages i.e.
va* , vb* , vc*.
4. The current references i.e. i*q and i*d have been
calculated by the DC and AC voltage controllers (PI) .
5. The voltage references i.e. v*q and v*d have been
calculated by current controllers (PI) .
V. RESULTS AND DISCUSSION
In this paper, initially the three phase test system without
the STATCOM has been observed under various load
conditions at 0.6sec of transient time. Later the same three
phase test system with STATCOM has been examined
under the respective load conditions.
Fig. 7. Transmission line voltage at bus no.3 without STATCOM.
The fig.7 shows the voltage response of the stepped up
transformer connected to bus no.3 to the test system in
absence of the STATCOM.
Fig. 5. MATLAB/SIMULINK model of the STATCOM controller.
IV. STATE SPACE VECTOR PWM
There are various PWM techniques which are used to
obtain variable voltage and frequency supply. The most
widely used PWM technique for three-phase VSC are
carrier-based sinusoidal PWM and SVPWM. The main
advantage of SVPWM is that it gives higher switching
frequency which is not possible by Sinusoidal PWM
technique and also there is degree of freedom of space
vector placement in the switching cycle. This improves the
harmonic performance of the SPVPWM [3]. The fig.6
shows the MATLAB/SIMULINK model of the State Space
Vector PWM:-
Fig. 8. Transmission line voltage at bus no.3 with the STATCOM .
The fig.8 shows the voltage response of the stepped up
transformer connected to bus no.3 to the test system in the
presence of the STATCOM.
Fig. 9. RMS voltage of the transmission line system at bus no.3
without the STATCOM.
The fig.9 shows the voltage dip at 0.6sec due to change
in load. At the first, load 1 of 1 MW with 0.8 p.f lagging
(reactive load) has been connected to the 33 kV
transmission line.
Fig. 6. MATLAB/SIMULINK model of the State Space Vector PWM.
219
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 5, May 2016)
At 0.6 sec the second load i.e. 4.25 MW load at 0.8 p.f
lagging has been introduced at receiving end side. Thus a
voltage dip at 0.6 sec due to change in load was observed at
bus no.3. In order to overcome the voltage dip STATCOM
has been used.
Fig. 13. RMS voltage of the transmission line at bus no 4 without the
STATCOM.
As shown in fig.13 a voltage dip at 0.6sec was observed
at the receiving end of the three phase transmission system
due to change in load in the system. But after connecting
STATCOM to the three phase transmission line the voltage
came back to its nominal value and the desired response
was achieved as shown in the fig 12.
Fig. 10. RMS voltage of the transmission line system at bus no.3 with
STATCOM
As shown in fig .10 when STATCOM was connected to
the test system it was observed that the voltage dip
occurred recovered and came back to its nominal value.
Thus no voltage dip was observed at bus no.3 .
Fig. 14. RMS voltage of the transmission line at bus no. 4 with the
STATCOM.
After connecting STATCOM to the three phase test
system it was observed that there was no voltage dip and
thus came back to its nominal value at bus no.4 as shown in
fig.14.
Fig. 11. Voltage of the transmission line (load side) at bus no.4 without
the STATCOM.
The fig.11 the response of the voltage at bus no. 4
connected to the test system in the absence of the
STATCOM in the transmission system.
Fig. 15. Reactive Power of the three phase transmission line.
Fig15 shows the reactive power of the three phase
transmission line.
Fig. 12. voltage of the transmission line (load side) at bus no.4 with the
STATCOM .
The fig.12 represents the voltage profile obtained of the
bus no. 4 in the presence of the STATCOM connected to
the three phase test system.
220
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 5, May 2016)
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[1]
Fig. 16. Reactive Power of the STATCOM
Fig16 shows the amount of reactive power inserted by
the STATCOM at the time of disturbances (change in load)
to the three phase transmission line.
VI. CONCLUSION
This paper explains about the implementation of the
STATE SPACE VECTOR PWM based STATCOM to the
three phase test system so that voltage matches its desired
response at the time of change in loads. In this paper the
stability improvement in the power system i.e. the voltage
level of the STATCOM has been described. A simple
Matlab/Simulink model of the SVPWM based STATCOM
has been designed for the three phase Voltage Source
Converter.
In this paper, the simulation results of the three phase
test system without the STATCOM have been compared
with the three phase test system with the STATCOM at
0.6sec of transient time. The results obtained shows that
that three phase test system model in presence of the
STATCOM has been able to improve the voltage stability
of the power system. Whereas, the three phase test system
model without the STATCOM showed the voltage dip at
0.6sec in the system when the respective loads were
connected to the system. From the above results shown it is
clear that the STATCOM successfully improves the voltage
dip in the power system.
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