 Open Access
 Total Downloads : 179
 Authors : Raj Kamal Kakoti
 Paper ID : IJERTV5IS050517
 Volume & Issue : Volume 05, Issue 05 (May 2016)
 DOI : http://dx.doi.org/10.17577/IJERTV5IS050517
 Published (First Online): 16052016
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
STATCOM Design for Voltage Control using Synchronous Vector PI Controller
Raj Kamal Kakoti Electrical Department GIMT
Azara, Guwahati, Assam 781017
Abstract The objective of this paper is to improve Voltage profile of the system by controlling reactive power using a synchronous vector PI controller. Voltage in a power system is mainly affected by reactive loads and faults. This paper proposes modeling of a STATCOM (STATic+ COMpensator) with a synchronous vector PI controller under a nonlinear load. The prototype has been programmed in MATLAB/SIMULINK environment which has been tested on a standard IEEE 14 bus system. It has been observed that after installing STATCOM the voltages of the buses are found to be within the acceptable limits.
Keywords FACTS, Reactive power, PI vector controller, STATCOM, Voltage control.

INTRODUCTION
After deregulation of the electricity market, the electrical utilities have been trying to improve the efficiency of the power system networks. One of the most concerned problems today is voltage swell or sag [1]. The voltage sag/swell magnitude is ranged from half cycle to one minute. System voltage is directly proportional to the reactive power of the system. Thus whenever there is a disturbance in the reactive power of the system, the system voltage profile gets disturbed. Moreover increased penetration of renewable energy sources such as wind and solar power plants is also disturbing the reactive power of the system. As for instance the solar power plant is employing a great number of semiconductor devices to convert the solar energy into electrical energy [23], the wind power plant is also absorbing great VARs during its running condition [34]. So all these problems are having a great impact on reliable and secure power supply which is very important in the world of Globalization and Privatization of electrical systems. New approaches have been coming up for power system operation and control for congestion management[56], reliable operation and counteracting of various dynamic disturbances[7] such as transmission lines switching, loss of generation, shortcircuits and load rejection. To counteract these problems, the power industries are shifting towards large deployment of FACTS devices [8]. The FACTS
mitigation of system oscillations [9]. Over the period of time various FACTS technologies were developed [10]. With respect to FACTS equipment, voltage sourced converter (VSC) technology, which utilizes selfcommutated technique such as GTOs, GCTs and IGBTs has been successfully applied in a number of installations worldwide for static synchronous Compensator [1112], unified power flow controllers [13] and back to back dc ties [14].
Amongst the FACTS devices, STATCOM is the most preferred device in the power industries for voltage and reactive power compensation [15]. Analogous to a synchronous condenser, STATCOM is operated as a shunt connected static VAR compensator whose capacitive and inductive output current can be controlled independent of ac system voltage. The converters used in STATCOM can be either voltage source or current source converters but voltage source converters are more economical [16] due to the high conduction losses in switches of current source converters
.The shunt connection of STATCOM to the Grid is shown in Fig.1
STATCOM provides the following advantages:

Quick response to system disturbance.

Smooth voltage control over a wide range of operating conditions.

Damping of power oscillations.

Transient stability improvement.

Alsop control of active power possible(with a DC energy source).

No Sub Synchronous Resonance.

Less space required due to availability in modules.
Vdc
Coupling Transformer
technology has the principal role of enhancing controllability and power transfer capability in power system. This technology enables the loading of transmission line closer to its thermal limit. FACTS devices can also be effectively used for power flow control, load sharing along parallel feeders, voltage regulation, and enhancement of transient stability and
AC system
Voltage sourced
converter
Fig 1: Connection of a STATCOM to ac system
The ideal VI characteristic of a STATCOM is shown in Fig. 2. STATCOM can be used both for leading VARs and
Vr
Vy
Vb
Vr
Vy
Vb
Rt Lt Er
lagging VARs compensation.
ing
voltage
swi
Rated capacitive current
Rated inductive current
ing
voltage
swi
Rated capacitive current
Rated inductive current
Limit for safe
switch
STATCOM
Limit for safe tching
Rt Lt
Rt Lt
Ey Eb
Voltage sourced
Rsc
Vdc
AC system
Coupling Transformer
equivalent
converter
Figure 4: Equivalent Diagram of a STATCOM
Control range
STATCOM
TABLE I NOMENCLATURE
Leading VARs Lagging VARs
current
Figure 2: Ideal VI characteristic of STATCOM
By implementing various hybrid converters the ideal characteristic can be made drooping as per the requirement for VAR compensation [17]. One of such modified drooping characteristic is shown in Fig.3.
Limit for safe switching
Rated capacitive current
STATCOM
voltage
Rated inductive current
Limit for safe switching
Control range
STATCOM
Symbol
Description
Vr, Vy, Vb
Line voltages
Rt
Equivalent resistance of the coupling transformer
Lt
Equivalent inductance of the coupling transformer
ws
System Frequency
Er , Ey , Eb
Voltages available in the STATCOM output
RSC
Switching loses equivalent resistance
VDC
Voltage available across the capacitor
C
Capacitance across the converter
s
Phase angle
Vs
R.M.S phase voltage
Firing angle
k
Factor relating Vdc and Vs
Symbol
Description
Vr, Vy, Vb
Line voltages
Rt
Equivalent resistance of the coupling transformer
Lt
Equivalent inductance of the coupling transformer
ws
System Frequency
Er , Ey , Eb
Voltages available in the STATCOM output
RSC
Switching loses equivalent resistance
VDC
Voltage available across the capacitor
C
Capacitance across the converter
s
Phase angle
Vs
R.M.S phase voltage
Firing angle
k
Factor relating Vdc and Vs
current
Leading VARs Lagging VARs
Figure 3: Drooping VI characteristic of STATCOM
The ac KVL equations for the circuit can be expressed in matrix form,
dir


MATHEMATICAL MODELLING OF THE SYSTEM
dt
i E V

Modelling of STATCOM
di
R r
r r
1
y t i E
V
(1)
The methodology proposed in this paper as with reference to [17], under the method of linearization. In fig 4. the
dt L y
t i
L y y
t E V
equivalent model of the STATCOM being connected to the AC supply has been shown.
dib
dt
b
b b
The AC system phase voltage and the output of the STATCOM (neglecting harmonics) is given by (2) & (3) respectively,
Vr
2Vs sin(wst s )
(2)
Vr kVDC sin(wst )
(3)
The above system is now transformed to a synchronous reference frame (on p.u. basis),
did
dt
controllers of current vector components in a rotating synchronous frame (dq) [21]. This controller has been used
id
Vs coss
as the control scheme in this paper as this scheme eliminates
diq
A * i
ws V
sin
(4)
the errors completely and also operates satisfactorily in high
dt
s q
L s s
frequency system. The only disadvantage of this controller is
V
t 0
that its dynamic properties are inferior to that of other non
dVDC
DC
linear controllers.
PWM
modulator
PWM
modulator
dt isdref
Current regulator
Coordinate transformation
where,
R w kw cos( )
PI
isq
isq
–
–
+
PI
PI
ref
DQ
– To RYB
– To RYB
to –
Three phase converter
w
w
t s
L
s s +
s L –
– To DQ
– To DQ
RYB
to –
RYB
to –
t t
isd ir
A w
Rt ws kws sin( s ) (5) iy
s s
Lt Lt
Cw
isq
ib
Non
M cos( )
M sin( ) s
linear
and
k s k s
Lt
Figure 5: Synchronous vector Controller (PI) Scheme
load
Mk 1.5* k * ws *C
(6)


IMPLEMENTATION
The injected active power and reactive power at the bus
connected to the STATCOM are given by,
The design of STATCOM with Synchronous Vector PI controller, presented in Fig. 4, has been implemented on a
P Vs *cossid sinsiq
Q Vs *sinsid cossiq
(7)
(8)
standard IEEE 14 bus system (Fig. 6) using MATLAB. Table II represents the data of IEEE 14 bus system [22].
The characteristic equation of the linearized system of (4) is 1
13 14
12
given by,
2R w w C R w R w 2w C 3k 2w2C
11 10 8
6 9
7
7
s3 s2 t s s s t s t s s w2 s
L R L L R
s 2L 5
t SC t
t SC t
w3C R2 3k 2 w3CR
(9)
s 1 t s t 0
2 3 4
R L2 2L2
SC t t
Now with reference to [18], the per unit values of
Rt = 0.01, Lt=0.15, C=0.88, Rsc =100/K, K=4/, ws=377
are taken and the characteristic equation (9) has been solved for the Eigen values in MATLAB.These parameters yield the following Eigen values for the Linearized system,
s=3.8 & s=25.35Â±j147
These values show that the STATCOM is highly overdamped and also has a high frequency of oscillations. Now in steady state (4) has been solved.
Figure 6: Standard IEEE 14 bus system
TABLE II
BUS DATA OF 14BUS SYSTEM
Bus
Voltage
Angle
PGi
QGi
PLi
QLi
1
1.0600
0.00
188.8
6
9.90
188.8
6
0.00
2
1.0450
4.05
40.00
34.63
21.70
12.7
0
3
1.0100
11.17
0.00
23.90
94.20
19.0
0
4
1.0202
8.237
0.00
0.00
47.80
– 3.90
5
1.0232
6.915
0.00
0.00
7.60
1.60
6
1.0700
10.32
20.00
15.83
8
11.20
7.50
7
1.0528
9.397
0.00
0.00
0.00
0.00
8
1.0900
7.638
20.00
23.31
1
0.00
0.00
9
1.0356
11.16
0.00
0.00
29.50
16.6
0
10
1.0341
11.30
0.00
0.00
9.00
5.80
11
1.0482
10.94
0.00
0.00
3.50
1.80
12
1.0537
11.20
0.00
0.00
6.10
1.60
13
1.0473
11.27
0.00
0.00
13.50
5.80
14
1.0225
12.23
0.00
0.00
14.90
5.00
td>
1.0900
Bus
Voltage
Angle
PGi
QGi
PLi
QLi
1
1.0600
0.00
188.8
6
9.90
188.8
6
0.00
2
1.0450
4.05
40.00
34.63
21.70
12.7
0
3
1.0100
11.17
0.00
23.90
94.20
19.0
0
4
1.0202
8.237
0.00
0.00
47.80
– 3.90
5
1.0232
6.915
0.00
0.00
7.60
1.60
6
1.0700
10.32
20.00
15.83
8
11.20
7.50
7
1.0528
9.397
0.00
0.00
0.00
0.00
8
7.638
20.00
23.31
1
0.00
0.00
9
1.0356
11.16
0.00
0.00
29.50
16.6
0
10
1.0341
11.30
0.00
0.00
9.00
5.80
11
1.0482
10.94
0.00
0.00
3.50
1.80
12
1.0537
11.20
0.00
0.00
6.10
1.60
13
1.0473
11.27
0.00
0.00
13.50
5.80
14
1.0225
12.23
0.00
0.00
14.90
5.00
(10)
(11)

Modeling of Control scheme
Various current control schemes for three phase voltage source PWM converters have been explained in [20]. Out of these schemes the Synchronous vector controller (PI) been implemented in this paper. The schematic diagram of this scheme is shown in Fig 5. The synchronous vector PI controller is used when the phase or amplitude errors are needed to be completely eliminated. It employs two PI


RESULTS
Load flow analysis is performed initially to identify bus voltages which are out of the tolerance level (5%). The results of load flow analysis in steady state show that the voltages in buses 9 and 13 violate the tolerance level. Hence the modeled STATCOM is connected to these buses to bring the voltage level within tolerance limit.
Table III shows the load flow analysis of 14bus system. It can be observed from Fig. 7 that the voltage profile is going out of the tolerance level in bus 9 and 13.
Table IV shows the load flow analysis of 14bus system with STATCOM been connected at buses 9and 13 respectively. It is evident from Fig. 8, that the voltage profile has been improved considerably. Thus it can be concluded that STATCOM results in improvement of voltage profile by controlling the reactive power.
TABLE III
RESULTS OF LOAD FLOW WITHOUT STATCOM
Bus
Voltages
Angle
1
1.0400
0.0000
2
1.0430
5.3543
3
1.0196
7.5308
4
1.0104
9.2841
5
1.0100
14.1738
6
1.0392
14.0644
7
1.0020
12.8649
8
1.0100
11.0581
9
0.9335
16.5031
10
1.0145
15.6550
11
0.9991
16.3007
12
0.9944
16.9077
13
0.9428
17.8067
14
1.0132
16.0084
Figure 7: Graph of load flow studies without STATCOM
TABLE IV
RESULTS OF LOAD FLOW WITH STATCOM
Bus
Voltages
Angle
1
1.0400
0.0000
2
1.0430
5.3543
3
1.0200
7.5318
4
1.0108
9.2848
5
1.0190
14.1692
6
1.0402
14.0508
7
1.0023
12.8655
8
1.0100
11.8168
9
1.0000
16.7794
10
1.0240
15.7112
11
1.0091
11.7434
12
1.0076
17.0359
13
1.0000
18.0205
14
1.0330
15.0935
Figure 7: Graph of load flow studies with STATCOM

CONCLUSION
This paper illustrates the design of a STATCOM model using a synchronous vector PI control technique to meet the voltage dip problem. The results obtained and the analyses are the justifications of the design.
The present work can be extended to incorporate the following sectors:

Conduction loss calculation of the switches.

Harmonic analysis and appropriate filter design.

Operation during various fault conditions.

Optimization of the capacitance value.

Dynamic performance.
New trends in the current control techniques have come up such as hysteresis controller neural networks and fuzzylogic based controllers. These controllers can also be implemented on the model discussed above.
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