 Open Access
 Total Downloads : 2548
 Authors : Habibur, Md. Fayzur Rahman, Harun
 Paper ID : IJERTV1IS7540
 Volume & Issue : Volume 01, Issue 07 (September 2012)
 Published (First Online): 03102012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Modelling & Performance Comparison of Different Types of SSSCBased Controllers
Habibur1, Md. Fayzur Rahman2, Harun3
1,3Dept. of EEE, Rajshahi University Of Engineering & Technology,Rajshahi6204,Bangladesh
2Professor & Head, Dept. Of ETE, Daffodil International University,Dhaka1207, Bangladesh
Abstract
This paper presents some new & different types of SSSC controller & compare their performance for different types of faults during transient conditions to improve the voltage level of a large scale power system. In this method, the network differential equations were replaced by a set of algebraic equations at a fixed frequency which dramatically reduced the simulation time. Moreover, this paper contributes to the improvement of transient stability of multimachine machines power system system by using different types of SSSC controllers i.e. POD, PI, PID, PLL & generic controller. The system response was simulated and evaluated during single and three phase faults applied to the terminals. This work is presented to improve the voltage stability & Damp out the oscillation by using SSSC with & without controllers & compare their performance to enhance the stability of power system. Simulation results show that SSSC with controllers enhance the stability of multimachine power system effectively.
Keywords Static Series Synchronous Compansator (SSSC), voltage regulator, PI,POD,PID, generic controller, IGBT, MATLAB Simulink.

Introduction
Stability improvements is very important for large scale power system. SSSC is one of the important members of FACTS family which can be installed in series in the transmission lines[1]. Traditionally, fixed or mechanically switched shunt and series capacitors, reactors and synchronous generators were being used to damped out oscillation[2]. However, there are some restrictions as to the use of these conventional devices. For many reasons desired performance was being unable to achieve effectively[3]. A SSSC is an
electrical device for providing fastacting reactive power compensation on high voltage transmission networks and it can contribute to improve the voltages profile in the transient state[5]. A SSSC can be controlled externally by designing PI, PID, POD, PLL & generic controller which can improve the dynamic & steady state performance of a large scale power system. The dynamic nature of the SSSC lies in the use of thyristor devices (e.g. GTO, IGCT) [4].Therefore, this paper presents thyristor based SSSC controllers to improve the performance the multimachine power system.

Control Concept Of SSSC
the SSSC does not use any active power source, the injected voltage must stay in quadrature with line current. By varying the magnitude Vq of the injected voltage in quadrature with current, the SSSC performs the function of a variable reactance compensator, either capacitive or inductive. The variation of injected voltage is performed by means of a VoltageSourced Converter (VSC) connected on the secondary side of a coupling transformer. The VSC uses forced commutated power electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage V_conv from a DC voltage source that shown in fig.1[6].
– Vs + I
I=Id(Iq=0)
Vs=V2V1=Vd+jXL Vd=0
Vq>0; sssc is capacitive Vd<0; sssc is inductive
Vconv
V
V2
1
VSC
Vdc
Fig.1 Connection diagram of SSSC with transmission Line
A capacitor connected on the DC side of the VSC acts as a DC voltage source. A small active power is drawn from the line to keep the capacitor charged and to provide transformer and VSC losses, so that the injected voltage Vs is practically 90 degrees out of phase with current I. In the control system block
G1
2100MVA
2100MVA
13.8/500KV B1
PI,PID,POD,PLL
Generic controller
T.F1
250MW
Load
B2 280KM
T.L.
SSSC
Fault B4
150KM
100MW
diagram Vd_conv and Vq_conv designate the components
of converter voltage Vq_conv which are respectively in phase and in quadrature with current.
T.F2
G2
1400MVA 1400MVA
13.8/500KV
50KM B3
50MW Load
Three phase dynamic Load
The control system consists of:
A phaselocked loop (PLL) which synchronizes on the positivesequence component of the current I. The output of the PLL (angle T=t) is used to compute the directaxis and quadratureaxis components of the AC threephase voltages and currents (labeled as Vd, Vq or Id, Iq on the diagram).Measurement systems measuring the q components of AC positivesequence of voltages V1 and V2 (V1q and V2q) as well as the DC voltage Vdc. AC and DC voltage regulators which compute the two components of the converter voltage (Vd_conv and Vq_conv) required to obtain the desired DC voltage (Vdcref) and the injected voltage (Vqref). Fig.2 represents that control concept[6]. The Vq voltage regulator is assisted by a feed forward type regulator which predicts the V_conv voltage from the Id current measurement.
Fig.2 Single line diagram of 2machine power system with different types of SSSC controller
The first power generation substation (G1) has a rating of 2100 MVA, representing 6 machines of 350 MVA and the other one (G2) has a rating of 1400 MVA, representing 4 machines of 350 MVA. The load center of approximately 2200 MW is modeled using a dynamic load model. The generation substation G1 is connected to this load by two transmission lines L1 and L2. L1 is 280km long and L2 is split in two segments of 150 km in order to simulate a threephase fault at the midpoint of the line. The generation substation G2 is also connected to the load by 50km line (L3). When the SSSC is bypass, the power flow towards this major load is as follows: 664 MW flow on L1 (measured at bus B2), 563 MW flow on L2 (measured at B4) and 990 MW flow on L3 (measured at B3). The SSSC, located at
I
Current measurement
Iq=0
PLL =t
V1 voltage
V1 measurement
V2 voltage
V2 measurement
Id
V1q
Control system
Vqref
Vq
Vdcref
Vq voltage Vq_conv regulator
bus B1, is in series with line L1. If it has a rating of 100MVA then it is capable of injecting up to 10% of the nominal system voltage. This SSSC is a phasor model of a typical threelevel PWM SSSC. Machine, POD & SSSC parameters value was taken from reference[6].
3
m Pm
Pref Vf
m A B
C
Pm
Vf _
–
B 1 CT 2
DC voltage measurement
Vdc
DC voltage Regulator
Vd_conv
A a + i
B b
C
C c
Vsc pulse
PWM
Modulator
Vd_conv Vq_conv
Pref 1
Reg _M1
2100 MVA
M1
2100 MVA
A B C
13 .8 kV/500 kV
2
250 MW
SSSC
m
A1
A2
B1
B2
C1
SSSC
L21 (150 km)
A
B C
Three Phase Fault A
Fig.2 SSSC based control system

Power System Model With SSSC
This example described in this section illustrates
Vpos. seq. B1 B2 B3 B4 6
V P Q
Measurements
P B1 B2 B3 B4 (MW) 5
Q B1 B2 B3 B4 (Mvar )
C 2
m CT 1
i
–
A
+
B
1 B2
L1
B4
L22
B C
A
B
A B C
100 MW
modeling of a simple tansmission system containing 2
C
m Pm
Pref Vf
m
Pm A
B
L3_50 km

a

b
(280 km)
(150 km)
hydraulic power plants [Fig.2]. The power grid consists
Vf _

C c
of two power generation substations and one major
Pref 2
Reg _M2
1400 MVA
M2
1400 MVA
13 .8 kV/500 kV
B3 A
A B C
B m
50 MW C
load center at bus B3.Complete simulink model is
Phasors
powergui
Three Phase Dynamic Load
shown in Fig.3.
Fig.3 Complete simulink model (without SSSC controller)
3.1. Simulation Results: Two types of faults: 3.1.1Single line to ground fault & 3.1.2 Threephase faults have been considered.
parameters becomes stable & its performance becomes higher then without controller.
Bypass
Bypass
A1
B1
C1
1
Bypass
3.1.1 Single line to ground fault: During single line to ground fault occurred at 0.1s & circuit breaker is opened at 0.2s (3phase 4cycle fault),If no SSSC is used then system becomes unstable[Fig.3(a)].But, If SSSC is applied then system voltage becomes stable within 0.65s[Fig.3(b)].
dw2
pm 2
m
dw1
K –
Gain
5
K –
Gain
5
5s Transfer Fcn
1
5s Transfer Fcn
Saturation
2
Saturation
20 MVA SSSC
m
Bypass 20 MVA
SSSC
SSSC
Vqref
A2
B2
Vqref
A1
B1
C1
A2
B2
C2
C2
SSSC
1.5
Voltage
1
0.5
0
Va
1
0.8
0.6
m
0 0.2 0.4 0.6 0.8 ti 1 e 1.2 1.4 1.6 1.8 2
Va
Fig.3(a) Bus voltage(B1) in p.u.( without SSSC)
m
pm1 m
2
Fig.4 Simulink diagram of SSSC P.I. controller
4.1 Simulation Results: Here also two types of faults: 4.1.1 Single line to ground fault & 4.1.2 Three phase faults have been considered.
4.1.1 Single line to ground fault: If PI controller is used as SSSC controller then, the system oscillation (delta d or pm) becomes stable within 8s with 0.01% damping[Fig.4(a)] & Bus voltage becomes stable within 0.6s with 0% damping [Fig.4(b)].
0 0.5 1 1.5
time
Fig.3(b) Bus voltage(B1) in p.u for 1phase fault (with SSSC)
3.1.2 Threephase faults: During 3phase faults, If SSSC is applied then at t=0.7s system voltage becomes stable within 6% damping[Fig.3(c)].
Va
Vb
Vc
Bus Voltage
1
0.8
0.6
0 0.5 1 1.5
time
Fig.3(c) Bus voltages in p.u for 3phase faults
Fig.4(a) Oscillation, Vqref in pu for 1phase faults
Va
1.2
1.1
1
Va
0.9
0.8
0.7
0.6


SSSC Model with PI controller
0.5
0 0.5
time 1 1.5
SSSC with proportional Integral (PI) controller is shown in Fig.4. The angular speed deviation d & mechanical power Pm has been taken as an input parameter. When any faults occurred in the network
,then both machines angular speed d mechanical power Pm & bus voltages will be changed & oscillated. But, when SSSC with PI controller is applied then all
Fig.4(b) Bus voltage in P.U. for 1phase faults
4.1.2 Threephase faults: Machines Oscillation (delta d or delta pm) becomes stable within 7s with 0.01% damping[Fig.4(c)] & Bus voltage becomes stable within 0.85s with 0% damping [Fig.4(d)]
Fig.4(c) Oscillation, Vqref in pu for 1phase faults
Fig.5(a) Oscillation, Vqref in pu for 1phase faults
1.1
Bus Voltage
1
0.9 Va
Vb
1.1
Va
1
Va
0.9
0.8
0.7
0.8 Vc
0.7
0.6
0.6
0.5
0 0.5 1 1.5
time
0.5
0 0.5
time 1 1.5
Fig.5(b) Bus voltage(B1) in p.u for 1phase fault
Fig.4(d) Bus voltages (in p.u.) for 3phase faults

SSSC Model with PID controller
Proportional Integral Derivative(PID) controller is one of the most power full controller which takes angular speed deviation(d),mechanical power difference Pm as input & after taking successively multiplication
,integration & derivative, the parameters related with this network becomes stable. The PID controller simulink model is shown in Fig.5
Bypass
5.1.2 Threephase faults: During 3phase faults, Oscillation (delta d or delta pm) becomes stable within 7s with 0.01% damping[Fig.5(c)] & Bus voltage becomes stable within 0.7s with 0% damping [Fig.5(d)]
dw2
pm 2
dw 1
K
Ga5in
K –
Ga5in
1
5s Transfer Fcn
1
5s Transfer Fcn
10 s+1
s
Transfer Fcn 1 Saturation
2
10 s+1
s
Transfer Fcn 1 Saturation
20 MVA SSSC
m
Bypass
20 MVA SSSC
SSSC
Bypass Vqref A1
B1 C 1
Bypass Vqref A1
B1 C 1
m
m
A2 B2
C2
A2 B2
C2
SSSC
Fig.5(c) Oscillation, Vqref in pu for 3phase faults
Bus Voltage
1.1
Va Vb
Vc
1
0.9
0.8
0.7
0.6
pm 1
0.5
m 0 0.5
2
time
1 1.5
Fig.5 Simulink model of SSSC with PID controller
5.1 Simulation Results: Two types of faults has been considered.
5.1.1 Single line to ground fault: During 1phase faults, the system oscillation (delta d or pm) becomes stable within 7s with 0.01% damping[Fig.5(a)] & Bus voltage becomes stable within 0.6s with 0% damping [Fig.5(b)].
Fig.5(d) Bus voltages (in p.u.) for 3phase faults

SSSC Model with POD controller
Power Oscillation Damping (POD) controller is also one of the most power full control system Which externally injects Vqref to the SSSC. The POD controller consists of an active power measurement system, a general gain, a lowpass filter, a washout
highpass filter, a lead compensator, and an output limiter. All parameter values has been taken from [6].
Bypass
P_MW
Vqref
Iabc
Vqref *
Vabc
Vabc _B2 Iabc _B2
Step Vqref POD Controller
P_B2
A1
B1
C1
20 MVA SSSC
SSSC
Bypass
Vqref
m
A2
B2
C2
m
Fig.6 Simulink model of SSSC with PID controller

Simulation Results: Two types of faults has been considered.

Single line to ground fault: During 1phase faults, the system power becomes stable within 0.2s with 0.05% damping[Fig:6(b)] & Bus voltage becomes stable within 0.52s with 0.05% damping [Fig.6(a)].
Va
1.2
Fig.6(d) Bus power (MW) for 3phase fault



SSSC Model with Generic controller
The block diagram of generic SSSC controller is shown in Fig:7
1.1
1
Va
0.9
0.8
0.7
0.6
0.5
0 0.5
time 1 1.5
Fig. 7 Generic SSSC controller block diagram The input of this controller is also the speed deviation of two machines & deviation of Pm. Here, T=10,T2=T4=0.3 has been taken as constant &
Fig.6(a) Bus voltage(B1) in p.u. for 1phase fault
gain,K,T1 & T3 can be selected by properly trail & error methods. For this network, the optimum value was, K=65.49,T1=0.5527 & T3=0.2563.
Bypass
Bypass Vqref
A1 B1 C 1
6
dw2
K
Gain 5
Transfer Fcn
TransferFcn 1
10 s+1
10 s+1
0.5527 s+1
0.2639 s+1
0.3s+1
0.3s+1
Transfer Fcn 2 Saturation
20 MVA SSSC
m
A2 B2
C2
SSSC
dw 1 m
Fig:6(b)Bus power (MW) for 1phase fault

Threephase faults: During 3phase faults, System power becomes stable within 0.2s with 0.05%
pm 2
K
Gain 5
Transfer Fcn
Transfer Fcn 1
6
10 s+1
10 s+1
0.5527 s+1
0.2639 s+1
0.3s+1
0.3s+1
Transfer Fcn 2 Saturation
Bypass
m Bypass
20 MVA SSSC
Vqref A1 B1 C 1
A2
B2
C2
SSSC
damping[Fig:6(d)] & Bus voltage becomes stable within 0.8s with 0% damping [Fig.6(c)]
Bus Voltage
1.1
1
pm 1 m
Fig.8 Simulink model of generic SSSC controller

Simulation Results: Two types of faults has been considered.

Single line to ground fault: During 1phase faults, if PI controller is used as SSSC controller then,
Va
Vb
Vc
data4
0.9
the system oscillation (delta d or p
m) becomes stable
0.8
0.7
0.6
0.5
0 0.5 1 1.5
time
within 2s with 0% damping[Fig:8(a)] & Bus voltage becomes stable within 0.6s with 0% damping [Fig.8(b)].
Fig.6(c) Bus voltage(B1 B2 B3) in p.u. for 3phase faults.
Fig.8(a) Oscillation, Vqref in pu for 1phase faults
Va Vb Vc
1.1
1
bus voltage
0.9
0.8
0.7
0.6
0.5
0 0.2 0.4 0.6 0.8 1
time
Fig.8(b) Oscillation, Vqref in pu for 3phase faults

Threephase faults: During 3phase faults, Oscillation (delta d or delta pm) becomes stable within 2.2s with 0% damping[Fig:8(c)] & Bus voltage becomes stable within 1s with 0% damping [Fig.8(d)]

Fig.8(c) Oscillation, Vqref in pu for 3phase faults
Bus voltage
1.1
1
Va
0.9 Vb
0.8 Vc
0.7
0.6
0.5
0 0.2 0.4 0.6 0.8 1
time
Fig.8(d) Bus voltages (in p.u.) for 3phase faults


Results & Discussions
The performance of different types of SSSC controller taking same 500KV transmission line are summarized below. In this table SSSC rating is represents in MVA, Syatem stability time is in Seconds, Damping is in percentage(%).
TableI
Performance Comparison of SSSC with Controllers
Stability Time
Damping
Controlle r
SSSC
Rating
Volt (3ph)
Volt (3ph)
Vqre
f
Volt (max)
Vqref(
(min)
Without
100
0.6s
0.7s
No
5%
0%
PI
80
0.6s
0.8s
8s
11%
0.01
PID
50
0.6s
0.7s
6.5
9%
0.01
POD
30
0.52s
0.8s
No
9%
No
Generic
20
0.55s
0.6s
2s
8%
0%

Conclusion
In this paper, the voltage level of two machines power system has been improved by using SSSC with different types of controller for 1phase & 3phase faults by Phasor simulation method. Same 500KV transmission line has been simulated & observed the transient response for different types of SSSC controller. Above all, SSSC with Generic controller is very suitable because of shorter stability time, small damping, small rating of SSSC , All controller parameters has been selected by trial & error methods normally, but those parameters can be selected by FSO, Neural network or Genetic algorithm techniques. Those controllers special advantages is that it can be used any robust multimachine power system network with very easily & cheaply. In this paper, only d & pm has been taken as input parameters of those controllers. But when any fault occurred, then voltage, current, power, pm, d everything will change. So, future work should be taken all of the above parameters as input parameters of those controllers & controller parameters can be tuned with any newly deigned algorithm.

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