Modelling & Performance Comparison of Different Types of SSSC-Based Controllers

Modelling & Performance Comparison of Different Types of SSSC-Based Controllers

Habibur1, Md. Fayzur Rahman2, Harun3

1,3Dept. of EEE, Rajshahi University Of Engineering & Technology,Rajshahi-6204,Bangladesh

2Professor & Head, Dept. Of ETE, Daffodil International University,Dhaka-1207, 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 multi-machine 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 multi-machine power system effectively.

Keywords- Static Series Synchronous Compansator (SSSC), voltage regulator, PI,POD,PID, generic controller, IGBT, MATLAB Simulink.

1. 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 fast-acting 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 multi-machine power system.

2. 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 Voltage-Sourced 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=V2-V1=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 phase-locked loop (PLL) which synchronizes on the positive-sequence component of the current I. The output of the PLL (angle T=t) is used to compute the direct-axis and quadrature-axis components of the AC three-phase voltages and currents (labeled as Vd, Vq or Id, Iq on the diagram).Measurement systems measuring the q components of AC positive-sequence 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 2-machine 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 280-km long and L2 is split in two segments of 150 km in order to simulate a three-phase fault at the midpoint of the line. The generation substation G2 is also connected to the load by 50-km 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 three-level 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

   L2-1 (150 km)

   A

   B C

   Three -Phase Fault A

   Fig.2 SSSC based control system

3. 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

   L2-2

   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

   1. a

   2. b

      (280 km)

      (150 km)

      hydraulic power plants [Fig.2]. The power grid consists

      Vf _

   3. 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 Three-phase 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 (3-phase 4-cycle 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 1-phase fault (with SSSC)

   3.1.2 Three-phase faults: During 3-phase 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 3-phase faults

   Fig.4(a) Oscillation, Vqref in pu for 1-phase faults

   Va

   1.2

   1.1

   1

   Va

   0.9

   0.8

   0.7

   0.6

4. 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 1-phase faults

   4.1.2 Three-phase 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 1-phase faults

   Fig.5(a) Oscillation, Vqref in pu for 1-phase 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 1-phase fault

   Fig.4(d) Bus voltages (in p.u.) for 3-phase faults

5. 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 Three-phase faults: During 3-phase 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 3-phase 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 1-phase 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 3-phase faults

6. 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 low-pass filter, a washout

   high-pass 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

   1. Simulation Results: Two types of faults has been considered.

      1. Single line to ground fault: During 1-phase 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 3-phase fault

7. 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 1-phase 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 1-phase fault

   1. Three-phase faults: During 3-phase 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

   1. Simulation Results: Two types of faults has been considered.

      1. Single line to ground fault: During 1-phase 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 3-phase faults.

         Fig.8(a) Oscillation, Vqref in pu for 1-phase 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 3-phase faults

      2. Three-phase faults: During 3-phase 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 3-phase 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 3-phase faults

8. 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(%).

   Table-I

   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%

9. Conclusion

   In this paper, the voltage level of two machines power system has been improved by using SSSC with different types of controller for 1-phase & 3-phase 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 multi-machine 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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