International Journal of System & Simulation (IJSS) Vol.6, No.2, July-December2012, pp.37-42 @ Serials Publications, (India)

ISSN: 0975-4814

On Line Voltage Level Improvement of Power System by Using Static VAR Compensator and PSS. Md. Habibur Rahman1, Md. Fayzur Rahman2 , Md.Harun-Or-rashid3 1,3

Department of Electrical and Electronic Engineering, *1 ,3Rajshahi University of Engineering & Technology, 2 Department of Electronics and Communication Engineering, 2Daffodil International University, Email: [email protected], [email protected], [email protected]

Abstract—This paper presents a new simple method of transient Stability and voltage level improvement of a large scale power system for different types of faults during transient conditions to improve the the stability of multi-machine large scale power system. This paper contributes to the improvement of transient stability of multi- machines power system by using PSS & SVC control. The system response was simulated and evaluated by using phasor simulation method during single and three phase faults applied to the terminals. This work is presented to improve the voltage stability & damped out oscillation by using power system stabilizer & SVC with and without P.I. controller & compare their performance to enhance the stability of power system. Simulation results show that PSS & SVC with controllers can enhance the stability of multi-machine power system effectively. Keyword- Static VAR compensator (SVC),TSR, Thyristor control reactor(TCR), automatic voltage regulator(AVR), PI controller, MATLAB Simulink.

I.

INTRODUCTION Stability augmentation is very important for large scale power system. Traditionally, fixed or mechanically switched shunt and series capacitors, reactors and synchronous generators were being used to enhance same types of stability augmentation [1]. However, there are some restrictions as to the use of these conventional devices[2]. For many reasons desired performance was being unable to achieve effectively. A static VAR compensator (SVC) 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 and therefore, it can improve the quality performances of the electric services[3]. A SVC can be controlled externally by using properly designed different types of controllers which can improve voltage stability of a large scale power system. The dynamic nature of the SVC lies in the use of thyristor devices (i.e. GTO, IGCT)[4].Therefore, This paper presents thyristor based SVC with PI controllers & PSS to improve the performance of multimachine power system.

conduction results in the reactor, and the current is the same as though the thyristor controller were short circuited. Bus Bar C.B

Transformer

Inductive reactive power absorbtion

(-Qind)

Thyristor Vulve

Reactor

P.T Isvc

Qnet=Qcap - Qind

Vsvc

SVC Controller AVR Capacitive reactive power injection (+Qcap)

Pulse generator Fixced Capacitor

TCR

Fig.1 SVC based control system A. Operation of TCR: The current is essentially reactive, lagging the voltage by nearly 90. Full conduction is shown by the current waveform in Fig. 3(a). If the gate signal is delayed by equal amounts on both thyristors, a series of current waveforms is obtained [Fig.2(a)-2(d)]. Full conduction is obtained with a gating angle of 90. Partial conduction is obtained with delay angles between 90 and 180. The effect of increasing the firing angle is to reduce the fundamental harmonic component of the current. This is equivalent to an increase in the inductance of the reactor, reducing its reactive power as well as its current. The instantaneous current „I‟ is given by

II. CONTROL CONCEPT OF SVC An SVC is a controlled shunt susceptance(B) which inject reactive power (Qnet) into thereby increasing the bus voltage back to its net desired voltage level. If bus voltage increases, the SVC will inject less (or TCR will absorb more) reactive power, and the result will be to achieve the desired bus voltage[Fig.1]. Here, +Qcap is a fixed capacitance value, therefore the magnitude of reactive power injected into the system, Qnet, is controlled by the magnitude of –Qind reactive power absorbed by the TCR. The basis of the thyristorcontrolled reactor(TCR) which conduct on alternate half-cycles of the supply frequency. If the thyristors are gated into conduction precisely at the peaks of the supply voltage, full

37

International Journal of System & Simulation (IJSS) Vol.6, No.2, July-December2012, pp.37-42 @ Serials Publications, (India) √

=0

ISSN: 0975-4814 𝐾𝑟 𝑠𝑇𝑟

(Vdesired-Vactual)

For, For,

Susceptance(B)

Fig.4 Transfer function of AVR control block. The error voltage, Verror = Vref – V – (Isvc.Xsl).

The fundamental component is found by, For,

SVC operating point

Blmax

Vref

Where , ;Where, σ is the conduction angle, and α+σ/2=π. We can write (2) as, I1 = BL(σ)V

dv Xslope=dv/Isvc-base Bcmax

Where BL(σ) represents an adjustable fundamentalfrequency susceptance, which is controlled by the conduction angle according to the law,

Isvc-base

Isvc-base:=MVA/KV2( base)

power system impedance bus

Isvc 0

Capacitive(+)

Inductive(-)

Fig.5 Steady state(V-I) characteristic of a SVC. B. SVC V-I Characteristic: The SVC can be operated in two different modes: a). In voltage regulation mode (the voltage is regulated within limits as explained below). b). In VAR control mode . From V-I curve of SVC, From Fig.6[5]. V=Vref+Xs.I : SVC is in regulation range (Bcmax
Fig.2 Phase & Line current wave for delta connected TCR. This control law is shown in Fig. 4. For the full conduction in the thyristor controller that is with σ=π or 1800, the maximum value of BL is obtained as 1/XL minimum value is obtained with σ=0 (α = 1800) as zero. This control principle is called phase control.

V=I/Bcmax, ,

: SVC is fully Capacitive(B=Bcmax)

V=1/Blmax,

: SVC is fully inductive(B=Blmax)

I. PERFORMANCE ANALYSIS An SVC in principle is a controlled shunt susceptance (+/-B) as defined by the SVC control settings that injects reactive power (+Q) or removes reactive power (-Q) based on the square of its terminal voltage. Bus Bar Qsvc

Isvc Isvc

1 1 + 𝑇𝑑

Xs1

SVC Controller (P.I.) Qmax 𝐾𝑝 +

Vsvc

1 1 + 𝑇𝑑

1 𝑇𝑑

1 1 + 𝑇1 Q min

Vref Pulse generator

Fig. 6 Block diagram of SVC with PI controller In this application Q=B×V2, and L and C are components which are sized such that Q≥0 is the only operating

Fig. 03 Susceptance Vs firing angle curve.

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International Journal of System & Simulation (IJSS) Vol.6, No.2, July-December2012, pp.37-42 @ Serials Publications, (India)

ISSN: 0975-4814

range. The output B of this control block diagram feeds into the pulse generator controller that generates the required thyristor firing signal for the light-triggered TCR [Fig.6]. The control objective is to maintain the system voltage at 115 kV bus at 1.02 p.u. voltage(Vref)[Fig.7]. If the bus begins to fall below 1.02 p.u., the SVC will inject reactive power (Q) into the system (within its controlled limits), thereby increasing the bus voltage back to its desired 1.02 p.u. voltage according to its slope setting, Xsl. On the contrary, if bus voltage increases, the SVC will inject less (or TCR will absorb more) reactive power (within its controlled limits)[Fig.8(d)], and the result will be the desired bus voltage. SVC with PI controller block diagram is shown in Fig.6. Considering different values of V svc, The SVC controller output has been shown sequentially in the Fig.[8(c)8(e)]. If Vsvc=0.75 i.e.27% voltage drop is occurred then Susceptance will increase [Fig.8(c)] & If line voltage will rise 27% (Vsvc=1.27),then Susceptance will decrease[Fig.8(d)]. If there will no fault in the transmission line (Vref=Vsvc=1.02) then, SVC supplied some susceptance to the bus because of some damping[Fig.8(e)].

Fig.8(b) Required pulse for thyristors control. 40 Susceptance when Vsvc=0.75
Susceptance,B

30 20 10 0

0

1 s

Slope ,Xs1

1

0.75

1/Tds

0.005 s+1 Transfer Fcn 2

Vsvc

1 s

3 Kp

Susceptance,B

1

0.006 s+1 Q limit Transfer Fcn 3 Susceptance , Pulse Gen Pulse (B)

-40 -60

0

Fig. 7 SVC controller block diagram

0

2

Error Signals

4 time

6

8

III. POWER SYSTEM MODEL This example described in this section illustrates modeling of a simple transmission system containing three hydraulic power plants. SVC and PSS are used to improve transient stability and power system oscillations damping. A single line diagram represents a simple 500 kV transmission system is shown in Fig. 9.

0.5 0

10

Fig. 8(a) Error voltage (Verror). Subsystem#1

0.8

Load

0.6

Pulse Signals

Subsystem#2

G1

1

G2

XT1

XT2

Fault

X13

0.2 0 2

time

4

6

8

C.B

SVC

X23

Load

G3

0

Load

X12

C.B

0.4

-0.2

10

Fig. 8(e) The susceptance (B) For Vref=Vsvc=1.02.

1

8

10

-4

Error Signals

6

8

Fig. 8(d) The susceptance(B) For Vsvc=1.27.

1.5

4 time

6

-2

-6

A. Simulation Results:

2

4 time

Susceptance when Vsvc=1.02=Vref

Vref

0

2

0

Verror

-0.5

10

-20

2

t 1.02

8

Susceptance when Vsvc=1.27

0

Susceptance,B

0.005 s+1 Transfer Fcn 1

6

Fig.8(c) The susceptance(B) curve For, Vsvc=0.75.

0.05

Isvc

4 time

20

1 0.7

2

Swing Gen

Fig. 9 Single line diagram of 3-machine power system.

10

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International Journal of System & Simulation (IJSS) Vol.6, No.2, July-December2012, pp.37-42 @ Serials Publications, (India)

ISSN: 0975-4814

A 1000 MW hydraulic generation plant (M1) is connected to a load centre through a long 500 kV, 700km transmission line. A 5000 MW of resistive load is modeled as performed on this system with plant M1 generating 950 MW so that plant M2 produces 4046 MW. The line carries 944 MW which is close to its surge impedance loading (SIL = 977 MW). To maintain system stability after faults, the transmission line is shunt compensated at its centre by a 200 MVAR Static VAR Compensator (SVC). The two machines are equipped with a hydraulic turbine and governor (HTG), excitation system, and power system stabilizer (PSS). Another machine is swing generator. The generic Power System Stabilizer (PSS) block [Fig.10] is used in the model to add damping to the rotor oscillations of the synchronous machine by controlling its excitation current[3]. Any disturbances that occur in power systems due to fault, can result in inducing electromechanical oscillations of the electrical generators. Such oscillating swings must be effectively damped to maintain the system stability and reduce the risk of stepping out of synchronism.

Fig. 11 Generic PSS block diagram. To ensure a robust damping, a moderate phase advance has to be provided by the PSS at the frequencies of interest in order to compensate for the inherent lag between the field excitation and the electrical torque induced by the PSS action. The general gain (K) determines the amount of damping produced by the stabilizer. The high-pass filter eliminates low frequencies that are present in the signal and allows the PSS to respond only to speed changes. The phase-compensation system is represented by a cascade of the two first-order lead-lag transfer functions used to compensate the phase lag between the excitation voltage and the electrical torque of the synchronous machine. Fig.10 shows the simulink of hydraulic turbine and governor (HTG), excitation system, and power system stabilizer (PSS) of synchronous machine.

wref

1 d_theta 1

the load centre. The remote 1000 MVA plant and a local generation of 5000 MVA (plant M2) feed the load. A load flow has been

2 Pref

wref

vs _qd

Pm

Pref we Pe0

1 Pm

gate

dw

d_theta

HTG 1 m

m

wm

1

dw

vref

Vref 1

vd

Demux

Peo

vq

Machines Measurement Demux

Vf

2 Vf

vs tab

IV. SIMULATION RESULTS The load flow solution of the above system is calculated and the simulation results are shown below. Two types of faults: (a)single line to ground & (b) 3-phase fault have been considered.

[PSS ]

Im

Re

EXCITATION 1

w1

0 no PSS

Pa = Pm -Pe

|u| Sum 2

Vt 1

In

Vs tab

Generic Power System Stabilizer dw

Vs tab

Multi -Band Power System Stabilizer

Fig.10 PSS, HTG and excitation system block diagram.

A. Single line to ground fault: The fault occurred at 0.1s & circuit breaker is opened at 0.2s (3-phase 4-cycle fault), Without SVC, the system voltage goes on oscillation [Fig.12] & If SVC exists in the line then system voltage becomes stable within 5s with 0.25% of damping[Fig.13].

The output signal of the PSS is used as an additional input (Vstab) to the excitation system block. The PSS input signal can be either the machine speed deviation, dw, or P a = Pm - Pe. it‟s acceleration power,

Fig:8; Complete Simulink Model of 3 machine power system

Fig.8 Complete Simulink Model of 3- machine power system Fig.11(a) Complete simulink model of Fig.9

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International Journal of System & Simulation (IJSS) Vol.6, No.2, July-December2012, pp.37-42 @ Serials Publications, (India)

ISSN: 0975-4814

If no PSS is applied then system parameters oscillates towards higher value & becomes unstable, But when PSS is applied, then the system oscillation damped out & system becomes stable within 5s with 0.25% of damping [Fig.14].

oscillation has been decreased & system becomes stable within 4s.[Fig.15]

2 Va Vb Vc

Bus Voltages

1.5 1

0.5 0

0

0.5

time,t

1

1.5

2

Fig.12 Bus voltage in p.u. at faulted condition (without SVC)

0.9

0.85

0

0.5

1

1.5

2

2.5 time

3

3.5

4

4.5

5 -K-

dw2

Fig.13 Output of SVC (Vm) in p.u for 1-phase fault (with SVC)

Gain

1 s Transfer Fcn Saturation SVC (Phasor Type )

B

0.95

C

1

A

1.05 Bus voltage

Fig.15 System response for 3-phase faults(with & without SVC) V. SVC MODEL WITH P.I. CONTROLLER The power system network with SVC proportional Integral (P.I.) controller is shown in Fig:4. The angular speed deviation dω & mechanical power deviation P m has been taken as an input parameter. When any faults occurred in the network, then both machines angular speed dω, mechanical power P m & bus voltages will be changed & oscillated. If SVC with P.I. controller [Fig.16] is used then every network parameters become more stable.

Va Vb Vc

Vref

1.1

SVC m

dw 1



SVC

pm 1

C

B

A

s Transfer Fcn Saturation SVC (Phasor Type )

SVC m

Gain

1 Vref

-K-

pm 2



SVC

Fig.16 Simulink diagram of SVC with P.I. controller A. Simulation Results: If uncontrolled SVC is replaced by PI controller SVC then the system response has been observed .Here also two types of faults:1)1-phase & 2)3-phase L-L fault has been considered. 1).1-phase fault: During 1-phase faults, after clearing the faults by opening circuit breaker, the system voltage becomes stable within 3s with 0.01% damping which is shown in Fig.17.

Fig.14 System response for 1-phase faults(with & without PSS) B). 3-phase fault: During 3-phase faults ,Without SVC, the system becomes unstable. But when SVC is applied then,

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International Journal of System & Simulation (IJSS) Vol.6, No.2, July-December2012, pp.37-42 @ Serials Publications, (India)

ISSN: 0975-4814

VIII. FUTURE WORKS Transient stability or voltage level can be improved by using following methods: Electromagnetic Transients Program(EMTP), Alternative transients Program(ATP),FSO, Another FACTS devices (SSSC, STATCOM), Fuzzy logic controller, Electromagnetic Transients Program for D.C.(EMTPDC), Decentralized nonlinear controller is one of the most advanced technique to improve the transients stability of a power system.

1.1 Va

1.08 1.06 1.04

Va

1.02 1 0.98 0.96 0.94 0.92 0.9

0

0.5

1

1.5

2

2.5 time

3

3.5

4

4.5

5

IX. REFERENCES [1] P. Kundur, Power System Stability and Control. Textbook ISBN 0-07-0359580-X, McGraw Hill, 1994. [2] Amit Garg ,”Modeling and Simulation of Static VAR Compensator for Improvement of Voltage Stability in Power System”ISSN:2249-071X,Vol.2,Issue-2 [3] Ali M. Yousef “Transient stability Enhancement of multi machine using Global deviation PSS” ” Journal of Engineering sciences, Faculty of Engineering, Assiut University, Vol. 32-No.2 April 2004 pp. 665-677. [4] A.E. Hammad, “Analysis of power system stability enhancement by static var compensator”, IEEE PWRS, vol 1, no. 4, pp. 222-227. [5] Nang Sabai, and Thida Win (2008) “Voltage control and dynamic performance of power transmission system using SVC” World Academy of Science, Engineering and Technology 42 Pp. 425-429. [6] " MATLAB Math Library User's Guide", by the Math Works. Inc. [7] Nang Sabai, and Thida Win (2008) “Voltage control and dynamic performance of power transmission system using SVC” World Academy of Science, Engineering and Technology 42 Pp. 425-429 [8] M.A Abibo ,”Power System stability enhancement using FACTS controllers “The Arabian Journal for Science and Engineering Volume 34, Pp. 153-161 [9] S. Sankar (2010),”Simulation and comparison of various FACTS Devices in power system” International journal of Engg Science And Technology Vol.2 (4),Pp. 538-547 [10] Haque M.H (1992).,” Maximum transfer capability with in the voltage stability limit of series and shunt compensation scheme for AC transmission systems”, Electric Power system research, vol. 24, pp. 227-235. [11] R.J. Koessler, “Dynamic simulation of SVC in distribution systems,” IEEE Trans. Power System, vol.7, no.3, pp. 1285-1291, Aug. 1992.

Fig.17.Bus voltage (in p.u.) for 1-phase fault 2).3-phase faults: During L-L faults, if SVC with PI controller is used then the system voltage becomes stable within 4s with 0.01% damping which is shown in Fig.18. 1.1 1.08 1.06

Bus Voltage

1.04 1.02 1 0.98 0.96 0.94 0.92 0.9 0

1

2

3

4

5

6

7

time

Fig.18 Bus voltage (in p.u.) for L –L faults

VI. PERFORMANCE COMPARISON The performance of PSS & SVC with & without controller taking same 500KV transmission line are summarized below: Stability . time Controller SVC Without control SVC with PSS SVC with P.I. controller

SVC Rating

1-Ø

L-L

Damping limit(1-ø)

200 MVA

5s

5.5s

7%~ 0.25%

100 MVA 50 MVA

4s 3s

4s 4s

7%~ 0.01% 3.5%~ 0%

VII. CONCLUSION In this paper, the voltage level of three machines power system has been improved by using PSS & SVC with P.I. controller for 1-phase & 3-phase faults by Phasor simulation method. Same 500KV transmission line has been simulated & observed the transient response for SVC with & without controller. Above all, SVC with P.I. controller are highly efficient for voltage stability & less damping for both steady state & dynamic conditions because of having shorter stability time with less damping. In this paper, Controller parameters optimum values has been selected by trial & error methods normally, but those parameters can be selected by FSO, Neural network or Genetic algorithm techniques. 42