Coordinated PSS and STATCOM Controller for Damping Low Frequency Oscillations in Power Systems

Coordinated PSS and STATCOM Controller for Damping Low Frequency Oscillations in Power Systems

Sundaraiah Nayini1 Ramakrishna Raghutu2

Department of Electrical Engineering Department of Electrical Engineering Mallreddy en gineering college for women GMR inistitute of technology

Abstract: The low frequency oscillations have become the main problem for power system small signal stability. They restrict the steady-state power transfer limits, which therefore affects operational system economics and security. In this work, using Parameters of the classic PSS and STATCOM internal AC and DC voltage controllers and AC damping stabilizer is designed in order to damp the Low Frequency Oscillations (LFO). The STATCOM is used for adjusting the voltage in line and the dynamic effect of STATCOM is proposed. The study is performed on the linearized power system model with presence of STATCOM and PSS. The designing results are performed using nonlinear simulation. The results show that the design according to simultaneous optimization is an effective procedure for power system stability improvement.

Keywords: STATCOM, PSS, FACTS devices, Power System Stability.

I.INTRODUCTION

The low frequency oscillations have become the main problem for power system small signal stability. They restrict the steady-state power transfer limits, which therefore affects operational system economics and security. Using PSS create change in oscillation stability. To increase power system oscillation stability, the installation of supplementary excitation control, power system stabilizer (PSS), is a simple, effective and economical method [1].

These oscillations may sustain and grow to cause system separation if no adequate damping is available. Although PSSs provide supplementary feedback stabilizing signals, they suffer a drawback of being liable to cause great variations in the voltage profile and they may even result in leading power factor operation under severe disturbances.

The recent advances in power electronics have led to the development of the flexible alternating current transmission systems (FACTS) [2]. Generally, a potential motivation for the accelerated use of FACTS devices is the deregulation environment in contemporary utility business. Along with primary function of the FACTS devices, the real power flow can be regulated to mitigate the low frequency oscillations and enhance power system stability. This suggests that FACTS will find new applications as electric utilities merge and as the sale of bulk power between distant and ill interconnected partners become more wide spread.

Recently, several FACTS devices have been implemented and installed in practical power systems such as static VAR compensator (SVC), thyristor controlled series capacitor (TCSC), and thyristor controlled phase shifter (TCPS) [3].

The emergence of FACTS devices and in particular gate turnoff (GTO) thyristor-based STATCOM has enabled such

technology to be proposed as serious competitive alternatives to conventional SVC. From the power system dynamic stability viewpoint, the STATCOM provides better damping characteristics than the SVC as it is able to transiently exchange active power with the system [4].

The method of phase compensation [5] and damping torque analysis method [6] are conventional methods for design and control of power system stabilizers [7]. Also multivariable design method has been performed on coordination between internal AC and DC voltage controllers [8]. The coordination between the AC and DC voltage PI controllers was taken into consideration. However, the structural complexity of the presented multivariable PI controllers with different channels reduces their applicability. Moreover, the utilization of damping capability of the STATCOM has not been addressed. The STATCOM damping characteristics have been addressed in [9]. However, the coordination among the STATCOM damping controllers and AC and DC voltage PI controllers has not been investigated. The coordination among different Controllers has been taken into consideration

1. where different damping channels of the STATCOM have been assessed in terms of their respective effectiveness on electromechanical modes controllability. In this study, to improve power system dynamic stability and voltage regulation, coordination among internal AC and DC voltage controller of STATCOM and AC-damping stabilizer and also PSS are performed. The studies are performed on a single machine infinite-bus power system. The linearized power system model is used for the studies [11].The parameters design is considered as an optimization problem and the genetic algorithm is used for searching optimized parameters [12].

   1. POWER SYSTEM MODEL WITH STATCOM

      Fig.1 shows a SMIB system equipped with a STATCOM. A single machine infinite bus (SMIB) system installed with STATCOM is considered for the analysis of stability.A simple STATCOM is incorporated which consists of a step down transformer (SDT) with a leakage reactance, three phase GTO based voltage source converters (VSCs), and a DC capacitor. The VSC generates a controllable AC-volt- age source behind the leakage reactance. The voltage difference between the STATCOM-bus AC voltage and produces active and reactive power exchange between the STATCOM and the power system, which can be controlled by adjusting the phase and the magnitude Vo.

      v sin xLB cv

      B x dc

      cos

      I sdt

      tlq x x

      x

      LB tL

      x

      q

      xL x x LB (1 LB )x

      SDT SDT

      Power System Linearized Dynamic Model

      The non-linear dynamic equations are linearized around a given operating point to have the linear model as given below

      Pe K1 K2Eq 'KpdcVdc KpcC Kp KpcuE KpmTm

      Fig.1. SMIB power system equipped with STATCOM

      Eq ' K4 K3Eq 'KqdcVdc Kqcc

      • Kq KqeUE KqmTm

   2. POWER SYSTEM NON-LINEAR DYNAMIC MODEL WITH STATCOM

      Vt K5 K6Eq 'KvdcVdc Kvc c

      Kv

      • Kve UE KvmTm

        V .dc K K E 'K V K

        C K

        K U

      • K T

        During low-frequency oscillations, the current induced in a damper winding is negligibly small hence the damper

        7

        Where

        8 q 9 dc DC D

        ce E

        cm m

        windings are ignored in system modelling. The natural oscillating frequency of the d and q armature windings of a

        synchronous machine is extremely high; their Eigen modes

        K1 to

        K9 , K pDC , K pc , K p KqDC Kqc Kq KvDC Kvc Kv Kdc Kd

        will not affect the low frequency oscillations hence they can

        K Eq 'Vb cos X q X d ' I

        V COS X q X d ' I

        V sin

        be described simply by algebraic equations. The field winding 1

        circuit of the machine must be described by a differential equation, not only because of its low Eigen mode frequency,

        X E

        X E

        tld b

        X d

        tlq b

        but also because it is connected directly to the excitation system to which the supplementary excitation controller is applied. The torque differential equation of the synchronous machine also must be included in the model.

        K2 I

        tlq

      • ( X q X d ' ) I

        X d

        ( X d X d ')

        tlq

        m

        . (P

        Pe D) / M

        K3 1

        X d

        o

        . ( 1)

        ( X X ' )

        The real power output of the generator is described as

        K4 d d Vb sin

        Pe Vtd

        <>Itd

        Vtq

        Itq

        X d

        Eq E'q

        ( X d

        • X d

        ')I

        tld

        K Vtd X q

        5 V X

        Vb

        cos Vtq

        V

        X d '

        X

        Vb

        sin

        Vt Vtd jVtq

        The real power output of the generator is described as

        t E

        K Vtq Vtq

        t d

        X d '

        6

        Vt Vt

        x

        X d

        Eq 'VB

        cos LB cv

        x dc

        sin

        K7

        Vb C cos sin

      • Vb C cos sin

      I sdt

      X d CDC

      X E CDC

      tld x x

      x x x LB (1 LB )x '

      C cos

      d

      tL LB

      tL x

      SDT

      xsdt

      K8

      X d

      CDC

      K C cos

      X LB

      C sin

      C sin

      X LB

      C cos

      d

      9 X

      CDC

      X SDT

      X E

      CDC

      X SDT

      KqDC

      ( X d X d ')

      X d

      X LB

      X SDT

      C sin

      0 0

      0 1

      0

      • K pc

        0

      • k p

      ( X X ') X

      M M M

      K d d LB V

      sin

      K K

      d

      X

      qc X

      DC

      SDT

      U 0 0

      • qc

        T '

        q

        T '

        • ( X d X d ')

      dpo

      dpo

      Kq X

      CVDC cos

      K A 0 K A Kvc

      K A Kv

      d

      V X X

      V X ' X

      TA

      TA TA

      K td q LB C cos tq d LB C sin

      0 0

      Kdc

      Kd

      VDC

      Vt X E

      XSDT

      Vt X d

      XSDT

      DC

      DC

   3. POWER SYSTEM STABILIZER

      KVC

      Vtd Vt

      X q

      X E

      X LB V X SDT

      cos Vtq

      Vt

      X d '

      X d

      X LB V X SDT

      sin

      One problem that faces power systems nowadays is the low frequency oscillations arising from interconnected systems. Sometimes, these oscillations sustain for minutes and grow to cause system separation. The separation occurs if no adequate

      • C cos X

      C sin X

      damping is available to compensate for the insufficiency of

      Kdc LB Vdc sin LB C cos

      the damping torque in the synchronous generator unit . This

      X d CDC X SDT

      K C cos XLB C V

      X E CDC X SDT

      X

      • cos C sin XLB C V

      • sin

        insufficiency of damping is mainly due to the AVR exciters

        high speed and gain and the systems loading. In order to overcome the problem, PSSs have been successfully tested

        d

        X

        d X

      • CDC

        dc

        SDT

        XE

      • CDC

      dc

      SDT

      and implemented to damp low frequency oscillations. The PSS provides supplementary feedback stabilizing signal in

      Are linearization constants The 20 constants of the model depend on the system parameters and the operating condition. The expressions for the constants are derived and are as follows.

      SMIB POWER SYSTEM STATE-SPACE MODEL

      WITH STATCOM

      The SMIB power system state-space model is obtained from the linearized dynamic equations as

      X AX BU

      the excitation system. The basic function of PSS is to damp electromechanical oscillations. To achieve the damping, the CPSS proceeds by controlling the AVR excitation using auxiliary stabilizing signal. The CPSSs structure is illustrated in Fig 2 shows

      Fig.2 PSS structure

      The block diagram of a linearized model of the SMIB power system with STATCOM is shown in fig 3

      q

      x [ E '

      E fd

      VDC ]

      U [U E Tm C ]

      0

      K1

      M

      b

      0

      • D M

        0 0

        K 2 0

        M

        0

        • k pDC

      M

      A

      K 4 '

      T

      dpo

      K 3 '

      T

      dpo

      1

      T

      '

      dpo

      K qDC

      M

      K A K 5

      0 K A K 6 1

      • K A K vDC

      TA

      K 7 0

      TA TA

      K8 0

      T

      A

      K 9

      Fig.3 Modified Heffron-Phillips transfer function model

   4. SIMULATION RESULTS

      The disturbance is given as step input and the output response is taken from omega, delta, change in power and DC voltage. The system shows with STATCOM and PSS in a time span of 10 seconds for light, nominal and heavy condition. From the Fig.4 to Fig.11 we concluded that STATCOM and PSS are more effective than the STATCOM.

      -4

      x 10

      -3

      x 10

      20

      15

      delta Pe (pu)

      10

      STATCOM and PSS STATCOM

      5

      STATCOM and PSS

      4 STATCOM 5

      3

      delta omega (pu)

      0

      2

      1

      -5

      0 0 1 2 3 4 5 6 7 8 9 10

      Time in S

      -1 Fig.6Time response of Pe

      -2 and PSS at light load.

      -4

      x 10

      with STATCOM & STATCOM

      1

      -3

      0 1 2 3 4 5 6 7 8 9 10

      Time in S

      STATCOM and PSS

      Fig.4 Time response of with

      and PSS at light load

      STATCOM & STATCOM

      0.5

      STATCOM

      0.06

      SATCOM and PSS 0

      delta Vdc (pu)

      STATCOM

      0.05

      0.04

      -0.5

      delta (rad)

      0.03 -1

      0.02

      0.01

      0

      0 1 2 3 4 5 6 7 8 9 10

      Time in S

      -1.5

      -2

      0 1 2 3 4 5 6 7 8 9 10

      Time in S

      Fig.5 Time response of

      and PSS at light load.

      with STATCOM & STATCOM

      Fig.7 Time response of Vdc

      with STATCOM &

      STATCOM and PSS at light load

      -4

      x 10

      6

      5

      STATCOM and PSS STATCOM

      0.04

      0.03

      STATCOM and PSS STATCOM

      4 0.02

      delta omega (pu)

      delta Pe (pu)

      3 0.01

      2 0

      1 -0.01

      0 -0.02

      -1 -0.03

      								9	10	-0.04 0	1	2	3	4	5 6	7	8	9	10
      					Time in S										Time in S

      -2

      0 1 2 3 4 5 6 7 8

      Fig.8 Time response of with and PSS at high load.

      0.09

      STATCOM & STATCOM

      SATCOM and PSS

      Fig.10Time response of with Pe

      and PSS at high load

      -4

      x 10

      2

      STATCOM & STATCOM

      STATCOM and PSS

      0.08

      0.07

      STATCOM STATCOM

      0

      0.06 -2

      delta (rad)

      delta Vdc (pu)

      0.05

      -4

      0.04

      0.03 -6

      0.02

      -8

      0.01

      0 0	1	2	3	4	5	6	7	8	9	10	-10 0	1	2	3	4	5	6	7 8	9	10
      					Time in S											Time in S

      Fig.9 Time response of and PSS at high load.

      with STATCOM & STATCOM

      Fig.11 Time response of with Vdc

      STATCOM and PSS at high load.

      STATCOM &

   5. CONCLUSIONS

This work the coordination among PSS and STATCOM was resented and discussed for power system dynamic stability improvement. The design results are performed for three loading conditions and also the nonlinear simulations are performed loading conditions. A MATLAB/SIMULINK has been developed for a single machine infinite bus power system with STATCOM and PSS. The design results are confirmed with nonlinear simulations. The results of coordinated design show dynamic stability improvement. With nonlinear simulation in coordinated design case has been shown that the oscillations are damped properly.

Appendix

,

System data: M=8 MJ/MVA, D=0, Tdo =5.044s,

,

X d =1pu, X q =0.6pu, X d =0.3pu, K A =10, TA =0.05s,

REFFERENCES

1. P. Kundur, M. Klein, G. J. Rogers, and M. S. Zywno, "Application of Power System Stabilizers for Enhancement of Overall System Stability, IEEE Trans.PWRS, Vol. 4, No. 2, 1989, pp. 614-626.

2. N.G. Hingorani, L. Gyugyi, Understanding FACTS: Concepts and technology of flexible AC transmission systems, Wiley-IEEE Press; 1999

3. N. Mithulananthan, C. A. Canizares, J. Reeve, and G. J. Rogers, "Comparison of PSS, SVC, and STATCOM Controllers for Damping Power System Oscillations" , IEEE Trans. PWRS, Vol. 18,No. 2, 1993, pp. 786-792.

4. H.F.Wang, "The phase compensation method to design FACTS-based stabilizer. Part I: single-machine infinite-bus power systems, Adv. Model. Anal., 1998.

5. H.F.Wang, "The phase compensation method to design FACTS-based stabilizer. Part I: single-machine infinite-bus power systems, Adv. Model. Anal., 1998.

6. H.F.Wang, "Interactions and Multivariable Design of STATCOM AC and DC Voltage Control, Elsevier, Electrical Power and Energy Systems, Vol. 25, 2003, pp. 387-394.

1. K. R. Padiyar and V. S. Parkash, Tuning and Performance Evaluation of

   XT X B X E =0.1pu,

   XT1 XT 2 =1pu,

   Damping Controller for a STATCOM, Int. J. of Electrical Power and Energy Systems, Vol. 25, 2003, pp. 155-166

2. Y.L.Abdel-Magid , M.A.Abido , "Optimal Multiobjective Design of

Operating conditions:

1)Nominal load P=0.8 Q=0.15 2) P=0.9 Q=0.17

3) P=1.0 Q=0.20

4) P=1.1 Q=0.28

5)Heavy load P=1.125 Q=0.285

6) P=0.7 Q=0.10

7)Very heavy load P=1.15 Q=0.3

Vt =1.032

Vt =1.032

Vt =1.032

Vt =1.032

Vt =1.032

Vt =1.032

Vt =1.032

Robust Power System Stabilizers Using Genetic Algorithms", IEEE Trans. on Power Systems, VOL. 18, NO. 3, AUGUST 2003 , pp. 1125- 1132.

10) Randy L. Haupt , Sue Ellen Haupt , "Practical Genetic Algorithms, chapter 2 , second edition , Copyright © 2004 by John Wiley & Sons.

1. H.F.Wang, "Phillips-Heffron model of power systems installed with STATCOM and applications, IEE Proc-Gener. Transm. Distrib. , Vol. 146, No. 5, September 1999, pp.521-527.

2. D.E Goldberg, Genetic Algorithms in search Optimization and Machine Learning: Addison-Wesley Publishing Company, Inc; 1989