torsional oscillations in alternators6

7/28/2019 torsional oscillations in alternators6

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Fuzzy Logic Bad Cotro of STAT COM forMitigatio of SSR

.T.NagaajanElectrical Engineering Department,

Delhi Technological University, Delhi, dia.email:[email protected]

A- Subsynchronous sonance is well knownphenomenon in Series compensated transmission line. Flexible

AC Transmission System (FACTS) devices can be used toenhance the performance of the transmission line and to mitigate

SSR with auxiliary controllers. The use of STATCOM a FACTS

device increasing in the power sstem for enhancing powertransfer capability and dynamic reactive power support in power

system. In this paper a Fuzzy logic control method forSTATCOM proposed and applied for damping oscillatnscased by SSR. The performance of the proposed controller hasbeen tested on Second bench mark model for SSR studies. Time

domain simulations have been carried out withMATLB/SIMULINK. Signicant improvement of damping oftorsional oscillatns has been achieved with the proposed fuzzy

logic supplementary controller.

K R; FU ;SACO;

I. ODUCO

Series compensation of the transmission line to improvethe power transfer capability may lead to the phenomenon ofSubsncronous resonance (SSR). The rst sha failure due

to SSR was experienced at Mohave Generating station inSouthe Nevada [1]. To prevent turbine generator sha formdaages caused by SSR vaious devices and tecniques have

been proposed in literature [2]. Flexible AC transmissionsystems (FACTS) have been employed in mode powersystem due to its capabilit to work as V generation andabsorption systems.

STATCOM a FACTS device is a second generation StaticV Compensator (SVC) based on voltage sourced inverterand has better reactive power support even at low voltages.STATCOM uses a self commutating device like GTOs andcan be designed as two level six-pulse bridge, tree level

twelve-pulse bridge, and forty eight pulse converter. The fortyeight pulse converter has superior performance as the outputcurrent of the converter almost nears the sinusoidal form and

the hamonic distortion is minimum compared to others [3].Hence forty eight pulse converter is selected for this paper.

Supplementary signals like speed deviation, thevenin'svoltage, hybrid Fuz/LQR method has been proposed forSTATCOM in literature [4-5] for the suppression ofoscillations due to SSR. Fuzzy logic controller (FLC) hasiherent capabilities to overcome the system nonlinearities, as

978-1-4673-0934-9/12/$31.00 2012 EE

Nanda KmaElectrical Engineering Department,

Delhi Technological University, Delhi, dia.email:[email protected]

the exact owledge of the system is not needed. li Ajami etal 5 have proposed a hybrid Fuzzy logic controller with LQRcontroller to a 3 level STATCOM.

this paper a Mamdani tpe Double input Single output(DISO) Fuzz controller has been proposed along with PIcontroller for fortyeight pulse converter STATCOM. Thestudy system has been derived om the EE second benchmark model for SSR studies. A STATCOM along with Fuzzyauxilia controller with speed deviation and terminal voltagedeviation of generator, has been considered to be conected atthe generator terminal for reactive power support andmitigation of SSR. Nonlinea time domain simulations havebeen caried out using MTL/SIML.

II. UBSCROOUS SOC

e phenomenon of SSR can be explained as follows:When a line is Series compensated it results in excitation ofSSR currents at electrical equencyVIn = sWhere = reactance of the series capacitor; = reactance ofthe line including generator and transformer; and f= the

nominal equency of the power system. ese currents resultin rotor torques and currents at the complementary equency:

Ie = Islnese currents results in subsyncronous amature voltagecomponents which may ehance subsyncronous amaturecurrents to produce SSR.

SSR manifests in tree fos [6].Self excitation, Torsionalinteraction and Transient SSR. this paper only Transient

SSR is analyzed. Transient SSR generally refers to transienttorques on segments of the Turbine-Generator (T-G) sharesulting om bsyncronous oscillating currents in thenetwork caused by faults or switching operations. is occurswhen the complement of the electrical network resonantequency gets closely aligned with one of the torsional naturalequencies.

III. DY SYSM

Design of Fuzzy logic controller does not depend on thesystem model. is eliminates the need to ineaise the power


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system model to tune the parameters of controller. e entire

power system components ae represented with their nonlinea equations/models for the EE second benchmark

power system model for SSR studies shown in Fig.. Thesystem parameters for generator, turbine sha and

transmission line are taken form ref [7] given in Appendi x.

R X X

X

R X

NNTE

Fig. 1. Sytem Mol

A. Generator Model

e Generator was modeled by Park's equation onedaper winding and eld winding in direct axis and with two

daper windings in quaature axis [8]. e mechanical

system was represented by a tree mass spring daper system,a high pressure (HP) trbine, a low pressure turbine (LP) and

the generator (GEN) as shown in Fig.2. The effect of Excitermass is assumed to be negligible and hence not considered in

this study.

Fig. 2. Tbine generator ha model

B STATCOM Controere Controller for fory-eight pulse GTO based

STATCOM is shown in Fig.3. The detailed explanation ofSTATCOM controller is given in reference [3]. The controller

has iner current control loop and an outer voltage controlloop. PI controller is considered for th the loops. V is the

reference voltage and V is the measured voltage for the outer

loop. Iqref and I is the quaature axis reference andquaature axis current for the iner loop.

ql

Fig. 3. STAOM controller trcture

C. Fuzzy logic controller

Fuzz logic control essentially involves the derivation of a

control law om heuristic and imprecise (zzy) rules. The

conguration of the Fuzz logic control system [9] that is

employed for designing the Fuzzy supplementary controller isshown in Fig.4. The FLC contains four main components, the

zzication interface (FUZZICA TION), the knowledge

base (FUZZY RULE BASE), the decision making logic(FUZZY FERENCE ENGE) and the dezzication

interface (DEFUZZICATION). A Mamdani type double

input single output (DISO) FLC has been designed.

C

ZCAO J ZY BASE I . ZY ERENCE ENGINE I 1 DCAO OU

C

Fig. 4. Fzzy logic contrl

e following steps are involved in the designing zzylogic controller.

1). Choose the inputs to FLC (PUT-CRISP): The inputs toFLC used in this study are generator terminal voltage

deviation ( V ) and generator speed deviation({) which are

given byLVt(pU) = Vre - Vtbw(pu) = w - woWhereV= generator terminal voltage; VReference voltage;

{=sncronous speed of generator; = speed of generator;

2). Choose membership nctions to represent the inputs and

outputs in zz set notation (FUZZICA TION): Triangula

membership nctions were selected for this study as shown in


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Fig.5 with ve linguistic variables chosen as Large positive

(LP), Small positive (SP), Very small (VS), Small negative(SN), Lage Negative (LN) for both inputs and outputs. e

values of the axes ae given in Appendix.

Fig. 5. Tringulr member ip nction for peed deiation nd voltagedeiation

3). Develop zz rules (FUZZY RULE BASE): A set of

decision rules relating the inputs to the controller with the

output ae compiled and stored in the memory in the form ofdecision table. Twenty ve rules for the present study aedeveloped as follows.

Rule: If is LN and V is LN, then change in susceptance

of SVS should be SN. e remaining twenty four rules areformed in the same way as shown in the Table I.

T . F RULE TALEW LN SN SP LPV.

LN SN SN LN LN LN

SN SN SN SN LN SN

VS SP SP VS SN SN

SP LP SP SP SP SP

LP LP LP LP SP SP

4). Since there are N (ve) membership nctions for eachinput, there are N (twenty ve) possible combinations

resulting in M (ve) values for the decision variable u. ll thepossible combinations of inputs, called states, and the

resulting control are arranged in a (NM) zz relationshipmatrix. e membership values for the output characterized by

the M linguistic variables are then obtained om theintersection of N values of membership nction (x) with the

corresponding values of each decision variables in the zzyrelationship matrix.

5). Dezz to obtain crisp output (DEFUZZICATION):

e output FLC is converted to crisp value by Centre orGravity (COG) method in this study. e crisp value of FLC

in COG is expressed as

Where b is the centre of the membership nction; is the

membership of member i of output zz set. e overallcontroller structre for STATCOM with FLC auxiliary

controller is shown in Fig.6

V,w

Fig. 6. STAOM controller trcte ith axiliry FC contrer.

SMUATO STUDY

A. Case Study

e system considered is a modied EE SBM [7] asshown in Fig. with ST A TCOM at the generator bus on high

voltage side of transformer for reactive power support. Thegenerator has ratings of 600MA, 22kV and 60Hz and is

conected to the innite bus trough a transformer of 22KV/

500KV and two transmission lines one of which is seriescompensated with a value of 55% of that transmission line

reactance. Load ow study was performed to keep thesncronous generator at no load at rated terminal voltage at

.pu and all the system variables were initialized.

B Analysis of study system

the generator sha has been represented with treemass system, there shall exit two torsional modes, mode (LP

turbine mode) and mode 2(HP Turbine mode). e study hasbeen performed for a compensation of 55% as the overall

undamping of mode 2 is negligible compared to mode 1,

which has maximum undamping at this level of compensation[7]. A damping of 0.4 ra/sec has been kept to make the unit

steady state stable for this case. A Fast Fourier Transform(FFT) analysis has been performed on the generator speed

signal to test the existence of this critical mode (mode 1)without STATCOM conected to the generator bus. e FFT

scan result shown in Fig.7 reveals the presence of dominant24.65 Hz mode (mode 1) in conformation. lso to test the

study system for the presence of sub sncronous resonance

condition, a equency scan of the study system wasperformed without the STA TCOM om generator terminals.

e result of the equency scan shown in Fig.8 reveals thepresence of sub syncronous resonance condition in the

system nea 36 Hz for 55% compensation. The resulting


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electromagnetic torque due to SSR oscillates at a equency f-

f (60-36=24Hz) which is nea modal (mode) equency ofthe turbine generator sha system leading to torque

aplication.

Fa.a lyi-x

I $ % o

f

Fig. 7. FFT nyi of generator ed i ith 55%compenton

Impedanceio 1 6FrBuy (Hz) 1 : . ..... . i . . . ... . . . .r l 1 t= l lJ .

Frquncy (Hz

Fig. 8. Frquency repone of the nmiion netork ied omgenerator terin ith 55% comnation

C. Transient analysis of study system

e transient analysis of the study system was performedwith tree cases, without STATCOM, with STATCOM PI

voltage controller, and with proposed supplementary FLC. Forall the cases the load ow stdy was performed to initialize all

system variables with no load on syncronous generator. A

tree phase fault is applied on the generator bus at timet=0.02s and removed aer a duration of 0.017s on the high

voltage side of the transformer. e inuence of the proposedFLC has been studied trough the following signals:

1. Generator speed deviation2. Mechanical torque between sha segments conecting

generator (GEN) and (LP) turbine.3. STATCOM reactive power.

4. Generator terminal voltage.

Case J Without ST ATCOM

e torsional oscillations in time domain with no load ongenerator (PG= 0 MW) is shown in Fig 9. was observed that

the rotor speed deviation, torsional oscillations and voltage at

generator terminals ae adually increasing in magnitude andhence the system is unstable.0.04-

S 0.02}

'0.02

rJ 10

i 1",.ar rwQ 10foc

L1.1

I 1l> 0.90.8" - mel)

Fig. 9. Tbinegenerator torque Ociation ithout ST ATOM

Case With STACOM-PJ voltage controller

STATCOM with a PI voltage controller was nowconected to the generator bus on high voltage side of the

transformer. The gain parameters for the controller wereobtained by hit and trial method without the multi mass sha

system to get reasonable settling time. e value used hasbeen given in Appendix.

i100'------'-- 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 1 HIM" 0 8OI I_ -_____-____'_

0.5 1.5 2.5

imel)3.5

Fig. 10. Tbinegenerar torque Ociation ith ST AOM

4.5


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can be observed om Fig.10 the torsional oscillations, rotor

speed deviation and voltage at generator terminals are stillgradually increasing in magnitude and the system is still

unstable. Only PI voltage Controller of STATCOM is not ableto suppress the torque amplication in the generator turbine

sha. is necessitates the demand for auxiliar controlsignals for the STATCOM to suppress torque amplication of

generator turbine sha.

Case 3 System with STACOM-PJ Voltage regulator and

supplementa FLC

e proposed supplementary FLC with membership values

given in Appendix was applied to STATCOM and simulation

was performed again.

DX

enerator data:

Ratings: 600M 22KV, 2PStability data:

X=0.14 pu R=0.0045 puX=1.65 pu X=1.59 pu

X'=0.25 pu X'=0.46 pu

X"=0.20 pu X"=0.20 puT'=4.5 s T' =0.67 s

T=0.040 s T=0.09 s

Torsional spring-mass system data

Ma Sha ta H(s)

HP 0.24894HP-LP

LP 15498LP-GEN

GEN 08788

Modal frequencies

Dpingu.T/rad005

00104004

Springcontnt/rad.

42702

847

2.5 3.5 4.5 Mode mrad/s freq (Hz) Daping factor Xc

0.5 .5 2.5 3.5 4.5! ;l 0.5 .5 2 2.5 3 3.5 4.5 5 1 154.82 203. 4Transmission line data500KV 24. 6532. 39Line with series capacitor: 04004R=0.0074pu, R=0.022pu, X=0.08pu, X=0.240puLine without series capacitor 55%5%R=0.0067pu, R=0.0186pu, X=0.0739pu, X=0.210pu

Tranormer data:

0.8"- 0.5 .5 2.5 3.5 4.5mel)

600M 60HZ, !y, 22KV/500KV, R=0.0006pu,R=0.0006 pu, X= pu, X=0.12pu

Fig. II Turbinegenerar torque cillato ith STAOM nd FC STATCOM data:

100 M (MA Reactive TO 100 M Capacitive) can be observed fom Fig.1that all SSR torsional PI Voltage Regulator Gains:

oscillations are damped and the system becomes stable. K=12, K=3000PI Current Regulator Gains:

V. OCUSO K=5, K=40STATCOM is a widely used FACTS device for reactive

power generation, voltage support and improving steady state Fuzzy logic controller:stability of power system. this paper a Fuzzy logic Iu: Tul bh u vlusupplementary controller for STATCOM to damp Sub

sncronous torsional oscillations has been presented forseries compensated system. e Fuzzy logic controller has

advantage over conventional controllers as the exact

owledge of the system is not required. ith the proposedFLC supplementary control signal for STATCOM it has been

V,

w

0. 30. 65I0. 0050. 01750. 03

S vs0 0. 030. 3 00. 6 0. 030 0.00 050. 003 00. 006 0. 0005

S0.60.300.0060.0030

shown trough time domain simulations that torsional Output: Triangularmembershipfunction values

oscillations due to SSR can be successlly damped. S VS SOUUT 0. 02 0 0.0 04 0.0 7

0.11 0. 035 0 0.03 50. 2 0. 07 0. 004 0

-I,0.650.30.030.01750.005

0.20.110.02


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FCS

[I] .M. nderon B..Ae nd J. Van e SubsnchronousResonance in Power System, re e York 1990.

[2] Working Comittee Report oth Supplement to aBiblioaphy for the Study of Subyncrono Reonnce tenRotating Macine nd or Sytem" IEEE Tra. On PowerSystems vol.2 no. 3 1997 pp.1 2761282.

[3] .. ingor nd . ygyi Understanding FACTS, re 1996.

[4] K.R. adiyr nd ageh rabhu Dei nd erfonce

auation of Subyncronou Dmping Controller ithSTATCOM" IEEE Trans on Power Dlive vo.21 .3 2006pp.1 3981405.

[5] Ajmi nd .Teheri A ybrid Fu/QR Baed Ocillaton

Dmping Controller Uing 3lel STAOM" roceeng ofSecond nternationa Conferee on Computer nd lecica

ngineering CC vol. 1 2009 pp 34835 2.[6] Comittee Rert eader' uide to Subyncrou

Reonnce" EEE Traactions on Power Syste, vol. 7 .1992 pp. 1 52157.

[7] SSR Working roup econd bencmrk model forimulation of ubyncrono ronnce" IEEE Tra. PASl. 104 no.5 1985 pp. 10571066.

[8] raue .C. Anasis of Elecic Mainery Mcrai 1986Secton 12.5.

[9] Zdeko Kocic nd Stjepn Bogdn Fuz Controer DsiCRC re Taylor& Frnci roup e York 2006.

torsional oscillations in alternators6

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