TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERINGIEEJ Trans 2006; 1: 116–120Published online in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/tee.20026

Letter

A Fuzzy Logic-Controlled SMES for Damping ShaftTorsional Oscillations of Synchronous Generator

Mohd. Hasan Ali∗Toshiaki Murata∗

Junji Tamura∗

This paper presents a fuzzy logic control scheme for the superconducting magnetic energystorage (SMES) based on a PWM voltage source converter and a two-quadrant chopperusing an insulated-gate-bipolar-transistor (IGBT) to dampen turbine-generator shaft torsionaloscillations. Simulation results of balanced faults in a single machine connected to an infinitebus system show that the proposed fuzzy logic-controlled SMES is effective in dampingshaft torsional oscillations of synchronous generators (GENs). 2006 Institute of ElectricalEngineers of Japan. Published by John Wiley & Sons, Inc.

Keywords: Index Terms—SMES, damping turbine-generator shaft torsional oscillations, fuzzy logic controller, PWMvoltage source converter, two-quadrant chopper, balanced fault, EMTP

Received 30 January 30 2006; Revised 1 March 2006

1. Introduction

Superconducting magnetic energy storage(SMES) is a large superconducting coil capableof storing electric energy in the magnetic fieldgenerated by a DC current flowing through it.The real power as well as the reactive power canbe absorbed (charging) by or released (discharg-ing) from the SMES coil according to systempower requirements. In Ref. [1] we reportedon the work for the fuzzy logic switching ofthe thyristor controlled SMES to enhance thetransient stability of an electric power system.However, in that work, the rotor of the turbinegenerator (GEN) was assumed to be made of a

∗ Department of EEE, Kitami Institute of Technology,165 Koen cho, Kitami, Hokkaido, 090-8507, Japan(email: hasan@pullout.elec.kitami-it.ac.jp)

single mass. In reality, a turbine-GEN rotor hasa very complex mechanical structure consistingof several predominant masses (such as rotorsof turbine sections, GEN rotors, couplings, andexciter rotors) connected by shafts of finite stiff-ness. Therefore, when the GEN is perturbed,torsional oscillations result between differentsections of the turbine-generator rotor. Thispaper analyzes the damping shaft torsional oscil-lations by fuzzy logic-controlled SMES, and thisis the novel feature of this work.

Another salient feature of this work comparedto our previous result [1] is that a pulse widthmodulation (PWM) voltage source converterand a two-quadrant DC–DC chopper using aninsulated-gate-bipolar-transistor (IGBT) insteadof a thyristor is used for SMES control. There-fore, the SMES can be operated to provideindependent control of real and reactive power.

2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

FUZZY LOGIC-CONTROLLED SMES

Charge and discharge of SMES are determinedby the chopper duty cycle, which is controlledby the fuzzy logic.

2. Model System

The power system model used for the sim-ulation of damping shaft torsional oscillationsis shown in Fig. 1. The model system consistsof a synchronous GEN, SG (1000 MVA), feed-ing an infinite bus through a transformer and adouble circuit transmission line. The SMES unitis connected to the GEN terminal bus. CB inthe figure represents a circuit breaker. The sameautomatic voltage regulator (AVR) and Gover-nor (GOV) control system models as those usedin Ref. [1] have also been included in the presentsimulation. The turbine-generator shaft modelhas six masses, namely high-pressure (HP) tur-bine, an intermediate-pressure (IP) turbine, twolow-pressure turbines (LPA, LPB), the GEN, andthe exciter (EXC) as shown in Fig. 2. Variousparameters of the synchronous GEN used for thesimulation are shown inside the area of Fig. 1,while rotor spring mass constants as shown inTable I are described in Ref. [2].

HP IP LPA LPB GEN EXC

Fig. 2 Turbine-generator shaft model

3. Control Scheme of SMES

Figure 3 shows the basic configuration of anSMES unit in the power system. The voltagesource converter consists of a 6-pulse PWMrectifier/inverter (60 MVA) using IGBTs. ThePWM converter and the DC–DC chopper arelinked by a DC link capacitor of 500 mF.The DC voltage across the capacitor is 7000V, which is kept constant throughout by the6-pulse PWM converter. The SMES has asuperconducting coil of 0.5 H. The modelingequations of SMES are described in detail inRef. [1]. To enable charging and discharging ofthe SMES, DC voltage of proper magnitude andpolarity are to be impressed across the coil. Thisis accomplished by the chopper duty cycle, D,controlled by the fuzzy logic.

The modulation index and phase angle of thecommand signal for the sinusoidal PWM con-verter are determined by the capacitor voltagedeviation as well as reactive power of the AC

#1

SG

FuzzyControl Device

Control input

j0.1(P/V = 0.9/1.0) 20/500 KV j0.2 0.04

I n f b u s#2

3L G fa ul t

CB

j0.2 0 .04j0.1

50 Hz, 1000 MVA Base V = 1.0 pu

Control output (D)

(∆ω)

SMESunit

Synchronous Generator Parameters: ra[pu] = 0.003, xa[pu] = 0.13, Xd[pu] = 1.79,

Xq[pu] = 1.71, X/d[pu] = 0.169, X/

q[pu] = 0.228,

X//d[pu] = 0.135, X//

q[pu] = 0.20, X0[pu] = 0.13,

T/do[sec] = 4.30, T/qo[sec] = 0.85,

T//do[sec] = 0.032, T//qo[sec] = 0.05, H[sec] = 2.894

Fig. 1 Power system model

117 IEEJ Trans 1: 116–120 (2006)

M. H. ALI, T. MURATA AND J. TAMURA

Table I. Rotor spring mass parameters

Mass Shaft Inertia H (s) Spring constant

K (pu) pu Torque/rad

HP 0.092897HP-IP 7,277 19.303

IP 0.155589IP-LPA 13,168 34.929

LPA 0.858670LPA-LPB 19,618 52.038

LPB 0.884215LPB-GEN 26,713 70.858

GEN 0.868495GEN-EXC 1,064 2.822

EXC 0.0342165

Y/∆transformer(500/5KV)

6-pulse PWMvoltage sourceconverter using

IGBT

Two-quadrantDC-DC chopper

using IGBTSMES

coil

Three-phase AC(from generator terminal bus)

DC link capacitor

Fig. 3 Basic components of an SMES control system

− .008 − .002 0.0 .002 .008

N Z P

Fig. 4 Membership functions of �ω (pu)

side of the converter. The frequency of the tri-angular carrier signal for the PWM converteris considered to be 1050 Hz. The frequency ofthe saw tooth carrier signal for the chopper is1000 Hz.

For the design of the proposed fuzzy logiccontroller (FLC), deviation of speed, �ω, of theGEN and chopper duty cycle, D are selectedas the input and output respectively. We haveselected the triangular membership functions for�ω as shown in Fig. 4 in which the linguisticvariables N, Z, and P stand for Negative, Zero,and Positive respectively.

The proposed control strategy is very simplebecause it has only three control rules as shown

Table II. Fuzzy rule table

�ω (pu) D

N 0.0Z 0.5P 1.0

in Table II where the numerical values of D

represent the output of the fuzzy controller.The parameters of the membership functionsand control rules have been developed from theviewpoint of practical system operation and bytrial and error method in order to obtain the bestsystem performance.

For the inference mechanism of the proposedFLC, Mamdani’s method [1] has been utilized.Also, the center-of-area method is the mostwell known and rather simple defuzzificationmethod [1], which is implemented to determinethe output crispy value (i.e. the chopper dutycycle, D).

118 IEEJ Trans 1: 116–120 (2006)

FUZZY LOGIC-CONTROLLED SMES

0 2 3 4 51−2

−1

0

1

2

3

4

HP-

IP s

haft

tor

que

[pu]

Time [sec]

2 3 410 5−2

−1

0

1

2

3

4

IP-L

PA s

haft

torq

ue [

pu]

Time [sec]

LPB

-GE

N s

haft

torq

ue [

pu]

Time [sec]

−2

−1

0

1

2

3

4

2 3 41 50

LPA

-LPB

sha

ft t

orqu

e [p

u]

2 3 41 5−2

−1

0

1

2

3

4

0

Time [sec]

(a) Without SMES

−4

−2

−3

−1

0

1

2

3

4

5

2 3 41 50

Ele

ctro

mag

netic

torq

ue [

pu]

Time [sec]

0 1 2 3 4 5−2

−1

0

1

2

3

4

HP-

IP s

haft

torq

ue [

pu]

Time [sec]

IP-L

PA s

haft

torq

ue [

pu]

−2

−1

0

1

2

3

4

0 1 2 3 4 5

Time [sec)

LPA

-LPB

sha

ft t

orqu

e [p

u]

−2

−1

0

1

2

3

4

0 1 2 3 4 5

Time [sec)

LPB

-GE

N s

haft

tor

que

[pu]

−2

−1

0

1

2

3

4

0 1 2 3 4 5

Time [sec)

(b) With SMES

Time [sec)

Ele

ctro

mag

netic

tor

que

[pu]

0 1 2 3 4 5−4

−3

−2

−1

0

1

2

3

4

5

Fig. 5 Shaft torsional torque responses for 3 LG fault

119 IEEJ Trans 1: 116–120 (2006)

M. H. ALI, T. MURATA AND J. TAMURA

4. Simulation Results

Simulations are performed by using Electro-Magnetic Transients Program (EMTP) [3]. Inthe simulation study, it is considered that abalanced (3 LG: Three-phase-to-ground) faultoccurs on the transmission line #2 as shown inFig. 1 at 0.1 s, circuit breakers on the faultedline are opened at 0.2 s, and circuit breakers areclosed again at 1.2 s. Step time and simulationtime have been chosen as 0.00001 s and 5.0 srespectively. Figure 5 shows the shaft torsionaltorque responses with and without the SMESin case of the 3 LG fault. It is clear fromthese responses that the SMES can significantlydampen the shaft torsional oscillations during abalanced fault.

5. Conclusion

This paper makes use of a fuzzy logic-controlled SMES for damping turbine-generatorshaft torsional oscillations. Simulation results of

a balanced fault clearly show that the proposedfuzzy logic-controlled SMES is an effectivemeans of damping turbine shaft torsional oscil-lations of a synchronous GEN.

Acknowledgement

This work was supported by the Grant-in-Aid for JSPSFellows from the Japan Society for the Promotion of Science(JSPS).

References

(1) Ali MH, Murata T, Tamura J. A fuzzy logic-controlledSuperconducting Magnetic Energy Storage (SMES) unitfor augmentation of transient stability. Proceedings ofPower Electronics and Drive Systems (PEDS 2005),Kuala Lumpur, Malaysia, 2005; 1566–1571.

(2) IEEE Subsynchronous Resonance Task Force of theDynamic System Performance Working Group PowerSystem Engineering Committee. First benchmark modelfor computer simulation of subsynchronous resonance.IEEE Transaction Power Apparatus and Systems 1977;PAS-96(5):1565–1572.

(3) EMTP Theory Book . 1994; Japan EMTP Committee,Tokyo.

120 IEEJ Trans 1: 116–120 (2006)