simulation and implementation of marine generator excitation system with pss

7/27/2019 Simulation and Implementation of Marine Generator Excitation System With PSS

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Proceedings ofthe2009 IEEEInternational Conference onMechatronics andAutomationAugust 9 - 12, Changchun,ChinaSimulation and Implementation ofMarine GeneratorExcitation System with PSSSun Caiqin, Li Chengqiu and Shi Chengjun

School ofMarine EngineeringDalian Maritime UniversityDalian 116026, China

[email protected] [email protected] - Small signal stability is the abi li ty of the

mar ine power system to maintain synchronism whensubjected to sma ll disturbance. To improve the stabilityperformance of marine power system, a power systemstabilizer (PSS) is introduced into the traditional marinegenerator excitation system with automatic voltageregulator (AVR). Load compensation is used to control avoltage which is representative of the voltage at a pointeither within or external to the generator. The PSS usesauxiliary stabilizing signals to control the excitation systemso as to more improve mar ine power system dynamicperformance. Marine power system dynamic performanceis improved by the damping of system oscillations. This isa very effective method of enhancing small signal stabilityperformance. By simulating of marine generator excitationsystem with PSS is presented and better stabilityperformance ar e acquired.

small perturbation, such as gradual changes in loads. Thisform of stability can be effectively studied with steady-stateapproaches that use linearization of the system dynamicequations at a given operating point.In this paper, power system stability performance isimproved by introducing power system stabilizer (PSS). Thebasic function of the PSS is to add damping to the generatorrotor oscillations by controlling its excitation using auxiliarystabilizing signal(s).II. OUTLINEOFPOWER STATION SYSTEM

Power Station System consists of generators, excitationcontrol system and bus. The dynamic process of thesynchronous generator is complicated.r---------------CT

S

- T

--I I( II \ I~ -

CCT

AVR

RT

................. . . - - - - r ~ - , . . . - - - - R

Fig.I Power station system ofcontainer ship

' - ---+--t ' ;o-iH 24v' ------t...!....o--< DC

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We will derive a dynamic model derived from the d, qcoordinate system for the synchronous generator. In certainspecial cases, a simplified model can be obtained from the

I. INTRODUCTIONPower system stability may be broadly defined as theproperty of a power system that enables it to remain in a state

of operating equilibrium under normal operating conditionsand to regain an acceptable state of equilibrium after beingsubjected to a disturbance. Instabili ty in a power system maybe manifested in many different ways depending on thesystem configuration and operating mode. The stabilityproblem can affect synchronous operation of all synchronousmachines. Since power systems rely on synchronous machinesfor generation of electrical power, a necessary condition forsatisfactory system operation is that all synchronous machinesremain in synchronism. This aspect of stability is influencedby the dynamics of generator rotor angles and power-anglerelationships. Instability may also be encountered without lossof synchronism.Instability that may result can be of two forms (I ) steadyincrease in generator rotor angle due to lack of synchronizingtorque, or (2) rotor oscillations of increasing amplitude due tolack of sufficient damp ing torque. In today's practical powersystems, the small-signal stability problem is usually one ofinsufficient damping of system oscillations.Small-disturbance (or small-signal) voltage stability isconcerned with a system's ability to control voltages following

Index Terms - Simulation; Marine generator; Excitation;Power system stabilizer(PSS).

978-1-4244-2693-5/09/$25.00 2009 IEEE 839


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complete dynamic model. With little disturbance, thecomplete non-linear model can be changed into a linear one atthe condition point. In some special cases, a simple linearmodel can be got.Electrical schematic diagram is a real power stationsystem in advanced large container ship (Fig. I ), and it is ablock diagram of the power station system (Fig.2). The maingenerator is revolving-magnetic pole type. Armature wingingof the main generator is on stator, and excitation winging is onrotor. The AC exciter is revolving-armature type. Armaturewinging of the AC exciter is on rotor, and excitation wingingis on stator. The output current of the revolving-armature typeAC exciter installed on the shaft of the rotating machine of themain generator excites the field system of the main generatorthrough the rotary rectifier on the shaft[I-3].The current signals transmitted from the currenttransformer CT and the voltage signals transmitted from thereactor RT are then put together to be rectified to DC by usingthree-phase rectifier (Si2) so that the excitation of the ACexciter needs can be achieved. Since the alternating exciter isrevolving-armature type, three-phase AC will be generated onthe rotating armature of the AC exciter, and the three-phaseAC is rectified to DC by using the coaxial rotating three-phaserectifier (SiI ) so that the excitation of the coaxial rotatingmain generator can be achieved.

main generator;AC exciter ;rotary rectifier;silicon rectifier;main generator field winding;Ex field winging;reactor;rheostat for voltage setting;automatic voltage regulator.current transformer;transformer for cross-current compensation;

III. MODELOF MARINE POWER STATIONSYSTEMA mathematical model is set up based on the marinepower station system. Marine power station computersimulation can be realized through operation on digitalcomputers. The most important task is to set up amathematical model that describes the marine power stationsimulation system. The marine power station system consists

of two main parts: part one is the generator set the dieselengine, the generator and accessories; part two includesmonitoring, protection, alarm, operation devices as well asdevices that regulate the operation of the generator. Therefore,the mathematical model has two parts: one is a dynamicmathematical model used to simulate diesel engines ,generators, accessories and a mathematical model for systemstatus; the other is a logical and monitoring mathematicalmodel used to simulate marine power station protection, alarmoperation and monitoring. On this, the power system stabilitymay be analyzed after being subjected to disturbance. Thegenerator set model and the excitation control system modelwill be set up.

reactance, between the generator terminals and the point atwhich the voltage is being effectively controlled. Thecompensator regulates the voltage at a point within thegenerator and thus provides voltage drop. With rotatingrectifier, the need for slip rings and brushes is eliminated, andthe DC output is directly fed to the main generator field. thevoltage regulator controls the AC exciter field, which in tumcontrols the field of the main generator. Brushless excitationsystems do not allow direct measurement of generator fieldcurrent or voltage. After adjusting the variable resistor VR, theoutput of generator terminal voltage can be manuallycontrolled.G:

Ex:s.,Si2:F1:F2:RT:VR:AVR:CT:CCT:

Ur

currentlimiter

WPrime Pm Rotor

wref mover and equation-. governorid, iq Stator

currentPe limiter

U re f hj r - -U - ,'--- - ..... IG-. Exci tat ion Genera tor Ud,Uq ,.-....;....---,U t system excitation

Fig.2 Power station system skeleton diagram

Therefore, the current signals transmitted from thecurrent transformer CT and the voltage signals transmittedfrom the reactor RT keeps the voltage at the main generatorterminal at a normal level. The AVR normally controls thegenerator stator terminal voltage. Load compensation is usedto control a voltage which is representative of the voltage at apoint either within or external to the generator. This isachieved by building additional circuitry into the AVR loop.The compensator has adjustable resistance and inductive

A. The MarineGenerator Sets ModelThe block diagram representation of the small-signalperformance of the system is shown in Fig.3.In this representation, the dynamic characteristics of thesystem are expressed in terms of the so-called K constants.

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. - - - - - - - - - - - - -1 K41+- - - - -+- - - --,

s,~ KlUg0.- KS

Fig.3 The Block diagram representation of the small-signalperformance of the system

brushes. Motor structures, excitation models, parts and circuitsmay vary, but there are mainly three types of excitationregulation principles for marine synchronous generators: AVRon disturbance principle, AVR on feedback principle andAVR on compound regulation principle.Excitation system comprised of elements with significanttime delays have poor inherent dynamic performance. This isparticularly true of DC and AC type excitation systems.Unless a very low steady-state regulator gain is used, theexcitation control(through feedback of generator statorvoltage) is unstable when the generator is on open circuit.Therefore, excitation control system stabilization, comprisingeither series or feedback compensation, is used to improve thedynamic performance of the control system. The mostcommonly used form of compensation is a derivative feedbackas shown in Fig. 4.

KF SI+ STF

B. The excitation control system modelThe advances in excitation control system over the last 20years have been influenced by developments in solid-stateelectronics. Developments in analog integrated circuitry havemade it possible to easily implement complex controlstrategies. The latest development in excitation systems hasbeen the introduction of digital technology. Thyristorscontinue to be used for the power stage. The control,protection, and logic function s have been implementsdigitally, essentially duplicating the functions previouslyprovided by analog circuitry. The digital controls are likely tobe used extensively in the future as they provide a cheaper andpossibly more reliably alternative to circuitry. They haveadded advantage of being more flexible, allowing easyimplementation of more complex control strategies, andinterfacing with other generator control and protectivefunctions.The quality of a marine electricity network is determinedby performance of the marine synchronous generator, factorssuch as voltage output precision, range of voltage regulation,the stabilization of the voltage when the load varies and thespeed for dynamic reaction will affect the power stability, andall these factors are related to the AYR. There are mainly twotypes of excitation models for the marine synchronousgenerator: one is with brushes and the other is without

Fig.4 Derivative feedback excitation control systemstabilization model

The effect of the compensation is to minimize the phaseshift introduced by the time delays over a selected frequencyrange. This results is a stable off-line performance of thegenerator, such as that existing just prior to synchronization orfollowing a load rejection. The feedback parameters can alsobe adjusted to improve the on-line performance of thegenerating unit. Depending on the type of excitation system,there may be many levels of excitation control systemstabilization involving the major outer loop and minor innerloops. Static excitation systems have negligible inherent timedelays and do not require excitation control-systemstabilization to ensure stable operation with the generator offline.Although most of the data related to excitation systemmodels can be obtained from factory tests, such as data canonly be considered as typical. The actual sett ings are usuallydetermined on site during installation and commissioning ofthe equipment. It is therefore desirable to determine the modelparameters by performing tests on the actual equipment onsite[4].Under is AVR on compound regulation principle in thelarge container ship. Its block diagram is as follows, Fig.5.

(00:~ ( O :H:K D:~ T~ T m :K 1-K 6:T3:'I'm:S:

rotor angle deviationrated speedspeed deviationinertia constantdamp ing torqueair-gap torquemechanical torquecoefficienttime constantthe field circuit d-axis flux linkageLaplace operator

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u u,: . . . . . . . ~ Exciterand AVR


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Fig.S The excitation control system model

Uref: voltage regulator reference voltageUpss: power system stabilizer output voltageKF: excitation control-system feedback gainTF: excitation control-system time constantSL: brushless exciter saturation functionTL: brushless exciter time constantKu: load compensator voltage gainK,: load compensator current gainUI: voltage-transducer output voltageTR: voltage-transducer time constantKA : amplifier gainTA: amplifier time constantS: Laplace operator

IV. POWER SYSTEM STABILIZER MODELExcitation systems comprised of elements with significanttime delays have poor inherent dynamic performance.Exci tat ion control system stabil ization, compris ing eitherseries or feedback compensation, is used to improve thedynamic performance of the control system.Field regulator's function is along with the generatorterminal voltage's change by adjusting the generator excitationunceasingly. The excitation adjustment will affect theelectromagnetic torque ~ T e 2 ( ~ T e 2 = K 2 ~ E q ' ) and the ~ E q 'will be changed by ~ ( ) . Between the voltage and the torquerelational vector graph is as follows, Fig.6.Electromagnetic torque ~ Te2 will be produce when the

voltage deviation signal pass through the field regulator, theexciter and the generator field. Some lag's phase lpl presencebetween the ~ T e 2 and ~ ( ) . Two parter of the ~ T e 2 are and~ T d ' is the synchronized torque. is the negativedamping torque. If the negative damping torque is bigger thanthe natural damping act ion, the generator rotor will vibratecontinuously.

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Fig.6 Generator voltage and torque relationsPower system stabil izer (PSS) provides an addit ional

input signal to the regulator to damp power systemoscillations. Some commonly used input signals of PSS arero tor speed deviation, accelerating power, and frequencydeviation. This is a very effective method of enhancing smallsignal stability performance [5].The basic function of the PSS is to add damping to thegenerator rotor oscillations by controlling its excitation usingauxiliary stabilizing signal(s). To provide damping, thes tabil izer must produce a component of electrical torque inphase with the rotor speed deviations. Since the purpose of aPSS is to introduce a damping torque component, a logicals ignal to use for control ling generator exci tat ion is the speeddeviation ~ ( ) ) f " If the exciter transfer function and the generatortransfer function between A'I', and ~ E f d (Fig.3) were puregains, a direct feedback of ~ w would result in damping torquecomponent. However , in prac tice both the generator and theexciter exhibit frequency dependent gain and phasecharacteristics. Therefore, the PSS transfer function, GPSS(S),should have appropriate phase compensation circuits tocompensate for the phase lag between the exciter input and theelectrical torque. In an ideal case, with the phase characteristicof Gpss(S) being an exact inverse of the exciter and generatorphase characteristics to be compensated, the PSS would resultin a pure damping torque at all oscillating frequencies.The PSS representa tion consists of three blocks: a phasecompensation block, a signal washout block, a gain block, andstabilizer limits. The power system stabilizer is shown inFig.?

['+7SJP'+i SFig.7 power system stabilizer

Ksr: gainTw: time constantT I: time constantT2: time constantp: constant (I or 2)


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The phase compensation block provides an appropriatephase-lead characteristic to compensate for the phase lagbetween the exciter input and the generator electrical torque.The phase characteristic to be compensated changes withsystem conditions, therefore, a compromise is made and acharacteristic acceptable for different system conditions isselected. Some under-compensation is desirable so that thePSS, in addition to significantly increasing the dampingtorque, results in a slight increase in the synchronizing torque.The signal washout block serves as a high-pass filter ,with the time constant Tw high enough to allow signalsassociated with oscillations 0) to pass unchanged. Without it,steady changes in speed would modify the terminal voltage. Itallows the PSS to respond only to changes in speed. From theviewpoint of the washout function, the valve of Tw is notcritical. The main consideration is that it is long enough topass stabilizing signals at the frequencies of unchangedinterest, but not so long that it leads to undesirable generatorvoltage excursions during system-islanding conditions.The stabilizer gain Ksr determines the amount ofdamping introduced by the PSS. Ideally, the gain should be setat a value corresponding to maximum damping; however, it isoften limited by other considerations in application. Thestabilizer gain is normally set to a value that results in as higha damping of the critical system modes as practical withoutcompromising the stability of other system modes or causingexcessive amplification of signal noise.The positive output limit of the stabilizer is set at arelatively large value in the range of 0.1 to 0.2 pu. This allowsa high level of contribution from the PSS during large swings.With such a high value of stabilizer output limit, it is essentialto have a means of limiting the generator terminal voltage toits maximum allowable value. The terminal voltage signal,however, contains small components of torsional components.Hence, feedback of this signal to the excitation system througha high gain may cause torsional mode instability.V. SIMULATION OF POWER STATION DYNAMIC RESPONSE AND

ANALYSISSeveral simulation calculations for power station dynamicresponses are completed based on the following parametersgiven.The marine generator sets model parameters:K]=1.59I ;K2=1.5;K 3=0.333;K4=1.8;K5=0. I2;K 6=0.3;T3=1.91;KR=0.003;O)o=377;The excitation control system model parameters:KF=O.OI;TF=0.52;SL=I.0;TL=15;TR=0.003.

Power system stabilizer model parameters:Ksr=9.5;T:1.4;T]=0.154;T2=0.033 ;p=1.The stability of the system has the following alternative formsof excitation control:(1) real marine generator excitation system, no PSS:response with 10% pulse excitation disturbance, disturbancetime equal to I second. As shown in Figure 8.

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response with 10% torque disturbance, disturbance time equalto 1 second. As shown in Figure 9.3

0r10 . ~.......... .... .... ......... .... .......... . 1 10- : , : --..,.:- - - - . . . . . . . .-05 ; , , , ; . . . . . . . . . . . . .1

:::rh i \ 'A k = . 1::: ............ .... 1 .4f. ~

I ~ ..... .. .\ ~


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response with 10% torque disturbance, disturbance time equalto 1 second. As shown in Figure 11.

r :01F I 1-...... .. . . . . . . .2 i .. . ................ ......... ... ............... ........ ; ; . . . . . . . . .4 . . . . ':' ': : : . . . . .6

..... ~ m m m m m m m m m m ~. . . .0.01 :.. . . . . . . . . . ... . . . . . . . . . . . . ....... ; : : . . . . . . . . . . . . ..,r ~ mmm immmm:2 .... . . . .... .... .. ... . . . . ..... ..... .. ...... ... ..... ...... ... ... . ., , . . . . ., ................. . . .. . . . .. . .. ..... ....;;. .o ............... .............................. . . . ..,t.iSJE1mmmmmmmmmmmmmmmS . . . . .. . . . . .. . . . . . . . . . . . . . . .. . . . . .. . . . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . .: . . .. . . .. . . . . ........ ... .......... . . . . . . . . . . . .2 i . ' ; : : . . . . . . . . . . . . .230 , 2 3 , 5 6Fig.IO The response with 10% pulse excitationdisturbance, time equal to I second, with PSS3 $.............................................. ......... ................

0 - ; ... .. .. ... . . .. ... .. ... .... ... . .. - .. . . . . . . .2 : ; ; : : . . . . . . . . . . . . .., '-----__ -- '---__ ---- '- L-__ ---'--- _..................$....................................................... .................02 : : : . ... ... . .. . .. . .. . . .. .... ... ... ... .. . . . . . . . . . . . .o ; : . ; ; . . . . . . . . . . .0.2 - .3,---------'----------'----'---------'---------l---:l'O .......... ..................... . .! .................0 - : ;. . .. . . .. . . . .. ... .- _ -'------':---- . . . . . . . . . . . . . . . ..2 '-----__--'---__----'- L-__ ---'--- _'o .:14 ; ~5 . . . . . . . . . . . . . . . ..... .... ........................................... : : .: == : 11 .o 1 2 3 , 5 6Fig.11 The response with 10% torque disturbance,time equal to I second, with PSSThe result of time responses computed with the fouralternative excitation controls are shown in figure 8, figure 9,figure 10 and figure 11 separately, which show time responses

of ~ w , ~ P e ~ U g and (), respectively. These results are

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calculated by using Matlab. The values of and ~ U gare per unit value. The value of () is actual rotor angle value. xaxis unit is second.From figure 8 and figure 9, no PSS, we can see that thetime responses of ~ P e , and () are stable within 2seconds to real marine generator excitation system. Theamplitude of ~ w , ~ P e and ~ U g are normal.From figure 10 and figure 11, with PSS, the ~ P eand~ U g and () during the time responses are stable, that is within 1second. The amplitude of and ~ U g are reduced half.Therefore, it can be concluded from the four results abovethat the use of a fast exciter with a high-ceiling voltage andequipped with the PSS contributes to the enhancement of theoverall system stability performance.

REFERENCES[I] Caiqin Sun, Chen Guo, Chengjun Shi. Modeling and Simulating to LargeIntelligent Marine Power Station SystcmjC] . Sixth World Congress onIntelligent Control and Automation , 2006: 6128-6132.[2] Sun Caiqin , Guo Chen, Shi Chengjun etc . Power Station SimulationSubsystem in Marine[J]. Journal of Dalian Maritime University. 2002.28(Suppl.): 31-34[3J Weifeng Shi, Tianhao Tang, Jianmin Yang. Simulation of A Large MarineContainer Ship Power SystemjC]. SICE Annual Conference in Sapporo,August 4-6, 2004: 39-44.[4] IEEE Tutorial Course Text, "Power System Stabilization via ExcitationControl - Chapter IV: Field Testing Techniques," Publication 81 EHO175-0 PWR.[5] Prabha Kundur, Power System Stability and Control[M]. Beijing: ChinaElectric Power Press, 2001.

simulation and implementation of marine generator excitation system with pss

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