A static synchronous compensator is one of the FACTS devices used to improve the transientstability of the power system. Flexible AC Transmission System (FACTS) devices are found tobe very effective in a transmission network for better utilization of its existing facilities withoutsacrificing the desired stability margin. The effect of STATCOM for improving the stability of thetwo machine power system at during fault condition is investigated. In this paper a Mamdanibased Fuzzy logic controller is designed, and Simulation results show that STATCOM is effectivein midpoint voltage regulation on transmission line. The study is thereby simulated using theMATLAB/SIMULINK software.
INTRODUCTIONModern electric power system is facing manychallenges due to day by day increasingcomplexity in their operation and structure. Inthe recent past, one of the problems that gotwide attention is the power system instability.With the lack of new generation andtransmission facilities and over exploitation ofthe existing facilities geared by increase inload demand make these types of problemsmore imminent in modern power systems.Demand of electrical power is continuouslyrising at a very high rate due to rapid industrial
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development (Hingorani and Gyungi, 2000).To meet this demand, it is essential to raisethe transmitted power along with the existingtransmission facilities. The need for the powerflow control in electrical power systems is thusevident. With the increased loading oftransmission lines, the problem of transientstability after a major fault can become atransmission power limiting factor. The Powersystem should adapt to momentary systemconditions, in other words, power systemshould be flexible. In an ac power system, theelectrical generation and load must balanceat all times up to some extent, the power
Research Paper
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Int. J. Elec&Electr.Eng&Telecoms. 2014 Poorinima S and Pushpalatha D, 2014
system is self regulating. If generation is lessthan load, the voltage and frequency drop, andthereby the load goes down to equal thegeneration minus transmission losses. Butthere are only a few percent margins for sucha self Regulation. Hence there is chance ofsystem collapse. Generator excitationcontroller with only excitation control canimprove transient stability for minor faults butit is not sufficient to maintain stability of systemfor large faults occur near to generatorterminals (Cong and Wang, 2002).
Thus, this requires a review of traditionalmethods and the creation of new concepts thatemphasize a more efficient use of alreadyexisting power system resources withoutreduction in system stability and security. In thelate 1980s, the Electric Power ResearchInstitute (EPRI) introduced a new approach tosolve the problem of designing and operatingpower systems the proposed concept isknown as Flexible AC Transmission Systems(FACTS). The two main objectives of FACTSare to increase the transmission capacity andcontrol power f low over designatedtransmission routes. FACTS are defined bythe IEEE as a power electronic based systemand other static equipment that provide controlof one or more AC transmission systemparameters to enhance controllability andincrease power transfer capability.
STATCOM CONTROLLERA static synchronous compensator is one ofthe FACTS device operated on principle ofreactive power compensation can use toimprove the transient stability of the system byincreasing (decreasing) the power transfercapability when the machine angle increases
(decreases) (Laszlo Gyugi, 1994). Staticsynchronous compensator’s three modes, i.e.,capacitive mode, inductive mode and no loadmode regulates voltage in transmissionsystem. When Converter a.c. output voltage(Vc) > transmission system voltage (Vs),STATCOM considered to be in Capacitivemode and when Vs > Vc, STATCOMconsidered to be in inductive mode and in No-Load mode Vs = Vc, no reactive Powerexchange takes place (Trainber et al., 1994).STATCOM mainly comprise of step downtransformer with leakage reactance, threephase GTO voltage source inverter and a dccapacitor voltage (Ekanayake and Jenkins,1996). Figure 1 shows equivalent circuitdiagram of STATCOM system (Yoon and Kin,1996).
Figure 1: STATCOM’s Basic Circuit Diagram
Voltage across capacitor, i.e., Vdc is givenby the following equation,
QDdcdc IImkGv
dtdvC cossin
...(1)
where G is conductance across the capacitorthat represented losses in the capacitor whileá and m are control variables of inverter whichaffected the magnitude and phase angle of thevoltage injected by the inverter (Padiyar andKulkarni, 1996). In STATCOM different
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Int. J. Elec&Electr.Eng&Telecoms. 2014 Poorinima S and Pushpalatha D, 2014
technologies used dependent upon the powerratings of STATCOM. For higher powerSTATCOMs GTO based technologies areused while for lower power STATCOMs IGBTbased technologies used (Nicolas Lechevinand Rajagopalan, 1998). In this paper GTObased technologies used. In GTO based staticsynchronous compensator m is normally keptconstant and angle is varied to controlreactive power (Padiyar and Kulkarni, 1996).Amount of real power generated or absorbedby STATCOM depends upon the size ofcapacitor.
As compare to SVC, STATCOM providesa number of performance advantages forreactive power control applications becauseof its greater reactive current output capabilityat depressed voltage, faster response, bettercontrol stability, lower harmonics and smallsize, etc. (Chun Li Ohrang and Jiang Wang,1998; and Moadders and Gole, 1999). In Thispaper a Mamdani based Fuzzy logic controlleris designed. The inputs to the Fuzzy logiccontroller are the alternator speed and itsderivative and output is firing angle of thevoltage source converter. The proposedcontroller is tested on a two machine powersystem under Matlab Simulink Environment.
OPERATING PRINCIPLE OFSTATCOMSTATCOM is made up of a couplingtransformer, a VSC and a dc energy storagedevice. STATCOM is capable of exchangingreactive power with the transmission linebecause of its small energy storage device,i.e., small dc capacitor, if this dc capacitor isreplaced with dc storage battery or other dcvoltage source, the controller can exchange
real and reactive power with the transmissionsystem, extending its region of operation fromtwo to four quadrants. A functional model of aSTATCOM is shown in Figure 2.
Figure 2: Functional Model of STATCOM
The relationship between fundamentalcomponent of the converter ac output voltageand Voltage across dc capacitor is given as
Vout = kVdc ...(2)
where k is coefficient which depends upon onthe converter configuration, number of switchingpulses and the converter controls. Thefundamental component of the converter outputvoltage, i.e., Vout can be controlled by varyingthe dc voltage across capacitor which can bedone by changing the phase angle of theoperation of the converter switches relative tothe phase of the ac system bus voltage. Thedirection of flow of reactive power whether itis from coupling transformer to the system orfrom system to the coupling transformerdepends upon the difference between theconverter output voltage and the ac system busvoltage. The real power flowing into the
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Int. J. Elec&Electr.Eng&Telecoms. 2014 Poorinima S and Pushpalatha D, 2014
converter supplies the converter losses dueto switching and charges the dc capacitor to asatisfactory dc voltage level. The capacitor ischarged and discharged during the course ofeach switching cycle but in steady state, theaverage capacitor voltage remains constant.If that were not the case, there would be realpower flowing into or out of the converter, andthe capacitor would gain or lose charge eachcycle. In steady state, all of the power from theac system is used to replenish the losses dueto switching.
The STATCOM’s ability to absorb/supplyreal power depends on the size of dc capacitorand the real power losses due to switching.Whenever the dc capacitor and the losses arerelatively small. The amount of real powertransfer is also relatively small. This impliesthat the STATCOM’s output ac current Iac, hasto be approximately + 900 with respect to acsystem voltage at its line terminals. Varying theamplitude of the converter three-phase outputvoltage Vout controls the reactive powergeneration/absorption of the STATCOM. If theamplitude of the converter output voltage Voutis increased above the amplitude of the acsystem bus voltage Vac then the ac current Iac,flows through the transformer reactance fromthe converter to the ac system generatingreactive power. In this case, the ac systemdraws capacitive current that leads by an angleof 900 the ac system voltage, assuming thatthe converter losses are equal to zero. The accurrent flows from the ac system to the voltage-sourced converter if the amplitude of theconverter output voltage is decreased belowthat of the ac system, and consequently theconverter absorbs reactive power. For aninductive operation, the current lags the acvoltage by an angle of 90°.
Assuming again that the converter lossesare neglected. If the amplitudes of the acsystem and converter output voltages areequal, there will be no ac current flow in/out ofthe converter and hence there will be noreactive power generation/absorption the accurrent magnitude can be calculated using thefollowing equation
XVVI acout
ac
...(3)
Assuming that the ac current flows from theconverter to the ac system. Vout and Vac arethe magnitudes of the converter output voltageand ac system voltage respectively, while Xrepresents the coupling transformer leakagereactance. The corresponding reactive powerexchanged can be expressed as follows:
XVVVQ acoutout cos2
...(4)
V-I CHARACTERISTICS OFSTATCOMWhen the STATCOM is worked in voltageregulation mode, It implements the V-ICharacteristics as shown in Figure 3. The V-Icharacteristics are depicted by the followingequation:
V = Vref + Xs.I ...(5)
where,
V = Positive sequence voltage
I = Reactive current (Pu/Pnorm)
(I > 0 indicates an inductive current and I <0 indicates capacitive current)
Xs = Slope (usually between 1% and 5%)
Pnorm = Converter rating in MVA
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Int. J. Elec&Electr.Eng&Telecoms. 2014 Poorinima S and Pushpalatha D, 2014
TWO MACHINE SYSTEMFig 4 shows single line diagram of two areasystem (area 1 and area 2). Area 1 (1000 MWhydraulic generation plant) connected to area2 (5000 MW hydraulic generation plant)through 500 kV, 700 km transmission line.
Both plants fed to a load center, modelledby a 5000 MW resistive load. System isinitialized so that line carries 950 MW whichis close to its surge impedance loading. Inorder to maintain system stability Static
synchronous compensator of 200 MVA isconnected at midpoint of transmission line.By connecting it at midpoint the powertransfers capability of system increasessignificantly (Yixin Ni and Mak, 1999; andChen et al., 2000).
FUZZY LOGIC CONTROLLERFuzzy modeling is the method of describingthe characteristics of a system using Fuzzyinference rules. The method has adistinguishing feature in that it can expresslinguistically complex non-linear system. It ishowever, very hand to identify the rules and tunethe membership functions of the reasoning.Fuzzy Controllers are normally built with Fuzzyrules. These Fuzzy rules are obtained eitherfrom domain experts or by observing thepeople who are currently doing the control.
The membership functions for the Fuzzy setswill be derive from the information availablefrom the domain experts and/or observedcontrol actions. The building of such rules andmembership functions require tuning. That is,performance of the controller must bemeasured and the membership functions andrules adjusted based upon the performance.This process will be time consuming.
The basic configuration of Fuzzy logiccontrol based as shown in Figure 5. Consistsof four main parts, i.e.
• Fuzzification,
• Rule base,
• Inference Engine, and
• Defuzzification
In Analytical approaches, Modelling andControl of Power Network requires
Figure 3: V-I Characteristics of STATCOM
Figure 4: Single Line Diagram of Two AreaInterconnected System
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Int. J. Elec&Electr.Eng&Telecoms. 2014 Poorinima S and Pushpalatha D, 2014
mathematical equations/models. As powersystem models are highly non linear, numberof assumptions need to be made beforederiving mathematical equations (PraneshRao and Crow, 2000). Fuzzy Logic is oneoption by which one can get rid from aboveproblem because Fuzzy logic is techniquewhich deals with human reasoning that can beprogrammed in to Fuzzy logic language, i.e.,is membership function, rules interpretation(Farsangi and Sang , 2002).
Function of fuzzification is mapping of inputof Fuzzy logic, i.e., is crisp value in to Fuzzyvariables by using membership functions whilefunction of Fuzzy logic engine to infer the propercontrol actions based on given Fuzzy rules.Under defuzzif ication, control actionstranslated into crisp values by using normalizedmembership functions (Cong and Wang, 2002;and Liu et al., 2003).
In this paper defuzzification of output signalis done by using centroid method. Fuzzy basedController has been designed by taking
generator speed and its derivative as inputwhile angle alpha as output. The Fuzzymembership functions of these variables areas shown in Table 1.
Figure 5: A Fuzzy System
FuzzyRule Base
FuzzyInferenceengine
DefuuzificationFuzzificationInput Output
NB PM PS NB NM NS Z PM
NM PS NM NM NB Z Z PS
NS PM NS NS Z NM PS NS
Z PB Z Z Z NM PS NM
PS Z Z PM NS NS PM NS
PM Z PM PM PS PB PM NS
PB PM PS PM PS PM PB NS
Table 1: Fuzzy Rules
dw/dt
NB NM NS Z PS PM PB
The logic is that when frequency is high andits rising fast, the system is in critical conditionbecause the input mechanical power ofgenerators is more than output electrical power.Therefore the STATCOM should inject bigcapacitive current into the network hence alphashould be small (Amit Jain and Aman Behal,2004). By this loaction the transmittable powercapacity of the line on which STATCOMinstalled will be increased and the transientstability will be improved. Other conditions canbe analyzed in a similar way.
SIMULATIONSimulink Model of two machines (M1 andM2) system,Each machine equipped with aGovernor, excitation system and Powersystem stabilizer. These components areincluded in Turbine and Regulator 1 andTurbine and Regulator 2. Both machineconnected through a 500 kv, 700 kmtransmission line. Resistive load of 5000 MWconnected on Machine M2 side. GTO basedSTATCOM having rating of 200 MVAconnected at midpoint of transmission line.
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Int. J. Elec&Electr.Eng&Telecoms. 2014 Poorinima S and Pushpalatha D, 2014
Given simulation model run for under discrete mode with sample time (Ts) set at 20 ×10-6sec.
SIMULATION RESULTSSystem Without StatcomA three phase fault having clearing time of 0.1sec is given at 0.2 sec. System installedwithout STATCOM becomes unstable on thefault time as shown by Figures 6 and 7.
System Installed with Fuzzy BasedStatcom ControllerNow system is installed with Fuzzy basedSTATCOM Controller and fault having clearingtime of 0.1 sec is given during time period of0.2 sec to 0.3 sec as shown in the Figures 8and 9.
Figure 6: Deviation of Rotor Anglewith Time (Under Fault)
Figure 7: Load Angle with Time (Under Fault)
Figure 8: Rotor Angle Deviation with Time(After Clearing Fault)
Figure 9: Load Angle with Time(After Clearing Fault)
The Figure10 shows that the voltage underfault conditions. When the three phase faultoccurred at the transmission line, the system
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Int. J. Elec&Electr.Eng&Telecoms. 2014 Poorinima S and Pushpalatha D, 2014
voltage gets unbalanced. Due to unbalance ofthe voltage, Transient stability, randomvariations of voltage magnitudes, mainly dueto loads, and involving speed variations. Toavoid these problems by installing theSTATCOM at the midpoint of transmission line,the voltage will be get balanced and aspossible to improve the transient stability andthe system becomes stable as shown inFigure 11.
the STATCOM for improving transient stabilityof the power system. Controller inputs arechosen carefully to provide better damping tothe system and its range are determined bythe simulation results of fuzzification process.Simulation results indicate that Fuzzy basedSTATCOM controller provides better transientstability and Simulation results indicated thatthe Fuzzy based STATCOM controller installedwith two machine system provides betterdamping characteristics and providesimproved transient stability.
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Figure 10: Three Phase Fault Voltage
Figure 11: Three Phase Clearing FaultVoltage
CONCLUSIONIn This Paper, Mamdani based Fuzzy logiccontroller is successfully designed to control
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Data for various components used in Matlab Simulink model of Figure 6 are as follows:
