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IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com

IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com

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  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326315 | P a g eAn Improved Electronic Load Controller for Isolated SmallHydro Wind Hybrid systemSamundraGurung, P.Somasundram(Power Systems Engineering, College of Engineering, Anna University, Guindy-25)(Associate Professor, Power Systems Engineering, College of Engineering, Anna University,Guindy-25)ABSTRACTA standalone hydro wind hybrid grid feeding acommon three phase four wire load is considered.There are three generators Synchronous andInduction Generator fed by a hydro-turbine and avariable Wind speed turbine driven PermanentMagnet Synchronous Generator (PMSG). AnElectronic Load Controller (ELC) based onStatcom principle is connected in shunt with thesystem. Vector control scheme for referencecurrent generation technique of Statcom isdeveloped. The major objectives in a standalonesystem are regulation of frequency and voltage. Abattery bank is used on the dc side of Statcomthus enabling it for active power control and thusregulating the frequency. The ELC also performsother functions such as voltage regulation,reactive power compensation, neutral currentcompensation and harmonic elimination. Theoutput from PMSG is connected to dc link viathree phase voltage source converter (VSC) whichis used for maximum power point tracking(MPPT). All models are developed using Matlab,Simulink.Keywords-Battery Energy Storage, Voltage andFrequency regulator, MPPT, Standalone HydroWind systemI. INTRODUCTIONStandalone generation with small hydro turbines iswidely used in developing countries. On average itcan feed as many as 600 houses which can be widelydispersed. However, only a single source makes theseschemes far from reliable. Normally SynchronousGenerators are preferred in small hydro plants due totheir better voltage regulation capability. For astandalone Wind system, Permanent MagnetSynchronous Generators are preferred due toreduction in gear box and non-requirement forexcitation capacitors. Thus a more reliable isolatedgrid can be developed if we can integrate these twosources.The major challenge in an isolated grid is voltage andfrequency regulation. The voltages are allowed +-5%variation and frequency +-2% variation from itsnominal value.An isolated grid can have an increase in load demandin future. Thus an additional generator is needed tomeet the extra demand. Induction Generator will bemore suitable for this purpose due to ruggedness andeconomic reasons.Unlike SG they do not have Automatic VoltageRegulators(AVR) which results in poor voltageregulation capability. They also need capacitor banksfor voltage buildup during no load condition. Whenthe IG is loaded, extrareactive power is needed for itscontinuous operation.Variable speed wind generators are very popular andare an ideal choice for standalone systems. PMSGbased schemes allow much lower cost and thus canbe considered for hybrid with the hydro schemes.However variable wind schemes suffer fromintermittency and thus a battery based schemes willgive more reliable operation.Parallel operation of Synchronous and InductionGenerator has been reported in literature[1]. APower-Quality Improvement of a Stand-AloneInduction Generator Using a Statcom with BatteryEnergy Storage System is also suggested [2].Avariable speed drive Permanent magnet SynchronousGenerator in hybrid with a hydro driven squirrel cageInduction Generator along with a battery is alsodiscussed [3].Analysis of voltage control for a self-excited induction generator using a current-controlledvoltage source inverter (CC-VSl) which alsoperforms functions of load balancing and harmonicelimination is also discussed [4]. A PermanentMagnet Synchronous Generator-Based StandaloneWind Energy Supply System has also been suggested[5].In this work an attempt is made to use Statcom asboth voltage and frequency controller which acts asan improved electronic load controller (IELC) for astandalone system feeding 3-phase 4-wire loadsdriven by uncontrolled micro hydroturbine and avariable wind speed turbine.The proposed controller is a 4 Leg IGBTs basedVSCs which are connected to the each phaseof thegenerator through interfacing inductors.The neutralpoint of the load is connected to neutral point of thefourth leg of the controller. A dc link capacitor and abattery are connected on the dc bus of the VSCs. Theoutput from variable wind speed driven PMSG isconnected to a three phase VSC which is then againconnected to the common dc bus. Maximum powerpoint tracking(MPPT) based on constant torquecontrol is used.The proposed IELC is also found
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326316 | P a g ecapable of reactive power compensation, harmonicelimination and neutral current compensation.The schematic diagram of the proposed ELC isshown in Fig. 1. There are two generators namelySynchronous and Squirrel Cage Induction Generatorfed by uncontrolled hydro turbines. These turbinesalways produce a rated power which is given to thegenerator. The exciter voltage is limited to 2pu whichis just sufficient for it to produce rated voltage duringfull load condition.The Induction Generator isconnected to an external capacitor bank which helpsin voltage buildup at no load condition. When there isan increase in load demand, additional reactive poweris needed for IG to supply real power. As the AVRoutput is limited, Statcom has to supply thisadditional reactive power to keep the voltage to itsrated value. The Statcom is implemented using threephase VSC with an additional leg for neutral currentcompensation. On the dc side of the Statcom acapacitor to filter voltage ripple and a dc battery isconnected. Thus, it can also provide or absorb realpower whenever needed. The amount of real power itcan provide depends on the capacity of the battery. Itcan also absorb excess power in case of lightly loadcondition and thus the Statcom acts both as a voltageand frequency controller.The control structure for a Statcom is based onVector Control principle where synchronouslyrotating reference frame is considered.This referenceframe is chosen as it gives the transformed quantitiesin dc variables which are ideal for compensation andalso filtering purpose.Besides functioning as voltageand frequency controller, it also performs otherfunctions like harmonics eliminator and neutralcurrent compensator. The third generator used isPermanent Magnet Synchronous Generator which isdriven by a variable speed small wind turbine. As thescheme is a variable speed wind system, a maximumpower point tracker (MPPT) must be employed forefficient power production. This is implementedusing Vector Control technique based on constanttorque angle control as shown in Fig. 4. Again a threephase VSC is used to track the reference speed basedon optimum tip speed ratio at certain wind velocity.Fig.1. Schematic diagram of Hydro-Wind Hybrid systemII. MODELING OF THE SYSTEM2.1 StatcomReference Current GenerationThe control structure for Statcom is based onsynchronously rotating reference frame theory asshown in Fig. 3.When the three phase voltages aretransformed in dq quantities, based on given directionof q axis leading d,peak voltage isobserved on the daxis only.A three phase PLL is used to synchronizeand thus calculate the position of transformationangle ϴs.Real power and reactive power in dq co-ordinates canbe written as𝑃 =32∗ 𝑉𝑑 𝐼 𝑑 + 𝑉𝑞 𝐼𝑞 (1)𝑄 =32∗ 𝑉𝑞 𝐼𝑞 − 𝑉𝑑 𝐼 𝑑 (2)As Vq=0 thus𝑃 = 𝑉𝑑 𝐼 𝑑(3)𝑄 = −𝑉𝑑 𝐼𝑞(4)where, Vd, Id, Vq, Iq are d-axis and q-axis voltage andcurrent respectively.Equation (3) and (4) shows the decoupling of activeand reactive power. Thus by controlling Id we cancontrol the real power and by controlling Iq we cancontrol the reactive power.Since the battery cannot produce reactive power andthe dc link voltage is held constant by the batteryunlike in other schemes where it is held constant bycapacitor, Id directly relates to the current that shouldbe produced or absorbed by the battery to keep thefrequency constant. This current is kept within a safevalue given by the battery rating using a limiter.d-axis reference current generation
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326317 | P a g eWhen the speed of the machine is at rated value andthus frequency is at its nominal value, there is noabsorption or production. The load current isdecomposed into its direct and quadraturecomponent. When harmonic loads are present, therewill also be an oscillating component in which isadded to frequency regulating component..Fig. 2. Block diagram of Statcom reference generation𝜔𝑒𝑟𝑟 = 𝜔 𝑟𝑒𝑓 𝑛 − 𝜔𝑟 (5)𝐼 𝑑𝑓 = 𝐼 𝑑𝑓 𝑛 − 1 + 𝐾𝑝𝑓 𝜔𝑒𝑟𝑟 − 𝜔𝑒𝑟𝑟 𝑛 − 1 +𝐾𝑖𝑓 𝜔𝑒𝑟𝑟 (n)(6)𝐼 𝑑𝑜 = 𝐼 𝑑𝑙 − 𝐼 𝑑𝑙𝑓 (7)𝐼 𝑑 = 𝐼 𝑑𝑜 + 𝐼 𝑑𝑓 (8)𝜔ref is the reference speed,𝜔is the actual rotorspeed, Kpf&Kifare PI controller gains respectively,Ido is the oscillating component of load current, Idlis the total d axis load current, Idlf is the filtered daxis load current.q-axis reference generationFor voltage regulation, peak voltage of three phasesystem (325 in this case) is compared with thedirect axiscomponent of the voltage which isobtained after passing through a low pass filter of10Hz. The error is passed through a PI controllerand subtracted from total load quadraturecomponent𝑉𝑒𝑟𝑟 = 𝑉𝑑𝑟𝑒𝑓 𝑛 − 𝑉𝑑 (9)𝐼𝑞𝑣 = 𝐼𝑞𝑣 𝑛 − 1 + 𝐾𝑝𝑣 𝑉𝑑 𝑛 − 𝑉𝑑 𝑛 − 1 +𝐾𝑖𝑣 𝑉𝑒𝑟𝑟 𝑛 (10)𝐼𝑞 = 𝐼𝑞𝑙 − 𝐼𝑞𝑣(11)WhereVdref is the rated d axis voltage, Kpv and Kivare PI controller values respectively. Iql is the q-axis load current.After obtaining reference currents from dq to abcconversion, these are compared with the actualStatcom currents and given to a PWM generator.Asimilar method of reference current generationtechnique can also be seen in [6].Fig.3. Vector control of Statcom
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326318 | P a g e2.2Vector control of PMSGThe stator flux linkage equations can be written as[7]𝑉𝑞𝑠𝑟= 𝑅 𝑞 𝑖 𝑞𝑠𝑟+ 𝑝𝜆 𝑞𝑠𝑟+ 𝜔𝑟 𝜆 𝑑𝑠𝑟(12)𝑉𝑑𝑠𝑟= 𝑅 𝑑 𝑖 𝑑𝑠𝑟+ 𝑝𝜆 𝑑𝑠𝑟− 𝜔𝑟 𝜆 𝑞𝑠𝑟(13)𝜆 𝑞𝑠𝑟= 𝐿 𝑠 𝑖 𝑞𝑠𝑐+ 𝐿 𝑚 𝑖 𝑞𝑟𝑐(14)𝜆 𝑑𝑠𝑟= 𝐿 𝑠 𝑖 𝑑𝑠𝑐+ 𝐿 𝑚 𝑖 𝑑𝑟𝑐(15)𝜆 𝑞𝑠𝑟= 𝐿 𝑞 𝑖 𝑞𝑠𝑟(16)𝑇𝑒 =32𝑃2𝑖 𝑞𝑠𝑟𝜆 𝑑𝑠𝑟− 𝑖 𝑑𝑠𝑟𝜆 𝑞𝑠𝑟(17)𝑇𝑒 =32𝑃2𝑖 𝑞𝑠𝑟𝜆 𝑎𝑓𝑟+ (𝐿 𝑑 − 𝐿 𝑞 )𝑖 𝑑𝑠𝑟𝑖 𝑞𝑠𝑟(18)𝑇𝑒 =32𝑃2𝑖 𝑞𝑠𝑟𝜆 𝑎𝑓𝑟(19)where Vqsr, Iqsrare rotor q axis voltage and current𝜆qsrand𝜆dsrare stator q and d axis flux linkages, Ld,Lq, Lm are d, q axis inductance and mutualinductance respectively, P is number of poles, Te iselectromagnetic torque .This vector control scheme is based on constanttorque angle control, where torque angle 𝛿 ismaintained at 90 degrees as shown in Fig. 4hence, the field or direct axis current is made tobe zero, leaving only the torque or quadratureaxis current in place.Reference speed can be found by𝜔𝑜𝑝𝑡 =𝜆∗𝑉𝑟(20)𝜔𝑒𝑟𝑟 = 𝜔 𝑟𝑒𝑓 𝑛 − 𝜔𝑟 (21)𝐼𝑞 = 𝐼𝑞 𝑛 − 1 + 𝐾𝑝𝑔 𝜔𝑒𝑟𝑟 − 𝜔𝑒𝑟𝑟 𝑛 − 1 +𝐾𝑖𝑔 𝜔𝑒𝑟𝑟 (n) (22)This speed is compared with the actual speed andpassed through a PI controller with limiter toproduce the reference torque.Fig. 4.Vector control of PMSGAs, Id=0𝐼𝑞 =4∗𝑇𝑒3∗𝑃∗𝜆𝑎𝑓(23)Ѳ 𝑚 =𝑃2∗ Ѳ 𝑒 (24)𝑖 𝑝𝑎𝑒 = 𝑖 𝑝𝑎∗− 𝑖 𝑝𝑎 (25)𝑖 𝑝𝑏𝑒 = 𝑖 𝑝𝑏∗− 𝑖 𝑝𝑏 (26)𝑖 𝑝𝑐𝑒 = 𝑖 𝑝𝑐∗− 𝑖 𝑝𝑐 (27)Where, 𝜔opt is the optimal speed, 𝜆 is the tip speedratio, V is the free wind velocity, r is the radius ofthe turbine, 𝜔ref is the reference speed,𝜔r is theactual rotor speed, Iq is quadrature currentcomponent, Kpg&Kigare PI controller gainsrespectively, i*pa, i*pb, i*pc are reference PMSG a, band c currents respectively, ipa, ipb and ipc are actualPMSG a ,b, c currents respectively, Ѳm and Ѳe aremechanical and electrical angles respectively.These currents are transformed into abc domainand compared with the actual currents and given tothe PWM generator and to the three phase VSC.Fig. 5. Control scheme of PMSGconverterIII. DESIGN OF COMPONENTS3.1DC link designFor proper PWM operation,𝑉𝑑𝑐 > 223∗ 𝑉𝑙 ∗ 𝑚 𝑎 (28)Taking line voltageVl as 400V and modulationindex, ma as 1, Vdc is chosen as 700V which is alsothe battery dc voltage.C >=𝑚 𝑎 ∗𝐼𝑠4∗2𝜋𝑓∗∆𝑉(29)C is the dc link capacitance, ∆V is the allowedvoltage ripple (5% in this case), Isis the Statcomcurrent, f is the supply frequency.The battery is chosen of 700V, 150Ah and C as8000μF.3.2 Synchronous link reactor designThe synchronous link reactor is chosen as [8]
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326319 | P a g e𝐿 =𝑉𝑎𝑛 +0.5∗𝑉𝑑𝑐4∗𝑓𝑡∗𝜉(30)Where, Van is the phase voltage, ft is the frequencyof the carrier wave, 𝜉 is the amplitude of carrierwave.Taking, Van=230V, ft=20 KHz,L=7.25mH3.3. Selection of wind turbineThe wind turbine used for simulation purpose istaken from Simpower systems, Matlab Simulinkwhich has maximum power coefficient cp=0.48and tip speed ratio 𝜆=8.1. Turbine rating is taken as5kW and radius can be found using𝑃 =12∗ 𝐶𝑝 ∗ 𝜋 ∗ 𝑅2∗ 𝜌 ∗ 𝑉3(31)Where R= radius of the turbine, 𝜌=air density, v iswind velocity.R=2.3mFig. 6. Turbine output characteristics3.4 Tuning of PMSG converterThe overall PMSG side with its controller can becan be modeled as [9]Fig. 7. Closed loop speed control of PMSG𝐾𝑠 =49∗𝐾𝑔∗𝑇𝑤𝑖(32)𝑇𝑠 = 6 ∗ 𝑇𝑤𝑖(33)Root locus method is used to tune the PI controllerwhere the design requirement taken are dampingratio, 𝜉=0.707, maximum overshoot, Mp = 2%, risetime, tr= 0.005s and settling time, ts= 0.01s.Fig. 8. Root locus plot of closed loopsystemFig. 9.Step input response for givensystem
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326320 | P a g eIV. RESULTS4.1 Statcom as voltage regulatorAt 0.5s an IG is connectedthrough a circuit breakerinparallel with the Synchronous Generator as shown inFig. 11.There is an injection of reactive power by theStatcom which quickly restores thedipping terminalvoltage back to its rated value.4.2 Statcom as frequency regulatorAs in Fig. 10.Initally as the total connected load is12kW and Synchronous Generator produces around15.4kW, remaining 3.4kW is consumed by theBattery leading to its charging.At 0.5s, Induction generator of 3.8kW is connectedthus changing total generating power (SG and IG) to19.2kW, thus causing a mismatch of 7.2kW. Thispower is again consumed by the battery.At 1s, an extra load of 12kW and 8kVAR isconnected. This causes deficiency in the generatingpower of around 4kW which is supplied by theStatcom. Thus we can see asStatcom keeps the totalgenerated and consumed power equal and thus thesystem frequency is kept at nominal value (50Hz).4.3 Statcom as reactive power compensatorIn Fig. 11.at 1s, an extra load of 12kW and 9kVAR,considering power factor of 0.8 lagging is connected.Since the Statcom is designed to keep the supply sidepower factor as unity, its reactive power injectionincreases from 0 to 8kVAR.4.4 Statcom as neutral current compensatorAt 1.5s, an unbalanced load of 6kW with two phaseswitched off is connected. Thus the load currents arehighly unbalanced as shown in Fig. 11. However, thesource currents are balanced as the Statcom providesa path for neutral current through its fourth leg.4.5 Statcom as harmonic eliminatorFig. 12 shows connection of only nonlinear load. Thenonlinear here considered is a three phase diodebridge rectifier with 50 ohms resistor. As we cannotice, the currents are highly distorted but the sourcecurrents are linear.Fig. 13 and 14 shows the Total harmonic distortion(THD) values of load and source current.Loadcurrent have THD equal to 25% which is very highwhereas source currents have their THD well withinallowed level of 3%.4.6 PMSG outputInitially, wind velocity is at 8m/s as shown in Fig. 17which gives reference speed of 34.11 rad/s formaximum power extraction. The electromagnetictorque developed is 45Nm and the generator output is2kW. At 0.5s, wind velocity increases to 12m/swhich correspond to reference speed of 51 rad/s. Theelectromagnetic torque developed is around 102Nmand the Power output is 4.7kW.
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326321 | P a g eFig. 10. Statcom as frequency regulator
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326322 | P a g eFig. 11. Statcom as voltage regulatorFig. 11. Statcomas voltage regulator
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326323 | P a g eFig. 12.Statcom as harmonic eliminator
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326324 | P a g eFig. 13. THD of source currentFig. 14. THD of source currentFig. 15. Speed of Induction Generator (pu)Fig. 16. Speed of Synchronous Generator (pu)
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326325 | P a g eFig. 17. Performance of PMSG converter
  • SamundraGurung, P.Somasundram/ International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.315-326326 | P a g eV. CONCLUSIONThe paper has divided its focus on mainly tworenewable sources, uncontrolled Hydro turbine andvariable Wind turbine. An Electronic load controlleror a Statcom is used interchangeably because itfunctions both as voltage and frequency control. It isbased on vector control scheme based onsynchronously rotating dq referenceframe.A battery is connected on the dc side which ischarged and discharged by the bidirectional activityof Statcom. This action helps to keep the frequencyconstant. Similarly, the battery is also charged fromthe output of PMSG converter. The Statcom alsoperforms other functions like voltage regulation,harmonic elimination and neutral currentcompensation.The PMSG machine converter is employed for MPPTtracking. Thus an overall hydro and wind hybrid hasbeen developed which can perform satisfactorilyeven in an isolated case. All these models aredeveloped using Matlab,Simulink.This paper can beextended to more detailedBattery EnergyManagement system. It can also be extended by usingMulti level Inverter for higher power ratings andlesser harmonic output.APPENDIXSynchronous Generator16kVA, 400V, 1500rpm, Xd=1.734pu,Xd’=0.177puXd’’ = 0.111pu, Xq = 0.861pu ,Xq’’=0.199pu,Xl=0.07pu,Td=0.018s,Td’’=0.0045s,Tq’’=0.0045,Rs=0.02pu,H=0.5s,P=2InductionGenerator4kVA,400V,50Hz,Rs=0.03513pu,Lls=0.04586pu,Rr’=0.03488pu,Llr’=0.04586pu,Lm=2.8pu,H=0.2s,Excitation capacitor=1.5kVARPermanent Magnet Synchronous GeneratorRs=0.085Ω,Ld=0.95mH,Lq=0.95mH, Fluxlinkage=0.192Vs, J=0.008kgm2,F=0.11Nms,P=8Synchronous link reactorRf=0.44Ω,Lf=7.25mHStatcom 40kVAVdc=700V,Cdc=8000μF,Battery=700V,150AHPI controllerFrequency controller, Kpf=9000,Kif=30000Ac voltage controller, Kpv=0.02,Kiv=0.02PMSG converter, Kpg=5, Kig=80REFERENCES[1] I. Tamrakar, L.B. Shilpakar, B.G. Fernandesand R. Nilsen, Voltage and frequency controlof parallel operated synchronous generator andinduction generator with STATCOM in microhydro scheme, IET Gener. Transm. Distrib, 1,(5), 2007, pp. 743–750[2] J. A. Barrado, R. Griñó, H. Valderrama-Blavi,Power-QualityImprovement of a Stand-AloneInduction Generator Using a STATCOMWith Battery Energy Storage System, IEEEtransactions on power delivery, vol. 25, no. 4,2010 , 2734- 2741[3] Puneet K. Goel, Bhim Singh, S.S. Murthy, andNavin Kishore, Autonomous Hybrid SystemUsing SCIG for Hydro Power Generation andVariable Speed PMSG for Wind PowerGeneration,IEEE International Conference onPower Electronics Drive Systems, 2009, 55-60[4] S.-C.KuoL.Wang, Analysis of voltage controlfor a self-excited inductiongenerator using acurrent-controlled voltage sourceinverter (CC-VSl),IEE Proc.-Genes. Transni. Distrrb., Vol.148, No. 5, 2001, 431-438[5] PrabhaKundur, Power System Stability andControl(New Delhi: Tata McGraw-HillEducation, 2006)[6] Muhammad H. Rashid, Power electronicshandbook (Burlington: Butterworth-Heinemann, 2011)[7] K. R. Padiyar, Facts controllers in powertransmission and distribution (Daryaganj,New Delhi: New Age International (P) Ltd,2007)[8] R. Krishnan,Electric Motor Drives: Modeling,Analysis and Control(New Delhi:PHILearning, 2009).