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Ch.Nayak Bhukya, S.Sankara Prasad, M.Venkateswarlu / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.300-306 Compensation Of AC And DC Loads Of A Three-Phase DSTATCOM By Using A Fast-Acting DC-Link Voltage Controller Ch.Nayak Bhukya1, S.Sankara Prasad2, M.Venkateswarlu3 1 Assistant Professor,Electrical & Electronics Engineering &NNRGI Hyderabad 2 Assistant Professor,Electrical & Electronics Engineering & GPCET Kurnool 3 Assistant Professor,Electrical & Electronics Engineering & GPCET KurnoolAbstract The DSTATCOM consists of a current- Keywords— DSTATCOM, Voltage-Sourcecontrolled Voltage-Source Inverter (VSI) which Inverter (VSI), Proportional-Integral (PI) controller,injects current at the Point of Common Coupling MATLAB.(PCC) through inductor. The operation of VSI issupported by a dc storage capacitor with proper INTRODUCTIONdc voltage across it. Now a days due to different types of power The transient response of the electronic based equipment, nonlinear and(DSTATCOM) is very important while unbalanced loads, has improving the power-qualitycompensating rapidly unbalanced and nonlinear problems in the power distribution network. Theyloads. Any change in the load affects the dc-link cause excessive neutral currents, overheating ofvoltage directly .The sudden removal of load electrical apparatus, poor power factor, voltagewould result in an increase in the dc-link voltage distortion, high levels of neutral-to-ground voltage,above the reference value, where as sudden and interference with communication systems. Theincrease in load would reduce the dc-link voltage literature records the evolution of different custombelow its reference value. The proper operation power devices to mitigate the above power-qualityof DSTATCOM requires variation of the dc-link problems by injecting voltages/currents or both intovoltage controller within the prescribed limits. the system.Conventionally, a Proportional-Integral (PI) To avoid these type of problems the shunt-controller is used to maintain the dc-link voltage connected custom power device, called theto the reference value. It uses deviation of the Distribution static compensator (DSTATCOM),capacitor voltage from its reference value as its injects current at the point of common couplinginput. However the transient response of the (PCC). The newly presented A Three phaseconventional PI dc-link controller is slow. DSTATCOM to Compensating AC and DC Loads To maintain the dc-link voltage at the with A Fast-Acting DC-Link voltage controller [1].reference value, the dc-link capacitor needs a So that harmonic filtering, power factor correction,certain amount of real power, which is and load balancing can be achieved. Theproportional to the difference between the actual DSTATCOM consists of a current-controlledand reference voltages. To overcome the voltage-source inverter (VSI) which injects currentdisadvantage of the conventional PI dc-link at the PCC through the interface inductor. Thevoltage controller, a fast-acting dc-link voltage operation of VSI is supported by a dc storagecontroller is proposed. To maintain the dc-link capacitor with proper dc voltage across it. Thisvoltage of fast acting controller reference value is process is called the compensation. For thisproportional to the difference between the compensation process is a dc-link voltage controllersquares of the actual and reference voltages. is should be used, which will regulate the dc voltageMathematical equations are given to compute the to the reference value.gains of the conventional controller based onfast-acting dc-link voltage controllers to achieve POWER QUALITY PROBLEMSsimilar fast transient response. The value of the Power quality and reliability cost thedc-link capacitor can be selected based on its industry large amounts due to mainly sags andability to regulate the voltage under transient short-term interruptions. Distorted and unwantedconditions.. voltage wave forms, too. And the main concern for The efficiency of the proposed controller the consumers of electricity was the reliability ofover the conventional dc-link voltage controller supply. Here we define the reliability as thehas been developed in MATLAB environment, continuity of supply. As shown in Fig.2.1, thesimulated and obtained related waveforms. problem of distribution lines is divided into two major categories. First group is power quality, second is power reliability. First group consists of 300 | P a g e
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Ch.Nayak Bhukya, S.Sankara Prasad, M.Venkateswarlu / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.harmonic distortions, impulses and swells. Second FACTS CONTROLLERS ANDgroup consists of voltage sags and outages. Voltage CONVERTERSsags is much more serious and can cause a large The most advanced solution to compensateamount of damage. reactive power is the use of a Voltage Source Converter (VSC) incorporated as a variable source of reactive power. These systems offer several advantages compared to standard reactive power compensation solutions. Reactive power control generated by generators or capacitor banks alone normally is too slow for sudden load changes and demanding applications, such as wind farms or arc furnaces. Compared to other solutions a voltage source converter is able to provide continuous control, very dynamic behavior due to fast response times and with single phase control also compensation of unbalanced loads. The ultimate aimFig.1 Power quality problems is to stabilize the grid voltage and minimize any transient disturbances. If exceeds few cycles, motors, robots,servo-drives and machine tools cannot providecontrol. Both the reliability and quality of supply areequally important. For example, a consumer that isconnected to the same bus that supplies a largemotor load may have to face a severe dip in hissupply voltage every time the motor load isswitched on. In some extreme cases even we have tobear the black outs which is not acceptable to theconsumers. There are also sensitive loads such ashospitals (life support, operation theatre, and patientdatabase system), processing plants, air trafficcontrol, financial institutions and numerous other Fig.2 Basic STATCOM configurationdata processing and service providers that requireclean and uninterrupted power. Some of the power STATCOM converters are designed to mitigate thequality disturbance wave forms are shown in Fig 1. described phenomena with solutions based onIn processing plants, a batch of product can be Power Converter System (PCS) platforms providingruined by voltage dip of very short duration. Such the following control features:customers are very wary of such dips since each dip Power factor correction (cosΠ control).can cost them a substantial amount of money. Even o Voltage control.short dips are sufficient to cause contactors on o Active harmonics cancellation.motor drives to drop out. Stoppage in a portion of o Flicker mitigation.process can destroy the conditions for quality o Unsymmetrical load balancing.control of product and require restarting of STATCOM solutions can be implemented alone orproduction. Thus in this scenario in which in combination with switched capacitor banks.consumers increasingly demand the quality power,the term Power Quality (PQ) attains increased DISTRIBUTION STATICsignificance. Transmission lines are exposed to the COMPENSATOR The Distribution Static Compensatorforces of nature. Furthermore, each transmission (DSTATCOM) is a voltage source inverter basedline has its load ability limit that is often determined static compensator that is used for the correction ofby either stability constraints or by thermal limits or bus voltage sags. Connection (shunt) to theby the dielectric limits. Even though the power distribution network is via a standard powerquality problem is distribution side problem, distribution transformer. The DSTATCOM istransmission lines are often having an impact on the capable of generating continuously variablequality of the power supplied. It is however to be inductive or capacitive shunt compensation at anoted that while most problems associated with the level up its maximum MVA rating. Thetransmission systems arise due to the forces of DSTATCOM continuously checks the linenature or due to the interconnection of power waveform with respect to a reference ac signal, andsystems, individual customers are responsible for therefore, it can provide the correct amount ofmore substantial fraction of the problems of powerdistribution systems. 301 | P a g e
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Ch.Nayak Bhukya, S.Sankara Prasad, M.Venkateswarlu / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.leading or lagging reactive current compensation toreduce the amount of voltage fluctuations. The major components of a DSTATCOM.it consists of a dc capacitor, one or more invertermodules, an ac filter, a transformer to match theinverter output to the line voltage, and a PWMcontrol strategy. In this DSTATCOMimplementation, a voltage-source inverter converts adc voltage into a three-phase ac voltage that issynchronized with, and connected to, the ac linethrough a small tie reactor and capacitor (ac filter). Fig.4 Single-line diagram of the test system for D-The shunt-connected custom power device, called STATCOMthe distribution static compensator (DSTATCOM),injects current at the Point of Common Coupling(PCC) so that harmonic filtering, power factorcorrection, and load balancing can be achieved. TheDSTATCOM consists of a current-controlledVoltage-Source Isnverter (VSI) which injectscurrent at the PCC through the interface inductor.The operation of VSI is supported by a dc storagecapacitor with proper dc voltage across it. The shunt voltage controller is a voltagesource converter connected in parallel with the loadbus bar through a transformer or a reactor, Fig.4.5.The difference between the DVR and the SVC isthat instead of injecting a voltage, the current Fig.5 Phasor Diagram for Shunt Voltage Controllerthrough the reactance is controlled. The shuntvoltage controller is normally used for power factorcorrection, voltage flicker, active filtering, etc.,rather than voltage mitigation. For faults originatedclose to the SVC, on the same voltage level or closeto the load, the impedance seen by the SVC will bevery low. Since the contribution to the bus barvoltage equals the injected current multiplied by theimpedance, a very high reactive current will bedrawn during such a fault. Fig.6 A radial distribution system with an unbalance load and a DSTATCOM Dc-Link Voltage Controllers: As mentioned before, the source supplies an unbalanced nonlinear ac load directly and a dc load through the dc link of the DSTATCOM, as shown in Fig. 1. Due to transients on the load side, the dc bus voltage is significantly affected. ToFig.3 Example of a standard configuration for regulate this dc-link voltage, closed-loop controllersDSTATCOM are used. The Proportional-Integral-Derivative (PID) control provides a generic and efficient solution to many control problems. The control signal from PID controller to regulate dc link voltage is expressed as Pdc= Kp(Vdcref-Vdc)+ Ki ∫(Vdcref-vdc)dt+kd(Vdcref- vdc)/dt kp,ki and Kd are Proportional,Iintegral, and Derivative gains of the PID controller, respectively. The proportional term provides overall control 302 | P a g e
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Ch.Nayak Bhukya, S.Sankara Prasad, M.Venkateswarlu / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.action proportional to the error signal. An increase measures the r.m.s voltage at the load point, i.e., noin proportional controller gain reduces rise time and reactive power measurements are required.steady-state error but increases the overshoot and An error signal is obtained by comparingsettling time. An increase in integral gain reduces the reference voltage with the voltage measured atsteady state error but increases overshoot and the load point. The PI controller process the errorsettling time. Increasing derivative gain will lead to signal and generates the required angle to drive theimproved stability. However, practitioners have error to zero, i.e., the load rms voltage is broughtoften found that the derivative term can behave back to the reference voltage.against anticipatory action in case of transport delay.A cumbersome trial-and-error method to tune itsparameters made many practitioners switch off oreven exclude the derivative term. Therefore, thedescription of conventional and the proposed fast-acting dc-link voltage controllers using PIcontrollers are given in the following subsections.Conventional DC-Link Voltage Controller: The conventional PI controller used formaintaining the dc-link voltage is shown in Fig.4.9To maintain the dc-link voltage at the referencevalue, the dc-link capacitor needs a certain amountof real power, which is proportional to thedifference between the actual and referencevoltages. The power required by the capacitor canbe expressed as follows Fig.7 Block diagram of the Fast acting DC-Link proposed DSTATCOM controller Pdc= Kp(Vdcref-Vdc)+ Ki ∫(Vdcref-vdc)dt The sinusoidal signal Vcontrol is phase-modulated by The dc-link capacitor has slow dynamics means of the angle.compared to the compensator, since the capacitor i.e., VA = Sin (ωt +δ)voltage is sampled at every zero crossing of phase VB = Sin (ωt +δ-2π/3)supply voltage. The sampling can also be performed VC = Sin (ωt +δ+2π/3)at a quarter cycles depending upon the symmetry ofthe dc-link voltage waveform. The drawback of this Selection of DC-Link Capacitor:conventional controller is that its transient response The value of the dc-link capacitor can beis slow, especially for fast-changing loads. Also, the selected based on its ability to regulate the voltagedesign of PI controller parameters is quite difficult under transient conditions. Let us assume that thefor a complex system and, hence, these parameters compensator in Fig.5.1 is connected to a systemare chosen by trial and error. Moreover, the dynamic with the rating of X kilovolt amperes. The energy ofresponse during the transients is totally dependent the system is given by X×1000 J/s. Let us furtheron the values of Kp and Ki when Pdc is comparable assume that the compensator deals with halfto Plavg . (i.e.X/2) and twice (i.e.2X,) capacity under the transient conditions for n cycles with the systemFast-Acting DC Link Voltage Controller: voltage period of T s. Then, the change in energy to To overcome the disadvantages of the be dealt with by the dc capacitor is given asaforementioned controller, an energy-based dc-linkvoltage controller is proposed. Which is ∆E=(2X –X/2)nTproportional to the difference between the squares ofthe actual and reference voltages, the power Now this change in energy should be supported byrequired by the capacitor can be expressed as the energy stored in the dc capacitor. Let us allowfollows the dc capacitor to change its total dc-link voltage from 1.4Vm to 1.8Vm during the transient conditionsPdc = Kp(V2dcref -V2dc)+Ki ∫(V2dcref -V2dc)dt where Vm is the peak value of phase voltage. Hence, we can write kp and ,ki are proportional and integralgains of the PI controller. The aim of the control 1/2Cdc[(1.8Vm )2 –(1.4Vm)2]= (2X –X/2)nTscheme is to maintain constant voltage magnitude atthe point where a sensitive load is connected, under Which implies thatsystem disturbances. The control system only Cdc= 3XnT/(1.8Vm )2 –(1.4Vm)2 303 | P a g e
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Ch.Nayak Bhukya, S.Sankara Prasad, M.Venkateswarlu / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.For example, consider a 10-kVA system (i.e., X=10kVA), system peak voltage Vm =325.2 V, n=0.5, andT= 0.02 s. The value of computed using is 2216µF.Practically, 2000µF is readily available and thesame value has been taken for simulation andexperimental studies.For computing proportional controller gainKp= Cdc/2TcWhere Tc =Capacitor voltage ripple period as0.01Sec Then Kp computed as kp=0.11 Fig.9 Simulation Diagram of VSI based Ki=Kp/2 DSTATCOM Therefore Ki=0.055. In the above figure DC-Link capacitor2000µF, Here DC-Link capacitor regulate the voltage for operate the VSI for inverting DC toSIMULATION RESULTS AND ANALYSIS AC. Here Scope is used to Observing the DC-linkSimulation Results: capacitor voltage in both methods. Fig.10 Simulation Diagram of Conventional PI- Controller It was comparing and subtract the Vdc reference Voltage Vdc actual Voltages and error is computed by PI controller. Here proportional gain Kp=40 and integral gain ki=20.Fig.8 Simulation Diagram of Three-phasecompensated system by using STATCOM Basic simulation diagram is same in boththe methods conventional and fast acting dc-linkcontrolling methods. The difference is at input givento the PI-controllers.It consists of 400 volts supplyfor the circuit operation, Unbalanced load Za=25Ω,Zb=44+j25.5Ω and Zc=50+j86.6Ω,Non-linear loaddrawing a current of 5A.Timer is used to half theload between 0.4sec to 0.8sec the load is and 0.8onwards its again Full load for observing settingtime and transient response of the system. Fig.11 Simulation Diagram of Fast-acting PI- Controller It was comparing and subtract the V2dc reference Voltage V2dc actual Voltages and error is computed by PI controller. Here proportional gain Kp=0.11and integral gain ki=0.055. 304 | P a g e
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Ch.Nayak Bhukya, S.Sankara Prasad, M.Venkateswarlu / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.Fig.12 Supply VoltagesAbove figure shows the supply voltages in the 3- ts=0.03 sec Fig.15 Transient response of Fast-actingphase compensated system. These voltag are controller Compensated Source Current in Phase ‘a’denoted by Vsa, Vsb and Vsc.The supply voltages are325 volts. Compared to conventional method settling time of DC the system reduced to ts=0.03sec by using fast acting LINK DC link controller is observed. V o Time (sec) l t a g e (v)Fig.13 Compensated Source currentsAbove figure shows the source currents in 3-phasecompensated system. These currents are denoted ts=0.24 secby i sa, isb and isc.The source currents in Three phasesis 22amperes Fig.16 Transient response of Conventional controller DC-Link voltage From 0.4sec to 0.8sec the load is halved whenever load is halved the dc link voltage is increases. It takes some time settle down this is called Settling time. From 0.8 onwards again Full load. The settling time of the system by using conventional controller, ts=0.24sec. ts=0.05sec D CFig.14 Transient response of Conventional LIcontroller Compensated Source Current in Phase ‘a’. N Time (sec) K From 0.4sec to 0.8sec the load is halved Vfrom 0.8 onwards again Full load. Whenever load ochanges occur fluctuations are generated. These l ts=0.12secfluctuations are settle down to take some time. This t Fig.17 Transient response of Fast-actingtime is called settling time. The settling time of the ag controller DC-Link voltagesystem by using conventional controller, t s=0.05sec e Compared to conventional method the settling timeis observed. (v of the system is reduced to ts=0.12sec by using fast ) acting DC link controller is observed. 305 | P a g e
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Ch.Nayak Bhukya, S.Sankara Prasad, M.Venkateswarlu / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.CONCLUSIONS AND FUTURE SCOPE Series), 2002. ISBN: 1-4020-7180-9, 460CONCLUSIONS: pages including index. The control of DSTATCOM are discussed [4] Instantaneous Reactive Powerand also presented the power quality problems, Compensators ComprisingSwitchingconsequences, and mitigation techniques, and the Devices without Energy Storagedesign and application of custom power device, D- ComponentsHIROFUMI AKAGI,STATCOM. The control scheme maintains the YOSHIHIRA KANAZAWA, ANDpower balance at the PCC to regulate the dc AKIRA NABAE, MEMBER, IEEE-1984capacitor voltages. A new PI control scheme has [5] Design of a Real-Time Digital Simulatorbeen implemented to control the VSI used in the D- for aD-STATCOM System VenkataSTATCOM. The dc bus voltage of the Dinavahi, Member, IEEE, Reza Iravani,DSTATCOM has been regulated to the reference dc Fellow, IEEE, and Richard Bonert,bus voltage under varying loads. Member, IEEE-2004The simulation results carried out showed that the [6] Instantaneous Power Compensation inD-STATCOM provides voltage regulation Three-Phase Systems by Using p–q–rcapabilities. It was also observed that the capacity Theory Hyosung Kim, Member, IEEE,for power compensation and voltage regulation of Frede Blaabjerg, Senior Member, IEEE,D-STATCOM depends on the rating of the dc Birgitte Bak-Jensen, Member, IEEE, andstorage device. In this Project an attempt was made Jaeho Choi, Senior Member, IEEEto simulate the performance of D-STATCOM [7] Tutorial on Voltage Sag Analysis- Math Hsystem for various power quality problems. J Bollen and Emmanouil StyvaktakisPrinciple of Fast acting DC link control is described. Chalmers University of TechnologyMatlab/Simulink base model is developed and Gothenburg, Sweden-2002simulation results are presented. From the results, [8] voltage source converter technology,the transient response by the conventional controller Different Custom devices by N.G.ts=0.05sec for current and ts= 0.24sec for DC link Hingorani and L. Gyugyi.voltage, where the transient response by using fastacting DC link controller is ts=0.03sec for current Mr. Ch Nayak Bhukya has completed hisand ts=0.12sec for voltage. Finally, it is concluded professional career of education in B.Tech (EEE) atthat by fast acting DC-link control the settling time KLCE Vijayawada in the year 2008. Later hefor DC link capacitor is reduced. successfully completed M.Tech in EPS in 2012 from JNTU Anantapur. His keep interests and6.2 FUTURE SCOPE: special focus his in POWER SYSTEMS & POWER Application of same principle for AC ELECTRONICS. He has 5 years of teachingvoltage regulation during faults conditions. experience. Present he is working as AssistantApplication of same principle for other FACTS Professor in the EEE Department in NNRGI,devices. Real time implementation of Fast Acting Hyderabad(AP).DC link voltage control scheme. The scope forfurther work is to minimize the power injection of Mr. S. Sankara Prasad has completed hisD-STATCOM for compensation of the voltage sags professional career of education in B.Tech (EEE) atby using FUZZY and combined NEURO-FUZZY JNTU Anatapur in the year 2009. Later hecontrol. successfully completed M.Tech in EPS in 2012 from JNTU Anantapur. His keep interests andBIBLIOGRAPHY special focus his in POWER SYSTEMS & POWER [1] A Fast-Acting DC-Link Voltage Controller ELECTRONICS. Present he is working as Assistant forThree-Phase DSTATCOM to Professor in the EEE Department in GPCET Compensate AC and DC LoadsMahesh K. Engg.College,Kurnool(AP). Mishra, Member, IEEE, and K. Karthikeyan-2009 IEEE M.Venkateswarlu was born in 1979. He received [2] A Novel Technique to Compensate Voltage his Graduate in Electrical & Electronics Engineering Sags in Multiline Distribution System— from G.Pulla reddy Engineering college in the year The InterlineDynamic Voltage RestorerD. 2003. He received his M.Tech in power & industrial Mahinda Vilathgamuwa, Senior Member, drives frm J.N.T.U.A, Anantapur, in the year 2011. IEEE, H. M. Wijekoon, and S. S. Choi, Presently he is working as a Sr.Assistant Professor Member, IEEE-2006 in Dept. of EEE at G.Pullaiah college of Engg & [3] Power QualityEnhancement UsingCustom Tech, Kurnool. He has 7 years of teaching Power Devices By Arindam Ghosh and experience. His area of interest is power electronics, Gerard Ledwich. Published by Kluwer converters, and hvdc transmission (Power Electronics and Power Systems 306 | P a g e
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