Dstatcom based voltage regulator for wind turbine driven self excited indu

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Dstatcom based voltage regulator for wind turbine driven self excited indu

  1. 1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 209 DSTATCOM BASED VOLTAGE REGULATOR FOR WIND TURBINE DRIVEN SELF-EXCITED INDUCTION GENERATOR Dr. RAJSEH KUMAR AHUJA, PRIYANKA PHAGERIA YMCA University of Science & Technology, Faridabad ABSTRACT Voltage sag is the most important powerquality problems faced by many industries and utilities.This paper presents the voltage sag mitigation usingDSTATCOM (Distribution Static Compensator).DSTATCOM is used for reactive power compensation. This paper presents an analysis of the three-phaseself-excited induction generator (SEIG) with DSTATCOM as a voltage regulator , which provides fast dynamic response tomaintain constant voltage at SEIG terminals during severeload perturbations and acts as a source and sink of reactivepower.Thispaper evaluates vector control algorithm forextracting reference source currents for voltage regulationat PCC (Point of Common Coupling) for DSTATCOM. Thevoltage source converter (VSC) is used with DC linkcapacitor as a DSTATCOM. 1. INTRODUCTION Power Quality problem such as Voltage sag is the most important power quality problems faced by many industries and utilities. It contributes more than 80% power quality (PQ) problems that exist in power systems. The majority of power consumption has been drawn in reactive loads.These excessive reactive power demandincreases feeder losses and reduces the active power flow capability of distribution system which also affects thevoltage profile. The reason for this is the majority of loads in the distribution systems are linear laggingpower factor and nonlinear, balanced and unbalanced loads. Such power quality problems can be mitigatedby the DSTATCOM (Distribution Static Synchronous Compensator) at the point of common coupling (PCC). Induction Generators used in stand- alone as well as grid connected system as recent development has done in pollution free power generation schemes.When a capacitor bank is connected across the Induction INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), pp. 209-219 © IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2013): 5.5028 (Calculated by GISI) www.jifactor.com IJEET © I A E M E
  2. 2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 210 Generator then it Known as a Self-Excited Induction generator. It is well known that when capacitors are connectedacross the stator terminals of an induction machine,driven by an external prime mover, voltage will beinduced at its terminals . The induced emf andcurrent in the stator windings will continue to rise untilsteady state is attained, it means reactive power supplied by capacitor bank is balanced by reactive power absorbed by load. In order for self- excitationto occur, for a particular capacitance value there is acorresponding minimum speed. The self-excited induction generator (SEIG) has some advantages over the synchronous generator, for example:brushless (squirrel-cage rotor), reduced size, rugged and low cost. However, the induction generator offers poor voltage regulation and its value depends on the prime mover speed,capacitor bank size and load characteristics. The generated voltage of the SEIG is mainly depends upon the excitation capacitance values, change in wind velocity and load conditions. The reactive power requirementby the induction generator can also be supplied by a group of capacitors. If the capacitance is insufficient,the induction generators will not build up voltage. The main drawback of the induction generator is need ofreactive to build up the terminal voltage. In the SEIG, the excitation current is supplied by the capacitors connected across its terminals. Self-excited induction generators are good candidates for wind powered electric generation application especially in remote areas, because they do not need external power supply to produce the magnetic field. By using the DSTATCOM the voltage across the SEIG remains constant at varying loads.Thesimulated results demonstrate the use of DSTATCOM as a voltage regulator across a SEIG network. 2. CONTROL SCHEME When used in low voltage distribution system, thestatic compensator (STATCOM) is identified asDistribution STATCOM (DSTATCOM). In generalDSTATCOM is used to generate or absorb reactivepower.The D-STATCOM is a three-phase and shunt connected power electronics based device. When a DSTATCOM is connected across the SEIG then the additional demand ofreactive power is fulfilled by the DSTATCOM under varyingloads. The DSTATCOM acts as a source of lagging or leadingcurrents to maintain the terminal voltage constant with variation in load.The DSTATCOM consists of a three-phaseIGBT (Insulated gate bipolar transistor) based currentcontrolled voltage source inverter, DC bus capacitor and ACinductors. The AC output of the inverter is connectedthrough the AC filtering inductor to the SEIG terminals.The DC bus capacitor is used as an energy storage deviceand provides self-supporting DC bus. The control technique to regulate the terminal voltage of theSEIG is based on the control of source currents (having twocomponents one in-phase and other quadrature). PLL is used to generate the sin(wt) and cos(wt). The two measurement systems(Vmeas and Imeas) blocks compute the d-axis and q-axis components of the voltages and currents by executing an abc-dq transformation in the synchronous reference determined by sin(wt) and cos(wt) provided by the PLL,this block calculate the mag V and IdIq current. TheAC terminal voltage (V) is compared with the reference voltage. The voltage error is processed in the PI controller. The output of the PI controller (lq ref) for AC voltage control loop decides the amplitude of reactive current required for the system.The DC bus voltage of the STATCOM is sensed and compared with DC reference voltage. The error voltage is processed in another PI controller. The output of this PI controller (Id ref) decides
  3. 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 211 the amplitude of active current, which is required by DSTATCOM to maintain constant DC bus voltage. The quadrature and in-phase components is thesource reference current, which is compared with the SEIG line current. The error current is processed in PID controller.The output of this PID controller maintains the primary voltage equal to the reference value. This VqVd is further used to generate reference voltage for Voltage source converter. 3. MATHEMATICAL MODELLING Modelling of SEIG The dynamic model of the three-phase SEIG is developedusing stationary d-q axes references frame is used because it provides a complete solution for dynamic analysis and control. The flux linkages per second taken into account the saturation effect are Circuit for q axis and q axis of IG Vqs=rsiqs+ωφds+d/dt(φqs) Vds=rsids-ωφqs+d/dt(φds) V’qr = r’ri’qr + (ω - ωr )φ’dr +d/dt(φ’qr) V’dr= r’ri’dr - (ω - ωr )φ’qr +d/dt(φ’dr) φqs = Llsiqs + Lm (iqs + i'qr) φds = Llsids + Lm (ids + i'dr) φ’qr = L’lriqr + Lm (iqs + i'qr) φ’dr = L’lri’dr + Lm (ids + i'dr)
  4. 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 212 In the above equations, all the rotor variables are referredto the stator side. The expression for the electromagnetic Torque of the SEIG is expressed as in per (unit can be written as:) Te=(3/2)(p/2)Lm(φdsiqs- φdsids) The electromagnetic torque balance equation of SEIG isdefined as Tshaft = TB + J(2/P)p(ωr) The derivative of rotor speed of the SEIG- pωr={P/(2J)}(Tshaft-Te) Modelling of wind turbine The wind turbine is characterized by no dimensional curves of the power coefficient Cp as a function of both the tip speed ratio, ë and the blade pitch angle, Â. In order to fully utilize the available wind energy, the value of ë should be maintained at its optimum value. Therefore, the power coefficient corresponding to that value will become maximum. The output power of the wind turbine, can be calculated from the following equation Pm=.5AρCp(λ,β)V3 wind where Pm=Mechanical output power of the turbine (W) Cp=Performance coefficient of the turbine Ρ=Air density (kg/m3) A=Turbine swept area (m2) Vwind=Wind speed (m/s) λ =Tip speed ratio of the rotor blade tip speed to wind speed β=Blade pitch angle (deg) Pm can be normalized. In the per unit (pu) system we have:- Pm_pu=.5AρCp_pu(λ,β)V3 wind_pu generic equation is used to model cp(λ,β). This equation, based on the modelling turbine characteristics of [1], is:-
  5. 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 213 The non-linear, dimensionless Cpcharacteristic is represented as .:- Cp(λ,β)=C1(C2/ λi-C3 β-C4)e-(C5/ λi)+ C6λi With (1/λi)=(1/( λ+0.08 β))-(0.035/( β3 +1)) where, C1 = 0.5176, C2 = 116, C3 = 0.4, C4 = 5, C5 = 21, C6 = 0.0068 Modelling of DSTATCOM Three-phase voltages at the SEIG terminals (va, vb and vc) are considered Convert this sinusoidal voltage into dq0. Vd = 2/3*[Va*sin(wt) + Vb*sin(wt-2pi/3) + Vc*sin(wt+2pi/3)] Vq = 2/3*[Va*cos(wt) + Vb*cos(wt-2pi/3) + Vc*cos(wt+2pi/3)] V0 = 1/3*[Va + Vb + Vc ] Amplitude is:- V=√(Vd2 +Vq2 ) Three phase current at the SEIG terminals(Ia,Ib,Ic) are considered.and convert this current into Id Iq I0. Id = 2/3*[Ia*sin(wt) + Ib*sin(wt-2pi/3) + Ic*sin(wt+2pi/3)] Iq = 2/3*[Ia*cos(wt) + Ib*cos(wt-2pi/3) + Ic*cos(wt+2pi/3)] I0 = 1/3*[Ia + Ib + Ic ] The active power component of the source current isId* which is required for the self supporting DC bus ofDSTATCOM. Moreover, the reactive component isIq* which is required for maintaining the amplitude of the PCC voltage. The active component of the load currentId* can be expressed as:- I* q = kpVdce+ki∫Vdcedt Vdce=Vdc*-Vdc
  6. 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 214 Vdce error in DC bus voltage.Vdc*and Vdc are the reference voltage and sensed filteredvoltage of DC bus of DSTATCOM respectively. Kp and Ki are the proportional and integral gains of the PI controller over the DC bus voltage of DSTATCOM. I*d=kpVte+ki∫Vtedt Vte=V*t-Vt Vte=error in amplitude of the PCCvoltage. Vt* and Vt are reference voltage and amplitude of PCC voltage respectively. Kpand Ki are proportional and integral gains of the PI controller over the PCCvoltage. Now this I*d I*q is compared with Id Iq and the error signal is given to the controller and the output of controller iusVdVq. Vd=KpIde+Ki∫Idedt+Kd(d/dt)Ide Where Ide=Id-I*d Vq=kpIqe+ki∫Iqedt+kd(d/dt)Iqe Where Iqe=Iq-I*q This VqVd is further used to generate reference voltage for Voltage source converter. 4. SIMULATION RESULTS AND DISCUSSION Simulations have been carried out for a 7.5kW threephase- SEIG with balanced, unbalanced resistive and reactive at 0.8 PF and nonlinear load. Disconnecting one or two phases of loads from the SEIG has created unbalanced conditions. The DC bus voltage of DSTATCOM is selected 700V for the source line voltage of 400 V. The VSC is connected to the network through the AC inductor of 800mH. For self- supporting DC bus of DSTATCOM, a capacitor of 6000µF is used. Initially considered linear reactive load is 3-single phase load.The PCC line voltage is considered 700 V,The simulation results are taken for the above mentioned load conditions. A. Performance of DSTATCOM with and Without in Balanced load condition The SEIG system under balanced load without connecting the FACT device i.e. DSTATCOM. Here asynchronous machine driven by wind turbine is connected across the various balanced loads. This asynchronous machine is converted into SEIG by connecting a capacitor bank.The balanced loads consist of two reactive loads (0.8 lagging) and one resistive load. The first reactive load is applied at the starting of the simulation, the second reactive load comes at 2 seconds after starting and the resistive load comes after 2.5 seconds. Fig. 1 (a), (b) & (c) shows the variations in load voltage, load current and stator current with respect to variations in applied load and wind speed. It is depicted that at instant 2 sec. as load increases from 7A to 9A the load voltage falls to 290V from 470V. Further at
  7. 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 215 instant 2.5 sec as load increases to 10A the load voltage falls to 300V from 340V. It is also clear that the voltage and currents are also changing with the variations in wind speeds. The load voltage drops marginally from 500V to 470V when wind speed changes from 12m/s to 10m/s at instant 1.95 sec. At 2.2 sec. the wind speed increases to 11m/s resulting in increase in load voltage to 340V from 290V. There are also marginal variations observed in load current and stator current. This shows that the Voltage across the SEIG not constant, so to maintain voltage constant across the SEIG DSTATCOM is connected. Fig. 1 (a) Load Voltage, (b) Load Current, (c) Stator Current and (d) Wind Speed Now, The DSTATCOM is connecting across the SEIG to improve the Voltage profile. By comparing the results of with and without DSTATCOM on SEIG system in has observed that the DSTATCOM maintains the voltage profile of load voltage as constant irrespective of changes in load and wind speed. The DSTATCOM injects sufficient reactive power into the system so as to maintain the load voltage constant. It has also observed that the profile of stator current has also got improved. When the load is increased across the generator then the reactive power is shared by IG and load so the IG needs some reactive power to maintain the voltage constant. This Reactive power demand fulfil by connecting the DSTATCOM across the generator.
  8. 8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 216 Fig. 2 (a) load voltage, (b) load current, (c) stator current and (d) wind speed B. Performance of DSTATCOM with and without in non-linear load condition Now connect a non-linear load across the SEIG system without connecting the FACT device i.e. DSTATCOM Fig. 3 (a) load voltage, (b) load current, (c) stator current and (d) wind speed The non-linear load injects the harmonics in system and distorts the currents & voltages and also de-rates the machine.
  9. 9. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 217 In Fig. 3 when wind speed is change the voltage across the IG is changes respectively and when load is increased across the IG the voltage is decreased.to make the voltage profile constant a FACTS device is connected across the SEIG.A FACTS device DSTATCOM is connected across the SEIG to improve the voltage profile of the system and also remove the harmonics in system By comparing the results of the effect of non-linear load on SEIG system after the inclusion of DSTATCOM it has observed that it not only fulfils the reactive power demand of the system but also minimises the harmonics in the system. It is clear from Fig. 4 that the effects of non-linearity on load voltage and stator current are considerably reduced. . Fig. 4 (a) load voltage, (b) load current, (c) stator current and (d) wind speed In this way DSTATCOM also restores the rating of the machine which has reduced due to the effects of harmonics. THD of non-linear load in system without DSTATCOM THD for Stator voltage = 42.18%
  10. 10. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 218 THD for stator current = 71.34% THD of non-linear load in system with DSTATCOM THD for Stator voltage = 9.39% THD for stator current = 6.98%
  11. 11. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 219 5. CONCLUSION A mathematical model of three-phase self-excited induction generator with DSTATCOM based voltage regulator under balanced/unbalanced/non-linear loads is studied in this project. It is concluded from the simulated results that the DSTATCOM acts as an ideal voltage regulator and load balancing device, which maintains the SEIG voltage constant and balances the SEIG system at varying balanced, unbalanced and non-linear loads APPENDIX A Parameters of machine Rated power 10 HP(7.5 KW) Voltage 400V Frequency 50HZ RPM 1440 Capacitor Bank 7.5 KVAR Parameters of wind turbine Nominal power 7650W Base power 7650(VA) Base speed 10 Max power at base wind speed 10m/s Base rotational speed 1.2 p.u REFERENCES [1]. S. S. Murthy, O. P. Malik. and P. Walsh, "Capacitive VAR requirements of self-Excited induction generators to achieve desired voltage regulation," IEEE Industrial and Commercial Power SystemConf, Milwaukee, USA, June 1983. [2]. M. B. Brermen and A. Abbondati, "Static exciter for induction generator," IEEE Trans, on Ind. Appl, Vol. 13, No. 5, pp. 422-428, Sep./Oct. 1977. [3]. Bhim Singh, Sunil kumar, “Modified power balance theory for control of DSTATCOM” IEEE Transaction 2010. [4]. K. R. Padiyar, “FACTS Controllers in Power Transmission and Distribution,” New Age International (P) Limited, Publishers, New Delhi, 2007. [5]. B.Singh, S.S.Murthy and S.Gupta, “Analysis and design of STATCOM Based Voltage Regulator for Self-Excited Induction generators,” IEEE Transactions on Energy Conversion, vol.19, no.4, Dec 2004, pp.783-790. [6]. B. Singh, P. Jayaprakash, T.R. Somayajulu, D.P. Kothari, A. Chandra, and K. Al-Haddad, “Integrated three-leg VSC with a zig-zag transformer based three-phase four-wire DSTATCOM for power quality improvement,” in Proc. of IECON 2008, Nov. 2008, pp. 796 -80. [7]. Haider M. Husen , Laith O. Maheemed and Prof. D.S. Chavan, “Enhancement of Power Quality in Grid-Connected Doubly Fed Wind Turbines Induction Generator”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 182 - 196, ISSN Print : 0976-6545, ISSN Online: 0976-6553. [8]. Youssef A. Mobarak, “Svc, Statcom, and Transmission Line Rating Enhancments on Induction Generator Driven By Wind Turbine”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 326 - 343, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

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