Synchronous Reference Scheme for Harmonic and Unbalance Compensation Using Active Power Filter

Transcription

1 International Journal of Electrical Electronics Computers & Mechanical Engineering (IJEECM) ISSN: Volume 2 Issue 6 ǁ Dec ǁ IJEECM journal of Electrical Engineering (ijeecm-jee) Synchronous Reference Scheme for Harmonic and Unbalance Compensation Using Active Power Filter NallamothuVenkataGowtam, NaraboinaSambasivaRao Electrical & Electronics Department, NRI Institute Of Technology,Vijayawada, A.P gowtam.venkat@gmail.com Abstract This paper suggestsan active power filter implemented with a four leg voltage-source inverter using DQ (synchronous reference frame) based Current Reference Generator scheme is presented. The use of a four-leg voltage-source inverter allows the compensation of current harmonic components, as well as unbalanced current generated by single-phase nonlinear loads.the grid interfacing can thus be utilized as:1)power converter to inject power generated from rest the grid, and 2)shunt APF to current unbalance, load current harmonics and load reactive power demand. The compensation performance of the proposed active power filterand the associated control scheme under steady state and transient operating conditions is demonstrated through simulations results. Keywords Active power filter, Current control, Predictive control, Four-leg converters, I. INTRODUCTION The widespread use of non-linear loads is leading to a variety of undesirable phenomena in the operation of power systems. The harmonic components in current and voltage waveforms are the most important among these. Conventionally, passive filters have been used to eliminate line current harmonics. However, they introduce resonance in the power system and tend to be bulky. So active power line conditioners have become popular than passive filters as it compensates the harmonics and reactive power simultaneously. The active power filter topology can be connected in series or shunt and combinations of both. Shunt active filter is more popular than series active filter because most of the industrial applications require current harmonics compensation. Different types of active filters have been proposed to increase the electric system quality; a generalized block diagram of activepower filter is presented in [2]. The classification is based on following criteria. a) Power rating and speed of response required in compensated system. b. System parameters to be compensated (e.g. current harmonics, power factor, voltage harmonics) c. Technique used for estimating the reference current/voltage. Current controlled voltage source inverters can be utilized with appropriate control strategy to perform active filter functionality. The electrical grid will include a very large number of small producers that use renewable energy sources, like solar panels or wind generators. One of the most common problems when connecting small renewable energy systems to the electric grid concerns the interface unit between the power sources and the grid, because it can inject harmonic components that may detoriate the power quality [1],[2]. However, the extensive use of power electronics based equipment and nonlinear loads at PCC generate harmonic ijeecm.org

2 currents, which may detoriate the quality power. In [3] an inverter operates as active inductor at a certain frequency to absorb the harmonic current. A similar approach in which a shunt active filter acts as active conductance to damp out the harmonics in distribution network is proposed in[4],[5] three phase converter with the fourth leg connected to the neutral bus of the system. The fourth leg increases switching states from 8 (23) to 16 (24), improving control flexibility and output voltage quality, and is suitable for current unbalanced compensation.. II. FOUR-LEG CONVERTER MODEL The proposed system consists of RES connected to the dc-link of a gridinterfacing inverter as shown in Fig. 1. The voltage source inverter is a key element of a DG system as it interfaces the renewable energy source to the gridand delivers the generated power. The RES may be a DC source or an AC source with rectifier coupled to dc-link. Usually, the fuel cell and photovoltaic energy sources generate power at variable low dc voltage, while the variable speed wind turbines generate power at variable ac voltage. Thus, the power generated from these renewable sources needs power conditioning (i.e., dc/dc or ac/dc) before connecting on dclink [6] [8]. The dc-capacitor decouples the RES from grid and also allows independent control of converters on either side of dc-link. Figure 2. Two-level four-leg PWM-VSI topology The voltage in any leg x of the converter, measured from the negative point of the dc-voltage (N), can be expressed in terms of switching states, as follows: The mathematical model of the filter, derived from the equivalent circuit shown in Fig. 1, is: Where Reqand Leq are the 4L-VSI output parameters, expressed as Thevenin impedances at the converter output terminals, Zeq. Therefore, the Thevenin equivalent impedanceis determined by a series connection of the ripple filter impedance Zfand a parallel arrangement between the system equivalent impedance Zsand the load impedance ZL (3). Fig.1.Schematic diagram of renewable based distributed generation system The four-leg PWM converter topology is shown in Fig. 3. This converter topology is similar to the conventional For this model, it is assumed that ZL >>Zf, that the resistive part of the system s equivalent impedance is neglected, and that the series reactance is in the range of 3-7% p.u., which is an acceptable approximation of the real system. Finally, inequation (2) Req = Rf and Leq = Ls + Lf III. CURRENT REFERENCE GENERATION

3 Adq-based current reference generator scheme[9]-[14] is used to obtain the active power filter current reference signals. This scheme presents a fast and accurate signal tracking capability. This characteristic avoids voltage fluctuations thatdeteriorate the current reference signal affecting compensation performance. The current reference signals are obtained from the corresponding load currents as shown in Fig. 3. This module calculates the reference signal currents required by the converter to compensate reactive power, current harmonic and current imbalance. The displacement power factor (sin (L) )and the maximum total harmonic distortion of the load (THD (L) )defines the relationships between the apparent power required by the active power filter, with respect to theload, as shown in below equation. where the value of THD (L) includes the maximum compensable harmonic current, defined as double the sampling frequency fs. The frequency of the maximum current harmoniccomponent that can be compensated is equal to two times theconverter switching frequency. The dq-based scheme operates in a rotating reference frame[15]- [17];therefore, the measured currents must be multiplied by thesin(wt) and cos(wt) signals. By using dq-transformation, thed current component is synchronized with the correspondingphase-to-neutral system voltage and the q current component isphase-shifted by 90. The sin(wt) and cos(wt) synchronizedreference signals are obtained from a Synchronous Reference Frame (SRF) PLL. The SRF-PLL generates a puresinusoidal waveform even when the system voltage is severelydistorted. Tracking errors are eliminated, since SRF-PLLs aredesigned to avoid phase voltage unbalancing, harmonics (i.e.less than 5% and 3% in 5th and 7th respectively), and offsetcaused by the nonlinear load conditions and measurement errors. Below equation shows the relationship between thereal currents ilx(t) (x = u, v,w) and the associated dqcomponents (id and iq). A low-pass filter (LFP) extracts the dc component of the phase-currents id to generate the harmonic reference components id. The reactive reference components of the phasecurrents are obtained by phase-shifting the corresponding AC and dc components of iq by 180. In order to keep the dcvoltage constant, the amplitude of the converter reference current must be modified by adding an active power reference signal (ie) with the d-component. The resulting signals i d, and i q are transformed back to a three-phase system by applying the inverse Park and Clark transformation, The cutoff frequency of the LPF used in this paper is 20 Hz. The current that flows through the neutral of the loadis compensated by injecting the same instantaneous valueobtained from the phase-currents, phase-shifted by 180, asshown in below One of the major advantages of the dq-based current referencegenerator scheme is that allows the implementation ofa linear controller in the dc-voltage control loop. However,one important disadvantage of the dq-based current referenceframe algorithm used to generate the current reference is thata second order harmonic component is generated in id and iqunder unbalanced operating conditions. The amplitude of thisharmonic depends on

4 the percent of unbalanced load current(expressed as the relationship between the negative sequencecurrent il,2 and the positive sequence current il,1 ). Thesecond order harmonic cannot be removed from id and iq,and therefore generates a 3rd harmonic in the reference currentwhen it is converted back to abc frame. Figure 6 showsthe percent of system current imbalance and the percent of3rd harmonic system current, in function of the percent ofload current imbalance[18],[19]. Since the load current does not havea 3rd harmonic, the one generated by the active power filterflows to the power system. show that the proposedcontrol scheme effectively eliminates unbalanced currents.additionally, Results shows that the dc-voltage remains stablethroughout the whole active power filter operation. IV.SIMULATED RESULTS A simulation model for the threephase four-leg PWMconverter with the parameters shown in Table II has been developedusing MATLAB-Simulink. The objective is to verify thecurrent harmonic compensation effectiveness of the proposedcontrol scheme under different operating conditions. A sixpulserectifier was used as a non-linear load. In the simulated results shown in Fig.8-15, the active filter starts to compensate at t =0.2. At this time, the active powerfilter injects an output current iou to compensate currentharmonic components, current unbalanced, and neutral currentsimultaneously. During compensation, the system currents (is)show sinusoidal waveform, with low total harmonic distortion. At t =0.4, a three-phase balanced load stepchange is generated from 0.6 to 1.0 p.u. The compensatedsystem currents remain sinusoidal despite the change in theload current magnitude. Finally, at t =0.6, a single-phase loadstep change is introduced in phase u from 1.0 to 1.3 p.u.,which is equivalent to an 11% current imbalance. As expectedon the load side, a neutral current flow through the neutralconductor (iln), but on the source side, no neutral currentis observed (isn). Simulated results Fig.8. phase to neutral source voltage Fig.9. Source currents Fig.10. Source currents at 0<t<0.2

5 Fig.11. Source currents at 0.2<t<0.4 Fig.12. Source currents at 0.4<t<0.6 converter current loop proved to be an effective solutionfor active power filter applications,improving current trackingcapability, and transient response. Simulated results have proved that the proposed control method is a good alternative to classical linear control methods. Simulated results have shown thecompensation effectiveness of the proposed active power filter. VII.References Fig.13. Load current Fig.14. Load current at 0<t<0.4 Fig.15. Load current at 0.4<t<0.6 VI.Conclusion Improved dynamic current harmonics and a reactive power compensation scheme for power distribution systems with generation from renewable sources has been proposed to improve the current quality of the distribution system. Advantages ofthe proposed scheme are related to its simplicity, modeling and implementation. The MATLAB/SIMULINK simulation model of the proposed system with the connection of renewable energy sources is shown and validated.the use of a dq-based current reference generation scheme forthe [1] J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodriguez,[2012]. Control of powerconverters in ac microgrids, Power Electronics, IEEE Transactions on,vol. 27, no. 11, pp , nov. [2] M. Aredes, J. Hafner, and K. Heumann, [1997]. Three-phase four-wire shuntactive filter control strategies, Power Electronics, IEEE Transactionson, vol. 12, no. 2, pp , March. [3] S. Naidu and D. Fernandes,[2009]. Dynamic voltage restorer based on a fourlegvoltage source converter, Gener. Transm.Distrib, IET, vol. 3, no. 5, pp , May. [4] N. Prabhakar and M. Mishra, [2010]. Dynamic hysteresis current control tominimize switching for three-phase four-leg vsi topology to compensatenonlinear load, Power Electronics, IEEE Transactions on, vol. 25, no. 8, pp , August. [5] V. Khadkikar, A. Chandra, and B. Singh,[2011]. Digital signal processorimplementation and performance evaluation of split capacitor, fourlegand three h- bridge-based three-phase four-wire shunt active filters, Power Electronics, IET, vol. 4, no. 4, pp , April. [6] F. Wang, J. Duarte, and M. Hendrix,[2011]. Grid-interfacing converter systemswith enhanced voltage quality for microgridapplication;concept and implementation, Power Electronics, IEEE Transactions on, vol. 26, no. 12, pp , dec. [7] X. Wei,[201]. Study on digital pi control of current loop in active powerfilter, Electrical and Control Engineering (ICECE), 2010 InternationalConference on, pp , June.

6 [8] R. de AraujoRibeiro, C. de Azevedo, and R. de Sousa,[2012]. A robustadaptive control strategy of active power filters for power-factor correction,harmonic compensation, and balancing of nonlinear loads, PowerElectronics, IEEE Transactions on, vol. 27, no. 2, pp , feb. [9] J. Rodriguez, J. Pontt, C. Silva, P. Correa, P. Lezana, P. Cortes, and U. Ammann,[2007]. Predictive currentcontrol of a voltage source inverter, Trans. Ind. Electron., IEEE, vol. 54, no. 1, pp , February. [10] P. Cortes, G. Ortiz, J. Yuz, J. Rodriguez, S. Vazquez, and L. Franquelo,[2009]. Model predictive control of an inverter with output LC filter for UPSapplications, Trans. Ind. Electron., IEEE, vol. 56, no. 6, pp , June. [11] R. Vargas, P. Cortes, U. Ammann, J. Rodriguez, and J. Pontt,[2007]. Predictivecontrol of a three-phase neutral-point-clamped inverter, Trans. Ind.Electron., IEEE, vol. 54, no. 5, pp , October. [12] P. Cortes, A. Wilson, S. Kouro, J. Rodriguez, and H. Abu- Rub,[2010]. Modelpredictive control of multilevel cascaded H-bridge inverters, Trans. Ind.Electron., IEEE, vol. 57, no. 8, pp , August. 13] P. Lezana, R. Aguilera, and D. Quevedo,[2009]. Model predictive control ofan asymmetric flying capacitor converter, Trans. Ind. Electron., IEEE,vol. 56, no. 6, pp , June. [14] P. Correa, J. Rodriguez, I. Lizama, and D. Andler,[2009]. A predictivecontrol scheme for current-source rectifiers, Trans. Ind. Electron., IEEE,vol. 56, no. 5, pp , May. [15] M. Rivera, J. Rodriguez, B. Wu, J. Espinoza, and C. Rojas,[2012]. Currentcontrol for an indirect matrix converter with filter resonance mitigation, Trans. Ind. Electron., IEEE, vol. 59, no. 1, pp , January [16] P. Correa, M. Pacas, and J. Rodriguez,[2007]. Predictive torque control forinverter-fed induction machines, Trans. Ind. Electron., IEEE, vol. 54, no. 2, pp , April [17] M. Odavic, V. Biagini, P. Zanchetta, M. Sumner, and M. Degano,[2011]. Onesampleperiod-ahead predictive current control for highperformanceactive shunt power filters, Power Electronics, IET, vol. 4, no. 4, pp , April. [18] R. de AraujoRibeiro, C. de Azevedo, and R. de Sousa,[2012]. A robustadaptive control strategy of active power filters for power-factor correction,harmonic compensation, and balancing of nonlinear loads, PowerElectronics, IEEE Transactions on, vol. 27, no. 2, pp , feb [19] M. Sumner, B. Palethorpe, D. Thomas, P. Zanchetta, and M. Di Piazza,[2002]. A technique for power supply harmonic impedance estimation using acontrolled voltage disturbance, Power Electronics, IEEE Transactionson, vol. 17, no. 2, pp , March [20] D. Driankov, H. Hellendoom and M. Reinfrank[1993]. An Introduction to Fuzzy Control, Springer-Verlag. [21] Karuppanan P., Mahapatra K.K.,[2011]. PLL with fuzzy logic controller based shunt active powerfilter for harmonic and reactive power compensation IEEE Conference, IICPT, Power Electronics, pp.1-6. [22] M. Rivera, C. Rojas, J. Rodriidguez, P. Wheeler, B. Wu, and J. Espinoza,[2011]. Predictive current control with input filter resonance mitigation fora direct matrix converter, Power Electronics, IEEE Transactions on,vol. 26, no. 10, pp , oct. NallamothuVenkataGowtamgraduated from the MenteyPadmanabham college of engineering & technology, Bhimavaram, in 2013.He is currently pursing master of technology in NRI institute of technology Vijayawada Andhra Pradesh. His research interests include converters, inverters and power quality issues of electrical systems Dr.NaraboinaSambasivaRao received his B.Tech degree from the GIET,Rajhmundry, Andhra Pradesh, India, in 1998, and his master degree and PH.D from the University of JNTUH, Hyderabad,India. From 2002 to 2008 he worked as a Associate Professor in DVR AND DR.HS MIC college of technology Vijayawada Andhra Pradesh India. He is currently a professor and Head Of Department with the department of electrical & electronics, NRI InstituteOf Technology Vijayawada Andhra Pradesh, India. His research interests include power quality, microgrids, and distributed generation systems..