A Comparative Study on Compensating Current Generation Algorithms for Shunt Active Filter under Non-linear Load Conditions

Transcription

1 International Journal of Scientific and Research Publications, Volume 3, Issue 6, June A Comarative Study on Comensating Current Generation Algorithms for Shunt Active Filter under Non-linear Conditions S. S. Wamane *, J.R. Baviskar **, S. R. Wagh ** Electrical Deartment,VJTI Mumbai Abstract- With the raid use of ower electronic converters which are considered as non-linear loads have roblems of drawing non-sinusoidal currents and reactive ower from source which in turn ollutes the ower uality. This aer resents the ower uality roblems and methods for its correction where two control strategies ( - instantaneous ower theory and d - synchronous reference frame theory) for extracting reference currents for shunt active ower filters (SAPF) have been evaluated and their erformances have been comared under distorted suly and non-linear load conditions. These theories are used to imlement the control algorithm of a shunt active filter which comensates harmonic currents. This aer imlements the three hase three wire shunt active ower filter to suress current harmonics. The SAPF erformance is validated using MATLAB/Simulink model showing comarisons for the two algorithms. Index Terms- Active filters, harmonic comensation, instantaneous ower theory, ower uality, synchronous reference theory, total harmonic distortion (THD). T I. INTRODUCTION herecent growth in the use of non-linear loads has resulted in the ower uality roblems like voltage sag, voltage swell, voltage notch, voltage flicker, current and voltage harmonics, unbalance, excessive neutral current and interference in communication network. These disturbances range from sub cycle duration to long term steady state roblems. Voltage and current harmonics causes reduction in the efficiency of the generation, transmission and utilization of electric energy. It causes a shar increase in the zero seuence current, and therefore, increases the current in the neutral conductor. Harmonic voltages can interfere the controllers used in electronic systems and its effects are classified into two tyes: instantaneous effects and long term effects. The instantaneous effects are maloeration and erformance degradation of electronic euiments. The long term effects are of thermal nature and are related to additional losses. Figure 1 resents a ower system with sinusoidal source voltage (Vs) oerating with linear and non-linear load. The non-linear load current (i L1 ) contains harmonic. The harmonics in the line current (i s ) roduces non-linear voltage dro ( V) in line imedance, which distorts the load voltage (V L ). As the load voltage is distorted, the linear load current (i L2 ) will also be a non-sinusoidal. The solid stateconverters draw harmonic and reactive ower comonents of current from AC mains which affect ower uality [1]. V S i S R R V L L V V L PCC i L1 i L2 NON Figure 1: Power System with Linear and Non-linear load The total harmonic distortion (THD) is the ratio of the RMS value of the sum of all harmonic comonents and the RMS value of the fundamental comonent, for both current and voltage. where, h is the order of harmonic. To overcome these roblems SAPF is used which satisfies the need of the dynamic and adjustable solution reuirement to the ower uality roblems. In order to comensate the distorted currents, SAPF injects currents eual but oosite with the harmonic comonents, thus only the fundamental comonents flows in the oint of common couling (PCC). V S i S R V L i FA V L SHUNT ACTIVE FILTER PCC i L1 i L2 (1) NON Figure 2: Power System with load and shunt active filter

2 International Journal of Scientific and Research Publications, Volume 3, Issue 6, June The SAPF connected in arallel to the disturbing loads, unbalanced and non linear, as seen in Figure 2, causes the suly currents to be near sinusoidal and balanced. SAPF can comensate current harmonics and erform ower factor correction furthermore; it allows load balancing by eliminating the current in the neutral wire [1]-[2]. This aer resents a comrehensive analysis of SAPF using two (- and d- theory) algorithms for derivation of comensating signals. These comensating signals are then comared in a hysteresis controller for generation of switching signals. In section II, the control strategy of active filters is resented. In section III and IV, basic of - theory and its control algorithm are resented. The d- theory and its control algorithm is resented in section V. A comarative analysis of simulation results is resented in section VI. Finally, section VII concludes the results. steady-state and transient oeration as well as for generic voltage and current waveforms, allowing the control of the active filters in real-time. Since only algebraic calculations are reuired the additional advantage of simlicity is achieved by using this method. It consists of an algebraic transformation (Clarke transformation) of three hase voltages and currents in the a-b-c coordinate to α-β-0 coordinate. The main advantage of using Clarke transformation is searation of zero seuence comonents [2]-[6]. The calculation of the instantaneous ower - theory comonents is given by = (2) II. PROCEDURAL STEPSTO CONTROL ACTIVE FILTERS The control strategy of active filters is a comlex rocess which suoses observing certain rereuisites. It may be looked uon as a three searate stes: signal monitoring, getting comensating signals and generating gating signals. 1. Signal monitoring For the urose of imlementation of the control algorithm, instantaneous voltage and current signals are reuired which are also useful to monitor total harmonic distortion (THD), ower factor, active and reactive ower. These signals, sensed by using otential transformer (PT) and current transformer (CT) resectively are then comared with comensating signals to generate gating signals. 2. Derivation of comensating signals The imortant art of SAPF control is the develoment of comensating signals in terms of voltages or currents. The control methods used to generate comensating commands are based on freuency or time domain techniues. The comensation in freuency domain is based on Fourier analysis of distorted voltage or current signals to extract comensating commands but it results in large resonse time. The comensation in time domain are based on instantaneous derivation of comensating commands in terms of voltage or current signals from distorted voltage or current signals. This uses simle algebraic calculations and transformations. There are many control methods in time domain, few of them are: - Instantaneous ower (-) theory - Synchronous reference (d-) method - Frize-Buchholz-Deenbrock (FBD) method. 3. Generation of gating signals The gating signals to control solid state devices of SAPF are generated by using ulse width modulation (PWM), hysteresis, sliding mode or Fuzzy logic based control techniues. A. The P-Q Theory The - theory roosed in 1983 by Akagi et al [7] to control active filter (AF) is based on the time domain. It is valid for both = (3) The instantaneous zero seuence ower is given by o = v 0 * i 0 (4) The instantaneous real ower is given by = v α i α + v β i β (5) and, the instantaneous imaginary ower is given by = v α i β - v β i α (6) so, the ower comonents and are given by = (7) These uantities are illustrated in Figure 3 for an electrical system reresented in a-b-c coordinates and have the following meanings: a b c N Figure 3: Power comonents of - theory

3 International Journal of Scientific and Research Publications, Volume 3, Issue 6, June where, = mean value of instantaneous zero seuence ower = alternated value of instantaneous zero seuence ower = mean value of instantaneous real ower = alternated value of instantaneous real ower = instantaneous imaginary ower. From all the ower comonents of - theory, only and are desirable. The other uantities can be comensated using a SAPF. From Figure 4 the can be comensated without the need of any ower suly in the SAPF. This uantity is delivered from ower suly to the load through active filter in balanced way [1]-[8]. The reactive and harmonic comensation is carried by injecting aroriate currents into the line through comensator i.e. SAPF. The dc link caacitor is reuired to comensate and. The instantaneous imaginary ower () is comensated without any contribution of caacitor; this means that, the size of the caacitor does not deend on the amount of reactive ower to be comensated. The can be comensated without any ower suly in active filter. This uantity is delivered from the source to the load through SAPF as shown in Figure 4. To calculate the reference comensation currents, in α-β-0 coordinates (7) is inverted; given by = (8) Filter current and voltage measurement Clarke / Park transformation - or d- theory calculations Comensation current calculation Clarke / Parke inverse transformation Filter Figure 5: Control algorithm for extraction of comensation currents To obtain the reference comensation currents in a-b-c coordinates the inverse transformation of (3) is alied which is given by Since, the zero-seuence current must be comensated, the reference comensation current in the 0 coordinate is: = (10) i c0 * = i 0 (9) a i a a b c b c N i b i c i N N a b c N SHUNT ACTIVE FILTER + _ C Figure 4: Comensation of ower comonents Phase Voltages (Va Vb Vc) (ia ib ic) Shunt Active Filter CONTROLLER Reference INVERTER C + - V dc Figure 6: Shunt active filter block diagram The - theory calculations are carried out in the shunt active ower filter block shown in Figure 6. From the received values of hase voltages (v a, v b, v c ), load currents (i a, i b, i c ) and DC

4 International Journal of Scientific and Research Publications, Volume 3, Issue 6, June voltage, the controller calculates the reference currents given by (10) based on control algorithm shown in Figure 7. The inverter uses these reference currents to roduce the comensation currents which are injected in ower system by the inverter. Phase Voltages V a V b V c i a i b i c Caacitor Voltage V dc + - V ref V α V β V 0 i α i β i 0 Gain K LPF LPF i cα * i cβ * P reg Figure 7: of - theory comonents i ca * i cb * i cc * i cn * i 0 (fundamental and harmonic comonents) and the erformance of the Phase Locked Loo (PLL). The PLL circuit rovides the rotation seed (rad/sec) of the rotating reference frame, where W is set as fundamental freuency comonent and 30 degree hase angle followed by sin and cos for synchronization. The (i d - i ) currents are sent through low ass filter (LPF) for filtering the harmonic comonents of the load current, which allows only the fundamental freuency comonents [3]. The LPF is second order butterworth filter used for eliminating the higher order harmonics. The PI controller is used to eliminate the steady-state error of the DC comonent of the d-axis reference signals. The DC side caacitor voltage of inverter is sensed and comared with desired reference voltage for calculating the error voltage. This error voltage is assed through a PI controller whose roagation gain (K P ) and integral gain (K I ) is taken as 0.1 and 1 resectively. i abc PLL abc to d LPF + - d to abc i abc * B. Synchronous Reference Frame Theory The synchronous reference frame theory or d- theory [8] is based on time domain reference signal estimation techniues. It erforms the oeration in steady state or transient state as well as for generic voltage and current waveforms. It allows controlling the active ower filters in real time system. Another imortant characteristic of this theory is the simlicity of the calculations, which involves only algebraic calculation. In this strategy, the reference frame d--0 is determined by the transformation angle θ with resect to the α-β-0 frame alied in the - theory [6]. It is based on the transformation of the stationary reference frame three hase variables (a,b,c) to synchronous reference frame variables (d,,0) whose direct (d) and uadrature () axes rotate in sace at the synchronous seed, which is the angular electrical seed of the rotating magnetic field of the three hase suly given by, where is the freuency of the suly. If θ is the transformation angle, then the current transformation from a-b-c to d--0 frame is calculated as V dc Gain K + - P-I Controller Figure 8: Synchronous reference theory based control algorithm III. SIMULATION RESULTS The urose of the simulation is to show the effectiveness of - and d- methods for maintaining sinusoidal source currents when the source sulying a non-linear load. The MATLAB/Simulink simulation tool is used to develo a model that allowed the simulation of the - and d- theory calculations, which are imlemented in the controller of the SAPF (Figure 7 and 8). The load is simulated to include harmonic distortion from a three hase uncontrolled rectifier. = (11) where, (12) The sine and cosine functions hel to maintain the synchronization with suly voltage and current. The d- transformation outut signals are deendent on the load current (a) Three hase source voltages

5 International Journal of Scientific and Research Publications, Volume 3, Issue 6, June TABLE I: Harmonic comensation by - theory method THD without comensation I a 20.25% 1.21% I b 21.07% 1.22% I c 21.69% 1.28% THD with comensation TABLE II: Harmonic comensation by d- theory method (b) currents without SAPF THD without comensation I a 20.25% 1.31% I b 21.07% 1.34% I c 21.69% 1.37% THD with comensation From Table I and II, it is seen that without comensation, the THD level of the source current was in between 20-22% which do not comly with the IEEE 519 harmonic standards [9]. After comensation by - and d- theory, the THD level of the source current is reduced considerably for all three hases which comly with the IEEE 519 standards. Comaring these tables it is confirmed that - theory gives better erformance than the d- theory method. (c) Three hase comensating currents IV. CONCLUSION In this aer the erformance analysis of - and d- methods of comensation current generation for SAPF under distorted suly and non-linear load conditions based on simulation studies is discussed. Comarative study brings that, the - theory gives a better aroach than synchronous reference frame (d-) theory for comensation ofharmonic currents and takes aroriate corrective measure for the THD in imrovement of the ower under distorted suly and non-linear load conditions. (d) Three hase load currents with SAPF Figure 9: Simulation results for ideal voltage source and thyristor rectifier load Because of non-linear load the source current contains harmonic comonents as shown in Figure 9 (b). To make the source current distortion free, SAPF oerate to comensate the current harmonics by injecting the current of variable magnitude and hase as shown in Figure 9 (c). The Figure 9 (d) indicates that after comensation the main currents are sinusoidal even when the load is non linear. REFERENCES [1] Bhim Singh, Kamal Al-Haddad and Ambrish Chandra, "Review of active filters for ower uality imrovement," SringerIEEE Transaction on Industrial Electronics, Vol.-46 No. 5, Oct [2] Joao L. Afonso, M. J. Seulveda Freitas, and Julio S. Martins, - theory ower comonents calculations, ISIE 2003, IEEE International Symosium on Industrial Electronics Rio de Janeiro, Brasil 9-11 Junho de 2003, ISBN: [3] Murat Kale and Engin Ozdemirc, Harmonic and reactive ower comensation with shunt active ower filter under non-ideal mains voltage, Electrical ower systems research 74(2005) [4] Jora M. Gonda, Anantha adithya and Sumam david S., Performance analysis of comensation current extraction circuit for three hase three wire shunt active ower filter under unbalance suly, IEEE / [5] P. Salmeron and S. Litran, Imrovement of the electric ower uality using series active and shuntassive filters, IEEE Transaction on ower delivery, Vol 25 No. 2, Ar. 2010

6 International Journal of Scientific and Research Publications, Volume 3, Issue 6, June [6] Joao L. Afonso, H.J. Ribeiro da silva and Julio S. Martins, Active Filters for ower uality imrovement, IEEE orto ower Tech set 2001 ISBN: [7] H. Akagi, Y. Kanazawa and A. Nabae, Generalized theory of the instantaneous reactive ower in three-hase circuits, IPEC'83 -Int. Power Electronics Conf., Tokyo, Jaan, 1983, [8] V. Soares, P. Verdelho and G. Marues, Active ower filter control circuit based on the instantaneous active and reactive current id-i method, Proc. of PESC'97, Vol. 2, [9] IEEE recommended ractices and reuirements for harmonic control in electrical ower systems, IEEE Std AUTHORS First Author S.S. Wamane, M.Tech, VJTI - Mumbai sudhirwamane@gmail.com Second Author J.R. Baviskar, PhD, VJTI - Mumbai jaywantbaviskar@gmail.com Third Author S.R. Wagh, PhD, VJTI - Mumbai sushamawagh@gmail.com Corresondence Author S.S. Wamane,sudhirwamane@gmail.com