Real Time Implementation of Shunt Active Power Filter (SAPF) for Harmonic suppression and Power Quality Improvement

Transcription

1 Real Time Implementation of Shunt Active Power Filter (SAPF) for Harmonic suppression and Power Quality Improvement B. Babes 1 L. Rahmani 2 A. Bouafassa 3 and N. Hamouda 4 1, 3 Department of Electrical Engineering, Setif University, Algeria 2 Automatic Laboratory of Setif, Setif University, Algeria 4 Centre National de Recherche Scientifique en Soudage et Contrôle CSC Route Dely Brahim Chéraga, Algerie elect_babes@yahoo.fr, lazhar_rah@yahoo.fr, amar.bouafassa@gmail.com, hammouda.nourou@yahoo.fr Keywords «Harmonics», «Power quality», «Active power filter», «Hysteresis comparator», «Real-time control» Abstract In this paper, A Shunt Active Power Filter (SAPF) is implemented using a dspace DS114 processor to compensate harmonics and reactive power produced by nonlinear load. The reference source current is computed based on the measurement of harmonics in the supply voltage and load current. A hysteresis based current controller has been implemented in a DSP processor for injecting the compensating current into the power system, so that SAPF allows suppression of the harmonics and reactive power component of load current, resulting in a supply current that is purely sinusoidal. Simulation and experimental results of the proposed SAPF to meet the IEEE-519 standards are presented. I. Introduction Harmonics contamination is a serious and a harmful problem in Electric Power System. Active Power filtering constitutes one of the most effective proposed solutions. A shunt active power filter that achieves low current total harmonic distortion (THD), reactive power compensation and power factor correction is presented. Hence, it is necessary to reduce the dominant harmonics below 5% as specified in IEEE harmonic standard [1]. Harmonic Amplification is one the most serious problem. It is caused by harmonic resonance between line inductance and power factor correction (PFC) capacitors installed by consumers. Active filters for damping out harmonic resonance in industrial and utility power distribution systems have been researched [1]-[2]. Traditionally based, passive L-C filters were used to eliminate line harmonics in [3]-[4]. However, the passive filters have the demerits of fixed compensation, bulkiness and occurrence of resonance with other elements. The recent advances in power semiconductor devices have resulted in the development of Active Power Filters (APF) for harmonic suppression. Various topologies of active filters have been proposed for harmonic mitigation. The shunt APF based on Voltage Source Inverter (VSI) structure is an attractive solution to harmonic current problems. The SAF is a pulse width modulated (PWM) VSI that is connected in parallel with the load. It has the capability to inject harmonic current into the AC system with the same amplitude but opposite phase than that of the load [1]-[3]. The principal components of the SAPF are the VSI, a DC energy storage device that in this case is capacitor, a coupling transformer and the associated control circuits. The performance of an active filter depends mainly on the technique used to compute the reference current and the control method used to inject the desired compensation current into the line.

2 There are two major approaches that have emerged for the harmonic detection [3], namely, time domain and the frequency domain methods. The frequency domain methods include, Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT), and Recursive Discrete Fourier Transform (RDFT) based methods. The frequency domain methods require large memory, computation power and the results provided during the transient condition may be imprecise [4]. On the other hand, the time domain methods require less calculation and are widely followed for computing the reference current. There are several current control strategies proposed in the literature [2]-[5], [6]-[7],[8], namely, PI control, average current mode control (ACMC), predictive current control, sliding mode control (SMC) and hysteresis control. Among the various current control techniques, hysteresis control is the most popular one for active power filter applications. Hysteresis current control [9] is a method of controlling a voltage source inverter so that the output current is generated whichh follows a reference current waveform. In this paper, a simple and straight hysteresis band current controller is implemented for an active shunt filter to compensate the harmonics and reactive power of a typical non-linear load. A dspace DS114 controller board with five ADC channels and high resolution Pulse Width Modulated (PWM) channels is used to implement the proposed control algorithm. An IGBT based VSI Bridge with common DC bus capacitor is employed to realize the SAPF. A three phase diode bridge rectifier with R-L loading is taken as a non-linear load. The control algorithm is tested through simulation and thereafter, experimental verification is also carried out. II. Active power filter principle Active power filters (APFs) are basically power electronic devices that are used to compensate the current or voltage harmonics and the reactive power flowing in the power grid. The APFs may be used as a controlled current source and it has to supply a current wave as close as possible to current reference. [1], [11] As it is shown in the Fig.1, the active filter is composed of a three phase voltage source inverter with an ac inductor (L f ) and a dc bus capacitor (C dc ) to provide a constant dc voltage and the real power necessary to cover the losses of the system. A three-phase AC supply system (V sa,v sb and V sc ) with line impedance (R s and L s ).[12],[13] Fig. 1: Shunt active power filter configuration

3 III. Control scheme The control scheme consists of PI controller, limiter, and three phase sine wave generator for reference current generation and generation of switching signals. The peak value of reference currents is estimated by regulating the DC link voltage. The actual capacitor voltage is compared with a set reference value. The error signal is then processed through a PI controller, which contributes to zero steady error in tracking the reference current signal. The output of the PI controller is considered as peak value of the supply current (I max ), which is composed of two components: (a) fundamental active power component of load current, and (b) loss component of APF; to maintain the average capacitor voltage to a constant value. Peak value of the current (I max ) so obtained, is multiplied by the unit sine vectors in phase with the respective source voltages to obtain the reference compensating currents. These estimated reference currents (I sa *, I sb *, I sc *) and sensed actual currents (I sa, I sb, I sc ) are compared at a hysteresis band, which gives the error signal for the modulation technique. This error signal decides the operation of the converter switches. In this current control circuit configuration the source/supply currents I sabc are made to follow the sinusoidal reference current I abc, within a fixed hysteretic band. The width of hysteresis window determines the source current pattern, its harmonic spectrum and the switching frequency of the devices. The DC link capacitor voltage is kept constant throughout the operating range of the converter. In this scheme, each phase of the converter is controlled independently. To increase the current of a particular phase, the lower switch of the converter associated with that particular phase is turned on while to decrease the current the upper switch of the respective converter phase is turned on. With this one can realize, potential and feasibility of PI controller. The conventional hysteresis band current control scheme used for the control of active power filter line current is shown in Fig.2 Fig. 2: Control scheme for APF III.1 Hysteresis current controller The schematic diagram of hysteresis controller is shown in Fig.3. The error between reference current I f * and real compensating current I f is the input to hysteresis comparator. And then the PWM signals are generated by the hysteresis comparator. The semiconductors of SAPF are controller by those PWM signals and then the control of compensating current I f is realized. Fig. 2: Block diagram of periodical sampling constant Hysteresis band

4 Defining HB as the width of hysteresis comparator, when I f < HB the output of hysteresis comparator will remain invariable, and when I f HB the output of hysteresis comparator will reverse. Then the direction of I f will change. Based on the analysis mentioned above, the I f will change between HB and HB, and I f will change between I f * - HB and I f * + HB with sawtooth-shape. Fig.4 shows the tracking process of compensating current. Fig. 4: The upper and lower bands of the reference compensation current From Fig. 4, the below relations can be obtained: V t V t (1) V t V t (2) Where and are the rising current and the falling current, respectively. Furthermore, the following relations can be form: t t f t 2 (3) t 2 (4) (5) Where t 1 and t 2 are switching intervals and f is the switching frequency. By substituting (1), (2) and (5) in (3) and (4), the hysteresis band (HB) can be achieved as follow: (6) III.2 DC Voltage controller When the SAPF is compensating the harmonic and reactive power components, the dc capacitor voltage V dc varies. Hence V dc is also sensed and regulated at a reference value in order to establish a self-sufficient energy at the dc bus. The regulation loop consists of the comparison of the measured voltage with the reference voltage, admitting that the function of the system to be controlled is given by [14]-[15]: (7)

5 The closed loop transfer function using a PI regulator is given by: The development of this equation gives: / / / / (8) (9) The development of this equation gives: (1) A second order characteristic equation of the closed loop system is deduced: Where: 2 (11), ( 1) (12) From (12) the proportional and integrator coefficient K p, K i of the controller can be deduced:, (1) (13) The expression of the current I sc to compensate the inverter losses and maintain the constant dc-link voltage is given by: Δ Δ (14) To obtain optimal dynamic performance for the system, the value of the damping ration ξ must be equal a.77. IV. System modeling and simulation To simulate the proposed control strategy of the SAPF, a model is developed in Matlab/Simulink environment using SimPower Systems Blockset. Fig.5 depicts the test bench to estimate the performance of the SAPF with proposed control scheme. The complete SAPF system is composed mainly of a three-phase source, a nonlinear load which is a three phase rectifier feeding an inductive load, a PWM voltage source inverter, and a control bloc.

6 I f (A) V dc (V) I S (A) V S (A) Fig. 6: Source voltage, Source current, DC side capacitor voltage and filter current waveforms. Filter switched on at.5s 1 Fig.5: Main block of proposed control scheme with SAPF under MATLAB First simulation is carried out with a fixed load and the SAPF is switched on at t=.5s. Fig.6 shows the source voltage V sa (V), source current i sa (A), DC side capacitor voltage V dc (V) and filter current i fa (A). The instant the filter is switched on the source current becomes sinusoidal from the stepped wave shape, the DC capacitor voltage achieve quickly the reference value V* dc (142V) after practically only one cycle (about 1ms). In Fig.7 one can see that the active power P s (W) joined its nominal value and that reactive energy Q s (VAR) becomes null when the active filter is activated at this moment P s (A)

7 Q s (A) Fig. 7: Active and reactive powers source. Filter switched on at.5s Fig.8 shows the source current spectrum analysis before and after filtering. Before filtering; one can see the current harmonics distortion value was 23. 7%= THD i and after filtering it will be 3.59% =THD i, which proves that the proposed SAPF control strategy has the capability of compensating for current harmonics successfully (a) I s (A) Fundamental (5Hz) = 3.113, THD= 23.7% (b) I s (A) Fundamental (5Hz) = 3.111, THD= 3.59% Mag (% of Fundamental) Mag (% of Fundamental) Harmonic order Harmonic order Fig. 8: Source current and its spectrum: (a) before filtering (b) after filtering V. Experimental validation The experimentation of this work is done using the test bench which was developed in LAS Laboratory, University of Setif.1 (Fig.9). Fig. 9: Photography of the APF system prototype

8 The input step-down transformer (12KVA, 38/22 V) is connected to the mains. The three phase parallel active filter is achieved with a voltage source inverter of 2 KVA. This VSI contains a three phase IGBT 12 V, 5 A (SKM 5 GB 123D). To ensure the insulation and the dead time of control signals a developed card based on the IXDP63 component and a special driver circuit (SEMIKRON, SKHI 22) are used. The control strategy is implemented using a dspace card DS114 developed under Matlab/SimulinkTM RTW environment. The sampling time using in practical tests for the proposed systems is 45 µs. In this control type, switching frequency is variable, although in our design this frequency is limited to 2 khz to not reach the IGBTs maximum switching frequency. The same experimental test bench parameters are used for simulation: V s = 5V (RMS), R s =.1Ω, L s =.1mH, R c =.8Ω, L c = 2mH, R f =.1Ω, L f = 1mH, R L = 3 Ω, L L = 5mH, V * dc= 142V, C = 11µF, HB = (hysteresis band) =.1, K i = and K p =.15 Vdc (V) I f (A) I Sa (A) I ca (A) Fig. 1: Steady state response of the SAPF with a current-source type of nonlinear load (a) (b) Fig. 11: Source current spectrum and vector diagram: (a) before filtering (b) after filtering

9 Fig. 1 shows steady state response of the SAPF for harmonic elimination with a current source type of nonlinear load. In this figure, all the quantities are shown for phase a and top to bottom waveforms are the load current (i ca ), supply current (i sa ), active filter current (i fa ) and DC side capacitor voltage (V dc ). These waveforms show the capability of the proposed SAPF to compensate harmonic currents of the load, the DC-bus voltage of the SAPF is regulated at its reference value (142v). The frequency analysis of supply current before and after compensation in phase a are shown in Fig. 11 a b. The SAPF is able to reduce harmonics in the supply currents from 26.4% before compensation to 4.6% after compensation. The THD of the supply voltage is 3%. VI. Conclusion A shunt active power filter based on a hysteresis current control algorithm has been studied in this paper to determine the reference current for the SAPF in order to improve the power quality and compensate reactive power required by nonlinear load. With the advanced control system designed in this paper the proposed SAPF can attenuate harmonics well and has a good dynamic performance, various simulation and experimental results of the proposed control algorithm are presented to confirm his validity and effectiveness. The THD i of the supply current after compensation is 4.6% which is less than 5%; the harmonic limit. Acknowledgements This work was supported by the 213 Research Fund of University of Setif-1. The authors would like to express their sincere gratitude. References [1] C.Dugan R. McGranaghan Mark F. Santoso S. and Wayne Beaty H.: Electrical Power System Quality, McGraw Hill [2] Kazmierkowsi M. Malesani L.: Current control techniques for three phase voltage source PWM converters, IEEE Trans on Industrial Electronics Vol.45 no5, pp [3] Singh B. Al Haddad K. and Chandra A.: A Review of active filters for power quality improvement, IEEE Trans on Industrial Electronics Vol.46 no 5 pp [4] Sato Y. Kawase T. Akiyama M. and Kataoka T.: A control strategy for general purpose active filters based on voltage detection, IEEE Trans. Ind. Appl. Vol. 36 no 5, pp [5] EL-Khoy E.E. EL-Sabbe A. El-Hefnawy A. and M.Mharous H.: Three phase active power filter based on current controlled voltage source inverter, Electrical Power and Energy Systems Vol.28, pp [6] Agelidis V. Calais M.: Application specific harmonic performance evaluation of multicarrier PWM techniques, IEEEPESC 98 Conference Record, pp [7] Carrara G. Gardelta S. Marchesoni M.: A new multilevel PWM method: theoretical analysis, IEEE Trans. On power electronics Vol. 7 no 3, pp [8] Jeong S.-Gi and Woo M.-Ho: DSP-based active power filter with predictive current control, IEEE Trans. on Industrial Electronics Vo1.44 no 3, pp [9] Buso S. Malesani L. Mattavelli P.: Comparison of current control techniques for active power filter applications, IEEE Transactions on Industrial Electronics Vol.45 no5, pp [1] Chaoui A. Gaubert J. P. Krim F.: Power quality improvement using DPC controlled three-phase shunt active filter, Electric Power Systems Research Vol.8, pp [11] Rahmani S. Al-Haddad K. Youssef Kanaan H.: A comparative study of shunt hybrid and shunt active power filters for single-phase applications: Simulation and experimental validation, Mathematics and Computers in Simulation Vol.71, pp [12] Labben-Ben Braiek M. Fnaiech F. Al-Haddad K. and Yacoubi L.: Study of two current control techniques applied to a shunt active power filter: Power quality and active filtering III, ELECTRIMACS 22, pp 1-6

10 [13] Hu J. Zhu Z. Q.: Investigation on switching patterns of direct power control strategies for grid-connected DC AC converters based on power variation rates, IEEE Transactions on Power Electronics Vol. 26 no1, pp [14] Chaoui A. Krim F. Gaubert J.P. Rambault L.: DPC controlled three-phase active filter for power quality improvement, Elsevier, Electrical Power and Energy System Vol3, pp [15] Chennai S. Benchouia M-T. Goléa A. Zouzou S.E. :Fuzzy logic current controller for shunt active filter to compensate harmonic currents based on ANN dc voltage regulator, International conference on electrical engineering, electronics and automatics 21,Bejaia, Algeria,2-3 November 21