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National Power Electronics Conference (NPEC-10) June 10-13, 2010. Three phase shunt active power filter with unipolar PWM technique to compensate reactive power and low order harmonics 1 J.Vikramarajan, 2Rasmi ranjan das and 3Razia sulthana. 1 PG Student, School of Electrical Engineering, VIT University, Vellore, India. 2 Assistant Professor (senior), School of Electrical Engineering, VIT University, Vellore, India. 3 Assistant Professor, School of Electrical Engineering, VIT University, Vellore, India. Abstract: Active power filter continues to economic concerns. Non-linear loads result in attract considerable attention. Because of voltage and current harmonics in distribution sensitivity of consumers on power quality and network. advancement in power electronics. Active Harmonics provide main problems in network power filter technology is the most efficient like power losses and excess heat. Thus, way to compensate reactive power and harmonic limitation seems to be vital. cancel out low order harmonics generated by Nowadays, active power filters (APF) play nonlinear loads. An active power filter is a effective role in distortion recognition and device that is connected in parallel to and elimination. These filters are classified with cancels the reactive and harmonic currents respect to distortion determination strategy, from the group of nonlinear loads so that the inverter control techniques, inverter resulting total current drawn from the ac main topologies and their connection types to the is sinusoidal. The shunt active power filter grid. Shunt filters are connected in parallel was consider to be the most basic with distribution networks. They recognize configuration for the APF.This project current distortions by sampling the line presents Three phase shunt active power current and compensate distorted current filter with unipolar pulse width modulation components to maintain sinusoidal source technique to make the total input current current. In most recommended grids, the drawn from the three phase AC main as reference signal can be extracted by sinusoidal, by compensating reactive power sampling and analyzing the input voltages and low order harmonics generated by the and load currents for harmonic and reactive three phase nonlinear loads. The power calculations. These control methods synchronous d-q-o reference frame algorithm require rapid and real time calculating blocks is also used to recognize and extract like high-speed digital microprocessors and harmonic distortions. The validity of proposed capable A/Ds, so cost increment, circuit method and achievement of desired complexity and system stability reduction compensation are shown by simulation would be inevitable. results. Current control technique based on unipolar Keywords: Three phase shunt Active power PWM provides better stability and filter, Unipolar PWM technique. compensation with loss reduction; efficiency increment and dc offset elimination compare I.INTRODUCTION to bipolar PWM technique. This paper presents a hysteresis current control Increasing demand of electric and electronic technique based on unipolar Pulse Width instruments including non-linear loads (e.g. Modulation (PWM) with time and magnitude computers, power electronic inverters and errors control to reduce switching losses and induction furnace) necessitates investigations to improve the quality of output current. about stability maintenance, continuous energy serving and power quality considering An active power filter is a device that is connected in parallel to and cancels the 1 National Power Electronics Conference (NPEC-10) June 10-13, 2010. reactive and harmonic currents from a group analyze the voltage at the point of installation of nonlinear loads so that the resulting total and is controlled as to present infinite current drawn from the ac main is sinusoidal. impedance for the fundamental frequencies A device which converts the pulsating output and a low impedance for the harmonic of a rectifier into steady state dc level is frequencies. In order to perform other known as Filter. This filters out the functions, as reactive power compensation, undesirable ac component and allows only dc Flicker/imbalance compensation etc., the component to reach the load. overall system must be equipped with other feedback or feed forward control loops. Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate, that combine with the fundamental voltage or current, and produce waveform distortion. It is caused by non- linear loads. Lower order harmonic is defined as the harmonic component whose frequency is closest to fundamental one and its amplitude is greater than or equal to 3% of Fig.1: Block diagram of the system the fundamental component. II.SHUNT ACTIVE POWER FILTER A shunt active filter is designed to be connected in parallel with the load. It detects the harmonic current of load and injects into the system a compensating current, identical with the load harmonic current but in opposite phase. Therefore, the net current drawn from the distribution network at the point of coupling of filter and the load will be a sinusoidal current of only fundamental Fig.3: Block diagram of a simple power frequency. Fig 2 shows the principle of system with APF OFF operation and the connection scheme of a shunt active filter. Fig.4: Block diagram of a simple power system with APF ON The above diagrams describe the function of Fig.2: shunt active filter Active Power Filter in a system. When the Harmonic current and/or negative sequence APF is OFF, the input current is distorted. current compensation and dc link voltage When the APF is ON, the input current is regulation between both active filters. The sinusoidal and the distortions are reduced, purpose of the shunt active filter is mainly to APF will inject the harmonic currents 180 compensate current harmonics generated degree out of phase in to the Point of from the distributed lines. Thus, it has to Common Coupling (PCC) as shown in Fig.1. 2 National Power Electronics Conference (NPEC-10) June 10-13, 2010. III.DISTORTION DETECTION BY considered in this paper. Finally, REFERENCE FRAME D-Q-O compensated currents are determined by adverse park application on d and q axis to Several methods are used to recognize and be injected to the network after tracing and extract harmonic distortions which are reconstruction as shown in fig.1. classified as frequency, time and time- frequency approaches. Fast Fourier Transformer (FFT)[3] and adaptive neural network [2] in frequency domain, synchronous reference frame theory d-q-o (SRF) [4] and instantaneous active and reactive power theory (pqr)[5] in time domain and the other methods such as small wave technique and one-cycle control or separation with suitable digital or analogue filters[6] have iL = ig + iapf (6) wide applications. In this approach, reference frame algorithm Conduction and switching losses of diodes is used due to simplicity in calculation and and IGBTs in inverters increase voltage ripple implementation. Having measured three- in DC-link which affects the performance of phase currents in a-b-c orientation, the filter. These effects are controlled by a transformed to d-q-o by park equation: feedback loop where PI regulator compares the DC-link voltage with a reference voltage to extract d component of current. Reference frame rotates synchronous with fundamental currents. Therefore, time variant currents with fundamental frequencies would be constant after transformation. However, harmonics with different speeds remain time Fig.5: Synchronous d-q-o reference frame variant in this frame. Thus, currents would be based compensation algorithm. separate simultaneously to DC and AC parts. IV.HYSTERESIS BAND CURRENT i d = id + i%d (3 ) CONTROL APPROACH i q = iq + i%q (4 ) Reference currents are generated by DC to AC converters using a current control technique such as a hysteresis control. The hysteresis band is used to control load AC part of d axis and whole current in q axis currents and determine switching signals for are used for harmonics elimination and VAR inverters gates. Suitable stability, fast compensation. Zero current is produced due response, high accuracy, simple operation, to a three-phase voltage imbalance or inherent current peak limitation and load waveform distortions which have not been parameters variation independency make the 3 National Power Electronics Conference (NPEC-10) June 10-13, 2010. hysteresis current control as one of the best A.Unipolar control current control methods of voltage source inverters. In this approach the current error, There are four switching states in this (difference between the reference and approach. In unipolar control, when Vg>0 inverter currents) is controlled in hypothetical (reference current in ascendant slope control band surrounding reference current. (dic/dt>0), S3 is on in the reference current half cycle, S1 and S2 turn on and off e = i*c - iapf ( actual ) (7) periodically in switching cycles. Thus V0 and When the load current exceeds the upper zero voltages produce between P and N so band, the comparator output activated so the error signal slope is negative in hysteresis output voltage is changed in such a way to band and periodically decreases and decrease the load current and keep it increase. Originated current variations in between the bands and deactivated at lower reference current half cycle is as follows: limit. Switching frequency varies with respect dic 1 to distance between upper and lower band. = ( vg + v0 )First half cycle of switching (9) The other parameters like inverter-network dt L inductance and DC link voltage affect dic 1 = ( vg ) Second half cycle of switching (10) significantly on the switching frequency. dt L Adaptive band definition considered as As the output voltages have three levels, +V0, proper solution in this problem to fix the -V0 and zero, the two switching states (similar switching frequency [7]. The highest to bipolar) cannot control the load current switching frequency is as follows [8]: sufficiently. In this case more bands are vdc required to achieve different switching states f sw (max) = (8) 9 HB.L corresponding to different output voltages. As Where, HB and L are hysteresis band and shown in Fig.5, the reference current has a load inductance, respectively. positive dic/dt from 0- 90° and the load current can follow the reference current V.UNIPOLAR PULSE WIDTH based on two voltage levels, +V0 and zero MODULATION CONTROL volts. But when the reference current has a negative dic/dt, (90- 270°), the output voltage Fig.6 shows a full bridge Three-phase of inverter has to be changed in such a case inverter schematic that is connected in to generate negative dic/dt for the load parallel to a non-linear load. The inverter can current, thus more band are required to be controlled in unipolar PWM method. change the voltage level from +V0&0 to –V0&0. Current variation in the second half cycle of the reference current is: dic 1 = (- vg - v0 ) First half cycle of switching (11) dt L dic 1 = (- vg ) Second half cycle of switching (12) dt L VPN has unit polarity in each half cycle of the reference current, so this approach called unipolar control. Fig.6: Typical shunt APF connected in ( ) = - vg - - vg - v0 = v0 (13) parallel with nonlinear load. 4 National Power Electronics Conference (NPEC-10) June 10-13, 2010. It is shown that the current control based on zero to –Vo at t5. In this case, when the slop unipolar PWM has a low switching losses or of reference current is close to the slop of better performance compare to the bipolar inverter output current, then the time error PWM due to less number of switching control improves the quality of the APF and transient per switching cycle. pushes the inverter current into the main area. Fig.7: Current and voltage waves with hysteresis band control (for APF under Unipolar control mode) VI.UNIPOLAR BASED ON TIME AND MAGNITUDE ERRORS CONTROL In hysteresis current control based on unipolar PWM, there are two upper bands and lower bands in order to change the slop of inverter output current based on there level voltages, +Vo, 0 and -Vo. The idea is to keep the current within the main area but the Fig.8: (a) noisy load current with upper second upper and lower bands are to change and lower bands (b) unipolar current the voltage level in order to increase or control based on time and magnitude decrease the dic/dt of inverter output current. error control. As shown in Fig.8.(a) Al cannot be very small For magnitude error control, two switches in as the noisy signal changes the switching inverter legs are controlled with a main band time due to instantaneous comparison and two other ones by the second band in a between the load and the reference currents unipolar PWM. Control signals originated by and increases the switching losses; and it the unipolar command circuit based on the cannot be big as the total harmonic distortion hysteresis current control are shown in Fig.9. may be increased. In APF, load current has several different slops within one cycle and to have a fast current tracking, the control algorithm in unipolar current control has been defined based on magnitude and time errors control as shown in Fig.8.(b). In this case, the second upper or lower band values can be big enough in order to remove the noise issue of the inverter output current but the second decision to change the level is based on time error. For example, when the load Fig.9: Proposed unipolar control circuit current exceeds the first upper band at t4, the In the first positive half cycle (Vg>0), S3 is output voltage of inverter is change from +Vo permanently on. Inductance voltage to 0. The controller waits for At, if the inverter calculated as follows: output current does not cross the second upper band within this period, then the vL (ON )= vg 0 < t < dTs controller changes the output voltage from 5 National Power Electronics Conference (NPEC-10) June 10-13, 2010. s1 : OFF , S 2 : ON (14) Fig.10,Fig.11,Fig.12 respectively.They are as follows; VL (OFF ) = Vg - v0 dTs < t < Ts s1 : ON , s2 : OFF (15) For constant-frequency operation and quasi- steady-state analysis, the average inductor voltage is approximately balanced during each switching cycle, that is: vL (ON ).dTs + vL (OFF ).(1- d )= 0 (16) Substituting (18) and (19) in (20): v g .d + (v g - v0 )( - d )= 0 When vg > 0 1 (17) v g .d + (v g + v0 )( - d )= 0 When vg < 0 1 (18) Combining above equations provide relationship between duty ratios of switches and input ac voltage as well as dc bus voltage of APF inverter, that is vge = v0 ( - d ) 1 (19) vge = vg vg > 0 (20) vge = - vg vg < 0 VII.SIMULATION RESULTS Simulations are performed using Matlab and Simulink platform for a non-linear load (a full bridge diode rectifier with RL-load) and a three-phase voltage source inverter produce and cancel the distorted currents as an APF with the specifications shown in table 1. TABLE I According to simulation results.the load current, inverter injected current and compensated supply current are shown in 6 National Power Electronics Conference (NPEC-10) June 10-13, 2010. VIII.CONCLUSION [7] K. Murat, O. Engin, “An adaptive hysteresis band current controller for shunt As we all know that nowadays the active active power filter,” Elsevier Electric Power power filter technology is the most efficient Systems Research 73(2005) pp. 113-119. way to compensate reactive power and low [8] M. Rukonuzzaman, and M. Nakaoka, order harmonics, which has been generated “Single-phase shunt active power filter with due to nonlinear loads of the systems. So harmonic detection,” IEEE proc. Electr. finally synchronous reference frame theory Power Appl. Vol. 149, n0.5, September 2002. for distortion detection and three phase shunt active power filter with unipolar pulse with [9] Moleykutty George, 2004. Modeling and modulation technique to compensate reactive simulation of a current controlled three-phase power and low order harmonics which has shunt active power filter using MATLAB/PSB: been generated by the three phase nonlinear AIUB Journal of Science and Engineering, 3: loads of the system has been successfully 11-18. implemented. So that the total current drawn [10] Janko Nastran, Rafael Cajhen, Matija from the three phase ac main is sinusoidal. Seliger and Peter Jereb, 1994. Active Power Thus problems like power losses and Excess Filter for Nonlinear AC Loads: IEEE Trans. heat caused by the harmonics in network has Power Electronics, 9: 92-96. been controlled and solved. [11] E. Destobbeleer and L. Protin, 1996. IX. REFERENCES On the Detection of Load Active Currents for Active Filter Control: IEEE Trans. Power [1] Ambrish Chandra, Bhim Singh, B.N. Electronics,11: 768-775. Singh and Kamal Al-Haddad, 2000. 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