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ISSN 2319-8885 Vol.03,Issue.41 November-2014, Pages:8401-8405 www.ijsetr.com Closed Loop Current Control Strategy for Active Power Filter Using Resonant Controller K. SABITHA1, E. NAGA PRABHA2 1 PG Scholar, K V Subbareddy College of Engineering for Womens, Kurnool, AP, India, E-mail: [email protected] Asst Prof, K V Subbareddy College of Engineering for Womens, Kurnool, AP, India, E-mail: [email protected] 2 Abstract: Novel simple indirect control concepts for an active power filter (APF) application are proposed here. The concepts are exemplarily presented to control a modified APF structure. The main advantage over other control strategies is the achieved excellent simplicity-to-performance ratio. The proposed control strategies are based on the concept of virtual impedance emulation to provide high power factor in a system. To validate the operating principle, a single-phase low-power prototype has been built and experimentally tested. This prototype operates at a remarkably low switching frequency of 5 kHz and is digitally controlled by a digital signal processor. Keywords: Active Power Filters (APF), DC–AC Converters, Implicit Control, Sensor Less Control Techniques. I. INTRODUCTION Power quality has become a major research topic in power distribution systems due to a significant increase of harmonic pollution caused by proliferation of nonlinear loads such as diode rectifiers, switching power supplied and other types of line connected power converters etc. The shunt APF is recognized as a cost effective solution for harmonic compensation in low and medium power systems. It composed of a PWM voltage source inverter, with a large dc link capacitor, and connected to the line by means of an inductor [2]. Now a day most of the loads are nonlinear due to usage of power electronic devices like semiconductor devices used in rectifiers and inverters, switching power supply and other power electronic converters etc... To overcome above problems shunt APF is recognized as cost effective solution for compensating harmonics in low and medium power applications. In general, PI controllers are played major role for controlling shunt APF. For three phase Systems synchronous frame PI controllers can be used but it requires computational burden in case of multiple frame transformations. To overcome the above problem Proportional resonant controller is used. The frequency response characteristic of a PR controller is similar to that of a PI controller. PR controllers are used for reference tracking in the stationary reference frame. The basic functionality of a PR controller is to introduce infinite gain at a selected resonant frequency for eliminating steady state error at that frequency. Basically, an integrator whose DC gain forces the DC steady state error to zero in the same way resonant portion of the PR controller whose AC gain (GI) forces the AC steady error to zero. All the advantages will give importance to PR controller. Under unbalanced load the fundamental negative sequence component appears in load current. It can result in the fundamental negative sequence component in the APF AC currents and APF AC voltages. The interaction of negative and positive sequence component of switching functions and ac currents of the APF produces a 2nd order harmonic ripple on the DC link of the APF, which will generate the 3rd order harmonic distortion in AC currents of the APF and line currents [3]. In addition, under a nonlinear load ac current of the APF contains high harmonics due to load current harmonics. The interaction of fundamental negative and positive sequence component of switching functions and high harmonics AC currents of APF produces high order even harmonics on the DC link voltage of the APF, which create high order odd harmonics in the APF AC currents such harmonics flow into the line, thereby lead to worsen the performance of the system. Moreover, the high harmonic voltage across capacitors in dc side results in high temperature due to their ESR. This leads to reduce their lifecycle of the capacitors. Some methods are proposed in the papers [3]-[5] to improve performance of the system. One method is to eliminate the 3rd harmonic of the APF ac current, thus to reduce the distortion of the line current. With this method only 3rd harmonic of APF current can be cancelled, the others high harmonics cannot be reduced. Moreover the even harmonics on the DC link side still exist. In [3] a DC link voltage control method is proposed. Although this control strategy can almost cancel the DC link ripple, the controller is complex. In order to avoid the disadvantages of the above control method and to eliminate a series harmonics at both DC side and AC side of the APF, PR control method is proposed. This method is implemented with a simple controller. Copyright @ 2014 IJSETR. All rights reserved. K.SABITHA, E.NAGA PRABHA II. DESIGN OF ACTIVE POWER FILTER A. Active Power Filter In [9] an inverter operates as active inductor at a certain frequency to absorb the harmonic current. But the exact calculation of network inductance in real-time is difficult and may deteriorate the control performance. The shunt active power filter acts as active conductance to damp out the harmonics in distribution network [11].The study of the Shunt APF can be classified into three categories; Harmonic Detection: Generally Shunt APF is connected across the load for compensation purpose. But now a day nonlinear loads are increased due to the usage of power electronic devices. These loads will produce higher order harmonics into the line current the harmonic detection method to calculate the reference currents of the shunt active power filter. Structure of Shunt APF: The Voltage source Inverter with six IGBTs is used for the Shunt APF. Switching pulses of six IGBTs can be generated by the PWM / Hysteresis controller. PWM / Hysteresis Control: This controller can be used to control the compensating currents. There are many techniques to control the compensating currents such as the Hysteresis current control, the Pulse width modulation control. B. Objective of Harmonic Compensation Objectives of harmonic compensation are as follows; Eliminate real power oscillations Power factor improvement Eliminate current harmonics Provide harmonic damping These objectives can be done by configure the Active Power Filter. C. Active Power Filter Configuration AF’s can be classified based on converter type, topology, and the number of phases. The converter type can be either CSI or VSI bridge structure. The topology can be shunt, series, or a combination of both. The third classification is based on the number of phases, such as two-wire (single phase) and three- or four-wire three-phase systems [8]. Coming to the unbalanced load issue are shown in Fig.1, the presence of fundamental negative sequence component in the load currents can affect the APF system performance. This negative sequence, if not blocked by the APF current reference generator, will produce a fundamental negative sequence component in the APF currents due to the proportional gain of the current controller. That will produce a 2nd order ripple in the DC-link voltage, which will generate harmonic distortion in the source currents. Therefore, if the unbalanced load compensation is not required, the circulation of the fundamental negative sequence currents in the APF must be completely blocked to avoid distortion of the source currents [3]. Fig.1. Basic current harmonic compression scheme of an unbalanced load using a shunt APF [3]. Voltage and current harmonics has become a serious problem in transmission and distribution systems in recent years. To solve the current harmonic related problems, passive filters connected in several circuit configurations present a low cost solution. However, passive filter implementations to filter out the current harmonics have the following disadvantages: Possibility of resonances with the source impedance Supply impedance dependent system performance Fixed compensation In order to diminish the preceding disadvantages of the passive filters, active power filters (APF) have been worked on and developed in recent years. Elimination of the current harmonics, reactive power compensation and voltage regulation are the main functions of active filters for the improvement of power quality is shown in Fig.2. There exist several active power filter topologies in the literature in accordance with their circuit configurations and connection types [2].Among these configurations, conventional parallel voltage source active power filter is widely used. The DC link capacitor voltage is required to be higher than the peak value of the utility voltage; otherwise, the generated compensation currents cannot be injected to the mains [3]. So, for high power applications, the required high DC link voltage restricts the active power filter implementations due to the increase in the losses, rating and the total cost of the APF. As a result, various hybrid filter topologies have been developed which combine the advantages of both the passive and the active filters [4]. Within these topologies, shunt hybrid filters formed with the use of a three phase voltage source PWM inverter and a series connected LC passive filter are superior to the conventional shunt APFs due to the reduced DC link voltage and the converter rating. The series connected LC filter absorbs the current harmonics arising from the non-linear load; however, the filtering characteristic of just the passive filter itself is not International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.41, November-2014, Pages: 8401-8405 Closed Loop Current Control Strategy for Active Power Filter Using Resonant Controller satisfactory. Hence, active filter is used to improve the active filters and shunt active filters are mainly used for current related compensations like reactive power, current filtering performance of the overall system. unbalance, etc,. In this paper, a hybrid filter formed by a low-rated APF and a LC passive filter tuned to 350 Hz is presented. The proposed filter ensures a low DC link voltage and a superior filtering performance by the applied feedback and feed forward control methods. In addition, no switching ripple filter is used since the LC filter also operates as a switching ripple filters at high frequency. The startup procedure used in the laboratory and the experimental results obtained from a 300V laboratory prototype are also presented to show the effectiveness of the overall system. Fig.3. Series-Shunt Active Filters. Fig.4. Voltage Source Inverters. Fig.2. (a) Voltage Source APF (b) Hybrid Power Filter. III. TOPOLOGY OF ACTIVE POWER FILTER Active Power filters are basically classified in to three types: Single phase, three phase three wire and three phase four wire systems to meet the requirements of the nonlinear loads in the distribution systems. Single phase loads, such as domestic lights, TVs, air conditioners, and laser printers behave as nonlinear loads and cause harmonics in the power system. Many configurations, such as active shunt filter as shown in Fig 2(a), the active series filter as shown in Fig.2 (b) , and combination of shunt and series filter as shown in Fig.3 has been developed. This topology has been called the Unified Power Quality conditioner. The above mentioned APLC’s either based on a Voltage source inverter (VSI) with capacitive energy storage Fig.4 or Current source inverter (CSI) Fig.5 with inductive energy storage devices. Both voltage source inverters and current source inverters are used to compensate voltage and current harmonics under all the three categories. They are also used for the compensation of reactive power, unbalanced current and voltage, neutral current voltage spikes, voltage flicker and for regulation. The voltage related compensations (voltage unbalance, voltage flicker, voltage regulation, etc) are carried out using series Fig.5. Current Source Inverters. A. Controlling System The main component in the APF is the control unit. Controlling of APF is implemented in three stages: First stage can be called as the signal conditioning stage. The essential voltage and current signals could be sensed using power transformers, Hall Effect sensors and isolation amplifiers to gather accurate system information. The instantaneous voltage and current signals are useful to monitor, measure and record International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.41, November-2014, Pages: 8401-8405 K.SABITHA, E.NAGA PRABHA various performance indexes such as Total Harmonic Distortion(THD), Power factor, active and reactive power, crest factor, etc,. Second stage is the derivation of compensation signals stage. In this stage compensating commands in terms of current or voltage levels are derived based on control methods and APF configurations. Compensations can be done either in time domain or frequency domain. Compensation in frequency domain is based on Fourier analysis of distorted signal. In time domain a number of control strategies such as instantaneous reactive power theory initially synchronous frame synchronous detection method notch filter and fuzzy logic controller method, sliding mode controller, etc. Are used in the development of three-phase AFs .Out of these theories, Fig.7. more than 60% research works consider using p-q theory and d-q theory due to their accuracy, robustness and easy calculation. The third stage is the generation of gating signals to the device of Active filters. The main component of APF is the solid state devices. Earlier BJTs and MOSFETs were used. Now-a-days IGBTs are used for medium ratings and GTOs are used for high ratings. The gating pulses are generated by current control technique like sinusoidal pulse width modulation (SPWM), triangular PWM, hysteresis current control technique, Space Vector current controller. Advancement in Microelectronics has motivated new directions for APF design starting from the use of analog and digital components to microprocessors, microcontrollers, digital signal processors (DSP’s) and FPGA implementation. The analog controllers have some disadvantages such as high cost, slow response, large size etc., during real-time implementation. FPGA is given the highest priority as compared to DSP because of its low cost, faster response, and application oriented selection of the bit width for the data resister. Fig.8. IV. SUMULATION RESULTS Fig.9. Fig.6. V. CONCLUSION Novel control strategies for modified shunt APF architecture have been proposed here. The proposed strategies were able to guarantee close-to-unity power factor. Both current and voltage sensor less versions have been presented, where the voltage sensor less control has been International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.41, November-2014, Pages: 8401-8405 Closed Loop Current Control Strategy for Active Power Filter Using Resonant Controller analyzed in detail regarding its performance, equivalent [10] M. Salo, ―AC current sensor less control of the currentcircuits, and stability. The main feature of the strategies is source active power filter,‖ in Proc. IEEE 36th PESC, Jun. their extreme simplicity since no complex current reference 2005, pp. 2603–2608. computations are required. The implicit control loop is [11] D. Wojciechowski, ―Sensorless predictive control of another innovative characteristic. In this sense, it was proven three-phase parallel active filter,‖ in Proc. AFRICON, Sep. that the proposed strategies are equivalent to explicit current 2007, pp. 1–7. control strategies employing a resonant-type controller. This [12] G.-Y. Jeong, T.-J. Park, and B.-H. Kwon, ―Line-voltageexplains the excellent sinusoidal tracking performance. In sensor less active power filter for reactive power addition to the resistive behavior, the APF emulates virtual compensation,‖ Proc. Inst. Elect. Eng.—Elect. Power Appl., capacitance and/or negative inductance. Thus, zero mainsvol. 147, no. 5, pp. 385–390, Sep. 2000. side current phase displacement is achieved. These characteristics were verified through circuit simulation and in an experimental setup including a nonlinear rectifier load, with the APF being switched at 5 kHz. This is a very low switching and highlights the achievable performance. The concepts can be easily extended to the control of PFC rectifiers and three-phase systems. VI. REFERENCES [1] Mauricio Angulo, Domingo A. 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International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.41, November-2014, Pages: 8401-8405