CURRENT HARMONICS REDUCTION IN 3 PHASES 4 WIRE SYSTEM USING HYBRID FILTERS R.Saravanakumar 1#, S.Amritha 2#

Transcription

1 CURRENT HARMONICS REDUCTION IN 3 PHASES 4 WIRE SYSTEM USING HYBRID FILTERS R.Saravanakumar 1#, S.Amritha 2# 1 rjsaravanakumar@yahoo.co.in 2 amritha2507@gmail.com # Department of Electrical and Electronics Engineering, Anand Institute of Higher Technology, Chennai Abstract - In this paper a topologies of current controlledvoltage source inverter (CCVSI) based activepower filter using direct current control technique for compensating unbalanced loads in a three-phase fourwire system is proposed. In this method the harmonic compensation current reference is generated without introducing any additional harmonic extraction filtering circuits. A control approach for balancing the DC voltage of the hybrid active power filter is incorporated to cover the system losses. The proposed Hybrid shunt active power filter compensates harmonics and reactive power in all three phases as well as the neutral current. To regulate and balance the dc capacitor voltage, a current control method using hysteresis controller is proposed. The simulation results based on MATLAB/ Simulink tool demonstrate the feasibility of the proposed topologies. The total harmonic distortion of source current has been calculated for proposed control methodology of hybrid shunt active power filter. I. INTRODUCTION A large number of solid state power converters such as diode bridge rectifiers and thyristor converters are used in industrial applications and transmission/distribution networks. All these breeds of power converters are nonlinear in nature and cause serious problems of current harmonics, poor power factor, non sinusoidal supply voltage, reactive power burden and low system efficiency. Hence, due to these serious issues there has been an increasing interest in the subject of power quality improvement techniques which can suppress supply harmonics, improve power factor and balance the input supply [1]. Many circuit configurations of filters have been suggested to limit harmonic current distortion. Passive filters which act as least impedance path to the tuned harmonic frequencies. We are used initially to reduce harmonics. This technique is simple and less expensive. But it has many drawbacks such as resonance, fixed compensation characteristics, bulky size, high no load losses etc. As a better option of complete compensation of distortions, active power filters [2, 3] have been researched and developed. Active filters overcome drawbacks of passive filter by using the switched mode power converter to perform complete harmonic current elimination. Shunt active power filters are developed to suppress the harmonic currents and reactive power compensation simultaneously by suitable control techniques to generate a compensating current in equal and opposite direction so that source current becomes harmonic free[2,4]. However, the power rating and construction cost of active power filters in a practical industry is too high. To avoid this limitation, hybrid filter topologies have been developed. Using low cost passive filters with the active filter, the power rating of active converter is reduced compared with that of pure active filters. This hybrid filter retains the advantages of active filters and passive filters. Also hybrid filters are cost effective and become more practical in industry applications [5-6]. In this paper, the hybrid filter 23

2 structure consisted of an active filter and a passive filter, connected in shunt is used for power quality improvement. The effectiveness of the hybrid filter configuration was verified with simulation and experimental results. The results prove that the proposed method can effectively eliminate harmonic currents, balance source currents, compensate reactive power i.e. in other words; power quality improvement of the power system is achieved by the proposed hybrid filter structure and control method. II. HARMONIC MITIGATION FILTERS Harmonic distortion in power distribution systems can be suppressed through three basic approaches namely: 1. Passive filter. 2. Active power filter. 3. Hybrid active power filter. This section discusses general properties of various approaches for harmonic distortion mitigation. The advantages, disadvantages, and limitations of these approaches are also compiled in this section combination of basic APFs and passive filters. Hybrid APFs, inheriting the advantages of both passive filters and APFs provide improved performance and costeffective solutions. There are various hybrid APFs reported in literature but the two most prominent ones are shown in Figure 1(a) is the system configuration of the hybrid shunt APF. Both the shunt APF and passive filter are connected in parallel with the nonlinear load. The function of the hybrid APF can thus divided into two parts: the low-order harmonics are cancelled by the shunt APF, while the higher frequency harmonics are filtered by the passive HPF. This topology lends itself to retrofit applications with the existing shunt APF. Figure 1 (b) shows the system configuration of hybrid series APF, in which the series APF is coupled to the distribution line by an interfacing transformer. The shunt passive filter consists of one or more single-tuned LC filters and/or a HPF. A. Hybrid Active Power Filter Previously, majority of the controllers developed for APF are based on analogue circuits. As a result, the APF performance is inherently subjected to signal drift. Digital controllers using DSPs or microcontrollers are preferable, primarily due to its flexibility and immunity to noise. However it is known that using digital methods, the high-order harmonics are not filtered effectively. This is due to the hardware limitation of sampling rate in real-time application. Moreover, the utilization of fast switching transistors (i.e. IGBT) in APF application causes switching frequency noise to appear in the compensated source current. This switching frequency noise requires additional filtering to prevent interference with other sensitive equipment. Technical limitations of conventional APFs mentioned above can be overcome with hybrid APF configurations. They are typically the Fig.1 Hybrid APFs: Combination of Shunt & series APF and Shunt Passive Filter B. Harmonic detection method The harmonic detection method has the task of detecting the harmonic currents/voltages that have to be compensated by the APF; hence, a reliable method must be used. Once the harmonics are detected, they are given as reference signals to the inner current/voltage controller, which has the task of producing an output signal identical to its reference. There are numerous published references that describe different topologies and different algorithms used for active filtering. Most of them present a single 24

3 type of harmonic detection method implemented in an APF which mitigates the existing harmonics according to the initial expectations. Therefore, it is difficult to assess if the overall performance of the APF is due to the selected harmonic detection method or due to another design choice. That explains and compares different harmonic detection methods, describing their advantages and drawbacks by giving as final indices the dynamics, the total harmonic distortion (THD) reduction, the inverter efficiency, or the cost of the entire active filter. Usually, the comparisons are made between different APF topologies; each being considered as a whole unit, where only the implemented harmonic detection method is stressed out and the rest of the control is left, however, the inner and outer loop controllers may have different performances depending on tuning, which influence also the harmonic compensation. Consequently, the comparison between different APFs that use different harmonic detection methods is not a direct effect of only the performance of the used harmonic detection block, but also of the quality of the existing controllers. The harmonic detection methods decoupled from the entire APF structure in order to reveal its contribution. At first, the work summarizes the theoretical background of several commonly used harmonic detection methods for APFs. The study separates the harmonic detection method from the APF control and studies its behaviour with respect to the detection accuracy of a specific harmonic component. The input signal from sensors is replaced with a prior known signal, artificially constructed, by summing different known characteristic harmonics to the fundamental frequency. Thus, the output obtained from each analyzed harmonic detection method is recorded and compared to the input harmonic signal. The results indicate that the choice of numerical filters is a key factor for obtaining good accuracies and dynamics. The harmonic detection method is the part of the APF s control that has the capability of determining specific attributes of the harmonics (frequency, amplitude, phase, time of occurrence, duration, energy) from an input signal (which can be voltage or current) by using a special mathematical algorithm. Then, with the achieved information, the inner controller is imposed to compensate for the existing harmonic distortion. The outer dc voltage control loop maintains the dc capacitor charged at the voltage reference (VDC * ) in order for the inverter to be able to operate against the line voltage. Thus, the dc voltage controller produces a * current reference (IDC ) that keeps the capacitor charged and covers the losses in the passive and active elements. The harmonic detection method receives at the input the current from the nonlinear load and extracts the harmonic content (IHarm*) according to the selected algorithm. Then, both currents are summed into a new reference (IF*), which is provided to the inner current controller. Errors given by the harmonic detection method degrade the overall performance of the APF. III. CONTROL TECHNIQUES AND CONTROL CIRCUIT The aim of APF control is to generate appropriate gating signals for the switching transistors based on the estimated compensation reference signals. The performance of an APF is affected significantly by the selection of control techniques. Therefore, the choice and implementation of the control technique is very important for the achievement of a satisfactory APF performance. A. Hysteresis Control Technique The control of APF can also be realised by the hysteresis control technique. It imposes a bang-bang type instantaneous control that forces the APF compensation current ( f i) or voltage ( f v ) signal to follow its estimated reference signal ( f ref i, or f ref v, ) 25

4 within a certain tolerance band. This control scheme is shown in a block diagram form in Figure 2. In this control scheme, a signal deviation (H) is designed and imposed on f ref i, or f ref v, to form the upper and lower limits of a hysteresis band. The f iorf v is then measured and compared with f ref i, or f ref v, ; the resulting error is subjected to a hysteresis controller to determine the gating signals when exceeds the upper or lower limits set by (estimated reference signal + 2H ) or (estimated reference signal - 2H ). As long as the error is within the hysteresis band, no switching action is taken. Switching occurs whenever the error hits the hysteresis band. The APF is therefore switched in such a way that the peak-to-peak compensation current/voltage signal is limited to a specified band determined by H as illustrated. IV. PROPOSED SYSTEM A. System Description Most commonly used three-phase four wire hybrid shunt active power filters are based on three phase converters [17]. The commercially used general topologies for implementing a four wire shunt active power filter is four leg topology as shown in fig 3The topology of switching devices having unidirectional voltage stress with a magnitude equal to the full dc side voltage V dc fourth leg providing a return path for the neutral current with a single capacitor. The PWM pattern is generated using a hysteresis band control technique. The ac side of the converter is connected to the PCC via filter inductor. Fig.2 Block Diagram of Hysteresis Control Technique In this particular work, a hysteresis current controller with a fixed H is implemented. To obtain a compensation current ( fi) with switching ripples as small as possible, the value of H can be reduced. However, doing so results in higher switching frequency. Thus, increases losses on the switching transistors. The advantages of using the hysteresis current controller are its excellent dynamic performance and controllability of the peak-to-peak current ripple within a specified hysteresis band [24], [25], [42]. Furthermore, the implementation of this control scheme is simple; this is evident from the controller structure shown in Figure However, this control scheme exhibits several unsatisfactory features. Fig.3 Four Wire Hybrid Shunt Active Power Filter Topologies B. DC-Link Voltage Since the active power filter compensates only the power ripples of the nonlinear load, a capacitor can be used to replace a real DC source. The voltage across the dc capacitor is controlled by adjusting the small amountof real power absorbed by the inverter in order to ensure a correct operation of the active power filter. By controlling the amplitude of the fundamental current, the current controlled voltage source inverter absorbs the real power required to cover the inverter switching losses and to maintain the steady state dc capacitor voltage constant. In the case of both three leg and four leg voltage source topology of active power filter, the 26

5 voltage balance of the dc link capacitors can be controlled by altering the zero sequence component of the converter voltage. A block diagram of the voltage balance controlleris shown. The dc-link capacitor voltages v dc are measured, and the error value fed into the Pi type controller is formed by calculating the difference between the capacitor voltages. The control bandwidth of the PI controller is limited so that the controller does not react to any oscillating components in the dc-link capacitor voltages. v =sin( θ ) (1) a v b v c =sin( θ -120 ) (2) =sin( θ +120 ) (3) The actual dc-link voltage (v dc ) is sensed and compared with the reference dc voltage signal. The difference of this actual dc-link voltage and reference dc-link voltage (v dc,ref ) is given to a PI controller to maintain a constant dc-link voltage under varying load conditions. The instantaneous values of reference three phase grid currents are computed as: Fig.4 Dc-link Voltage Controller A. Control of CCVSI The control circuit of current controlled voltage source inverter for a 3-phase 4-wire system. The fourth leg of inverter in topology is used to compensate the neutral current of load. The main intend of proposed approach is to regulate the power at PCC. The given control technique compensates the harmonics, unbalance, and neutral current when the load connected at PCC is nonlinear or unbalanced or the combination of together. The modulation ratio of inverter switches are isa=i m.v a (4) isb=i m.v b (5) i sc =I m.v c (6) The source neutral current should not be drawn from the source and is considered as zero and can be expressed as i sn =0 (7) The reference fundamental source currents isa,b,c and neutral current i sn is subtracted with the actual load currents i la,b,c to compute the reference * compensation filter current i fa,b,c as: ifa * =i la- i sa (8) ifb * =i lb- i sb (9) ifc * =i lc- i sc (10) ifn =i ln -i sn* (11) Then the compensating current error is computed by comparing the compensating reference current i fa,b,c * with the actual filter compensating current. controlled ifa,err in =i a power fa - i fa cycle such that the (1 combination of, i fc err =i * fb - i fb (12) load and inverter injected power appears as balanced ifc err =i * fc - i fc (13) resistive load to the source. The output of dc-link ifn,err = i * fn - i fn (14) voltage regulator results in an active current component These current errors are given to hysteresis I m. The generated active current component I m is current controller. The hysteresis controller then multiplied with unity source voltage vector templates generates the switching pulses for the gate drives of (v a,v b, v c ) generates the reference source currents(i s CCVSI. a,b,c * ). The reference source neutral current (i sn ) is set to zero, which is the instantaneous sum of balanced source V. EXPERIMENTAL VALIDATION currents. The generated unit sine vector reference from In order to verify the conformity of the the three phase source with synchronizing angle θ is proposed control algorithm applied to topology of given by: 27

6 current controlled voltage source inverter connected to a 3-phase 4-wire power distribution network, various simulation studies is carried out using MATLAB/Simulink. The system parameters to make simulation studies are listed in table I. Topology of CCVSI is controlled actively to achieve balanced sinusoidal source currents with unity power factor regardless of either balanced/unbalanced nonlinear load connected on PCC. The dc link voltage of active power filter is always maintained constant in order to achieve effective control of active and reactive power flow between source and load. TABLE 1 SYSTEM PARAMETER topology of CCVSI respectively. The natural current compensation also obtained effectively VI. MATLAB SIMULATION RESULTS FOR THE PROPOSED SYSTEM Simulations were performed by using MATLAB-Simulink to verify that the proposed system can practically be implemented in a renewable energy system. A 4-leg current controlled voltage source inverter is actively controlled to achieve balanced sinusoidal grid currents at unity power factor (UPF) despite of highly unbalanced nonlinear load at PCC under varying renewable generating conditions. Ac Line voltage and frequency Source impedance R S, L S Unbalanced nonlinear load DC link capacitor:c dc 400V, 50Hz 0.02ohm,.0.5mH for each phase R:50ohm,C:1000uF(phase-a) R:80ohm,C:1000uF(phase-b) R:120ohm,C:1000uF(phase-c) 1000uF Interface filter inductance L V dc _Ref f 2mH 600V Hysteresis band 0.01 Fig.5 Simulation Circuit for 3- phase 4-wire Thus under various simulation studies, it is clear that the CCVSI can be effectively used to compensate current harmonics, current unbalance and load reactive power. This facilitates the source always to supply or receive sinusoidal and balanced power at unity power factor. From those results it is obtained that the harmonic spectrum of source current before and after active power filter is installed and THD obtained is 110% for without compensation and 2.98% while using four-leg topology. The four-leg topology has better compensation characteristics. From this simulation analysis the source current reaches to its steady state value for a time period of 0.3s and 0.65s for four-leg 28 Fig.6 Simulation Circuit for CCVSI

7 Fig.7 Simulation Circuit for Controller Fig.10 Simulation Results after Compensation Fig.8 Simulation Results before Compensation Fig.11 Compensated FFT Analysis Fig 9 Uncompensated FFT Analyses Fig.12 Dc Link Voltages 29

8 VII. CONCLUSION This paper presents efficient control algorithm using direct current control technique topology of current controlled voltage source inverter based hybrid shunt active power filter. In this method the harmonic current reference generated using direct current control without introducing any electronics filters is an added advantage. With the proposed approach of inverter can be used to improve the quality of power at PCC for a 3- phase 4-wire distribution system network. Proposed topology of inverter can efficiently compensate the current harmonics, current unbalance, and load reactive power due to unbalanced and non-linear load connected to the PCC. Also with the proposed topology of four leg-inverter prohibits the load neutral current flowing into the source side. In this topology supply side currents are always maintained as sinusoidal and balanced with unity power factor. [7] IEEE Guide for harmonic control and reactive compensation of Static Power Converters, IEEE Standard [8] Izhar, M.,.Hadzer, C.M., Syafrudin, M.,Taib, S., Idris, S., Performance for passive and active power filter in reducing harmonics in the distribution system ", Proc. National Power and Energy Conference, PECon 2004, Nov2004, pp [9] Leow, P and Naziha, A., SVM Based Hysteresis Current Controller for a Three Phase Active Power Filter. Proceedings of the IEEE National Conference on Power and Energy Conference (PECon). Nov , Kuala Lumpur, Malaysia: IEEE [10] Manjula, Nair, G., and Bhuvaneswari, G., Design, Simulation and Analog Circuit Implementation of a Three-phase Shunt ActiveFilter using the ICOS Ǿ Algorithm IEEE PEDS [11] Sella,S., R.Penzo, etal."hybrid active filter for parallel harmonic compensation", The European Power Electronics Association Journal 1993.pp REFERENCES [1] Akagi, A. Nabae, and S. Atoh. Control strategy of active power filters using multiple Voltage source P WM converters. IEEE Transactions on Industry Applications,22(3):460-5, 1986 [2] Akagi, H., Kanazawa, Y., and Nabae, A. Instantaneous Reactive Power Compensators Comprising of Switching Devices without Energy Storage Components. IEEE Trans. on Industry Applications (3): [3] Bhim Singh, Kamal Al-Haddad and Ambrish Chandra, A Review of Active Filters for Power Quality Improvement,IEEE Trans. On Industrial Electronics, Vol. 46, No. 5, October1999. [4] Blaabjerg,F., Teodorescu,R., Liserre,M., and Timbus,A.,V., Overview of control and grid synchronization for distributed power generation systems, IEEE Trans. Ind. Electron., vol. 53, no. 5, pp ,Oct [5] Bor-Ren Lin.et.al. "Analysis and operation of hybrid active filter for harmonic elimination", Electric Power SystemsResearch 2002, Vol.62, pp [10] Blaabjerg,F., Teodorescu,R., Liserre,M., and Timbus,A.,V., Overview of control and grid synchronization for distributed power generation systems, IEEE Trans. Ind. Electron., vol. 53, no. 5, pp ,Oct [6] Buso, S., Malesani, L., and Mattavelli, P. Comparison of Current Control Techniques for Active Filter Applications. IEEE Trans. on Industrial Electronics (5):