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MODELLING OF HYBRID FILTER FOR HARMONIC COMPENSATION IN POWER SYSTEMS R. Ramesh, V.Ramachandran, P.Chandrasekar, K.Nithiyananthan, J.R.Maglin Department of Electrical and Electronics Engineering Anna University, Chennai -24 India Abstract - The need for effective utilization of electrical power has increased the area of application of power electronics. Due to extensive use of power control devices, an increasing deviation of the system voltage and current waveforms is observed. The presence of harmonics in the power lines results in greater power losses and excessive heating of electrical equipment in the power distribution system. An attempt is being made in this paper to model a hybrid active filter to compensate for harmonics in power systems. The passive filter removes load produced harmonics just as a conventional one does. On the other hand, the active filter plays a role in improving the filtering characteristics of the passive filter. The hybrid active filter which is a combination of both active and passive filters is modeled using P-Q theory and simulation studies are performed using PSIM-SIMCAD software. Experimental results obtained from a prototype model are shown to verify the theory developed in this paper. Key – Words: - Hybrid filter, Resonance, Instantaneous reactive power theory 1 passive filters are bulky and they are susceptible to Introduction The need for effective utilization of electrical power has increased the area of application of power electronics. But non-linear loads draw non-sinusoidal currents, injecting current harmonics into the distribution system. The voltage drop across the impedance due to these current harmonics gives rise to voltage harmonics. Harmonics have damping effects on electrical equipment such as transformers, rotating machines, switchgear, capacitor banks, fuses and protective relays. The effects are in the form of increased losses and excessive heating that ultimately lead to the equipment’s failure. Shunt passive filters have been initially used to compensate the current harmonics injected by large industrial non-linear loads. The most common types of shunt passive filters are the single tuned filter and the high pass filter as they are the simplest to design and the least expensive to implement [1]. But the MAIN AUTHOR AFFILIATION INFORMATION GOES HERE. series and parallel resonance with the supply and the load. Also the source impedance influences their filtering characteristics. The large ratio of the source impedance to the filter impedance will lead to better filtering of load current harmonics. A single active filter can be used for compensating multiple harmonics as well as for damping resonance in a power distribution system [1]. An active filter can be a VSI or CSI operating on PWM and requires a high bandwidth because of the non-sinusoidal nature of its reference. The VSI is preferred over the CSI, due to its lower initial costs and higher efficiency. Active filters are currently expensive for larger ratings as compared to passive filters due to the higher prices of the switching devices, but it is admissible if several other features are added in them to improve the power quality of a distribution system. 2 State of the art the efficiency. The advantages of both the active filter and the passive filter are combined in a hybrid active The control of the hybrid active filter requires extraction of the harmonic component of the source current. However the filter current has also been used for the control of the hybrid active filter [1]. The traditional method of extracting the harmonic component in time domain is a notch filter at the fundamental frequency. But this method gives rise to errors due to changes in the system frequency. Dr.Akagi et al [3] proposed the IRP theory that has been widely used for harmonic extraction. The hybrid active filter shown in fig.1.1 (b) was proposed by Peng et al [4]. The extraction of harmonics is filter. Hybrid active filters are shown provide a practical and cost effective harmonic filtering solution for non-linear loads in the range of 1MVA to 30 MVA. A single phase equivalent circuit of HAFS is shown is fig.1. The source voltage (Vs) and the source impedance (Zs) are approximated, as the Thevenin’s equivalent as seen from the PCC and the load as a current source (IL). The hybrid active filter consists of an active filter (Vaf) in series with a shunt passive filter (Zf) in order to compensate the harmonics in the distribution system. based on the IRP theory. The active filter is a voltage controlled VSI that inserts a resistance (K) at harmonic frequencies. This ensures damping of resonance along with the filtering of harmonics in the system. The compensation characteristics and the stability of this hybrid active filter system were discussed in [4]. The active filter acts as a harmonic isolator by forcing the load current harmonics to flow into the passive filter. The hybrid active filter shown in fig.1.1. (c) Fig 1: single phase equivalent circuit of the HAFS. was proposed by Dr.Akagi et al [3] with a control scheme based on the IRP theory. The IRP theory has The control strategy of a hybrid active filter been modified by the use of fundamental component system depends on the harmonic detection method of the voltage at the PCC in place of its instantaneous used in determining the reference to the value. The active filter is controlled as a voltage active filter. The harmonic detection method used here source to insert a resistance (K) at harmonic for control of a hybrid active filter is Supply current frequency. It acts as a harmonic compensator and detection method ,In this method, Is is detected and its improves the filtering characteristics of the passive harmonic component Ish is extracted. The active filter filter. In [5,6] illustrates the methods of simulation is controlled to satisfy Vaf = Kc.Ish. and the guidance. This thesis deals with the hybrid active filter shown 3 in fig.1in which the active filter consisting of a VSI Hybrid active filter based on PWM is connected in series with the shunt It contains both an active filter and a shunt passive filter to reduce the initial costs and improve passive filter. The supply voltage is assumed to be sinusoidal and the non-linear load considered for compensation is a 3-phase, 6-pulse diode bridge It is observed that the lead characteristics rectifier. The shunt passive filter consists of a single extend the filtering capacity of the hybrid active filter tuned filter for 5th harmonic, which is the dominant from that of a single tuned shunt passive filter to that harmonic in the rectifier load current. For the control of a wide band filter .But the phenomenon of parallel of active filter, the harmonic component of the source resonance current is extracted by the IRP disproportionately. The resonant frequency ( Wp) theory [3]. shifts towards lower side for higher values of T1. The appears if T1 is increased magnitude of the resonant peak at W=Wp can be 4 Filtering characteristics The filtering characteristics of the hybrid reduced for a particular values of T1 and T2 by increasing the value of K. In contrast, the lag characteristics reduce the active filter system compensating harmonics generated by a non linear load are analyzed. The harmonic equivalent circuit of hybrid active filter in the lag-lead characteristics has been shown in Figure 2 and the filtering characteristics are shown in Figure 3. filtering characteristics of the hybrid active filter from that of a single tuned shunt passive filter to that of a narrow band filter. Increasing the value of K can increase the width of the pass band for a particular T 1 and T2. 5 control scheme The computation of the reference to the active filter involves the extraction of the harmonic component of a signal. This extraction can be done Fig 2 .Harmonic equivalent circuit of HAFS using frequency-domain or time-domain technique [7]. Hence time-domain techniques are preferred due The above circuit represents a generalized to their ease of implementation and little case that includes lead characteristics and lag computational burden. The harmonic extractions characteristics. The lead characteristics are obtained at based on the IRP theory have advantages over the either T2=0 or T1>T2(when T2≠0).The active filter notch acts as a series RL circuit with a resistance of K ohm performance from the system frequency variations. and an inductance of KT1 henry for the lead Hence, this is presently the most popular method of characteristics when T2=0. harmonic extraction. The P-Q theory is one of the filter in the independence of filtering Similarly lag characteristics are obtained at several methods that can be used in the control of either T1=0 or T1<T2 (when T1≠0). The active filter active filter [1]. The instantaneous voltages and acts as a parallel RC circuit with a resistance of K currents in three-phase circuits can be mathematically, ohm and a capacitance of T2 / K farad for the lag expressed as the instantaneous space vectors [3]. For characteristics when T1=0. simplicity, the three-phase voltages and currents excluding zero-phase sequence components are current harmonics produces voltage harmonics at the considered. PCC. A hybrid active filter is used to filter these 6 Modeling of hybrid filer harmonics. It consists of an active filter connected in The system consists of a power circuit and a control circuit. The power circuit is composed of the series with a shunt passive filter through a coupling transformer. various components of the hybrid active filter system. The control circuit deals with the interfacing of a micro-controller in order to generate the switching signals for the active filter. The simplified schematic of the hybrid active filter system is shown in a singlephase representation in Fig. 3 The system is categorized into three broad sections. The source comprises of an ideal voltage source in series with an Fig.3 : Simplified schematic of the HAFS inductive reactance. The non-linear load considered is a 3-phase, 2.5 kW, 6-pulse diode bridge rectifier.This lLoad injects currents harmonics into the system. The 7 Results The modeling of HAFS is done by using C++ programming .The results are tabulated in Table 1 voltage drop across the line impedance due to the Table 1: Parameters of HAFS Fundamental frequency f = 50 Supply voltage Vs = 415 Load Power P = 2500 1 p.u. = 68.89 Source inductance Ls = 0.0091 Source inductance Ls = 0.0183 Source resistance Rs = 0.3588 Load Current Idc = 4.56 Load Voltage Vdc = 547.7505 Load resistance Rdc = 120.1207 Load inductance Ldc = 0.0300 Passive filter inductance Lf5 = 0.0260 Passive filter capacitance Cf5 = 1.6586 Passive filter resistance Rf5 = 11.3068 Switching ripple inductance Lr = 0.0116 Switching ripple capacitance Cr = 1.3875 Switching ripple resistance Rr = 0.3484 Switching frequency fs = 5600 Corner frequency fc = 3138.7488 * Switching harmonics are attenuated. Leakage inductance L1 = 0.0030 Leakage resistance R1 = 0.0785 * Pulse number = 6 Order of the harmonics 5 7 11 13 17 19 23 25 Hz Volts Watts Ohm Henry for SCR = 24 Henry for SCR = 12 Ohm Amps Volts Ohms Henry Henry e-05 Farad Ohm Henry e-06 farad Ohm Hz Hz Henry Henry A detailed simulation is made for the diode rectifier 8 Protection scheme for the HAFS A protection scheme is analyzed and suggested for the hybrid active filter system to Isolate the active filter from the remaining system when a fault occurs in the PWM inverter. Provide a starting sequence for its operation. When a fault condition occurs in the MOS gate driver, a fault latch is set and all its six output drive signals are shunt down. It is necessary to isolate the inverter to attend to the fault condition without load with an RL circuit on its DC side. The effect of commutating inductance is also considered. The supply voltage is assumed to be sinusoidal in nature and the harmonics are injected into the system by the rectifier load. The coupling transformer is modeled as an ideal transformer with a turns ratio of N1:N2 in series with a leakage inductance along with its resistive component. The results were compared for different source impedance . disturbing the remaining part of the hybrid active filter system. This is done by connecting the switches NO1 and NC1 to the coupling transformer as shown in fig.4 HAFS Without any Filter: HAFS without the Active Filter: HAFS With the Active Filter: Table 2: Attenuation in the source current due to the shunt passive filter for Ls = 9mH. 5 7 11 13 17 19 H 21.54 15.80 32.57 18.76 0.6 6 4.3 3 Ishp (%) 12.19 9.82 22.81 13.36 0.4 8 3.2 9 Ahp 0.57 0.62 0.70 0.71 0.7 2 0.7 3 ILh (%) Fig 4. Circuit for isolating PWM inverter under fault / starting conditions The overall protection scheme for the hybrid active system also considered for discussed. 9 Modeling and simulation result The single-phase equivalent circuit of the hybrid active filter system adopted as the simulation model. The digital simulation is performed in MATLAB – SIMULINK software using differential equations to describe the working of the power circuit Table 3 give the attenuation factor for Ls = 18mH. It is observed that attenuation factor for Ls = 18mH is less as compared to attenuation factor for Ls = 9mH. Table 3 Attenuation in the source current due to shunt passive filter for Ls= 18mH. H 5 7 11 13 17 19 ILh (%) 21.54 15.80 32.57 18.76 0.6 6 4.5 3 Ishp( %) 7.46 7.08 17.53 10.37 0.3 8 2.5 9 Ahp 0.35 0.45 0.54 0.55 0.5 9 0.5 7 Table 4 gives the comparison of the current harmonics between Ls= 9mH and Ls = 18mH. It is observed that the harmonics are decreased due to change in the source inductance. Table 4: Comparison of 5th order harmonics due to the shunt passive filter Ls = 9 mH Source current 1.1 harmonics (Amps) 10 0.35 The DC bus voltage control of active filter is to be incorporated into the control scheme. The simulation of three-phase equivalent of HAFS is to be performed using MATLAB Simulink software. The modeled HAFS can be fabricated and experimentally tested. The results can be compared with the results obtained through the simulation. References [1] [2] [4] Z.Peng, H.Akagi, A.Nabaz, “ COMPENSATION CHARACTERISTIC OF THE COMBINED SYSTEM OF SHUNT PASSIVE AND SERIES ACTIVE FILTERS”, IEEE trans. on industry applications, 1993, vol.29, no.1, pp. 144-152. [5] Dr.P.S.Bimbhra, “ POWER ELECTRONICS”, Khanne publishers, 1998. [6] GUIDELINES TO MATLAB-SIMULINK SOFTWARE PACKAGE. [7] Z.Peng, H.Akagi, A.Nabze, “A NEW APPROACH TO HARMONIC COMPENSATION IN POWER SYSTEMS. A COMBINED SYSTEM OF SHUNT PASSIVE AND SERIES ACTIVE FILTERS”, IEEE Trans. on applications, 1990, vol.26, pp 983-990. Conclusions and future research The following are some of the problems, which need further investigation: H.Akagi, Y. Kanazowa, A.. Naba, “INSTANTANEOUS REACTIVE POWER COMPENSATORS COMPRISING SWITCHING DEVICES WITHOUT ENERGY STORAGE COMPONENTS”, IEEE Trans on industry application, 1984, vol. IA- 20, pp.625-630. Ls = 18 mH The modeling of HAFS had been done. The protection scheme for isolating the active filter from the remaining system under fault/starting conditions is described. The control schemes are described for the extraction of harmonic component of the source current using IRP theory. The system was simulated using MATLAB software package. The simulation results for the control of the hybrid active filter shows that the attenuation in the 5th order harmonic is the maximum due to the single tuned shunt passive filter. [3] J.Afonso, C.Couto, J.Martins, “ACTIVE FILTERS WITH CONTROL BASED ON THE P-Q THEORY”, Conf at Universidade do Minho, Portugal, Jan 2000. H.Peng, H.Akagi, “NEW TRENDS IN ACTIVE FILTERS FOR POWER CONDITIONING”, IEEE trans. on industry applications, 1996,vol .32, no.6, pp. 13121320.