INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) INDIRECT CURRENT CONTROLLED SINGLE PHASE SHUNT ACTIVE FILTER

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1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 ISSN (Print) ISSN (Online) Volume 4, Issue 4, July-August (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJEET I A E M E INDIRECT CURRENT CONTROLLED SINGLE PHASE SHUNT ACTIVE FILTER Narayan G. Apte 1, Dr. Vishram N. Bapat 1 (Lecturer, Department of Electrical Engineering, Walchand College of Engineering, Sangli, Maharashtra State, India) 2 (Director, Ganga Instt. Of Technology & Management, 20km, Bahadurgarh-Jajjhar Rd., Kablana, Dist. Jajjhar, Haryana, India) ABSTRACT This paper presents indirect current control scheme for single phase shunt active filter. There are two control loops, the outer loop for dc bus voltage control and inner loop for tracking current. The shunt active filter (SAF) reference current, which is the desired instantaneous supply current, is extracted by using sine signal integrator (SSI) considering the fundamental active current required by load and that required by SAF to meet losses and maintain dc capacitor voltage. A novel phase locking technique for single phase based on SSI is also proposed. Simulation results are reported to validate effectiveness of the proposed technique. Keywords: Shunt Active Filter (SAF), Sine Signal Integrator (SSI), Instantaneous Reactive Power (IRP), indirect control, reference current generation, harmonics. 1. INTRODUCTION Power electronics based systems form a major constituent of today s power processing used at transmission and distribution and domestic levels. These devices/equipment draw chopped/distorted current from the utility which gives rise to presence of unwanted frequency components (harmonics) in the current drawn. Power quality problems arise due to harmonics. Shunt active filters, series active filters, and their combination are employed to mitigate power quality problems. With emergence of distributed generation (DG), which are essentially single phase networks, there has been significant research and development on single phase active filters. Harmonic compensation and reactive power support are the principle issues of a single phase system that need to be handled by a single phase SAF. Fig. 1 shows basic schematic for single phase SAF. Shunt active filters (SAF) basically works by controlling switching of the inverter switches with an objective of minimizing harmonic and reactive currents in supply current. 264

2 The critical task of the SAF controller is to generate reference current. The SAF control consists of two loops. The outer loop controls dc voltage across capacitor. The inner current loop controls switching of inverter switches in order to track SAF reference current. The accuracy of the SAF current reference determines performance of SAF. Reference current generation needs particular attention since the theories are developed for three phase Figure 1: Basic Schematic block diagram of single phase SAF applications and the same are not directly applicable to single phase case. Different techniques for reference current generation for single phase SAF applications are reported in [1-12]. In [3], the methods are classified in direct and indirect methods. The direct method involves sensing load current and extracting harmonic and reactive component. The current controller injects these harmonic and reactive current components with same magnitude but opposite phase. The direct methods include the Instantaneous Reactive Power (IRP) theory [5-8], the Synchronous Reference Frame (SRF) theory [12-15] and the Fourier transform method [1]. In indirect method, a sinusoidal reference is generated by means of grid voltage sensing. The inverter switching is controlled in such a way that supply current is dictated to follow sinusoidal supply voltage reference. The main advantages of this method are that it requires only low bandwidth sensor and has a faster transient response [14]. The indirect methods reported in literature are based on controllers such as Proportional-Integral (PI) or an Enhanced Phase Locked Loop (EPLL) to find the reference current [2][3]. Some of these methods are sensitive to voltage distortion, require several control parameters and introduce delays on the reference current [13]. The IRP and SRF theories are the most sought after techniques addressed in relevant literature but originally developed for three phase systems. In three phase case, both IRP and SRF techniques operate in a reference system with two orthogonal axes (αβ for IRP and dq for SRF). The same logic is not directly applicable for single phase as the system possesses only one variable. Therefore, it is necessary to have a fictitious or imaginary variable which is phase shifted by 90 0 at all frequencies. To maintain orthogonality of the original and imaginary variable at all frequencies poses problem particularly for load current with high harmonic content. IRP and SRF theories for single phase applications are discussed in [7][15][16] and for creating orthogonal variable, delay blocks have been adopted. In this paper, SSI based reference current generation has been used for indirect control of single phase SAF. Fundamental 265

3 active component of the load current is extracted from load current signal. Additionally, the outer voltage control loop decides the active component to be absorbed by SAF. The linear current regulator tracks the supply current and making SAF to indirectly compensate for the reactive and harmonic current of the load. An SSI based single phase PLL has also been proposed. Use of SSI for detecting fundamental component of supply voltage makes the system insensitive to supply voltage distortion. This paper is organized in following sections. Section 2 describes the basic SSI control technique. Section 3 proposes a technique for SAF reference current generation. The design of phase locking system based on SSI is discussed in section 4. Section 5 presents simulation results and section 6 presents conclusion of the paper. 2. SAF REFERENCE CURRENT GENERATION USING SSI In a single-phase system, the use of a rotating frame is not possible unless a virtual system is coupled to the real one in order to simulate a two-axis environment. One common requirement of single phase IRP and SRF reference current generation technique is necessity to create a fictitious orthogonal or imaginary signal in which all frequency components are phase shifted through 90 0 electrical degrees with respect to original variable. Most common techniques to create such orthogonal component are to use Hilbert Transformation [7] or to use FIR filter. The drawback of Hilbert transformation is that it leads to non-causal system and cannot be implemented directly. FIR filter may cause some phase delays in the orthogonal variable. Alternatively, computation of the SAF reference current can be performed using Sine Signal Integrator (SSI) with less computational burden [17]. The SSI presents finite gain at the desired frequencies and adjustable bandwidth through the gain k a. The block diagram of the SSI is shown in Fig. 2 Figure 2: Block diagram of the SSI The input signal is x i (t), while the output signal in phase with the input signal is x o (t). The output quadrature signal is x oq (t). The transfer function for each output correspond to (1) and (2) respectively and their bode diagrams are as shown in Fig.3. In this diagram, a difference of phases of π x is observed at o( s) ω = ω. Besides that, behaves as a band pass filter and the bandwidth is 2 0 xi( s) xoq( s) determined by value of k a and behaves as a low pass filter. xi( s) xo( s) 2ks a H1( s) = = (1) 2 2 xi( s) s + 2ks a + ω0 xoq( s) 2kaω0 H2( s) = = (2) 2 2 x( s) s + 2ks + ω i 266 a 0

4 In steady state operation the relationship between the phases of the transfer functions and in frequency domain is, π H1( s) = H1( s) + (3) 2 The inherent capability to produce quadrature component is utilized in generating instantaneous fundamental reactive power component of the load. 3. SAF REFERENCE CURRENT GENERATION (a) As per IRP theory, the instantaneous powers in a single phase system can be expressed as, (b) Figure 3: Bode Diagrams of (a) H1 ( s)and (b) H ( 2 s ) p( ωt) vα( ωt) vβ( ωt) iα( ωt) q( ωt) = vβ( ωt) vα( ωt) iβ( ωt) (4) The instantaneous powers can be expressed in terms of dc and ac quantities as, p( ωt) p( ωt) + p% ( ωt) q( ωt) = q( ωt) + q( ωt) % (5) For indirect control, the quantity of interest is p( ωt), which is fundamental active power. The fundamental active power, p( ωt)can be calculated by knowing fundamental voltage components v1 α( ωt), v1 β( ωt) and current components i1 α( ωt), i1 β( ωt) and the resulting SAF current reference can be expressed as, v1 α( ωt) α ω α ω + β ω β ω + α ω α ω = v1 ( t) i * 1 ( t) v1 ( t) i1 ( t) v1 ( t) Pdc( wt) i ( t) v ( ωt) + v ( ωt) 2 2 1α 1β (6) 267

5 The block diagram of the reference current generation technique is shown in Fig. 4. The feedback for instantaneous frequency ω is taken from a SSI based phase synchronization block discussed later in this paper. P dc is the required power to be absorbed by SAF to meet switching losses and maintain constant dc bus voltage. The current reference i * α( ωt) goes to linear current controller. Figure 4: Block of the reference current generator for indirect control 4. PHASE SYNCHRONIZATION A rigid phase angle detection mechanism is an integral part of grid tied VSIs. Conventionally, Phase locked loops (PLL) have been used for the same. The PLL structure is a feedback control system that automatically adjusts the phase of a locally generated signal to match the phase of an input signal. Among the various solutions to compute instantaneous phase angle SRF method is more preferred for single phase systems. SRF based single phase PLLs are presented in [18-21]. In general, In general, a specific "filtering" techniques has been used in order to deliver a non distorted signal to the SRF-PLL. In the present work, a single phase PLL based on Sine Signal Integrator (SSI) has been proposed. Use of supply frequency tuned SSI eliminates the need of additional filters to generate orthogonal component. The proposed PLL is simple in structure with better performance in case of distorted supply and frequency variations. The block diagram of the proposed PLL structure is shown in Fig. 5. SSI V block generates αβ components of the supply voltage. Theαβ components are further transformed to dq frame by using appropriate transformation. The closed loop control system tries to maintain V =0. The V represents peak of the fundamental supply voltage. The frequency tracking performance is demonstrated in Fig 6. q d Figure 5: Block Diagram of SSI based single phase PLL 268

6 Figure 6: Frequency tracking performance of SSI based single phase PLL Simulation of SAF The general schematic diagram of the simulated SAF for compensation of non-linear loads is shown in Fig. 7. It consists of a single phase supply, a generic load and SAF unit. The procedure of the design of SAF is summarized in following steps: Collection of system parameters Define performance specifications Design of power circuit Control system parameter setting Since SAF should meet the IEEE-519 (1992) requirements, the harmonic levels of the supply voltage and current after compensation should lie below the levels prescribed in the standard. Design of power circuit is based on following points mentioned below. Table I lists parametric values of the simulated model. Selection of VA rating of SAF. Selection of switching frequency. Selection of dc bus voltage. Selection of coupling inductance. Selection of dc bus capacitance. Figure 7: Schematic diagram of simulated model of single phase SAF 269

7 Table I: Single phase SAF parameters Supply parameters Load Parameters SAF power circuit parameters Switching ripple filter DC voltage controller gains Current controller Sine signal integrator Source voltage V s 240V Nominal Frequency f 50Hz Source impedance Z s j Ω Solid state relay R L1 15 Ω Diode bridge rectifier with R-L load. Diode bridge rectifier with R-C load. R i2 L i2 R L2 L 2 R i3 L i3 R L3 C 3 0.1Ω 0.8mH 13 Ω 107mH 0.25Ω 0.7mH 70 Ω 2000 µ F Inductive Load Q L 10 kvar SAF VA rating S SAF 12.5 kva Switching frequency f sw 5kHz Dc bus voltage V dc 700V Coupling inductor L ac 2.5mH (0.2Ω ) Dc capacitor C dc 3500 µ F Filter resistance R f 5 Ω Filter inductor L f mh Filter capacitor C f 20 µ F Proportional gain Integral gain Sampling time Proportional gain Integral gain Proportional gain Proportional gain K p K i T s K pi 4e-03 5 %/% 13e-03 %s -1 4e-06 s 7 %/% K ii 1e05 %s -1 K vv 10 K vi 10 The computer simulation is performed using MATLAB/SIMULINK power system block set. SAF reference current generation is based on the structure shown in Fig. 4. Various parameters, listed in Table I, are used for simulation. The VSI switches are modeled as IGBTs with anti-parallel diodes. The IGBTs are assumed ideal except their on-state resistance which amount to switching loss in the device. The current ripple generated by the VSI of the SAF power circuitry can spread to the power line through the PCC where the SAF system is connected to the power system. High frequency switching harmonics create noise problems for other loads connected to the same PCC. To filter the high frequency switching ripple currents due to the switching of the inverter, passive switching ripple filters are placed at the PCC as an integral part of the SAF. Linear current regulators generate regular PWM ripples around the switching frequency and its multiples and sidebands over the frequency spectrum. The M-PWM switching technique ca [22] uses the switching ripple to be at 2f sw. By shifting the switching frequency harmonics to 2f sw, it becomes easy to design switching ripple filter. A broad-band tuned type switching ripple filter has been used to filter switching ripple as shown in Fig. 7. The L F, C F branch is a series resonant branch which is tuned to frequency f sw. R d is a damping resistor which limits the filter current. 270

8 The responses of the system using indirect current control are shown in Fig.7. It also exhibits start-up behavior of the SAF. The harmonic spectrum of source current before and after compensation is shown Fig. 9 and Fig. 10. It shows significant improvement on the THD of supply current from 71% to 1% with SAF compensation. The reactive power of the load is completely compensated by SAF resulting in near unity pf (0.996). Dc bus regulator performance is shown in Fig. 11. Performance of SAF under distorted utility conditions is shown in Fig. 8. Here the supply voltage THD is 12 %. The proposed SAF effectively compensates for the load harmonics in presence of distortion in supply voltage. Figure 9: Frequency spectrum of load current Figure 7: Time response of SAF Figure 10: Frequency spectrum of supply current after compensation Figure 8: SAF performance in distorted supply voltage conditions Figure 11: DC bus voltage controller response 271

9 5. CONCLUSION In this paper, an indirect controlled single phase SAF has been presented. The effectiveness of the SSI based reference current generation and performance of the SAF has been tested by computer simulation. A novel phase and frequency detection technique is also proposed. The results of computer simulations show significant improvement in frequency spectrum of mains current after SAF compensation which satisfies IEEE standard requirements. The proposed SSI based reference current generation technique also tested under distorted utility conditions and the time response of the same indicate that the SAF compensation is insensitive to supply voltage distortion. 6. REFERENCES [1] L. P. Kunjumuhammed, M. K. Mishra, A control algorithm for single-phase active power filter under non-stiff voltage source, IEEE Trans. Power Electron., Vol. 21, No. 3, May 2006, pp [2] M. K. Ghartemani, H. Mokhtari and M. R. Iravani, A signal processing system for extraction of harmonics and reactive current of single phase systems, IEEE Trans. Power Electron., Vol. 19, No. 3, July 2004, pp [3] L. P. Kunjumuhammed, M. K. Mishra, Comparison of single phase shunt active power filter algorithms, Conf. Ref. IEEE Power India Conference 06, April [4] M. T. Haque, Single-phase pq theory for active filters, Conf. Rec. IEEE TENCON 02, Oct. 2002, pp [5] M. T. Haque, Single-phase PQ theory, Conf. Rec. IEEE PESC 02, June 2002, pp [6] M. T. Haque, T. Ise, Implementation of Single-phase pq Theory, Conf. Rec. IEEE PCC 02, April 2002, pp [7] M. Saitou, T. Shimizu, Generalized theory of instantaneous active and reactive powers in single-phase circuits based on Hilbert transform, Conf. Rec. IEEE PESC 02, June 2002, pp [8] M. Saitou, N. Matsui, T. Shimizu, A Control Strategy of Single phase Active Filter Using a Novel d-q Transformation, Conf. Rec. IEEE IAS 03 Vol. 2, Oct. 2003, pp [9] J. Liu, J. Yang; Z. Wang; A new approach for single-phase harmonic current detecting and its application in a hybrid active power filter, Conf. Rec. IEEE IECON 99, Nov. 1999, pp [10] Y. J. Kim, J. S. Kim, Y. S. Kim, Single-phase Active Power Filter based on Rotating Reference Frame Method, Conf. Rec. IEEE ICEMS 05, Sept. 2005, pp [11] M. Gonzalez, V. Cardenas, F. Pazos, DQ transformation development for single-phase systems to compensate harmonic distortion and reactive power, Conf. Rec. IEEE CIEP 04, Oct.2004, pp [12] T. Tanaka, E. Hiraki, N. Ishikura, Y. Omura, M. Yamamoto, A Novel Real-Time Detection Method of Active and Reactive Currents for Single-Phase Active Power Filters, Conf. Rec. IEEE PESC 07, June 2007, pp [13] S. M. Silva B. M. Lopes B. J. C. Filho R. P. Campana W. C. Bosventura, Performance Evaluation of PLL Algorithms for Single-phase Grid connected Systems, Conf. Rec. IEEE IAS 04, 3-7 Oct. 2004, pp [14] B. N. Singh, Sliding mode control technique for indirect current controlled active filter, in Proc. IEEE Region 5 Annu. Tech. Conf., New Orleans, LA, Apr. 2003, pp

10 [15] B.N. Singh, V. Khadkikar, A. Chandra, Generalised single-phase p-q theory for active power filtering: simulation and DSP-based experimental investigation, IET Power Electronics, Vol. 2, No. 1, Jan 2009,pp [16] V. Khadkikar, B. Singh, et. al., Implementation of Single-Phase Synchronous D-Q Reference Frame Controller for Shunt Active Filter Under Distorted Voltage Condition, Joint International Conference on Power Electronics, Drives and Energy Systems (PEDES-2010), 2010, pp. 1. [17] L. R. Limongi, R. Bojoi, A. Tenconi, L. Clotea, Single-phase inverter with power quality features for distributed generation systems, Conf. Rec. IEEE OPTIM 2008, May 2008,pp [18] M. Ciobotaru, R. Teodorescu and F. Blaabjerg "Improved PLL structures for single-phase grid inverters", Conf. Rec. PELINCEC, CDROM, [19] S. M. Silva, M. Sidelmo, L. N. Arruda and B. C. Filho "Wide Band-width Single and Three- Phase PLL Structures for Utility Connected Systems", Conf. Rec. EPE '91, Florence, September 1991, pp ,. [20] L. N. Arruda, S. M. Silva, B. J. C. Filho; "PLL Structures for Utility Connected Systems", Industry Applications Conference, 2001, Vol. 4, pp [21] S. M. Silva B. M. Lopes B. J. C. Filho R. P. Campana W. C. Bosventura "Performance Evaluation of PLL Algorithms for Single-phase Grid connected Systems", Conf. Rec. IAS'04, Vol. 4, Oct. 2004, pp [22] S. Rahmani, K. Al-Haddad, H.Y. Kanaan, Two PWM techniques for single-phase shunt active power filters employing a direct current control strategy, IET Power Electronics, vol. 1, no. 3, pp , Sept [23] Dr. Leena G, Bharti Thakur, Vinod Kumar and Aasha Chauhan, Fuzzy Controller Based Current Harmonics Suppression using Shunt Active Filter with PWM Technique International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 1, 2013, pp , ISSN Print : , ISSN Online: , [24] Dr. R Prakash and Kiran R, Modelling & Simulation of Active Shunt Filter for Compensation of System Harmonics, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 4, 2013, pp , ISSN Print : , ISSN Online: , 7. BIOGRAPHIES N. G. Apte, received B.E (Electrical) and M.E. (Electrical) in 1993 and 1995 respectively from Walchand College of Engineering, Sangli, Inidia. He is working as senior lecturer in Electrical Engineering department for last 13 years. His areas of interest include embedded systems, power electronics applications to power systems. Dr. V.N. Bapat, received B.E. (Electrical) and M.E.(Electrical) 1983 and 1985 respectively from Walchand College of Engineering, Sangli, India. He received Ph.D. from Indian Institute of Technology, Kharagpur in He has 30 years of teaching experience. Presently, he is working as Director, Ganga Instt. Of Technology, Dist. Jajjhar, Hariyana, India. His areas of interest are control systems, electrical machine design, power quality. 273