A Hysteresis based Active Shunt, Passive Series Hybrid Filter for Power Quality Improvement

Transcription

1 INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 72132, DECEMBER 27-29, A Hysteresis based Active Shunt, Passive Series Hybrid Filter for Power Quality Improvement Shailendra Kumar Jain, Pramod Agrawal, and H. O. Gupta Abstract-- In case of medium and high power nonlinear industrial loads such as phase controlled converters THD increases with the increase in firing angle, which increases the bandwidth of the active filter for compensation. Also, because of the presence of sharp transients in non-linear currents, active filter does not perform satisfactorily. A series line smoothing reactance installed in front of target load limits the slope of the falling and rising edges of the load current and simplifies the task of active filter by reducing its bandwidth. This paper presents the simulation and experimental investigations on a series passive/shunt active hybrid filter for highly nonlinear loads. The compensation process is based on sensing line current only, which is simple and easy to implement. PWM pattern generation is based on carrierless hysteresis based current control. The spectral performance shows that the active power filter brings the THD of the source current well below 5%, the limit imposed by IEEE-519 standard. Index Terms Active power filter, hysteresis controller, phase-controlled converter, power quality. I. INTRODUCTION N recent years the application of power electronics I converters including switching devices have grown tremendously. These converters are widely used in large power industrial applications like variable speed drives to medium and low power commercial and household applications. Apart from the numerous advantages offered by them, they suffer from the problem of drawing harmonics and reactive power from the source, which causes different undesirable features like Low system efficiency and poor power factor. It also causes disturbance to other consumers and interference in nearby communication networks. Recently active power filters are researched and developed as a viable alternative over the classical passive filters to improve the power quality, by compensating harmonic and reactive power requirement of these nonlinear loads. Among the various topologies of the active power filter developed so far, the shunt active power filter based on the current controlled voltage source type PWM converter has been proved to be effective even when the load is highly nonlinear [3,4,6,7,8]. But, most of the active filters developed are based Shailendra Kumar Jain is with the Electrical Engineering Department, Maulana Azad National Institute of Technology, Bhopal, presently persuing Ph.D. under QIP at Dept. of Elect. Engg., I. I. T., Roorkee ( shailjain2@rediffmail.com). Pramod Agarwal, is Asso. Professor, with the Electrical Engineering Department, Indian Institute of Technology, Roorkee ( pramoda@nde.vsnl.net.in). H. O. Gupta is Professor, with the Electrical Engineering Department, Indian Institute of Technology, Roorkee ( harifee@iitr.ernet.in). [5,8,9] on sensing harmonics and reactive volt-ampere requirements of the non-linear load and require complex control [1,4,1,11,12]. Duke and Round [9] have proposed a scheme in which the required compensating current is determined using a simple synthetic sinusoid generation technique by sensing the load current. However, it is simple and more efficient to regulate all the system non-linearity at the supply side. Hence, this scheme is further modified by sensing line currents only [6,13]. It is observed in case of medium and high power industrial loads such as phase controlled converters, that with the increase in firing angle THD increases, which increases the bandwidth of the active filter for compensation. Also, because of the presence of sharp transients in non-linear currents, active filter does not perform satisfactorily. Many hybrid active filter configurations are proposed [14,15], which combine tuned passive filters and active filter in series or parallel to improve the filtering characteristics are complex and expensive. A simple and more practical configuration has been proposed in [16] which is comprised of a three phase shunt active power filter and series line smoothing reactance installed in front of target load. The passive element limits the slope of the falling and rising edges of the load current and simplifies the task of active filter by reducing the bandwidth. However, reference vector estimation and the control algorithm implementation of space vector PWM controller is some what complex and require a fast processor. A simple hysteresis based series passive/shunt active hybrid filter is investigated here. Series smoothing reactor is connected in front on nonlinear load to reduce the bandwidth of the active filter. The reference current is estimated by regulating the dc side capacitor voltage of the PWM converter, which is simple and easy to implement. Various simulation results are presented under both transient and steady state conditions. A laboratory prototype is developed to verify the simulation results. PWM pattern generation is based on carrierless hysteresis based current control to obtain the switching signals of the PWM converter. Based on simulation and experimental results it can be concluded that the compensation is simple, easy to implement and effectively control all the non-linearity from supply side. The spectral performance shows that the active power filter brings the THD of the source current well below 5%, the limit imposed by IEEE-519 standard. The smoothing process performed by the series passive filter reduces the bandwidth requirement of the shunt active filter.

2 ila 8 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 22 II. CONFIGURATION AND CONTROL SCHEME Fig. 1 shows the schematic diagram of the proposed hybrid filter. A series ac line smoothing reactor named as passive series filter proposed by [ ] is placed in front of the nonlinear load. This smoothing reactor limits the slope of the rising and falling edges of the load current, while the active filter suppresses the residual harmonic currents to the recommended limits. Hence, the filter bandwidth requirement is reduced. Reference current is estimated by regulating the dc link voltage. The actual capacitor voltage is compared with a set reference value. The error signal is then fed to a PI regulator. The output of PI controller is considered as peak value of the supply current. This peak value of the current is multiplied by the unit sin vectors (u sa, u sb ) in phase with source voltages to obtain the reference currents (i sa * & i sb * ). Third reference current (i sc *) is obtained by a negative adding circuit. These reference currents and actual currents are given to a hysteresis based, carrierless PWM current controller to generate the switching signals of PWM converter. These switching signals after proper isolation and amplification are given to the switching devices. Due to this switching action current flows through the inductor L c, to compensate the harmonic current and reactive power of the load, so that only active power is drawn from the source. A detailed design criteria for the selection of power and control circuit parameters is given in [17]. Input power factor and % THD of the phase controlled converters as a function of firing angle is plotted in fig.2. Input power factor becomes poor with the increase in firing angle, as fundamental reactive power demand increases with the increase in firing angle. Also, % THD increases with the increase in firing angle, as the current commutation process shifts towards the region of low control margin, i.e., the region around the peak of the line to line voltage (away from the zero crossing of the phase voltage). Fig. 3 shows the nonlinear load current drawn by a phasecontrolled converter without smoothing reactor, and its frequency spectrum. All odd harmonics, up to 49 th harmonic, exceeds the IEEE standard recommended limits excluding triplens. While nonlinear load current and its frequency spectrum with the smoothing reactor are shown in fig. 4. It is observed that the slope of the rising and falling edge of the load current is reduced, easing the task of active filter. Also, significant reduction in current harmonics is achieved by inserting the smoothing reactor at the front end of nonlinear load. However, the major harmonics (5 th, 7 th, and 11 th ) still exceed the IEEE standard limit. THD of the load current reduces from 3.52% to 23.96%. Magnitudes of various frequency components with and without smoothing reactor are given in table I. Non-linear Load is PCC il Rp, Lp Rs, Ls ic Rc, Lc is is* Hysteresis controller s1 - s6 Unit sine vector Analog multiplier I max PI controller Vdc Vdc,ref III. SMOOTHING PROCESS OF SERIES PASSIVE FILTER Medium and high power industrial power systems contain nonlinear loads with sharp transients like uncontrolled rectifiers for dc based loads, phase controlled converters for ASD applications, uncontrolled rectifiers with RC loads for power supplies, or some combinations of them. These converters have following distinct features - - Sharp rising and falling edges, which complicates the task of active filter, so that APF does not perform satisfactorily. - Phase controlled converters draw significant fundamental reactive power, whose magnitude depends on firing angle of the converters. Fig.1 Schematic diagram of the shunt active power filter Order of harmonics Table I Magnitude of harmonic components (% of fundamental) Without smoothing reactor 22.5% 11.48% 9.6% 6.53% 5.68% 4.56% 4.14 With smoothing reactor 2.% 1.25% 5.7% 4.% 1.6% 1.23%.3% % THD 3.52% 23.96%

3 INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 72132, DECEMBER 27-29, % % THD 45 4 % ila Firing Angle (degree) Fig. 2 (a) Variation of % THD with firing angle 1 PF DPF Fig. 3 Nonlinear load current (i La ) without smoothing reactor and its frequency spectrum, i La : A, THD : 3.52% Order of harm onics.8 Input PF & DPF.6.4 ila (amps) Firing Angle (degree) Fig. 2 (b) Variation of input power factor (PF) and displacement factor (DPF) with firing angle The size of the smoothing reactor depends on the maximum allowable load current slope. The value of smoothing reactance is determined as a function of the filter inductor L c as [ ] L P where, 3m L c (4 2 3m) (1) V LL(max) m = V dc For 1 kva compensation capacity, L c and hence L p are selected as Load current (amps) 5 L c 1.67 mh L p 3.66 mh = 3.7 mh time (sec) time (sec).8.85 % ila % Order of harmonics Fig. 4 Nonlinear load current (i La ) with smoothing reactor but without active filter and its frequency spectrum, i La : A, THD : 23.9% IV. SIMULATION RESULTS In order to investigate the performance of the APF, various simulation results are presented both in steady state and transient conditions, with and without smoothing reactor. Simulation parameters selected for 1 kva compensation capacity are given in appendix I. Fig 5(a) shows the source voltage, load current, source current, APF current and dc link voltage for phase A, in steady state, with active filter but without smoothing reactor. Frequency spectrum of compensated source current is shown in fig. 5(b). Significant amounts of harmonic components are present in compensated source current. Sharp rising and

4 82 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 22 falling edges of load current makes the task of APF difficult. However, THD of the source current reduces from 3.52% to 9%, which is more than the 5% limit imposed by IEEE standard. Fig. 6(a) shows load, source, APF current and dc capacitor voltage waveforms in steady state, with active filter and smoothing reactor for phase A. After compensation source currents becomes sinusoidal, balanced, and are in phase of their respective voltages. Smoothing reactor reduces the bandwidth of the active filter and brings the THD of the input current well below 5%. With smoothing reactor and active filter, THD of the source currents is reduced to well below 5% (3.52% to 1.81%). Power factor of the input current improves from.8 to almost unity (.9996). APF current contains loss component and fundamental reactive power as the fundamental component, and absorbs all other harmonic components. Table II gives the various parameters before and after compensation, with and without smoothing reactor. Table II (Various parameters with and without smoothing reactor, for α=3 ) 1 8 Parameter % THD PF DPF Input kva Filter kva Before compens -ation 3.52% With smoothing reactor 1.81% After compensation Without smoothing reactor 9.1% % isa Order of harmonics Transient behavior is investigated by switch-in and load perturbation response. Fig. 7 shows the switch-in while fig. 8 shows the response of the APF during sudden load change. Load current is suddenly changed from 14.47A to 6.33A. Change in source current is found smooth. It is observed that, dc capacitor voltage changes to absorb the energy during load change. With a smoothing reactor, APF perform satisfactorily for entire range of firing angle, however, rating of the APF increases with increase in α, as the fundamental reactive power compensated by APF increased. Table III gives the rating of the APF for different firing angle with similar loading. Table III (Rating of APF for different firing angle) Firing Rating of APF Angle α Load Filter kva % of load VA in degree kva % 64.9% 93.27% Fig. 5(a) Simulation results with active filter but without smoothing reactor, THD : ila 3.52%, isa 9% (b) Frequency spectrum of compensated source current Vsa x.1(volts) Vsa x.1(volts) & ila (amps) & isa (amps) ica (amps) Vdc (volts) time (sec) Fig. 6(a) Simulation results with active filter and smoothing reactor THD : ila 28.15%, isa 1.81% Vsa x.1(volts) & ila (am ps) % isa

5 INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 72132, DECEMBER 27-29, of load current is reduced; therefore the THD of the load current is reduced from 27.5% to 22.2%, easing the task of the active filter. (a) (b) Fig. 9 Load current drawn by the phase controlled converter (a) without smoothing reactor, THD 27.5% (b) with smoothing reactor, THD 22.2% i La -2 amps/div, time 5 ms/div Fig. 1(a) shows the source voltage and load current drawn by a phase-controlled converter for α 5. Compensated source current with active filter but without smoothing reactor are shown in fig. 1(b). Although, compensated source current is sinusoidal and in phase with the source voltage, it has sharp peaks at the rising and falling edges of the load current and therefore THD is above the 5% limit. While compensated source current with active filter and smoothing reactor are shown in fig. 1(c). Sharp peaks in the compensated source current as present in fig. 1(b) are disappeared now and the THD of the compensated current is reduced to 3.9%, below the recommended limit. V. EXPERIMENTAL VERIFICATION A prototype model of the hybrid shunt active power filter is developed and tested in the laboratory to verify the simulation results. Fig. 9(a) shows the waveform of the load current drawn by the phase-controlled converter for a firing angle of 3. Fig. 9(b) shows the current waveform with a smoothing reactor connected in front of the converter. It is clearly observed that the slope of the rising and falling edges VI. CONCLUSIONS A series passive, shunt active, hybrid filter is investigated in this paper for medium and high power, high distortion loads with sharp rising and falling edge. THD of the load current is significantly improved by introducing a smoothing reactor in front of nonlinear load, therefore reduces the bandwidth of the active filter. The compensation is based on the sensing line currents only which is simple and easy to implement therefore enhances the system reliability. A laboratory prototype is developed to verify the simulation results. Spectral performance shows that APF successfully meet the requirements of IEEE 519 standard to reduce the THD below 5% limit. However, the drawback of using this smoothing reactor is that - Displacement angle of the input current increases, therefore i.e. reactive power demand increases and hence capacity of APF increases. - System overall efficiency decreases due to the losses in the smoothing reactor.

6 84 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 22 Fig. 1(a) Source voltage (v sa ) and load current (i La ) drawn by phase controlled converter (α 5 ), THD 29% x-axis : time - 5 ms/div y-axis : v sa -16 volts/div, i La - 5 amps/div (b) (c) Fig. 1 Source voltage (v sa ) source current (i sa ) and APF current (i ca ) drawn by phase controlled converter (α 5 ) (b) with active filter but without smoothing reactor, THD % (c) with active filter and smoothing reactor, THD - 3.9% x-axis : time - 5 ms/div VIII. REFERENCES [1] H. Akagi, Y. Kanazawa and A. Nabae, Instantaneous reactive compensators comperising switching devices without energy storage components, IEEE Trans. on Industry applications, vol. IA-2, No.3, May/June 1984, pp [2] W.M. Grady, M. J. Samotyj and A. H. Noyola, Survey of active power line conditioning methodologies, IEEE Trans. on Power Delivery, Vol. 5, No. 3, 199, pp [3] H. Akagi, Trends in active power line conditioners, IEEE Transactions on Power Electronics, Vol. 9, No. 3, 1994, pp [4] Fang Zang Peng, H. Akagi and A. Nabae, A study of active power filters using quad series voltage source pwm converters for harmonic compensation, IEEE Transactions on Power Electronics, Vol. 5, No. 1, Jan 199, pp [5] L. A. Morgan, J. W. Dixon and R. R. Wallace, A Three-Phase active power filter operating with fixed switching frequency for reactive power and current harmonic compensation, IEEE Trans. on Industrial Electronics, Vol. 42, No.4, August 1995, pp [6] Bhim Singh, Ambrish Chandra & K. Al-haddad, Computer-Aided modelling and simulation of active power filters, Electrical Machines and Power Systems, 27, 1999, pp [7] Bhim Singh, K. Al-Haddad & Ambrish Chandra, A review of active filters for power quality improvement, IEEE Transaction on Industrial Electronics, Vol. 46, No. 5, Oct. 1999, pp [8] J. W. Dixon, J. J. Garcia & L. Morgan, Control system for three phase active power filter which simultaneously compensates power factor and unbalanced loads, IEEE Trans. on Industrial Electronics, Vol. 42, No. 6, 1995 pp [9] R. M. Duke and S. D. Round, The steady state performance of a controlled current active filter, IEEE Trans. on Power Electronics, Vol. 8, April 1993, pp [1] E. H. Watanabe, R. M. Stephan and M. Aredes, New concepts of instantaneous active and reactive powers in electrical systems with generic loads, IEEE Trans. on Power Delivery, Vol. 8, No. 2, April 1993, pp [11] J. C. Montano and P. Salmeron, Instantaneous and full compensation in three phase systems, IEEE Trans. on Power Delivery, Vol. 13, No. 4, Oct. 1998, pp [12] V. Soares, P. Verdelho and G. D. Marques, An instantaneous active and reactive current component method for active filter, IEEE Trans. on Power Electronics, Vol. 15, No. 4, July 2, pp [13] K. Chatterjee, B. G. Fernandes and G. K. Dubey, An instantaneous reactive volt-ampere compensator and harmonic suppressor system, IEEE trans. on Power Electronics, Vol. 14, No. 2, March 1999, pp [14] F. Peng, H. Akagi, and A. Nabae, Compensation cfharacteristics of the combined system of shunt passive and series active filters, IEEE Trans. Ind. Applicat., vol. 29, pp , Jan/Feb [15] S. Bhattacharya, P. Cheng, and D. Diwan, Hybrid solutons for improving passive filter performance in high power applications, IEEE Trans. Ind. Applicat, vol 33, pp , May/June [16] A. M. Al-Zamil and D. A. Torre, A passive series, active shunt filter for high power applications, IEEE Trans. Power Electronics, vol 16, no.1, Jan. 21. [17] S.K. Jain, P. Agrawal, and H.O. Gupta, Design simulation and experimental investigations on a shunt active power filter for harmonic and reactive power compensation, Electric Power Components and Systems, scheduled for publication in vol. 32, no.7. VII. APPENDIX Parameters for simulation study V s = 23 V, f = 5 Hz, L s =.15 mh, L c = 1.67 mh, L P = 3.7 mh, C dc = 2 uf, V dc,ref = 68 V Parameters for experimental study V s = 4 V (peak), L s =.15 mh, L c = 3.3 mh, L P = 4 mh, C dc = 12 uf, V dc,ref = 9 V