Shunt Active Power Filter with Dynamic Output Current Limitation

Transcription

1 Shunt Active Power Filter with Dynamic Outut Current Limitation R. Pregitzer, J. G. Pinto, Luís F.C. Monteiro, João L. Afonso Industrial Electronics Deartment University of Minho Guimarães, Portugal {rregitzer ginto lmonteiro Abstract - This aer resents a Shunt Active Filter with a controller able to dynamically limit the outut current, which allows an effective rotection of the inverter semiconductors without diminishing the active filter erformance. The control strategy is based on the -q Theory exanded for three-hase four-wire systems and the ower stage of the active filter is comosed by a two-level four-leg inverter. Simulation results on PSCAD /EMTDC are resented to demonstrate the erformance of the dynamic current limitation. Keywords - Active Power Filter, Instantaneous Power Theory, Digital Signal Processing, Comuter Simulations, Power Quality. I. INTRODUCTION The increasing use of rectifiers, thyristor ower converters, arc furnaces, switched ower sulies and other non linear loads is known to cause serious roblems in electric ower systems. These roblems can be artially solved with the use of assive filters. However, this kind of filtering cannot adat to variations of the loads, and they also can roduce undesired resonances. One solution to avoid resonance roblems is the shunt active ower filter [1].These filters work as current sources, connected in arallel with the electric grid, and they are caable of roviding the harmonics and the ower factor required by the loads. Three hase four wire active filters are also caable of comensating unbalance in order to minimize the neutral current []. This way, the mains only suly the fundamental, balanced currents with an unitary ower factor, avoiding roblems in the transmission lines. This aer aears in sequence of the work develoed in the ower electronics and actuators laboratory of the University of Minho (Portugal) that have as a main objective to develo a shunt active ower filter rototye that will be used to demonstrate the alicability and advantage of these kind of equiments in an industrial environment. Two of the most imortant asects when an equiment rototye is installed in an industrial environment are security and reliability. The security of the human oerators, the security of the industry lant and the integrity of the equiment are factors that must be evaluated carefully. Therefore it is very imortant to rotect the active ower filter against henomena that usually do not exist in a laboratory environment, but may occur in the industry. Although the resented results are achieved from simulation, the resented strategy will be imlemented in the hardware rototye that is in imrovement hase. From revious exeriences it is ossible to verify that if the simulation model is accurately modeled and the details of hardware imlementation are taken into consideration, the simulation results are very similar to the ones measured in the develoed rototye [3]. A summary involving the major toics of this aer is described as follows: asects related to the configuration of the roosed active ower filter are detailed in Section II, a set of equations describing the control system based on the instantaneous ower theory for three-hase four-wire systems are resented in Section III, and in Section IV the simulation arameters are introduced. The roosed system is analyzed by means of simulation results on PSCAD /EMTDC in Section V. Finally, conclusions and suggestions for further works are resented in Section VI. II. ACTIVE POWER FILTER CONFIGURATION The ower stage of the Active Power Filter (Fig. 1) is comosed by a standard two-level Voltage Source Inverter that uses eight IGBTs and a caacitor at the dc side. Fig. 1. Two level, four leg inverter configuration

2 The caacitor voltage can be ket constant by a control algorithm that exchanges the required energy from the electric grid. The inductances are used to connect the inverter to the electric grid and the RC filters are tuned to minimize the transmission of noise due to the switching of the semiconductors. The develoed rototye uses IGBTs with a maximum collector current of 1 A and V CE =17 V controlled by a four-hase PWM switching technique using a 16 khz triangular carrier (Fig. ). Only a roortional gain k is used, because this way the resonse of the active filter is imroved when it is comensating loads with a high harmonic content. Details regarding the tuning of this controller can be found in [4]. average value of the instantaneous real ower. Corresonds to the energy er time unit transferred from the source to the load, in a balanced way through the 3 hases; ~ Oscillating value of the instantaneous real ower. It is the energy er time unity that is exchanged between the ower source and the load, through the 3 hases; q The instantaneous imaginary ower, q, has to do with ower (and corresonding undesirable currents) that is exchanged between the system hases, and which does not imly any transference or exchange of energy between the ower source and the load; i cx i cx * _ + error k Comarator g 1x Mean value of the instantaneous zero-sequence ower. It corresonds to the energy er time unity that is transferred from the ower source to the load through the zero-sequence comonents of voltage and current; Triangular Carrier g x x ~ Oscillating value of the instantaneous zero-sequence ower. It means the energy er time unity that is exchanged between the ower source and the load through the zero-sequence comonents of voltage and current. Fig.. PWM switching technique, without integral gain. The couling between the inverter and the electric grid is made by 1,5 mh inductors. In order to attenuate the high-frequency noise, RC assive filters with a cutoff frequency of about 4 khz are used. III. THE CONTROL STRATEGY The strategy alied in the active ower filter control is based on the -q Theory. This theory was introduced by Akagi et al. [5] and exanded to three-hase four-wire systems by Aredes et al. [6]. It consists in an algebraic transformation (Clarke transform) of the three-hase voltages and currents in the a-b-c coordinates to the -- reference frame. After the transformation, the -q theory comonents are achieved by the exressions (1-3), where is the instantaneous real ower, q is the instantaneous imaginary ower (by definition) and is instantaneous zero-sequence ower. = v i + v i (1) q = v i v i () v i = (3) Each one of the instantaneous owers can be searated into an average value and an oscillating value. The hysical meaning of each of the instantaneous owers is: In addition to the standard owers, there is also a comonent, reg, which is used to regulate the caacitor voltage in the dc side of the active ower filter. reg ref DC = k ( V V ) (4) The instantaneous owers that will be injected by the Shunt Active Filter, x and qx, are comosed by the undesired comonents (harmonics, unbalance and reactive ower). ~ (5) x = reg q x = q (6) Finally, it is ossible to calculate the reference currents, in the -- coordinates, by alying the exressions (7) and (8). The reference currents are then translated to the a-b-c reference frame through the inverse Clarke transform. i i c c = v 1 + v v v 1 i = i = + 3 ( i + i i ) v x v qx (7) (8) c a b c The average values of the instantaneous real ower and the instantaneous zero-sequence ower are calculated using a sliding window, instead of the usual Butterworth filters.

3 Isa Ia Rs_A Ls_A Isb Ib RLOAD CLOAD Rs_B Ls_B Isc Ic Electric Grid Rs_C Ls_C Rs_N Ls_N Isn Icn Icc Icb Ica In Bridge Rectifier CB1 R R R C C C L L L L Linear Load G1 G3 G5 G7 CDC VDC G G4 G6 G8 Fig. 3. Electrical system to test the Shunt Active Filter Usually, when using a sliding window, it is necessary to have a whole cycle ( ms on the 5 Hz system) of information in order to determine the average value of a given variable. However, deending on the characteristics of the loads, it is ossible to use a sliding window that requires only the values of half a cycle or even just one third or one sixth of the cycle. In this secific case, where unbalance is resent, the sliding window requires half a cycle. More information regarding this toic can be found on [7]. In terms of imlementation, using a sliding average has the advantage that the calculation is very simle and requires only an array with the values of the last 1 ms. Other simlifications were done, namely by exanding the calculations of the -q Theory in order to remove square roots and other mathematical oerations that a fixed oint microrocessor unit is not efficient at. Another imortant art consists in comensating the delays roduced by the control system, by exloring the half wave symmetry of the reference currents, otherwise when the load currents change fast, the source currents will have notches that will increase the THD [8]. IV. SIMULATION PARAMETERS A test case was conceived to analyze the erformance of the roosed system through digital simulations, with PSCAD /EMTDC 4.. In order to get realistic simulation results, all the blocks of the control strategy are discretized, and are introduced delays, fixed oint variables, errors in analogue to digital conversions according to the electronic comonents used in the hardware imlementation of the active filter [9]. In terms of hardware comonents, the diodes, IGBTs and transformers were modeled according to the datasheets of the real comonents of the laboratory rototye, and care was taken in order to accurately model the assive comonents. In Fig. 3 it is ossible to see the electric circuit used in simulations. This circuit is comosed by an unbalanced RL linear load with the characteristics resented in Table I and by a diode bridge rectifier with a RC load in the dc side (R = 18 Ω,C = 8 μf). TABLE I LINEAR LOAD CHARACTERISTICS Phase a Phase b Phase c Neutral R,5 Ω 1, Ω 5,3Ω ---- L,3 H, H, H ---- Current 4,3 A 3,6 A 3,6 A,9 A Power Factor,8,16, The voltages shown in Fig. 4 have a 6% THD and a 1.6 % unbalance because of the source imedance, comosed by a resistance and an inductance ,16,17,18,19, Fig. 4 System Voltages

4 Fig. 5 shows the source currents and its harmonic contents before the connection of the active ower filter. The initial control strategy used does not aly any kind of current limitation. If the non-linear load is connected while the Shunt Active Filter is comensating the control theory will try to comensate the transient that occurs because of the load connection. In Fig. 7 it is ossible to see the load currents when the non linear load is connected at.1 s. It can be seen that the current transient is very high because there is no soft-start, however this was done in order to effectively test the control algorithm for limitation of the inverter current. Ph. a Ph. b Ph. c 4 5 THD = 63,1 % Neut. 4 5 THD = 1,1 % 3 THD = 6,3 % 3-5, THD = 51,7 % 3 B. Connection of a Non-Linear Load Without Current Limitation,4,5,6-1,3 F ,16 Harmonics Fig. 5 Load Currents,1,6,31 Fig. 7 Load currents when the bridge rectifier is connected V. SIMULATION RESULTS The first test consists in evaluating the system behavior when the active ower filter is connected to the system oerating in steady state, and the following tests show the erformance when the filter starts comensating the non-linear load. The analysis will haen with various versions of the control strategy. The transient eriod lasted aroximately 1 ms and during this time the load currents reach values u to 6 A. Fig 8 shows that the filter attemts to resond to this transient, however by looking at Fig 9 it is ossible to see that the hardware doesn t have the ability to roduce currents with a slew rate sufficiently high to comensate this roblem. 6 In Fig. 6 it is ossible to see the source currents and the harmonic contents when the Shunt Active Filter is comensating the load currents in steady state. The THD in all the three hases is lower than 4%. The ower factor and the current unbalance are also comensated and therefore the neutral current is ractically zero. 4 A. Steady State Performance ,17,7,3 Fig. 8 Comensation currents calculated by the controller 6 Ph. a Ph. b THD = 3,9 % 3-6,17 1 Ph. c,,7,3 Fig. 9 Comensation currents roduced by the active ower filter 4 5 THD = 3,4 % Neut. 4 5 THD = ,3,4,5, THD = 3,7 % 5,, F Harmonics Fig. 6 Source currents when the Shunt Active Filter is in steady state. 3 Although the currents injected by the active ower filter are significantly smaller than the currents calculated by the controller (Fig. 8 and Fig. 9), its value clearly exceeds the maximum allowed by the ower inverter IGBTs. In the real hardware this fact could damage the inverter, or at least it the active ower filter rotections would tri, which would disrut the comensation.

5 The transient caused by the connection of the non linear load also causes instability in the dc side of the active filter inverter. In Fig. 1 it is ossible to see the voltage waveform at the dc side of the filter inverter. Until the connection of the active filter, the dc voltage is stable around 5 V. When the active filter is turned on at.14 s the dc voltage increases and after the initial transient eriod remains stable around 8 V. At.1 s the connection of the non-linear load causes a large oscillation and the dc voltage reaches values greater than 1 V. This high dc voltage also tris the active ower filter rotections and can damage the dc side caacitor or the semiconductors ,1,15,,5,3 Fig. 1 Voltage at the dc caacitor of the active ower filter C. Connection of the Non-Linear Load with a Simle Limitation Technique To mitigate the roblems caused by the transients occurred during the connection and disconnection of non linear loads in the facility it is necessary to ugrade the controller. A very simle algorithm that limits the maximum reference currents at 95% of the maximum value allowed by the inverter IGBTs was imlemented. This was done by means of a hard limiter. In this simulation the active ower filter is already running when the non linear load is connected at.1 s. Fig. 11 shows the reference currents calculated by the filter controller. In this figure it is erfectly visible the limitations alied to the reference currents ,17,,7,3 Fig. 11 Comensation currents calculated by the controller In Fig. 1 the currents roduced by the active ower filter are resented. Although the limitation alied to the reference currents was imlemented, the active filter currents exceeds the value of 1 A allowed by the active filter IGBTs ,17,,7,3 Fig. 1 Comensation currents roduced by the active ower filter As it is ossible to see in Fig. 13, in the dc side of the filter the roblem remains unsolved. The dc voltage reaches values of about 1 V, and the time that the voltage requires to stabilize is the same as before ,1,15,,5,3 Fig. 13 Voltage at the dc caacitor of the active ower filter D. Connection of the Non-Linear Load with Dynamic Current Limitation As it can be concluded by the revious simulation, the limitation of the current is not a very simle task and requires some secial attention. Since the delay comensation delays the active filter resonse by a half cycle, it is ossible to use this time to determine the highest eak values between the four comensation currents calculated by the controller. Once the highest value is determined it is a simle matter to determine how much all the reference signals should be attenuated in order to revent an overcurrent at the inverter. Fig. 14 shows that the reference currents are now below the maximum currents allowed at the inverter, and Fig 15 demonstrates that the inverter current follows the reference, keeing the current below 1 A ,17,,7,3 Fig. 14 Comensation currents calculated by the controller

6 1-1 -,17,,7,3 Fig. 15 Comensation currents roduced by the active ower filter The dc voltage is also more stable with the current limitation. It is ossible to see on Fig. 16 that the dc voltage has a smaller transient in terms of amlitude and also in terms of length ,1,15,,5,3 Fig. 16 Voltage at the dc caacitor of the active ower filter When the Shunt Active Filter reaches stability, the source currents will be exactly like the ones shown in Fig. 6. VI. CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORKS A strategy to dynamically limit the outut currents of an active ower filter was resented. The simulation results show a good erformance of the Shunt Active Filter, when the load currents have a very high current transient. This strategy can be imroved, because ideally the limitation should be made on the comensation owers x and q x. In future work the current limitation will be controlled directly through the limitation of the comensation owers (x and qx). This way the limitation strategy will be more comlex because each one of the outut currents deends of the combination of the two owers and is necessary to limit both values simultaneously. On the other hand with this technique it will be ossible to limit only certain ower comonents, keeing others fully comensated. For instance, for an industry it will be usually more imortant to comensate the ower factor and therefore, if the active filter is overloaded, all the other ower comonents could be limited, excet for the one resonsible for the ower factor. ACKNOWLEDGMENT This work was suorted by the FCT (Fundação ara a Ciência e a Tecnologia), roject funding POCTI/ESE/4117/1 and POCTI/ESE/484/. The authors are also grateful to PRIME (Programa de Incentivos à Modernização da Economia) for funding the Project SINUS. Luís F. C. Monteiro is grateful to Alban rogram for suorting his education roject at the University of Minho. REFERENCES [1] L. Gyugi and E. C. Strycula, Active AC Power Filters, IEEE-IAS Annual Meeting Record, 1976, [] Verdelho, P., Marques, G., An Active Power Filter and Unbalanced Current Comensator, IEEE Trans. Ind. Electronics, vol. 44, no. 3, June 1997, [3] R. Pregitzer, J.C. Costa, Júlio S. Martins, J. L. Afonso, Simulation and Imlementation Results of a 3 Phase 4 Wire Shunt Active Power Filter, CDROM Proceedings of the ICHQP 6 International Conference on Harmonics and Quality of Power, Cascais, Portugal, 1-5 October 6. [4] S. Buso; L. Malesani; P. Mattavelli; Comarison of Current Control Techniques for Active Filter Alications; IEEE Transactions on Industrial Electronics, vol. 45, Issue 5, Oct. 1998; Page(s):7 79. [5] H. Akagi, Y. Kanazawa, A. Nabae, Generalized Theory of the Instantaneous Reactive Power in Three-Phase Circuits, IPEC'83 - Int. Power Electronics Conf., Tokyo, Jaan, 1983, [6] M. Aredes; E. H. Watanabe; New Control Algorithms for Series and Shunt Three-Phase Four-Wire Active Power Filters ; IEEE Transactions on Power Delivery, vol. 1, no. 3, Jul. 1995;.: [7] J. L. Afonso; M. J. S. Freitas; J. S. Martins; -q Theory Power Comonents Calculations; 3 IEEE International Symosium on Industrial Electronics, ISIE '3, vol. 1, 9-11 Jun. 3;.: [8] J. L. Afonso, Filtro Activo Paralelo com Controlo Digital ara Melhoria da Qualidade de Energia Eléctrica, Ph.D. dissertation, Det. Industrial Electronics, Univ. Minho, Portugal,. [9] R. Pregitzer; Comuter Simulations of Active Power Filters in Real Oerating Conditions; MSc. Thesis (in Portuguese); Universidade do Minho; Portugal; Setember 6.