Improvement of the Electric Power Quality Using Series Active and Shunt Passive Filters P. Salmerón and S. P. Litrán

Transcription

1 1058 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 2, APRIL 2010 Improvement of the Electric Power Quality Using Series Active and Shunt Passive Filters P. Salmerón and S. P. Litrán Abstract A control algorithm for a three-phase hybrid power filter is proposed. It is constituted by a series active filter and a passive filter connected in parallel with the load. The control strategy is based on the vectorial theory dual formulation of instantaneous reactive power, so that the voltage waveform injected by the active filter is able to compensate the reactive power and the load current harmonics and to balance asymmetrical loads. The proposed algorithm also improves the behavior of the passive filter. Simulations have been carried out on the MATLAB-Simulink platform with different loads and with variation in the source impedance. This analysis allowed an experimental prototype to be developed. Experimental and simulation results are presented. Index Terms Active power filters, harmonics, hybrid filters, instantaneous reactive power, power quality. I. INTRODUCTION T HE increase of nonlinear loads due to the proliferation of electronic equipment causes power quality in the power system to deteriorate. Harmonic current drawn from a supply by the nonlinear load results in the distortion of the supply voltage waveform at the point of common coupling (PCC) due to the source impedance. Both distorted current and voltage may cause end-user equipment to malfunction, conductors to overheat and may reduce the efficiency and life expectancy of the equipment connected at the PCC. Traditionally, a passive LC power filter is used to eliminate current harmonics when it is connected in parallel with the load [1]. This compensation equipment has some drawbacks [2] [4], due to which the passive filter cannot provide a complete solution. These disadvantages are mainly the following. The compensation characteristics heavily depend on the system impedance because the filter impedance has to be smaller than the source impedance in order to eliminate source current harmonics. Overloads can happen in the passive filter due to the circulation of harmonics coming from nonlinear loads connected near the connection point of the passive filter. They are not suitable for variable loads, since, on one hand, they are designed for a specific reactive power, and on the other hand, the variation of the load impedance can detune the filter. Manuscript received October 24, 2008; revised May 07, First published December 28, 2009; current version published March 24, Paper no. TPWRD The authors are with the Electrical Engineering Department, Huelva University, Huelva, Spain ( patricio@uhu.es; salvador@uhu.es). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TPWRD Fig. 1. Series active filter and parallel passive filter. Series and/or parallel resonances with the rest of the system can appear. An active power filter, APF, typically consists of a threephase pulsewidth modulation (PWM) voltage source inverter [5]. When this equipment is connected in series to the ac source impedance it is possible to improve the compensation characteristics of the passive filters in parallel connection [6], [7]. This topology is shown in Fig. 1, where the active filter is represented by a controlled source, where is the voltage that the inverter should generate to achieve the objective of the proposed control algorithm. Different techniques have been applied to obtain a control signal for the active filter [8] [10]. One such is the generation of a voltage proportional to the source current harmonics [11], [12]. With this control algorithm, the elimination of series and/or parallel resonances with the rest of the system is possible. The active filter can prevent the passive filter becoming a harmonics drain on the close loads. Additionally, it can prevent the compensation features depending on the system impedance. From the theoretical point of view, the ideal situation would be that the proportionality constant, k, between the active filter output voltage and source current harmonics, had a high value. However, at the limit this would be an infinite value and would mean that the control objective was impossible to achieve. The chosen k value is usually small so as to avoid high power active filters and instabilities in the system. However, the choice of the appropriate k value is an unsolved question since it is related to the passive filter and the source impedance values. Besides, this strategy is not suitable for use in systems with variable loads because the passive filter reactive power is constant, and therefore, the set compensation equipment and load has a variable power factor. In another proposed control technique, the APF generates a voltage waveform similar to the voltage harmonics at the load side but in opposition [13]. This strategy only prevents the parallel passive filter depending on the source impedance; the other limitations of the passive filter nevertheless remain /$ IEEE

2 SALMERÓN AND LITRÁN: IMPROVEMENT OF THE ELECTRIC POWER QUALITY USING SERIES ACTIVE AND SHUNT PASSIVE FILTERS 1059 Fig. 2. Transformation from the phase reference system (abc) to the 0 system. Other control strategies combining both the above have been proposed to improve the parallel passive filter compensation characteristics [14], [15], but they continue to suffer from the difficulty of finding an appropriate value for the APF gain k. Finally, another approach has recently been proposed [16]. It suggests that the active filter generates a voltage which compensates the passive filter and load reactive power, so it allows the current harmonics to be eliminated. The calculation algorithm is based on the instantaneous reactive power theory [17]. There, the control target is to achieve constant power in the source side. In this paper a new control strategy based on the dual formulation of the electric power vectorial theory [18], [19] is proposed. For this, a balanced and resistive load is considered as reference load. The strategy obtains the voltage that the active filter has to generate to attain the objective of achieving ideal behavior for the set hybrid filter-load. When the source voltages are sinusoidal and balanced the power factor is unity, in other words, the load reactive power is compensated and the source current harmonics are eliminated. By this means, it is possible to improve the passive filter compensation characteristics without depending on the system impedance, since the set load-filter would present resistive behavior. It also avoids the danger that the passive filter behaves as a harmonic drain of close loads and likewise the risk of possible series and/or parallel resonances with the rest of the system. In addition, the compensation is also possible with variable loads, not affecting the possible the passive filter detuning. Although the APF series control based on the instantaneous reactive power theory is not new, in this paper the authors propose a new formulation that has consequences in the control loop design. In fact, the instantaneous reactive power here is defined from a dot product whereas in [16], [17] it is defined as a cross product; this results in a remarkable simplification in the implementation of the reference generation method. The final development allows any compensation strategy to be obtained, among them, unit power factor. The compensated electric system was simulated in MATLAB-Simulink, and the strategy was applied to a three-phase system with balanced and unbalanced loads. The simulation results used to verify the theoretical behavior are presented. Finally, an experimental prototype was manufactured and its behavior checked. Experimental results are also presented. II. THE DUAL INSTANTANEOUS REACTIVE POWER THEORY The instantaneous reactive power theory is the most widely used as a control strategy for the APF. It is mainly applied to Fig. 3. Three-phase system. compensation equipment in parallel connection. This theory is based on a Clarke coordinate transformation from the phase coordinates (see Fig. 2). In a three-phase system such as that presented in Fig. 3, voltage and current vectors can be defined by The vector transformations from the phase reference system a-b-c to coordinates can be obtained, thus frame is cal- The instantaneous real power in the culated as follows: This power can be written as (1) (2) (3) where is the instantaneous real power without zero sequence component and given by It can be written in vectorial form by means of dot product where is the transposed current vector in coordinates is the voltage vector in the same coor- In the same way, dinates (4) (5) (6) (7) (8) In (5), is the zero sequence instantaneous power, calculated as follows: (9) (10) In a three-wire system there are no zero-sequence current components, that is,. In this case, only the instantaneous

3 1060 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 2, APRIL 2010 Fig. 4. Decomposition of the voltage vector. power defined on the axes exists, because the product is always zero. The imaginary instantaneous power is defined by the equation (11) In accordance with (7), this can be expressed by means of the dot product where is the transposed current vector perpendicular to and it can be defined as follows: (12) (13) Both power variables previously defined can be expressed as (14) In the plane, and vectors establish two coordinates axes. The voltage vector can be decomposed in its orthogonal projection on the axis defined by the currents vectors, Fig. 4. By means of the current vectors and the real and imaginary instantaneous power, the voltage vector can be calculated (15) In a four-wire system, the zero sequence instantaneous power,, is not null. In this case, (15) would have to include an additional term with the form, where is the zero sequence current vector. III. COMPENSATION STRATEGY Electric companies try to generate electrical power as sinusoidal and balanced voltages so it has been obtained as a reference condition in the supply. Due to this fact, the compensation target is based on an ideal reference load which must be resistive, balanced and linear. It means that the source currents are collinear to the supply voltages and the system will have unity power factor. If, in Fig. 3, voltages are considered as balanced and sinusoidal, ideal currents will be proportional to the supply voltages is the equivalent resistance, the load current vector. (16) the load voltage vector and Fig. 5. System with compensation equipment. The average power supplied by the source will be (17) In this equation, is the square rms value of the fundamental harmonics of the source current vector. It must be supposed that when voltage is sinusoidal and balanced, only the current fundamental component transports the power consumed by the load. Compensator instantaneous power is the difference between the total real instantaneous power required by the load and the instantaneous power supplied by the source (18) In this equation, the average power exchanged by the compensator has to be null, that is (19) When average values are calculated in (18), and (17) and (19) are taken into account Therefore, the equivalent resistance can be calculated as where is the load average power, defined as (20) (21) (22) Fig. 5 shows the system with series active filter, parallel passive filter and unbalanced and nonsinusoidal load. The aim is that the set compensation equipment and load has an ideal behavior from the PCC. The voltage at the active filter connection point in coordinates can be calculated as follows: (23) is the source current in coordinates. In this equation, the restriction of null average power exchanged by the active filter is imposed.

4 SALMERÓN AND LITRÁN: IMPROVEMENT OF THE ELECTRIC POWER QUALITY USING SERIES ACTIVE AND SHUNT PASSIVE FILTERS 1061 The load voltage is given according to (15) by (24) where is the real instantaneous power and is the load imaginary instantaneous power. The reference signal for the output voltage of the active filter is (25) Considering (23) and (24), the compensation voltage is (26) When the active filter supplies this compensation voltage, the set load and compensation equipment behaves as a resistor. Finally, if currents are unbalanced and nonsinusoidal, a balanced resistive load is considered as ideal reference load. Therefore, the equivalent resistance must be defined by the equation (27) Here, is the square rms value of the positive sequence fundamental component. In this case, (26) is modified, where is replaced by, that is (28) Reference signals are obtained by means of the reference calculator shown in Fig. 9 and Fig. 10. In the case of unbalanced loads, the block fundamental component calculation in Fig. 9 is replaced by the scheme shown in Fig. 17, which calculates the current positive sequence fundamental component. The compensation target imposed on a four-wire system is the one presented in (16). However, a modification in the control scheme of Fig. 9 is necessary. This consists in including a third input signal from the zero sequence power in the control block where is generated. The proposed control strategy may be suitable in a stiff feeder, where voltage could be considered undistorted. IV. SIMULATION RESULTS The system shown in Fig. 6 has been simulated in the Matlab- Simulink platform to verify the proposed control. Each power device has been modeled using the SimPowerSystem toolbox library. The power circuit is a three-phase system supplied by a sinusoidal balanced three-phase 100-V source with a source inductance of 5.8 mh and a source resistance of 3.6. The inverter consists of an Insulated Gate Bipolar Transistor (IGBT) bridge. On the dc side, two 100-V dc sources are connected. An LC filter has been included to eliminate the high frequency components at the output of the inverter. This set is connected to the power system by means of three single-phase transformers with a turn ratio of 1:1. Fig. 6. Series active power and passive filter topology. TABLE I PASSIVE ELEMENT VALUES The passive filter is constituted by two LC branches tuned to the fifth and seventh harmonics. Each element value is included in Table I. The selection criteria to fix the ripple filter were, in the case of low frequency components, that the inverter output voltage be almost equal to voltage across. However, in the case of high-frequency components, the reduced voltage in must be higher than in the capacitor. Furthermore, and values must be selected so as not to exceed the transformer burden. Therefore, the following design criteria must be satisfied., to ensure that inverter output voltage drops across at the switching frequency;, to ensure that voltage divider is between and, where is the source impedance, the shunt passive filter, reflected by the secondary winding. At 20-kHz switching frequency,,,,. Two load types were simulated: nonlinear balanced load; nonlinear unbalanced load. A. Case 1: Nonlinear Balanced Load In this case, the nonlinear load consists of an uncontrolled three-phase rectifier with an inductance of 55 mh and a 25 resistor connected in series on the dc side. Fig. 7 shows the phase a load current. The load current total harmonics distortion (THD) is 18.6% and the power factor 0.947, when the system is not compensated. The 5th and 7th harmonics are the most important in the current waveform. They are 16.3% and 8.4% of the fundamental harmonic, respectively. Two LC branches were connected to mitigate the fifth and seventh harmonics. The source current waveform with the passive filter connected is shown in Fig. 8. The THD falls to 4.7%. The 5th and 7th harmonics decrease to 3.6% and 0.9%, respectively. The passive filter was designed only to compensate the

5 1062 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 2, APRIL 2010 Fig. 10. Calculation fundamental component. Fig. 7. Load current of the phase a. Fig. 11. Source current when the active filter is connected. Fig. 8. Source current when the passive filter is connected. Fig. 9. Control scheme. source current harmonics; the reactive power was not considered. The power factor of the set load and passive filter is To simulate the active filter series connection with a passive filter, Fig. 6, it is necessary to determine the APF reference signal. The control scheme used to calculate the active filter compensation voltage is shown in Fig. 9. The voltage vector on the load side and the source current vectors are the input signals. By means of a calculation block, e vectors in coordinates can be determined. The product of these vectors allows the instantaneous real power to be calculated, obtaining its average value with a low-pass filter (LPF). This is the power required by the set passive filter and load. The average power is divided by the average value of the sum of the square of the instantaneous values of the current fundamental component. The fundamental component is obtained by means of a block with the scheme shown in Fig. 10. Each component of the source current vector is multiplied by and where is the fundamental frequency in rad/s. The average values of the results are obtained using two low-pass filters. They are multiplied by and again and then by 2. This allows the current vector fundamental component to be obtained; conversely, the real instantaneous power is divided by. The result is multiplied by the current vector, which allows the first term in the compensation voltage in (26) to be determined. On the other hand, the imaginary instantaneous power is obtained and divided by and finally multiplied by the current vector. This determines the second term in the compensation voltage (26). When the active filter is connected, the source current THD falls to 1.30%. The waveform is shown in Fig. 11. Now, the power factor rises to This allows the proposed control to be verified, the passive filter compensation characteristic to be improved and unity power factor is practically achieved. The passive filter impedance has to be lower than the system impedance in order to be effective. When the branch LC or impedance source quality factor is low, the harmonics filtering deteriorates. To verify the behavior of the compensation equipment in this situation, the source impedance is modified. It is changed from 3.6 and 5.8 mh to 1.3 and 2.34 mh. The source current has a THD of 10.1%, when the active filter is not connected and the compensation equipment is only the passive filter. The source current waveform is shown in Fig. 12. The 5th and 7th harmonics are 8.2% and 2.5% of the fundamental harmonic and the power factor is 0.97.

6 SALMERÓN AND LITRÁN: IMPROVEMENT OF THE ELECTRIC POWER QUALITY USING SERIES ACTIVE AND SHUNT PASSIVE FILTERS 1063 Fig. 12. Source current when passive filter is only connected. Source impedance, 1.3 and 2.34 mh. Fig. 15. Source current when the active filter is connected and the resistor on the dc side is 50. TABLE II PASSIVE ELEMENT VALUES OF THE LOAD Fig. 13. Source current when the active filter is connected. Source impedance, 1.3 and 2.34 mh. Fig. 16. Source current without filters. Unbalanced load. This control algorithm can compensate reactive power and can improve the behavior of the passive filter. Fig. 14. Source current when only the passive filter is connected and the resistor on the dc side is 50. When the active filter is working with the proposed control algorithm, the THD of the source current improves to 1.6% and the power factor is This current waveform is shown in Fig. 13. In a distribution system, variations may appear in the load power. To verify the behavior of the proposed control algorithm, the resistor value connected at the dc side of the uncontrolled three-phase rectifier was changed from 25 to 50. The source impedance is 3.6 and 5.8 mh. When the passive filter is connected, the source current waveform is shown in the Fig. 14, which has a THD of 4.9%. In this case, the power factor is With the active filter connected the source current has the waveform shown in Fig. 15. The THD falls from 4.9 to 1.8% and the power factor rises to 0.99 Therefore, with the proposed control algorithm, the set active filter and passive filter allow the compensation of variable loads. B. Case 2: Non-Linear Unbalanced Load In this case, the three-phase load is built with three singlephase uncontrolled rectifiers with capacitors and resistors connected in parallel at the dc side with the values shown in Table II. Fig. 16 shows the source currents in the uncompensated system. Current THDs of a, b, and c phase are 18.8%, 35.0%, and 37.6%, respectively. The control scheme for the active filter shown in Fig. 9, is modified for unbalanced loads. The block fundamental component calculation in Fig. 9 is replaced by the scheme shown in Fig. 17. Now the average power P is divided by the square rms value of positive sequence fundamental component. In this case, the positive sequence component is calculated by means of the block positive sequence component, where the operator necessary to implement the Fortescue transformation is obtained with an all pass filter. Subsequently, its fundamental value is calculated and the Fortescue inverse transformation applied. Fig. 18 shows the three source currents when this control is applied to the active filter. The system presents a behavior similar to a resistive and balanced load. The source currents THD are 1.4%, 0.85%, and 1.3% in phases a, b, and c.

7 1064 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 2, APRIL 2010 Fig. 17. Modification in the control scheme for unbalanced load. Fig. 20. Source voltage and current, phase a, uncompensated system. Voltage 48 V/div and current 4 A/div. Fig. 18. Source current with active filters and unbalanced load. Fig. 19. Experimental prototype. In (16), the compensation aim is to obtain unity power factor. When the voltage is sinusoidal and balanced, a sinusoidal and balanced source current is obtained. When the voltage is distorted, unity power factor is achieved, although the source current is distorted. V. EXPERIMENTAL RESULTS The experimental prototype which has been developed is shown in Fig. 19. It is a three-phase system supplied by a sinusoidal balanced three-phase 100-V rms source. Fig. 5 shows the power circuit scheme. The converter is a Semikron SKM50GB123-type IGBT bridge at the dc side, two 100-V capacitors are connected. At the ac side, an LC filter has been included to eliminate high switching frequency. This set is matched to the power system by means of three single-phase transformers with a turn ratio of 1:1 to ensure galvanic isolation. Two dc sources in the APF simulation model were considered. It simplifies the control scheme and makes the simulation easier. The prototype also includes two capacitors. The control scheme includes a loop with PI controller to ensure the capacitor voltages remain constant [11]. The passive power filter compensation is formed by two LC branches tuned to the fifth and seventh harmonics. Each passive element values are included in Table I. The control strategy was implemented in a control and general application data acquisition cards compatible with Matlab- Simulink and developed by dspace were used. dspace Real-Time Interface (RTI) together with Mathworks Real Time Workshop (RTW) automatically generate real time code in dspace. This allows the processor board to be programmed and I/O boards to be selected. It is based on the DS 1005 PPC placed in a dspace expansion box. The input board was the dspace DS 2004 A/D and the output board the DS DWO. The control board has a PowerPC 750GX processor running at 1 GHz. For the experimental prototype the sampling rate was limited to 50 in order to avoid overrun errors. A hysteresis band control was developed. The IGBT gating signals were generated comparing the reference signal with the inverter output voltage considering a hysteresis band. This method switches the transistor when the error exceeds a fixed magnitude: the hysteresis band. Therefore, the switching frequency is variable, although in our design this frequency is limited to 20 khz so as not to reach the IGBTs maximum switching frequency. The prototype was verified using two different loads: a nonlinear balanced load and a nonlinear unbalanced load. A. Case 1. Nonlinear Balanced Load In the first experiment the source impedance has a inductance of 5.8 mh and resistance of 3.6. The nonlinear load consists of an uncontrolled three-phase rectifier with an inductance of 55 mh and a 25 resistor connected on the dc side. Fig. 20 shows the phase a load current and voltage, measured without compensation. This waveform is obtained using a LECROY Wavesurfer 424- type oscilloscope. The current THD measured using the Fluke 43 power quality analyzer is 21.5% and the voltage THD 11.2%. The measured power factor is Fig. 21 shows source current and voltage at the connection point when the shunt passive filter is connected. This current and voltage have a THD of 5.1% and 4.3% respectively. The passive power filter was designed to eliminate source current harmonics; the reactive power was not taken into account. The

8 SALMERÓN AND LITRÁN: IMPROVEMENT OF THE ELECTRIC POWER QUALITY USING SERIES ACTIVE AND SHUNT PASSIVE FILTERS 1065 Fig. 21. Source voltage and current, phase a, in the system with passive filter. Voltage 48 V/div and current 4 A/div. Fig. 23. Source voltage and current, phase a, system with passive filter and source impedance modified. Voltage 48 V/div and current 10 A/div. Fig. 22. Source voltage and current, phase a, system with active filter. Voltage 48 V/div and current 4 A/div. Fig. 24. Source voltage and current, phase a. System with active and passive filter and variation in source impedance. Voltage 48 V/div and current 4 A/div. power factor corresponding to the set load and passive filter is When the series active filter is connected, the source current THD falls to 1.0%. This current waveform is shown in Fig. 22. Now, the power factor rises to 0.99, practically achieving unity power factor. The voltage THD in the PCC is 1.3%. Its waveform is shown in Fig. 22 with the phase a current. These results verify the resistive behavior of the system; voltage and current are sinusoidal and they are in phase. Another experiment to verify the behavior of the system was to modify the source impedance. This was changed from 3.6 and 5.8 mh to 1.3 and 2.34 mh. Fig. 23 shows the voltage and source current waveforms at the point of common coupling when the load is compensated with only a passive filter. The waveforms are worse than the previous experiment. The voltage and current THD are 5.5% and 10.8%, respectively. The power factor is When the active filter is connected, the voltage THD at the point of common coupling is 0.8% and the source current THD is 0.9%. The power factor rises to This waveform is shown in Fig. 24. Another typical situation in a power system is that corresponding to variable load. To verify hybrid filter behavior, the resistance at the dc side is changed from 25 to 50. Fig. 25 shows the voltage and current waveforms with the passive filter connected. When the active filter is connected, the voltage THD Fig. 25. Source voltage and current, phase a, system with passive filter and variation in load. Voltage 48 V/div and current 10 A/div. falls from 7% to 1.5% and the current THD changes from 5.7% to 0.5%. Fig. 26 shows the waveforms. Finally, to verify the system dynamic response experimentally, load impedance was changed and current waveform was recorded. The APF transient performance was mainly determined by the dynamic response of the low-pass filters in the control scheme. The control circuit was automatically implemented with MATLAB-Simulink and the toolbox Real Time Workshop (RTW) together with the Real Time Interface (RTI) from dspace, which generated the code to program the control board. Therefore, the low-pass filters were implemented within

9 1066 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 2, APRIL 2010 Fig. 26. Source voltage and current, phase a. System with active filter and variation in load. Voltage 48 V/div and current 10 A/div. Fig. 28. Source currents, system without compensating, unbalanced load. 4 A/div. Fig. 27. Dynamic response of the compensated system. Current 4 A/div. Fig. 29. Source currents, system with active filter, unbalanced load. 4 A/div. a Simulink block. This block was modeled as a second order filter with a cut-off frequency fixed to 100 Hz and the damping factor to It allowed a settling time of 8 ms to be achieved. Fig. 27 shows the dynamic response of the source current when a sudden change of load happens. The resistance at the dc side of the three-phase rectifier changes from 50 to 25. The response time is about 4 cycles. B. Case 2. Nonlinear Unbalanced Load The nonlinear unbalanced load consisted of three single-phase uncontrolled rectifiers with a capacitor and a resistor connected in parallel at the dc side. The capacitor was the same in the three phases, 2200, however, the resistor was different for each one. It was 16.7 for phase a, 25 for phase b and 50 for phase c. Fig. 28 shows the three source currents. The THD measured was 15.6%, 38.7%, and 42.2% in each phase. When the active filter was connected using the proposed control strategy for unbalanced loads, the source currents waveforms were almost sinusoidal and balanced, which is the aim of the control strategy. Fig. 29 shows these currents. The THD values are 1.8%, 1.0%, and 1.7% in each phase. Fig. 30 shows the common connection point voltage and source current in phase a. Both signals are almost sinusoidal and in phase. When only phase a is considered the power factor measured is Fig. 30. Common coupling connection voltage and source current in phase a. system with active filter. Unbalanced load. Voltage 48 V/div and current 10 A/div. VI. CONCLUSIONS A control algorithm for a hybrid power filter constituted by a series active filter and a passive filter connected in parallel with the load is proposed. The control strategy is based on the dual vectorial theory of electric power. The new control approach achieves the following targets. The compensation characteristics of the hybrid compensator do not depend on the system impedance. The set hybrid filter and load presents a resistive behavior. This fact eliminates the risk of overload due to the current harmonics of nonlinear loads close to the compensated system.

10 SALMERÓN AND LITRÁN: IMPROVEMENT OF THE ELECTRIC POWER QUALITY USING SERIES ACTIVE AND SHUNT PASSIVE FILTERS 1067 This compensator can be applied to loads with random power variation as it is not affected by changes in the tuning frequency of the passive filter. Furthermore, the reactive power variation is compensated by the active filter. Series and/or parallel resonances with the rest of the system are avoided because compensation equipment and load presents resistive behavior. Therefore, with the proposed control algorithm, the active filter improves the harmonic compensation features of the passive filter and the power factor of the load. Simulations with the MATLAB-Simulink platform were performed with different loads and with variation in the source impedance. This fact allowed an experimental prototype to be developed. Experimental and simulation results are presented. They allow the verification of the developed theoretical analysis. REFERENCES [1] F. Z. Peng and D. J. Adams, Harmonics sources and filtering approaches, in Proc. Industry Applications Conf., Oct. 1999, vol. 1, pp [2] J. C. Das, Passive filters-potentialities and limitations, IEEE Trans. Ind. Appli., vol. 40, no. 1, pp , Jan [3] H. L. Ginn, III and L. S. Czarnecki, An optimization based method for selection of resonant harmonic filter branch parameters, IEEE Trans. Power Del., vol. 21, no. 3, pp , Jul [4] J. A. Pomilio and S. M. Deckmann, Characterization and compensation of harmonics and reactive power of residential and commercial loads, IEEE Trans. Power Del., vol. 22, no. 2, pp , Apr [5] H. Akagi, Active harmonic filters, Proc. IEEE, vol. 93, no. 12, pp , Dec [6] B. Singh, K. Al-Haddad, and A. Chandra, A review of active filters for power quality improvement, IEEE Trans. Ind. Electron., vol. 46, no. 5, pp , Oct [7] J. W. Dixon, G. Venegas, and L. A. Moran, A series active power filter based on a sinusoidal current-controlled voltage-source inverter, IEEE Trans. Ind. Electron., vol. 44, no. 5, pp , Oct [8] S. George and V. Agarwal, Optimum control of selective and total harmonic distortion in current and voltage under nonsinusoidal conditions, IEEE Trans. Power Del., vol. 23, no. 2, pp , Apr [9] H. Yang and S. Ren, A practical series-shunt hybrid active power filter based on fundamental magnetic potential self-balance, IEEE Trans. Power Del., vol. 23, no. 4, pp , Oct [10] A. Luo, Z. Shuai, W. Zhu, R. Fan, and C. Tu, Development of hybrid active power filter based on the adaptive fuzzy dividing frequency-control method, IEEE Trans Power Del., vol. 24, no. 1, pp , Jan [11] F. Z. Peng, H. Akagi, and A. Nabae, A novel harmonic power filter, in Proc. IEEE/PESC, Apr. 1988, pp [12] F. Z. Peng, H. Akagi, and A. Nabae, A new approach to harmonic compensation in power systems-a combined system of shunt passive and series active filters, IEEE Trans. Ind. Appl., vol. 26, no. 6, pp , Nov./Dec [13] H. Zhong, P. Chen, Z. Lu, Z. Qian, and H. Ma, Three-phase fourwire series hybrid active power filter with fundamental current bypass channel, in Proc. Industrial Electronics Society (IECON), Nov. 2004, vol. 1, pp [14] Z. Wang, Q. Wang, W. Yao, and J. Liu, A series active power filter adopting hybrid control approach, IEEE Trans. Power Electron., vol. 16, no. 3, pp , May [15] G.-M. Lee, D.-C. Lee, and J.-K. Seok, Control of series active power filters compensating for source voltage unbalance and current harmonics, IEEE Trans. Ind. Electron., vol. 51, no. 1, pp , Feb [16] Y. S. Kim, J. S. Kim, and S. H. Ko, Three-phase three-wire series active power filter, which compensates for harmonics and reactive power, IEE Proc. Electric. Power Applications, vol. 151, no. 3, pp , May [17] F. Z. Peng and J. S. Lai, Generalized instantaneous reactive power theory for three phase power system, IEEE Trans. Instrum. Meas., vol. 45, no. 1, pp , [18] R. S. Herrera and P. Salmerón, Instantaneous reactive power theory: A comparative evaluation of different formulations, IEEE Trans. Power Del., vol. 22, no. 1, pp , Jan [19] P. Salmerón, R. S. Herrera, and J. R. Vázquez, Mapping matrices against vectorial frame in the instantaneous reactive power compensation, IET Elect. Power Applic., vol. 1, no. 5, pp , Sep P. Salmerón was born in Huelva, Spain. He received the Ph.D. degree from the Electrical Engineering Department, University of Seville, Seville, Spain, in In 1983, he joined the University of Seville as an Assistant and then Associate Professor in the Department of Electrical Engineering. Since 1993, he has been a Professor, and is currently Dean, at Escuela Politécnica Superior, University of Huelva, Huelva, Spain. He has joined various projects in connection with research in power theory of nonsinusoidal systems and power control in electrical systems. His research interests include electrical power theory, electrical power systems, active power filters, and power conversion systems. S. P. Litrán was born in Sanlúcar de Barrameda, Spain. He received the B.Sc. degree in automation and industrial electronics engineering from the University of Seville, Seville, Spain, in Since 1989, he has been teaching circuit theory and power electric systems with the Escuela Politécnica Superior, Department of Electrical Engineering, University of Huelva, Huelva, Spain. His current research includes power quality, electrical power theory, and active power filters.