A comparative study of shunt hybrid and shunt active power filters for single-phase applications: Simulation and experimental validation

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Abstract

The aim of this paper is to compare the performance of the single-phase shunt active power filter (SPSAPF) and the single-phase shunt hybrid power filter (SPSHPF) that adopt both an indirect current control scheme with a unipolar pulse width modulation (U-PWM) strategy. The SPSHPF topology includes, in addition to the components of the SPSAPF, a power factor correction capacitor connected in series with a transformer. The primary winding of the transformer is connected to the single-phase voltage–source inverter, which is the main part of the filter. The indirect current control technique that is implemented for both filters is based on extracting the source current reference from the distorted waveform of the load current. The U-PWM control technique is based on comparing simultaneously a triangular high frequency carrier signal with a slow-varying regulation signal and its opposite. The double comparison process results in the gate signals for the semiconductors. A laboratory prototype for each filter is built. It is demonstrated that the rating of the inverter used in the SPSHPF is three to four times lower than the one corresponding to the SPSAPF. In addition, the performance of the SPSHPF is found to be much better than that of the SPSAPF as far as the line current distortion is concerned.

Introduction

Active power filters are basically power electronic devices that are used to compensate the current or voltage harmonics and the reactive power flowing in the power grid. The reduction of the harmonic and reactive currents becomes an increasingly required issue owing to the wide use of power electronic equipment. Traditionally, passive LC filters have been used to remove line current harmonics and to improve the power factor. However, when implemented, these passive filters present many drawbacks such as tuning problems, series and parallel resonance. Other industrial applications use power factor correction (PFC) devices for reactive power and current harmonics compensation. In these circuits, switched capacitors banks are typically connected in parallel to current–source-type loads. Seen from the load side, the capacitance of the PFC and the source inductor create a parallel resonant circuit. Looking from the source side, the PFC capacitors and the line inductor represent a series resonant circuit. To prevent resonance due to current harmonics in power systems with PFC equipment, typical shunt or series active topologies have been proposed [3], [10], [11]. However, these topologies suffer from a high kVA-rating of the filter power stage [1], [5]. The boost-type rectifier that constitutes the shunt active filter requires generally a high DC-link voltage [4], [9], [11] in order to compensate effectively the high order harmonics. On the other hand, a series active filter needs a transformer capable to withstand full load current in order to compensate the voltage distortion. Therefore, a hybrid filter topology has been developed achieving the desired damping performance with a significant reduction of the kVA-rating required by the power active filter [3], [4], [6], [11].

In voltage–source inverter applications, the majority of the published papers use a cutting first-order filter Lc through which the inverter is connected to the power grid. This inductor ensures, firstly, the controllability of the active filter current and, secondly, acts as a first-order passive filter attenuating the high frequency ripples generated by the PWM inverter. A relatively low value of Lc improves the dynamics of the active filter but, on the other hand, does not attenuate sufficiently the switching frequency component in the line current. Conversely, a relatively high value of Lc will prevent these high frequency harmonics from flowing through the network but will affect negatively the dynamics of the active filter and degrade, consequently, the quality of compensation. Moreover, these papers do not consider the high frequency harmonics in the evaluation of the total harmonic distortion (THD) of the line current. They give theoretically a THD after compensation lower than 5%. Indeed, a more accurate calculation of the THD that take into account the high frequency harmonics would give easily more than 10%, which is higher than the limit required by international standards such as IEEE-519 and IEC-555-2. However, in this paper, the suggested unipolar PWM control strategy naturally compensates these high frequency harmonics.

In this paper, the operating characteristics of a SPSAPF and a SPSHPF are presented and compared on an economical basis. Both topologies use the indirect current control strategy with the U-PWM technique for current wave shaping. The SPSHPF uses a power factor correction capacitor connected in series with the secondary winding of the transformer. The primary winding of the transformer is connected to a single-phase voltage–source inverter. The compensation capacitor needed for PFC is used as part of the filter to create a single harmonic trap. A relatively low-rated active filter compensates the residual harmonics and prevents the excitation of the resonant circuit [2]. The indirect current control technique is based on the classic demodulation method that allows extracting easily the source current reference from the distorted waveform of the load current. Contrary to the existing methods, the indirect method eliminates switching spikes in the mains current [7], [8] and allows a straightforward determination of the proportional gain Kp, which is considered as a hard task for the direct method, especially when high quality compensation is required. The adopted unipolar PWM control technique is based on comparing simultaneously a triangular high frequency carrier signal with a slow-varying regulation signal (given by the current regulator) and its opposite. This comparison process generates the gate signals for the power switches. The U-PWM pushes back the first significant harmonics towards twice the switching frequency. Furthermore, it eliminates the families of rays that are centered on the odd multiples of the switching frequency. Computer simulation and experimental results will show that better results are achieved with the SPSHPF topology.

Section snippets

Single-phase shunt active power filter topology

The SPSAPF consists of a single-phase full-bridge voltage–source PWM inverter, a DC bus capacitor CDC and an inductor Lc. The inductance, through which the inverter is connected to the power supply network, ensures, firstly, the controllability of the active filter current and acts, secondly, as a first-order passive filter attenuating, thus, the high frequency ripples generated by the inverter. Fig. 1 shows the system under study where the SPSAPF is connected in parallel with the non-linear

Single-phase shunt hybrid power filter topology

Fig. 2 shows the configuration of the SPSHPF. It consists of a full-bridge voltage–source PWM inverter, a DC side capacitor CDC, an inductor Lc, a transformer and a PFC capacitor Cc. The primary winding of the transformer is fed by the inverter. The PFC capacitor and the secondary winding of the transformer are connected in series to form a branch parallel to the non-linear load. The iron core of the transformer contains an air-gap in order to reduce its magnetizing inductance Lμ. The PFC

Extraction of current reference

The performance of an active filter is greatly influenced by the method used for extracting the current reference. There are two control techniques [7], [8] for line current wave shaping in an active filter: the direct current control and the indirect current control. In the direct current control technique, the closed-loop current error is the difference between the desired current ic* and the real current ic at the AC input of the filter. Whereas in the indirect current control strategy, the

Design of the unipolar PWM control

The principle of the unipolar PWM control technique applied with the indirect current control strategy is illustrated in Fig. 4. It is based on two comparisons: firstly, between a slow-varying regulation signal (+β) and a triangular high frequency carrier and, secondly, between the opposite of the regulation signal (−β) and the same carrier. Considering the frequency range of the regulation signal at the carrier, we may consider that the regulation signal (which is also called the modulating

Harmonic analysis of APF inverter input voltage

According to Fig. 4, the voltage vc at the input terminal of the inverter can take two values +VDC and −VDC during both half periods of the modulating signal β. The fundamental frequency of voltage vc is equal to the modulating one; its high frequency components are, however, at twice the switching frequency of T1 or T3. In the following, let us define vc=vc1 when standard PWM is used and vc=vc2 when unipolar PWM is used. Applying Fourier transform to vc1 and vc2 over a switching period Tsw = 1/f

Generation of the gate signals with the U-PWM

The current reference is* (obtained from the extracting algorithm described above) is compared with the sensed current is. The error signal is fed to the current controller having a limiter at its output. The regulation signal delivered by the controller and its opposite are then compared simultaneously with a triangular carrier resulting in the switches gate signals (see Fig. 6).

Equivalent circuit of the SPSHPF

An equivalent circuit of a single-phase power system with the current harmonic producing load and the PFC is shown in Fig. 7. The load is simply represented by the current source iL. The PFC equipment is connected in parallel with this load, and is represented by the capacitor Cp. The PFC creates a resonant circuit together with the line inductance Ls. The transfer function between a source current harmonic (ish) and a load current harmonic (iLh) is given by:H(s)=11+s2LsCc

The resulting resonant

Simulation results with the shunt active power filter

The simulation results of the system with the shunt active power filter are presented in Fig. 10, Fig. 11, Fig. 12, Fig. 13. The load current (iL), SPSAPF current (ic), supply voltage (vs), supply current (is) and DC bus voltage of the SPSAPF (vDC) are depicted in Fig. 10.

The DC-link voltage VDC is set to 350 V and its capacitance has a value of CDC = 2000 μF. The converter output inductance Lc, used to smooth the filter output current ic, equals 1 mH. The average switching frequency of the

Compensation with the shunt passive filter

Fig. 14 shows the load current (iL), the shunt passive filter current (ic), the supply voltage (vs) and the supply current (is) of a non-linear load connected to the supply voltage.

The harmonic spectra of the supply current before and after compensation with the shunt passive filter are shown in Fig. 15, Fig. 16, respectively. The total harmonic distortion is decreased from 27.33 to 14.92%.

Compensation with the shunt hybrid power filter

The load current (iL), the SPSHPF current (ic), the SPSAPF current (iF), the supply voltage (vs), the

Experimental results

In order to verify the results obtained by simulation, a 1 kVA laboratory prototype for each topology has been built.

Conclusion

In this paper, a comparative analysis of the performance of a single-phase shunt active power filter (SPSAPF) and a single-phase shunt hybrid power filter (SPSHPF) is presented. Simulation and experimentation results show that the performance of the SPSHPF is much better than the SPSAPF. Furthermore, the DC-link voltage for the SPSHPF is twice smaller than in a SPSAPF. The filter current of the SPSHPF is also reduced by a factor of 2, and the switching frequency is reduced by a factor of 3

Acknowledgements

The authors wish to thank NSERC and Canada Research chair in Energy conversion and power electronics for supporting this work.

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