Study On Performance Optimization Technology Of Harmonic Suppression With Shunt Active Power Filter

The power quality of power distribution system has aroused much attention from all around the world, which stimulates the development of power quality conditioning methods. As a high-efficient and flexible harmonic compensation device, shunt active power filter (SAPF) has been widely studied and applied, and it becomes more and more muti-functional nowadays, various power quality problems can be dealt by the active power filters. This paper concentrates on the target of improving the composite harmonic suppression performance of shunt active power filter, some key issues about the performance optimization technology are deeply investigated, including the improved output filter, compensation of output current errors, resonance damping composite control strategy and harmonic compensation characteristics to different nonlinear loads.To improve the harmonic compensation performance of the shunt active power filter, its output current waveform quality must be guaranteed, and a proper designed output filter is a key factor for that. LCL filter is widely used for its excellent switching ripple attenuation ability with relatively small output inductor, but active damping or passive damping methods are needed to suppress the LCL filter resonance. Extra sensors may be needed for voltage and current detection by the active damping control algorithm, and the damping performance will be limited by the control system bandwidth; while the power losses of traditional passive damping methods are too high, and the switching ripple attenuation performance will be degraded by the damping resistor. To avoid such problems, a new kind of LCFL type output filter based on the selective passive damping idea is proposed in this paper. This LCFL filter is constructed by adding an LC resonance branch in parallel with the damping resistor of the traditional passive damped LCL filter. The resonance frequency of the LC branch is set at the switching frequency, so it can bypass most of the switching harmonic current and thus reduce the power losses of the damping resistor, furthermore, it can eliminate the affection of damping resistor to the switching harmonic attenuation performance. The impedance of the LC branch is much higher than the damping resistance at the resonance frequency, so the passive damping performance is guaranteed. The shunt active power filter with a LCFL output filter can work steady and reliable with good switching harmonic attenuation ability but relatively lower damping resistor losses, which could improve the output waveform quality of the shunt active power filter.To ensure good harmonic compensation performance of the SAPF, high-performance output current control strategy must be adopted, and meanwhile the factors that cause output errors must be eliminated. In this paper, a PI and repetitive composite controller is adopted for the output current tracking control of the SAPF, which can make sure good steady accuracy and dynamic response performance. The factors that cause the output current errors are identified, and the influences of the current amplitude and phase errors to the harmonic compensation performance are analyzed. Based on the analyses, two kind of output current error compensation methods are proposed. The first method is a improved recursive Discrete Fourier Transformation (RDFT) algorithm with phase delay compensation function, which compensates the output current phase delay in the reference-extraction process and thus imroves the harmonic compensation accuracy of the shunt active power filter. This algorithm is easy to be implemented, and it is highly flexible and accurate that the phase errors of different harmonic components can be adjusted independently. Furthermore, another method to diminish the output current error is proposed too, which is based on closed-loop regulation of current reference, which is implemented in the harmonic synchronous rotating reference (SRF). The dc component of harmonic errors are extracted in the hamronic dq-SRF, and then the error compensation reference is obtained by a PI regulator. The complete output current reference is composed by the harmonic current reference that directly extracted from the nonlinear load and the error compensation reference. By compensation from the harmonic current reference, the output current error can be effectively diminished, and the harmonic compensation accuracy of the SAPF can be improved.It is easily to cause harmonic resonance when shunt capacitors and harmonic producing loads are installed in the power distribution system, and it is hard to obtain good harmonic suppression performance for the SAPF if it only compensate harmonic current of the nonlinear load. To improve the harmonic suppression performance of the SAPF in such resonance condition, this paper proposes a harmonic current compensation and resonance damping composite control strategy. The SAPF extracts the harmonic compensation current reference from the nonlinear load curren and extracts the resonance damping current reference from the PCC voltage, then the total output current reference is composed by the two components. The mechanism of resonance generation and damping is analyzed through equivalent circuit model, and the influence of load current detection position to the harmonic suppression performance is investigated, and the effectiveness of the harmonic compensation and resonance damping composite control strategy is examined.There are two kinds of typical nonlinear loads that widely used in the power distribution system, namely the voltage-source type and current-source type nonlinear loads. This paper analysis the harmonic compensation characteristic of shunt active power filter to these two kinds of nonlinear loads. It is revealed that when SAPF compensating voltage-source type nonlinear load, it is easily to cause load harmonic current amplification effect. Normal operation of the nonlinear load may be affected, and harmonic compensation performance of the SAPF will be degraded meanwhile. The relationship of the load harmonic current amplification ratio to the system impedance and equivalent impedance of the nonlinear load is revealed by analyses. Methods about adjusting system impedance relationship to diminish the harmonic amplification effect are proposed, which can make sure good harmonic compensation performance of the SAPF to both kinds of nonlinear loads.Based on the analyses and research, a three-phase three-wire SAPF prototype and test platform were built in the laboratory, and the SAPF devices have been put into practice in some industry fields. The results of experiment and industry field tests have shown that the harmonic suppression optimization technologies proposed in this paper can be properly applied in the SAPF and improve its composite harmonic suppression performance. It is hopefully that the work of this paper can simulate the widespread application of shunt active power filter.