Research On The Key Issues Of The Four-leg Active Power Filter

Technological and economic development has stimulated the demand in electrical energy. In the three-phase four-wire low-voltage power supply system, further application of non-linear loads has brought greater challenges to power quality, coupled with the increasingly prominent problems of harmonic current and unbalanced issue in three-phase four-wire system. Four-leg active power filters (APF) has become one of the effective measures to solve the problem of harmonic in three-phase four-wire system. This paper studied the key technologies of four-leg active power filters.Accurate and rapid detection of harmonic currents is the premise to ensure the good compensation performance of active power filter. Based on the analysis of the traditional FBD algorithm and the harmonic detection method of the generalized integrator, this paper proposed the detection scheme combining the generalized integrator and FBD. The frequency-selective characteristics of the generalized integrator were utilized to extract the components of positive sequence fundamental voltage, combined with the FBD power theory to realize harmonic current detection, The method can reduce the impact of the attenuation of output signal amplitude caused by the generalized integrator itself, and improve the execution speed of the original FBD detection algorithm. Simulations and experiments verified the rationality and feasibility of the method.Four-leg converter has so many switch states and the modulation voltage is also complex, so in order to simplify the modulation algorithm, this paper presented a new strategy, that is, three-dimensional space vector pulse width modulation (3D-SVPWM). Based on the classification algorithm, the region judgment of the reference vector was realized and the duty ratios of the switching vectors were given by the linear operation of the inner products. Simulations and experiments were carried out for two situations on reference voltages without distortion and reference voltages including harmonics. The results show that the proposed modulation algorithm can obtain the output voltage in agreement with reference voltage which verify the correctness and validity of this proposed scheme. The adaptability of different over-modulation strategies for the four-leg APF was studied and the influence of dead-time effect on the output voltage of four-leg active filter was analyzed. In addition, a pulse adjustment method applicable to the dead-time compensation of four-leg inverter was proposed and the simulation results show that this method can effectively compensate for dead-time effect of four-leg converter.The current loop control of active power filter (APF) requires the compensation current to track the given signal without any error, while it is difficult to eliminate steady-state error by using the traditional PI controller in synchronous d-q frame. In this paper, two tracking control schemes were proposed. As to the grid frequency fluctuation and the unbalanced problem of three-phase system, the parallel improved controller in theαβγcoordinate was proposed to obtain zero steady-state error. The paper concluded the effect of PR controller parameters on the performance index and the debugging methods of the parameters. The deadbeat control strategy with plug-in repetitive controller was proposed to realize the current loop control of the four-leg APF and eliminate periodic steady-state error. The design method of the low-pass filter and the compensator were given which were used to improve the system stability. The simulation and experiment were carried out to verify the proposed control strategies. The results suggest that the two proposed methods can both effectively restrain harmonics and solve the unbalanced problem excellently.The parameters selection of power circuit and stability of DC-link voltage are inflence factors to the performance of APF also. The relationship of parameters was analized and the power circuit parameters were determined by using the vector locus analysis method. According to the analysis of energy transfer between the source and DC-link capacitor, control scheme of DC-link voltage were presented by compensating instantaneous active current. Parameters of the controller were designed. Simulation and experimental results show that the method of the power circuit parameters selection is resonable and the DC-link voltage control scheme is effective.