Study On An Active Power Filter Based On P-cell And N-cell Unit

The rapid development of power electronics technology not only significantly improves the level of modern electrification, but also inevitably bring about all kinds of power quality problems. Among them, the problem of harmonic and reactive currents injected into the power system is the most serious, which is caused by using nonlinear loads. It will not only reduce the overall operating efficiency of the system, but also affect and disturb other devices, and even cause that custom equipment cannot be safely and stably operate. Active Power Filter has significant advantages for solving the problems above, such as: rapid response, good real-time, not influenced by system parameters, high controllability, so, in its own field, it is always irreplaceable.In this paper, the main focus of the research is on the topology structure of the main circuit, the design of the output filter and current tracking control strategy for the parallel three-phase three-wire APF.Whether the main circuit design is reasonable directly affect the detection and control of the harmonic current. There exists a shoot-through problem of the switch devices in each leg of the traditional converter with only one solution by setting dead time. So, this paper presents a new converter topology based on P-cell and Ncell unit from the basic switch module. The current changing rate is limited by placing an inductance in the public side of the arm, so that the waveform distortion caused by dead zone can be solved. Both the reliability of the system and the quality of the output waveform can be improved by using this topology in APF. Based on this, the mathematical model is established and the working mode of APF is analyzed in this article.The use of the output filter is mainly to suppress the high frequency harmonics, especially the switching frequency harmonics. In this paper, the LCL filter is briefly analyzed, including the establishment of its model, the derivation of transfer function and the analysis of damping loss. However, the LCL filter can only filter part of the high frequency harmonics, especially the switch ing harmonics. In order to further enhance the high frequency attenuation capability of the filter and reduce the volume of the filtering device, a higher order filter LCL-LC filter is proposed in this paper. The resonance characteristics of the filter and the related factors which affect the filtering effect are analyzed, and the filter parameters are designed. By adding passive damping, the stability of the system is improved. Simulation experiment verifies the proposed filter has a stronger suppression for switching harmonics and can better reduce the total harmonic distortion rate of the system.It raises higher mandates for the provided control system to use a high-order filter. The conventional PI control cannot track any other signal except DC signal without error, but the repetitive control has a high steady state accuracy, but its speed is poor. Therefore, the two methods above were combined into a compound control method in this paper, which is able to meet the mandates better of both the speed of response and the steady state accuracy. In this paper, the control model is analyzed and the parameters of the composite controller are designed. The analysis of the stability and steady state characteristics of the system has verified the superiority of the proposed control strategy. Simulation results also show that the harmonic compensation effect of compound control strategy is better than that of PI control.