Research On Control Strategy Of Three-phase Four-wire Three-level APF

Due to more and more nonlinear loads being used, it leads to a lot of harmonic current injecting into the grid and people pay more attentions to the harmonic problem in the 3-phase 4-wire system. The 3-phase 4-wire 3-level active power filter(APF) has the characteristics of fast response and dynamic continuous real-time compensation and can effectively compensate the harmonic current caused by the nonlinear loads. It is an ideal project to solve the harmonic problem of gird, so this paper focuses on the topology of 3-phase 4-wire neutral-point clamped(NPC) 3-level three-leg structure, and the following key issues of the control algorithm of 3-phase 4-wire 3-level APF is researched:Firstly, the operational principle of NPC 3-level 3-leg structure is analyzed. Single bridge is taken as an example to analyze all kinds of switch states and neutral current’s effect on the voltage of DC capacitor. Then, the mathematic model of 3-phase 4-wire NPC 3-level 3-leg APF is deduced.Secondly, the current control strategy of APF is researched. The ability of current control loop with PI regulator to track the compensation harmonic current instruction is not enough. To solve this problem, the repetitive controller is introduced to the current control loop. A double-loop current control system is put forward by a PI inner loop and repetitive controller external loop. The repetitive controller is given in detail. The analysis shows superiority of double-loop current control system in steady state.Thirdly, the total voltage and the voltage difference of DC capacitance are analyzed. In order to satisfy the stability of DC-bus voltage when the loads change, two high-efficient voltage regulators are designed. In order to make sure that the DC-bus voltage can smoothly up to the pre-value and the whole process don’t have phenomena of over-voltage and over-current, a simple and practical soft-up method of DC-bus voltage is designed.Finally, the relation between DC-bus voltage and power loss, compensation affect and modulation ratio is analyzed. Fluctuations of grid voltage and switching of loads can be reflected by the 3-phase maximal modulation ratio. In order to optimize the power loss and compensation affect synthetically, a method to calculate the DC-bus voltage reference by the maximum modulation ratio is put forward.