Research On The Key Technologies Of Three-level Active Power Filter In Threephase Four-wire System

With the widespread application of power electronic techniques, the harmonic has become the major factor that affects the power quality. Adopting active power filters(APF) is one of the efficient methods to compensate the harmonics in the power grid. Compared with two-level APF, three-level APF has smaller current ripple, lower total harmonic distortion(THD), more compact hardware structure, and cheaper passive devices with smaller volume. Therefore, it is more suitable for medium/high-voltage large capacity applications.This thesis proposed an active power filter with neutral point clamped(NPC) three-level topology applied in three-phase four-wire system. The operation principle of shunt active power filter and NPC bridge were analyzed. The dynamics models in both stationary coordinates and rotating coordinates were established. Based on the system model, the design method of main circuit and passive devices was explained. The harmonic current detection strategy, current control strategy, DC-link voltage with neutral point balance control strategy were studied and then analyzed with Matlab/Simulink. Simulation results showed the effectiveness of the control strategies.A prototype with 50 A nominal current has been built. The circuit design including power supplies, IGBT drive circuits, sampling circuits, hardware overcurrent protection circuits and control circuits was elaborated in the thesis. The software design based on TMS320F28335 DSP and CPLD was explained. An experiment platform was set up to verify the proposed control strategies and system design. Two current control methods, proportional-resonance control and PI-repetitive control, were compared in the experiments. The experimental results showed the effectiveness of the prototype. The THD of grid current was declined from over 20% to lower than 5%.