Research On Control System For Three-level Active Power Filter

Due to the limitation of the voltage capability of the power devices, it is very difficult to handle the nonlinear loads for the traditional active power filter(APF) with two-level inverter in high voltage grid. In recent years, multilevel technology has become an effective and practical solution for high-voltage high-power application field. As described in many literatures, using multilevel technology, the voltage stress on switches will be reduced, the shape of output waveform will be improved and the rate of voltage and power can be increased too. Diode-clamped three-level inverter is the most widely used and investigated topology at present.The paper describe the theoretic bases of the proposed APF. The mathematic model in ABC three-phase stator coordinate of the three-level APF is set up. Using coordinate transformation, the mathematic model in dq two-phase rotating coordinate is established. To control the waveform shape of output current, this paper presented a repetitive prediction based deadbeat control scheme. In this scheme, the predictive harmonic current value is obtained by the repetitive predictor. The repetitive predictor can achieve better prediction of the harmonic current with same sampling frequency, thus improves the overall performance of the system.Three-phase command voltages can synthesize a rotate space voltage vector. In this paper, the method of how to attain space voltage vector by SVPWM modulation strategy in three-level APF is discussed. And a modulation strategy that can reduce switching loss is introduced, too. This paper researches the issues of neutral-point voltage control from the space vector PWM(SVPWM)point of view in detail. The influences of every switching state on neutral-point are analyzed. Then a neutral-point control method, which is based on the detection of the three-phase output current and the neutral-point ripple's directions, is adopted in this paper.This paper adopts an optimal approach of calculating the command current under capacity-limitation. In this APF, if the detected harmonic current exceeds the limit current of the APF, the compensating current can be limited with the optimal approach under capacity-limitation. The APF has expansion flexibility when larger APFs arc required for a larger system.A three-level APF prototype is constructed, which uses TMS320F2407 DSP as the heart of control system. Experimental results illustrate that the performance of the proposed APF is suitable for harmonic loads with high voltage and large capacity.