A Study On Three-phase Four-Leg Active Power Filter Based On DSP

With the rapid development of the power electronics and the increasing of power loads, the power grid harmonic pollution becomes more and more serious. At present, harmonic current of typical nonlinear load causes some serious power quality problems, including low power factor, voltage waveform distortion, voltage fluctuation and flicker and three-phase unbalance. Therefore, solving the power grid harmonic is widely concern. Active Power Filter (APF) is a new kind of power electrical device that dynamically restrain the harmonic and compensate the reactive power, with high degree of controllability and fast responsiveness, which get more attention. Therefore, scholars at home and abroad attaching more and more importance on fully digital control technology of APF,and harmonic suppression and reactive power compensation have become a major issue in the field of power electronics and power system. This thesis focuses on the three-phase four-wire active power filter to study. The theoretical analysis, simulation and experiment are taken on some related questions.Firstly, this thesis introduces the reason, harm and the compensation standard of harmonic in power system and the commonly used compensation methods are compared, Then describes the development background and previous research work of APF, including the research situation, the topology structure, the harmonic detection algorithm and current control algorithm, etc. Take the three-phase four-bridge APF as the study object and established the mathematical model, which lays the foundation for later research work.Secondly, this thesis analyzes the principle and characteristic of traditional harmonic detection algorithms. Compared with other detection algorithm, the detection accuracy of abc detection algorithm and ip-iq-0algorithm is affected by the grid voltage distortion slightly. Based on this advantage, the direct projection harmonic detection algorithm is used, which can omit the matrix transformation and can be used in any harmonic and unbalance current detecting. To improve the PLL, the mean filter with phase shift algorithm is used instead of Butterworth filter so that the detecting accuracy and response speed are improved.Then, the advantage of PI controller is tracking dc reference without static error in a fast speed, while PR controller can track the ac reference volume errorless and fast. Combining the advantages of them both, this thesis puts forward a proportional-integral-dual frequency resonance (PI-DFR) current controller to realize sufficient harmonic compensation, better steady compensation results and faster dynamic response. In addition, the dc voltage control strategy is researched, showing that using the specific harmonic detecting method can reduce the dc voltage fluctuation caused by load changing. As the compensation current and capacitor voltage over shoot during start up, a double-loop soft-start control strategy. Simulation and experimental results show the validity and effectiveness of mentioned strategyFinally, a220V/20A three-phase four-bridge APF prototype was designed and manufactured. According to the above-mentioned research results to design the active filter, prototype experiments show the validity of harmonic detection algorithm, current control algorithm and the dc voltage control strategy mentioned in this article, which has good industry application value.