The Control And Experiment Research On Three-Phase Four-Arm Active Power Filters

With the inherent ability to handle the unbalanced current, three-phase four-leg Active Power Filters (APF) are often chosen to compensate the negative and/or zero sequence harmonic current in three-phase four-wire system.This paper establish the mathematical models of three-phase four-leg APF in both a-b-c and d-q-0 coordinates, and proposes the cross-decoupling and grid voltage feedforward control strategy under the rotating d-q-0 coordinate.Three-Dimensional Space Vector Pulse Width Modulation (3D-SVPWM) strategy is chosen due to the structural characteristics of four-leg active power filters. The principle and digital control algorithm of three-dimensional space vector modulation, based on the a-b-c coordinate, therefore, are described in details. The theoretical analysis and simulation results show that three-dimensional space vector modulation, compared with the traditional SPWM modulation, has a higher utilization of DC voltage and lower harmonic content. With the development of microprocessor, the execution time will also be perfectly acceptable.Harmonic reference detection and current tracking control are the two key techniques for APF. The d-q-0 detection method ,based on the simultaneously rotating coordinate ,and harmonic extraction algorithm, based on the Discrete Fourier Transform ,are employed together to realize flexible compensation under different load occasions. The control diagram under the rotating d-q-0 coordinates, with outer voltage loop and inner current loop is obtained after the analysis of the control objectives and tasks for the three-phase four-leg APF. The design process and results of digital PI controller of voltage and current loop are also described in detail. Theoretical analysis shows that PI controller, employed in APF applications, can't get zero steady-state error. With the harmonic order of the reference current increased, the performance of compensation become worse. Since the steady-state performance under PI control is not acceptable, the repetitive control is introduced to improve the steady-state performance, where the PI controller and parallel together to keep the satisfied dynamic performance at the same time. The structure, design process and stability analysis of the repetitive controller are described in detail. Theoretical analysis shows that, the composite control strategy of the current loop, with PI control parallel with repetitive control, has high steady-state accuracy and fast dynamic response.The topology, modulation strategy, harmonic detection, control strategy and compensation performance are verified by software simulation using the Matlab simulation tools. The simulation results accord with the basic theory.Finally, the experimental system of the three-phase four-leg APF is established. The control program is realized using the digital signal processor TMS320F2812 in C language. The experimental results verify the correction of theory and simulations and get to the conclusion, that the three-phase four-leg APF, based on the three-dimensional vector modulation, as well as PI control parallel with repetitive control, has the high steady-state accuracy , fast dynamic response and good compensation performance.