Research On Open-Circuit Fault Diagnosis For Three-Phase Four-Wire Active Power Filters

With a large number of nonlinear single-phase loads continuously connected to the grid, the problem of power quality for four-wire system becomes more and more serioue. Active Power Filter(APF) can effectively compensate the harmonic and reactive power in three-phase four-wire system, and improve the power quality. Three-phase four-wire three-leg APF system is selected as the study object in this paper to research the mid-point potential balancing control of DC side, extraction of harmonic instruction, open-circuit fault diagnosis and so on.Harmonic sources and harm as well as the commonly treatment schemes have been introduced briefly in this paper; and a brief overview of research status of three-phase four-wire APF system is provided, including harmonic instruction extraction, harmonic current tracking control, main topology of APF system and IGBT open-circuit fault diagnosis.The mathematical model of three-leg APF, which is suitable for all topology, is proposed by analyzing working principle of different topological structure; and the mathematical model of DC voltage is deduced as well. The mid-point potential balancing control of DC side is one of the key technologies of three-leg APF, and the mid-point potential balancing control of two-level and three-level are achieved respectively by adjusting the duty cycle of the zero vectors and optimizing redundant vectors. The traditional ip-iq harmonic detection needs a large number of coordinate transformations and mathematical calculations. A harmonic instruction extraction algorithm is obtained by combining separated-phase ip-iq harmonic extraction with instantaneous symmetrical components, which is suitable for three-phase four-wire system and can realize specific and all harmonic compensation, as well as positive, negative and zero sequence harmonic extraction. The effectiveness of the algorithm is verified by the results of simulation.Detecting and locating the faults of IGBT rapidly and accurately is the key to ensure the security and reliability of the APF system. A fault diagnosis algorithm based on the mathematical model of AC side output voltages for three-phase four-wire three-leg two-level shunt APF is proposed in this paper. According to the different relationships between the APF’s AC side output voltages and turn-on duty cycle of the corresponding IGBT under normal condition and fault condition, the open-circuit faults can be detected and located. All the signals used in this algorithm have already been employedin the APF system controller, and no additional hardware circuits or sensors are required. In addition, the algorithm has a good performance in rapidity and reliability, and can be used during the dynamic process of load changes. The excellent performance of the algorithm is verified by the results of simulation.The diagnosis algorithm proposed above is improved and applied to three-phase four-wire three-level APF. The bridge arm location of open-circuit faults are detected by using the above diagnosis algorithm. Then, the precise location of open-circuit faults can be determined by analyzing the different current conduction paths under the different open-circuit faults in the same bridge arm. The feasibility of the algorithm is verified by the results of simulation.Finally, experiments are achieved in the four-wire APF experimental platform, and experimental results are provided in the paper.