Study On Control System Of Three-level Active Power Filter

Active power filter (APF) is a new power electronics device used to restraining harmonics and compensating reactive power. By realtime detecting harmonics and reactive current in the power line, then it generates the compensating current which is equal to harmonic current in amplitude and opposite in polarity, aim at improvement and advancement of electric quality. When frequency and amplitude of harmonic and reactive current vary, this kind of technique can achieve dynamic compensation effectively. Moreover, the compensation performance is not affected by the impedance of power system, it has advantages as follows, fast response, high controllability and adaptability, etc. So, more and more importance is attached to it. Most previously proposed APF are based on two-level converters and thus are suitable for low voltage use. However, in a high voltage system, the circuit designers are often confronted with a serious challenge: there are no semiconductors capable of sustaining the desired voltage. Multilevel converters that provide more than two voltage levels, less distorted waveform and that have merits of low sustaining voltage, low switching losses, high efficiency and low EMI and that are called "perfect zero-harmonic"converters offer many benefits for higher voltage, higher power and especially higher frequency applications. In this thesis, the circuit topology and control strategy of multilevel converters previously reported are analysed and compared with each other, and their merits, defects and applications are presented respectively. Further, diode-clamp three-level converter is used to main circuit of APF. The principle and characteristic of one-cycle control are analysed, and its applications in single-phase, three-phase three-wire and three-phase four-wire two-level system is discussed. Hereby, this thesis applies one-cycle control to three-level three-phase three-wire APF and three-level three-legs three-phase four-wire APF for the first time respectively. Based on the analysis of equivalent average modes in switching period, the key control equations are derived and circuit modes are created respectively. Simulation is carried out using PSIM simulation software, furthermore, the different compensation performance between three-level APF and two-level APF is compared. The simulation results verify that three-level APF based on one-cycle control is simple and reliable, which make it be realized easily. The switch frequency of the main circuit is constant, which is desirable for engineering and industrial applications. They all can dynamically and effectually compensate the harmonics and reactive current, and zero sequence currents are eliminated in three-level three-legs three-phase four-wire APF, and they all have excellent compensating capability.