Study On Three-phase Three-level Active Power Filter

For the wide application of the nonlinear loads and all kinds of converter equipments making the power system polluted with a mass of harmonic current, it is urgent to eliminate harmonic and improve power quality. As an ideal device to eliminate harmonic current and improve power factor, active power filter(APF) can compensate the harmonic and reactive power with variable both of frequency and amplitude, and it makes up the drawback of the passive power filter. Therefore, the development of active power filter is rapidly studied and has been applied in practice in some developed countries. Nowadays, in China, the exploitation of active power filter is at the stage of experiment and high power products have not been used up to now yet, so the research of active power filter has very important significance.Most of the previously proposed APFs are based on two-level converters and thus are suitable for low voltage use. However, in a high voltage system, there are no semiconductors devices 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 becomes the research focus of scientific and technological personnel.This paper applies one-cycle control to three-level two-leg three-phase three-wire APF. Based on the analysis of equivalent average modes in switching period, the key control equations are derived and circuit modes are created. The simulation results verify that three-level two-leg 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, and compensation performance is close to three-level three-leg three-phase three-wire APF based on one-cycle control.A novel control strategy is proposed for three-level APF based on direct source current control in this paper. With the ip-iq detecting method based on the instantaneous reactive power theory, reference current is to be get. Thus APF can compensate harmonic component, reactive power component respectively, harmonics and reactive power component together. In addition, the three-level converter that provides more than two voltage levels and less distorted waveform offers many benefits for higher voltage or higher power applications. This strategy needs simple controller, and is very easy to be implemented by the existing hard ware. The simulation results of an example show that proposed strategy has well control effect and can be used in practice.