Research On Cascaded Three-Phase-Bridge Active Power Filter

The application of various nonlinear load to the power grid has brought lots of harmonic and reactive power, causing the deterioration of power quality and influencing the security and stabilization of power system, it is urgent to take measures to govern harmonic and reactive power. Active power filter(APF) is used for suppressing harmonic dynamically and compensating reactive power. It compared with the traditional passive filter, it responses quickly, controls easily, and it is not easy to resonance with the power grid impedance, etc. Therefore, it has become an important means of governance harmonic and reactive power. With the constant improvement of the power grid capacity, the requirement of APF has become higher. Especially in some high power occasion, traditional APF is limited by pressure level and working frequency of switching devices that can not meet the requirement. In view of this problem, multilevel APF is proposed. At present, the cascaded H-bridge APF is the most widely used, but when used in three-phase system, cascaded H-bridge should be delta or star-connected. In this paper, we study the cascaded three-phase-bridge APF that can be directly applied to high-power three-phase high voltage, and it’s easy to modularized design, which has a very good application prospect in power quality research field of high power.The carrier phase-shifted PWM technique combines the advantages of both sinusoidal pulse width modulation and multi-modular technique, and it has become the optimal modulation strategy due to the superior harmonic characteristic and switch load balancing. This paper studied cascaded three-phase-bridge APF with CPS-PWM technique, which has the advantages of cascaded three-phase-bridge converter and CPS-PWM. And it can obtain higher output voltage level and a good harmonic performance.There are many capacitors on the DC side of cascaded three-phase-bridge APF and a constant value of DC side capacitor voltage largely decided the compensation result of the whole system. Based on the analysis of cascaded three-phase-bridge converter power relationship, it can be found that the instantaneous power of each module transmission is fluctuating and it will have a bad influence on the constant value of capacitor voltage. So in this paper, according to the characteristics of cascaded three-phase APF and combine the characteristics of control method of traditional APF and cascaded H-bridge APF, two kinds of DC side capacitor voltage control method is got, called mode one and mode two. Finally, with the ip-iq detecting method which is based on the instantaneous reactive power theory, the two level cascaded three-phase-bridge APF simulation model is built in matlab/simulink platform to analyze the static and dynamic performance under the two control modes of APF. The simulation result shows that the DC side capacitor voltage with a constant value and the whole system has good static and dynamic compensate performance based on the two control modes. And the total harmonic distortion with mode one is smaller than mode two, and its control method is more complicated than mode two.