Research On Active Power Filter Based On Cascaded H Bridges

ABSTRACTAs modern industrialization is developing, nonlinear grid load will continue to increase, resulting in increasingly prominent harmonic problems. As a power electronic device that can compensate harmonic current dynamically, active power filter is considered to be the most effective way to control the harmonic pollution in power system and now it has gradually become a hot research topic. As to reduce the influence of nonlinear load harmonic current in power system on the quality of power grid, this paper developed the active power filter based on cascaded H Bridges.As it adopts modern power electronic and digital signal processing technology and have mature control theory, APF based on cascaded H Bridges can play an important role in the governance of harmonics.This paper firstly describes main means of relevant knowledge of harmonic and filtering, and analyzes research status and characteristics of the active power filter.Then this paper detailed introduces three kinds of commonly used harmonic detection algorithm of — the harmonic detection based on Fourier transform, the harmonic detection based on instantaneous reactive power theory and adaptive harmonic detection method,and makes a comparative analysis of the superiority and inferiority of these three methods respectively.This paper builds the mathematical model of shunt single-phase and three-phase APF,and deduces the current tracking strategy of single-phase and three-phase APF. Then make an analysis of PWM modulation of APF based on cascaded H Bridge, design a improved carrier control method, which not only has advantages of the carrier phase shift that makes the power device switching equilibrium, but also has advantages of carrier with laminated phase that has good harmonic elimination characteristics. For cascaded H bridge DC bus capacitors voltage balancing control, this paper puts forward a kind of control algorithm which can effectively guarantee the DC voltage balance, and it consists of two parts: the module of the mean voltage controller and the DC bus voltage controller. Analyze the dead time effect in the practical application when PWM driving signal triggers switch devices,and proposed a compensation algorithm that can effectively inhibit the effect of dead time through the voltage on the equivalent disturbance values doing a feedback compensation to compensate the dead time objective.Finally, build the experimental prototype,a device of a single-phase APF 380V/50 A three unit cascaded H bridges, using the FPGA+DSP control platform and carrying out two groups of contrast experiments:one is the chain APF absorbing device or generate reactive current experiment, the other is the chain APF device compensating specific harmonic current experiment, and set the switching frequency of the 5k Hz and the 10 k Hz as a comparison experiment to analyze the compensation effect of the APF device, the result also proves the correctness of the theory proposed in this paper.