Research On Control Strategy Of Cascaded H-bridge Rectifier

In recent years, with the constructions of smart grid, distributed generation continuous promoted, and applications in industry of intelligent power electronic equipment of high voltage more and more widely. In high voltage and power field, because of the voltage level limit of single IGBT power switch device, cascade H-bridge multi- level converter becomes a promising topology. As cascade H-bridge converter has the advantages of simple control, modular structure, easy expansion and less power switching devices, etc.First of all, this paper studies the working principle of cascaded H-bridge rectifier and carrier phase-shifted PWM modulation technology, and analyzes the principle of carrier phase-shifted modulation and harmonic characteristics. Then the average switching cycle mathematical model of cascaded H-bridge rectifier in the stationary coordinate is established. The two phase PWM rectifier system is constructed by the virtual orthogonal component, and cascaded H-bridge rectifier is controlled integrally in DQ rotating coordinate adopting the voltage and current double closed- loop decoupling control strategy, which makes the system can run in unit power factor.Besides, the parameters of PI controller are designed based on the logarithmic expected frequency characteristics of voltage loop and current loop.Then the DC side voltage balancing mechanism of each module of the H bridge rectifier is analyzed. The dq synchronous system is built by constructing virtual orthogonal voltage and current based on seco nd order generalized integrator. The predictive power model is derived to construct main control system of cascaded H-bridge rectifier combining the instantaneous power theory, which takes the duty cycle of active and reactive power as control object. A new voltage balancing control strategy is proposed dealing with the problem of DC voltage imbalance in cascaded H-bridge rectifier. The modification of active power is obtained by the voltage balance controller when the objective function of coupling effect achieves minimum and the active power is distributed by reconstructing the duty cycle of active power ineach modular, which eliminates the coupling effect between the voltage balance controller and the main control system on the basis of ensuring the DC vo ltage balance.Finally, two module cascaded H-bridge rectifier power experimental prototypewas built based on TMS320F2812 DSP software development platform, and introduces the design of hardware circuit and control system of software. The correctness and feasibility of proposed strategy was proved by building the simulation model and experimental prototype.