Control Strategy Of Bi-directional AC/DC Converter Parallel System In DC Microgrid

Bi-directional AC/DC converter is the interface between the utility grid and the DC Micro-grid.It plays a key role in controlling the energy exchange between DC Micro-grid and the utility grid,maintaining the voltage of the DC bus and improving the system operation efficiency.DC Micro-grid can have better redundancy,reliability and the ability to expand its capacity by adopting bi-directional AC/DC converter parallel system.Therefore,control strategy of bi-directional ac/dc converter parallel system in DC Microgrid has always been one of focuses in the field of DC Micro-grid.In this paper,different types of circuit structures and their characteristics are compared and analyzed and the single-phase voltage-source bi-directional AC/DC converter is chosen as study object.Then,three types of sinusoidal PWM method are studied and bipolar SPWM is adopted.Also,the open function model and state space averaging model of the bi-directional AC/DC converter are built.Simulation results prove the validity of the selected topology.Three traditional control strategies for bi-directional AC/DC converter are introduced.Particularly,the contradiction between the equal current sharing and voltage adjustment rate using droop control is analyzed in detail.Aiming at the disadvantage of the droop control,a low-voltage-deviation equal-power-sharing control strategy for bi-directional AC/DC converters is proposed.By feeding back the average current of the DC bus as the global variable and introducing integrating element,the power can be shared equally between each converter without being infected by line parameters.By applying average voltage PI control,the DC bus voltage deviation is reduced.The impact on the power control of the parallel system caused by second-ripple current is discussed.And the distortion rate of parallel current caused by second-ripple current and voltage is inhibited by using band rejection filter.The system transfer function and the ole-zero distribution are derived and then the stability of the proposed control strategy is analyzed.Combining the system stability and dynamic,the design method of control parameters is proposed.A DC Micro-grid simulation model with two bi-directional AC/DC converters is built in PSIM and the simulation results prove the validity of the proposed control strategy.At last,a DC Micro-grid experiment platform with two bi-directional AC/DC converters is designed.The design of the main hardware circuit,the whole structure of the software and important subprogram flow charts are elaborated.The traditional droop control and the proposed strategy in this paper are tested on the experiment platform.Thus,the results of voltage and power on the DC side using these two methods are compared and studied.From the experiment results,the proposed low-voltage-deviation equal-power-sharing control strategy for bi-directional AC/DC converters proves to be helpful to improve the effect of equal power sharing and the DC bus voltage adjustment rate,which means the theoretical study and the design in this paper are reasonable and practical.