Bipolar DC Bus Power Balance Control And Load Distribution Control For Energy Storage System

The global energy crisis and environmental pollution have promoted the rapid development of distributed generation and Microgrid.However,distributed generation has the disadvantages of intermittence and randomness,which will cause power fluctuation and threaten the stable operation of microgrid.As an essential part of microgrid,energy storage system plays an important role in stabilizing energy fluctuation and solving the imbalance between power supply and demand.This paper will focus on the research of the bipolar DC bus power balance control and the power allocation control between the energy storage modules of the distributed energy storage system.Considering the three-wire structure of bipolar DC microgrid,this paper selected the non-common-ground three-level bidirectional DC/DC converter as the interface converter of energy storage module.The working process of the converter under different modes and the principle of partial voltage equalizing voltage are analyzed.In order to achieve bipolar DC bus power balance in energy storage system,the control strategy of three level bidirectional DC/DC converter is studied.The three frequency division method is used to design the regulator in the control loop,and the adjustment ability of the equalizing loop is explained in detail.The State-of-Charge(SOC)of each energy storage module in distributed energy storage system is not necessarily equal.In order to realize the efficient utilization and safe operation of the energy storage system,this paper studied the power allocation strategy of parallel energy storage modules.On the basis of traditional droop control and double-quadrant State-of-Charge-Based droop control,an improved droop control method based on SOC without communication interconnections is proposed.By establishing the new functional relationship between the droop coefficient and SOC of energy storage module,this method realized that each energy storage module can adjust the output or absorption power in real time according to their SOC.Thus,the reasonable allocation of the power between parallel energy storage modules can be realized.On the basis of theoretical research and simulation analysis,a distributed energy storage system composed of two energy storage modules is built,and the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified by experiments.