| The energy storage system has the function of stabilizing the power fluctuation of the microgrid and maintaining the stability of the bus voltage,at the same time,it can provide the electric energy for the load to ensure the uninterrupted power supply of the DC microgrid.In this paper,the power sharing and voltage management of low voltage DC microgrid distributed energy storage system are studied.The distributed energy storage system uses the traditional droop control strategy,which has two problems:(1)The energy storage device has a fixed output power due to the influence of the traditional droop coefficient,which causes the battery overcharge and over discharge problem.(2)The traditional droop coefficient is too large,causing the DC microgrid bus voltage to deviate.In order to solve the above problems,a self-compensating dynamic droop control strategy based on SOC of energy storage battery is proposed in this paper.Designing improved dynamic droop coefficient based on SOC state quantity and initial droop coefficient.The improved droop coefficient is proportional to the SOC~n of each storage battery,and the output power(or input power)of the energy storage device is proportional to the droop coefficient,Therefore,the power sharing of the energy storage device is adjusted in real time according to the SOC state quantity.The problem of overcharging and over-discharging caused by the fixing of droop coefficient is solved.In order to eliminate the problem of bus voltage deviation,the mathematical model of source load power balance is establish,and the voltage compensation algorithm is deduced to eliminate the bus voltage deviation caused by droop coefficient.Compared with the traditional secondary control method,this method reduces complexity of the control.The focus of the improved droop control strategy above is the design of the droop coefficient and the voltage compensation algorithm,and both design parameters include the initial droop coefficient.As for the selection of the initial droop coefficient,the fuzzy control is used to calculate the bus voltage deviation value and the bus voltage change rate as the input quantity,and the initial droop coefficient is the output quantity.The initial droop coefficient is adjusted in real time according to the system operation state.A self-compensating dynamic droop control strategy based on Fuzzy is designed,which can adjust the power sharing according to the change of dynamic droop coefficient and improve the power sharing accuracy of the energy storage system.At the same time,the voltage compensation algorithm based on Fuzzy control can better realize the bus voltage unbiased control.From what has been discussed above,the DC microgrid model is built.The feasibility of self-compensation dynamic droop control and a self-compensation dynamic droop control strategy based on Fuzzy control are verified in three conditions of power shortage,power surplus and energy storage system operation state of microgrid system.Both control strategies can solve the power sharing and voltage management problems,while the self-compensated dynamic droop control based on Fuzzy control has a faster dynamic response and a larger control range. |
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