Optimal Control Of The DAB-based DC Transformer In DC Distribution Grids

With the rapid growth of distributed generation and DC load,DC distribution grid,as an ideal access mode for distributed generation and DC load,has been widely concerned.As the key interface equipment of DC distribution grid,DC transformer has the technical characteristics of electrical isolation,voltage flexible control and so on.Among many DC-DC topologies,dual active bridge(DAB)is the most suitable topology for DC transformer because of its advantages of high power density,bidirectional energy flow and fast regulation speed.The DC bus voltage is relatively stable,while the voltage at the outlet of new energy and energy storage equipment may fluctuate greatly,and different scenarios have different requirements for the control method of DAB based DC transformer.This paper researches the control strategy of DAB based DC transformer for DC distribution grid,which is used to improve its steady-state characteristics and broaden the application scenarios.The main work of this paper is as follows:(1)the relevant modes of DAB under single phase shift control and extended phase shift control are analyzed in detail,and the mathematical models of power characteristics,current stress characteristics and soft switching characteristics of DAB under two control methods are established.When DAB based DC transformer is used in DC distribution grid,the degree of freedom of single phase shift control is low,and the steady-state characteristics can’t be optimized.The extended phase shift control improves the degree of control freedom,but the effect of reactive power optimization is not good.(2)by comparing the reactive power suppression ability and control complexity,the dual phase shift control is selected as the control method to explore the principle of reactive power generation.By establishing the mathematical model of reactive power,a method of minimum reactive power control is proposed,which can simultaneously realize the minimum reactive power of the primary side and the secondary side of the transformer and improve the operation characteristics of the transformer.This method is simple to control and easy to implement in engineering.(3)in order to meet the requirement of wide port voltage variation for DC transformer in DC distribution network,this paper analyzes all working modes of triple phase shift control in detail,and proposes a minimum current stress control method.This method can reduce reactive power and make all switches realize soft switching while optimizing current stress.Compared with the single phase shift control,the proposed method is suitable for the situation where the input and output voltage of DC transformer is not fixed.(4)the DAB experimental platform is built to verify the minimum reactive power control method and the minimum current stress control method proposed in this paper.The experimental results show that the proposed minimum reactive power control method can minimize the bilateral reactive power of DAB and improve the power quality.The minimum current stress control method can determine the only operation point when the input and output voltage do not match,and achieve the minimum current stress in the full power range.