Research On Power Allocation Algorithm Based On The Variable Virtual Impedance

As the development of new energy techniques,the microgrid is applied more and more widely.For the microgrid in island mode,it is a very important control target to distribute the load proportional among the DGs according to the capacity.The droop control can allocate the power without communication system,so it receives widespread attention from the relevant experts and scholars,and has become the mainstream strategy of microgrid control.Due to environmental factors such as feeder impedance unbalance,it is difficult for droop control to proportionally distribute the microgrid load among the DGs.In this paper,the power sharing characteristics of the parallel DGs in the microgrid is studied deeply,and a power allocation strategy based on variable virtual impedance is designed.Firstly,the transmission characteristic of DG output power is analysed,and the difference of the power transmission characteristics,when the feeder impedance is resistive and the feeder impedance is inductive,is discussed.The basic principle of droop control is analysed based on that background,and the droop control model is built.Secondly,the power sharing characteristics of active power,reactive power,harmonic power and negative power are analysed respectively,and the conditions of the DG output power proportionally distributed is summarized.The analytic results show that the microgrid cannot realize the power distributed proportionally in the practical application because of the unbalanced feeder impedance.Thirdly,the characteristics of the voltage and current of the DG interface inverter output are analysed,and then the calculation formulas of instantaneous positive power,negative sequence power and harmonic power are introduced.The basic principle of virtual impedance is analysed and the virtual impedance control strategy is designed,the virtual impedance control based on the fundamental positive sequence current,negative sequence current and harmonic current is added to the traditional droop control.The simulation analysis verify the fundamental positive sequence,negative sequence and harmonic virtual impedance can adjust the distribution characteristics of the reactive power,the negative sequence power and the harmonic power respectively,and they are not influenced by each other.Last,a power allocation algorithm based on variable virtual impedance is designed.The algorithm adds the difference among the DGs output power into active frequency droop control,and uses the dynamic changes of active power to regulate the virtual impedance value,and the DGs in microgrid output reactive power,negative power and harmonic power are distributed according to capacity ratio.Moreover,the control effect of this power allocation algorithm is verified by simulation analysis.