Mutual Power Support Strategy For Isolated Hybrid AC/DC Microgrid

In order to implement the China’s sustainable development strategy,the penetration levels of renewable distributed generation(DG)such as photovoltaic power and wind power in China’s power system is higher and higher.However,the large-scale integration of distributed generation with the disadvantages of intermittence and randomicity into power system brings many new challenges.The hybrid AC/DC microgrid as the organic combination of AC and DC DGs and loads,is an effective way to solve the problem caused by the large-scale integration of DG into modern power system.Compared with pure ac microgrid or DC microgrid,hybrid AC/DC microgrid can also stabilize power fluctuation through mutual power assistance between sub-microgrids,which is also a unique regulation method of hybrid AC/DC microgrid.The power cooperative control for isolated hybrid AC/DC microgrid system is comprehensively studied from two aspects:the mutual power assistance effect of steady-state and the reasonable support of transient inertia to realize the reasonable mutual power assistance between AC and DC sub-microgrids.First,the droop control and virtual inertia control of dispatchable distributed generation in hybrid microgrid are introduced,and the instantaneous power balance characteristics of hybrid microgrid under different control strategies are analyzed.The traditional bidirectional droop control adopted by interlinking converter(ILC)between AC bus and DC is reviewed,and its limitations are analyzed.Second,focusing on the steady-state mutual power assistance strategy of hybrid microgrid,and aiming at the hybrid microgrid with AC and DC sub-microgrids differing greatly in some aspects like the critical load proportion and dispatchable capacity,this paper proposes a mutual power assistance strategy for islanded hybrid AC/DC microgrid,which considers the conditions of AC and DC sub-microgrids and the state of charge(SOC)of battery.In order to avoid the power loss caused by the frequent actions of the ILC,a hierarchical control strategy is proposed,which includes power autonomy model and power interaction model,and the changing between system operating models is designed reasonably.Then the goal of mutual power support based on the conditions of the sub-microgrids and the SOC of the battery is proposed.Meanwhile,a power cooperation control algorithm,which gives consideration to both AC frequency and DC voltage,is designed to achieve the goal.Third,this paper focus on the transient performance of hybrid microgrid while considering the steady-state mutual power assistance goal.This paper mainly studies how to realize the reasonable inertial support between AC and DC sub-microgrids during the transient response,when AC and DC sub-microgrids both have certain inertia due to dispatchable distributed generators of hybrid AC/DC microgrid adopt virtual inertial control strategy.On the basis of the virtual synchronous generator,a bidirectional virtual inertia control strategy for ILC is proposed.The control strategy is able to respond to the deviation of DC voltage and AC frequency in steady state,which can realize the objective that both the sub-microgrids can share the system power according to requirement of the deviation of DC voltage and AC frequency.Also,the control strategy is able to respond to the rate of change of DC voltage and AC frequency,which can realize the reasonable inertial support between the sub-microgrids during transient response.The small-signal model of the proposed strategy is built to analyze the system stability and dynamic response of frequency and DC voltage during active power changing.In order to juggle both steady and transient objective of the proposed strategy,this paper gives the design method for the parameters in the proposed control.Finally,a simulation model is built in PSCAD/EMTDC to verify the effectiveness of all the proposed control strategies.