Research On Voltage Stability Control Strategy Of Photovoltaic-energy Storage DC Distribution Network

Compared with AC distribution network,DC distribution network is more suitable for large-scale new energy access in terms of operation mode,no frequency and phase synchronization problem,simple structure and high controllability,so it becomes an important development direction of future distribution network.However,as a typical application of power electronic converter,DC distribution network has almost no inertia,which makes the sudden change of load and the random fluctuation of new energy output directly affect the stability of DC bus voltage.Aiming at the problem of poor voltage stability of photovoltaic-energy storage DC distribution network,this paper proposes a flexible virtual inertia control strategy using energy storage equipment to provide inertia.The control strategy can solve the problem that energy storage can flexibly adjust the inertia according to the change of DC voltage.The main work of this paper are as follows:(1)Aiming at the problems of small inertia and poor voltage stability of photovoltaicenergy storage DC distribution network,a flexible virtual inertia control strategy for photovoltaic-energy storage DC distribution network is proposed.Based on the analysis of the basic concept and principle of virtual inertia of DC system,the function of battery state of charge and DC voltage is established,so that the battery can respond to the change of DC voltage and adjust the inertia flexibly.Because the battery has the problem of charge and discharge limit,by introducing the arctangent function,the deep charge and discharge is avoided,and the safety of the battery is ensured on the premise of making full use of the battery capacity.By establishing the small signal model of photovoltaic-energy storage DC distribution network and analyzing its stability,the variation range of virtual inertial control parameters and its influence on system stability are defined.(2)In view of the steady voltage deviation caused by droop control,a voltage optimization control strategy is proposed.By adding PI control to the voltage deviation of droop control of grid connected inverter,the purpose of secondary voltage regulation of DC distribution network is realized by using the power of AC system side.Moreover,the proposed flexible virtual inertia control can reserve enough time for secondary voltage regulation,reduce the variation amplitude of DC voltage,and make the DC voltage quickly and smoothly return to the rated value.(3)A four-terminal DC distribution network simulation system was built on MATLAB/Simulink simulation software to verify the effectiveness of the proposed control strategy.When the load increases or decreases and the photovoltaic output fluctuates,the DC voltage of the system fluctuates greatly due to the lack of inertia.The battery can quickly respond to the change of DC voltage,and the controllable inertia can be simulated.But too large virtual inertial control parameters will also lead to the loss of static stability of the system.The results of secondary voltage regulation show that the combination of secondary voltage regulation and flexible virtual inertia can reduce the amplitude of voltage fluctuation and restore time.