Research On Virtual Inertia Control Method For DC Grid

With the continuous development of distributed generation technology,DC grid has been paid more and more attention and research.Compared with the traditional AC power grid,the DC grid is more suitable for the access and transmission of new energy sources,and can reduce the amount of DC/AC converters.DC grid does not exist the problem of reactive power circulating current,frequency stability and power angle stability.But due to the isolation of the converters,the DC grid has lost the support of the AC grid to the stability of the DC side.Aiming at the problems of low inertia and poor DC voltage regulation,this paper takes the DC power grid as the research object,and make a deep research on the virtual inertia control technology of DC power grid.This strategy is to improve the ability of DC power grid to suppress the DC voltage mutation under disturbance and provide virtual inertial support for DC power grid,so as to improve the voltage quality and stability of the DC system.The main work and achieved results are shown as follows:(1)A mathematical model of wind-PV-battery-based DC power network system is established.Power generation principle of wind power,photovoltaic power and battery power storage is analyzed in this paper.Then a six-terminal DC loop network system is established in MATLAB/Simulink,which includes wind power generation unit,photovoltaic power generation unit,battery energy storage unit,grid connected converter unit and AC/DC load unit.All above lay the foundation for the future work.(2)A virtual inertia control method of DC power grid based on the variable droop coefficient is proposed.In order to enhance the DC power grid inertia to improve the power quality,the concept of DC grid virtual inertia is put forward and explained based on the in-depth analysis of DC voltage droop control strategy;The inherent inertia and virtual inertia of DC power grid are analyzed from the virtual capacitive point,and the relationship between virtual inertia and droop coefficient and DC bus voltage is obtained.Thus the virtual inertia control method based on the variable droop coefficient is proposed,which provides inertia for DC side while preserving converter droop characteristic.(3)An adaptive virtual inertia control strategy considering the virtual inertia margin is proposed.By improving the virtual inertia control method based on the variable droop coefficient,the adaptive virtual inertia control strategy considering the virtual inertia margin is proposed;At the same time,the theoretical analysis of the specific virtual inertia margin under different operating conditions is provided,so that the control strategy can adjust the droop coefficient on the basis of the allowed swing range adaptively,which allows battery to provide variable inertia support in accordance with its virtual inertia margin.This ensures the full and rational use of inertia margin.(4)At last,in order to verify the effectiveness of the proposed control strategy,a DC power network is established in MATLAB/Simulink.The results sho w that,under different operating conditions,the proposed control strategy are able to improve the system’s transient response,improve the quality of DC bus voltage,smooth the power fluctuation between AC grid and DC grid and improve the stability of th e system operation.