Research on influencing factors and improving methods for DC distribution system stability

In this paper, the Nyquist admittance ratio criterion was used to analyse the influencing factors on the stability of the system, such as the parameters of DC line, the intensity of interconnected AC power grid, the droop coefficient and etc. The theoretical analysis showed that the increase of the DC line resistance would decrease the system stability margin, while the increase of the DC line inductance had small effect on the system stability margin; the weaker the gird intensity was, the smaller the system stability margin was, while the larger the droop coefficient was, the larger the system stability margin was. In the case that AC and DC line parameters were the same, the system stability margin under U(dc)-I(dc) droop control mode was larger than that under U(dc)-P droop control mode. According to these influencing factors, suggestions were given to improve system damping and system stability through reducing the impedance of DC line, enhancing the interconnection of AC power grid and optimising the droop coefficient. Finally, a detailed simulation model was built with DIgSILENT simulation tool to verify the correctness of the above theoretical analysis by time-domain simulation results