Stability Analysis And Control Strategy Research Of Flexible DC Power Distribution System

The continuous development of the social economy,the massive access to renewable energy and distributed power sources,and the extensive application of DC loads have brought greater challenges to the development of the power system.Due to the tremendous advantages of the flexible DC technology in terms of flexibility,controllability and reliability,its application in DC power distribution system demonstrates a greater potential.Compared with the traditional AC power distribution system,the flexible DC power distribution system with flexible power supply,high power quality and convenient access to renewable energy has become an essential part of domestic and international research.However,there are still several important technical challenges that need to be addressed in the flexible DC distribution system,such as the small-signal stability problem due to the interaction of the converter and other components in the system.The results of numerous researches show that when the system converter station adopts constant power control,the external display of constant power load characteristics can have a serious impact on the system stability.Based on the above problems,this paper investigates the issues related to system stability from the small-signal mathematical model of the flexible DC distribution system.Firstly,the mathematical model of the flexible DC distribution system is derived,the small-signal model of the flexible DC distribution system is introduced,and the accuracy of the small-signal model of the system is validated by time-domain simulation.Meanwhile,this paper also establishes a variable closed-loop feedback control model for the flexible DC distribution system and derives the model transfer function in detail.Combined with the frequency domain analysis method,this paper analyzes the system in the frequency domain from the perspective of the stability margin of the open-loop transfer function,investigates the influence of converter power,grid strength of the AC system,and the main parameters of the DC grid on the system stability,and explains the mathematical mechanism of the dynamic interaction of the system.Secondly,this paper investigates the application of the superconducting magnetic energy storage(SMES)devices in flexible DC power distribution systems.This paper investigates the influences of the SMES device on power distribution system stability by combining frequency domain analysis and time domain simulation analysis.The SMES device is controlled by the deadbeat control strategy to promote its control performance.The simulation results demonstrate that the SMES device can effectively improve the stability of the flexible DC distribution system,and the SMES device with deadbeat control has better dynamic performance than it with conventional PI control.Finally,this paper investigates the impact of the SMES device adopting a modular multilevel DC/DC converter on system stability.The SMES device enables to set the ratio between the input and output voltage by flexibly configuring the number of sub-modules in the DC/DC converter,which enable to reduce the voltage at both ends of the superconducting magnet and avoids the problem that the switching devices and superconducting magnet in devices are subjected to higher voltages when the SMES connected to the DC grid.It is suitable for accessing a wide range of grid voltages.The simulation results demonstrate the effectiveness and good dynamic performance of the proposed method.