Impedance Modeling And Stability Analysis Of Medium Voltage And Low Voltage DC Distribution System

In recent years,with the continuous growth of DC load for electric vehicles such as DC charging piles and communication facilities,people’s demand for DC distribution system is also increasing.Compared with AC distribution network,DC distribution network does not need to consider the phase,harmonics and other issues,just to keep the bus voltage stable.However,when some parameters in the system are changed,the DC bus voltage may oscillate and threaten the stability of the system.Thence,it is meaningful to study the stability of medium and low voltage DC distribution system.The bus voltage stability of medium voltage and low voltage direct-current is the key to the medium voltage and low voltage direct-current power distribution system in the stable index.In order to analyze the small signal stability impedance of the system,which is the winding transformer isolation converter and the input serial output parallel multi-module Boost + LLC in type of direct-current transformer in the medium voltage and low voltage direct-current power distribution system.The flexible converter,the direct-current transformer and the Buck converter,whose small signal models are established.The medium voltage and the low voltage direct-current port impedance expressions are derived.The characterization parameters of the cascade system stability is analyzed by using the method of impedance.Based on the Bode diagram and Nyquist curve,the influence of the variation of circuit parameters and system control parameters on the system impedance characteristics and the stability of the system is analyzed.The specific research contents are as follows:Firstly,the topological structure of the medium and low voltage DC distribution system and the control strategy are introduced.According to the topology and control strategy of the subsystem,the small-signal models of the VSC,DC transformer,Buck converter and line impedance model are set up by adopting the small-signal modeling method,which lay the foundation for the subsequent derivation of the impedance model.Secondly,according to the small signal model and a series of derivations,the expressions of the output impedance of the DC side of the flexible converter,the input impedance of the medium voltage side of the DC transformer,the output impedance of the low voltage side of the DC transformer and the input impedance of the Buck converter are obtained.A simulation model is built on the MATLAB/Simulink platform.The disturbance current is injected into the system.According to the ratio of the response disturbance voltage to the disturbance current,the Bode diagram of the simulated actual measured impedance is obtained.The Bode diagram of the theoretical impedance model is compared with the Bode diagram of the simulated actual measured impedance model to verify the correctness of the impedance model.According to the impedance model expression,the influence of circuit parameters and controller parameters on the impedance of the medium voltage DC port and the low voltage DC port is analyzed,and the impedance characteristics of the flexible converter,the DC transformer and the Buck converter are obtained.Finally,according to Thevenin’s equivalent principle and Norton’s equivalent principle,the equivalent circuits of flexible converters,DC transformers and Buck converters are acquired,and then the equivalent circuits of medium-voltage DC distribution systems and low-voltage DC distribution systems are acquired.According to the equivalent circuit,the stability criteria characterizing the medium-voltage DC port and the low-voltage DC port are obtained,and the influence of the circuit parameters and controller parameters on the stability of the medium-voltage DC distribution system and the low-voltage DC distribution system is analyzed.