Research On Power System Vulnerability Analysis Method Considering Electric Vehicle Accessed

As a green,environment-friendly and new type of power grid load,electric vehicles show the uncertainty of random load affected by multiple random factors.Large scale electric vehicles access to the power grid,which has a great impact on the safe operation of the power system: changing the network structure and power flow distribution of the power grid,causing load fluctuation,increasing the peak valley difference of the power grid load,reducing the voltage and frequency of the power grid,which has a great impact on the safe and stable operation of power grid.The research shows that some vulnerable lines show high fault frequency in the process of cascading failure propagation.Uncertain electric vehicle charging load may cause some unpredictable impacts on vulnerable lines.Therefore,it is necessary to deeply study the power grid vulnerability analysis under uncertain electric vehicle charging load.Firstly,considering the influence of various random factors,a calculation model of space-time distribution of electric vehicle charging load demand is constructed.According to the operation law of different types of electric vehicles,electric vehicles are classified.Considering the influence of time,space and state variables on the charging load,the time and space distribution of electric vehicle charging load is calculated based on the actual urban geographic information and Monte Carlo method.Secondly,a vulnerability assessment method of power system based on the branch potential energy entropy is proposed.According to the power grid topology and the potential energy accumulation of the remaining branches after the occurrence of the two-level cascading failures,a two-level state transition network is generated to comprehensively reflect the topological structure and state characteristics of the power grid.The Theil entropy standard is introduced into the power system vulnerability assessment to characterize the disorder and imbalance degree of the system.The branch vulnerability is evaluated by synthesizing the global vulnerability of the branch,which can effectively reflect the impact of the branch fault on the power grid and the ability of the branch to withstand the impact of the power grid.The simulation of the IEEE-39 bus system and an actual regional power grid proves the effectiveness of branch potential energy Theil entropy index to identify vulnerable transmission lines in the power grid.Finally,a power system vulnerability assessment method based on an improved H-index is proposed.Under the N-1 safety verification principle,considering the occurrence probability of hidden fault,a two-level cascading fault model is constructed.According to the power flow increment before and after failure,a bidirectional weighted state transfer network is constructed and decomposed into a state transfer propagation network and state transfer infection network.The traditional H-index is improved and applied to the state transfer transmission diagram to calculate the transmission index H-out and the infection index H-in.The simulation analysis of the IEEE-39 system and an actual power grid shows that the H-out propagation index can effectively identify the vulnerable lines of the system,and the H-in infection index can identify the lines with frequent secondary faults,that is,the lines with weak ability to withstand the impact of power flow.