Risk Assessment Of Cascading Failures In Interconnected Distribution Networks With Flexible Multi-state Switch

In order to cope with the energy crisis,China will continue to deepen the energy reform plan during the "fourteenth five year plan" period,and continue to promote the vigorous development of clean energy.Therefore,renewable energy access technology and problem governance are still the key issues for the future development of distribution network in China.With the increase of new energy penetration,the impact of high proportion of distributed generation on the safe and stable operation of distribution network is increasingly prominent.At the same time,the load is gradually developing,and the power quality,power supply reliability and other customized power demand are improved.The increase of electric vehicles,electric heating and irrigation load greatly reduces the utilization efficiency of distribution network equipment.For the current distribution network,the contradiction between load demand growth and technical constraints is increasingly prominent.The application of Flexible Multi-state Switch in distribution network can replace the traditional tie switch to form a flexible interconnected system with closed-loop power supply,which can realize uninterrupted power supply to important users in case of fault.Its function of continuous power flow regulation can effectively deal with the voltage fluctuation,feeder power flow imbalance and other problems caused by the grid connection of distributed generation,which has become a comprehensive solution.However,most of the power electronic equipment is not strong enough to withstand the over-current,the control system is more complex,and the introduction of flexible equipment will make each link of the distribution network coupling,which may lead to cascading failures and increase the risk of large-scale outage events.Therefore,considering the uncertainty of operation state in flexible interconnected distribution system,this paper studies the reliability model of FMS,analyzes the reaction mechanism of cascading failure in flexible interconnected distribution system,and puts forward the risk assessment process of cascading failure.The specific research contents are as follows:Firstly,the physical structure,steady-state operation characteristics and transient characteristics of FMS are introduced.Based on the common double closed-loop control,the control and protection strategy of FMS is proposed;Then,based on the traditional FMS structural reliability model,the influence of random current load on IGBT reliability is quantitatively analyzed.The operational reliability model based on Bayesian network is established by using directed acyclic graph and conditional failure probability table,and the reliability indexes of each component and subsystem are obtained;Then,the reaction mechanism of cascading failure in flexible interconnected distribution system is analyzed,and the uncertain factors that affect the cascading failure are pointed out.The probability models are established respectively,and the risk assessment process is proposed.The risk index is obtained through the calculation of probability and consequence;Finally,based on MATLAB/PSCAD dual platform co simulation,the proposed modeling method and risk assessment process are simulated and verified.The equipment reliability parameters are derived from the statistical data of dajiangdong area in Hangzhou.Three improved ieee-33 bus systems are used in the test system.The calculation results of the traditional model and the model in this paper are compared respectively to verify the feasibility of the modeling method in this paper,and at the same time The risk index of cascading failure is calculated,and the influence of various uncertain factors on the probability of cascading failure is discussed.