Research On Security Assessment Method For Interregional Loop Closing Operation In Medium-voltage Distribution Network With Distributed Generation

The loop closing operation for load transfer(or loop closing operation for short)in medium voltage distribution network can reduce the outage time of load and improve the reliability of power supply in the case of planned maintenance of equipment or local fault treatment of power grid.However,due to the influence of the regional operation of the power grid,there may be large loop closing current caused by the interregional loop closing operation in medium-voltage distribution network,which is easy to cause overload of electrical equipment and switch tripping,and even cause expanding accidents.In addition,due to the random fluctuation of the output of distributed generation,the access of distributed generation to the medium-voltage distribution network increases the risk of loop closing operation.In view of the actual security problems of the loop closing operation in medium-voltage distribution network,the purpose of this paper is to study the security assessment method for interregional loop closing operation in medium-voltage distribution network with distributed generation according to the measurement configuration and state estimation level of distribution network,so as to provide scientific decision-making basis for operators,and improve the security of loop closing operation and the reliability of power supply.In this paper,the main modes of loop closing operation in medium-voltage distribution network is analyzed firstly.Based on the superposition theorem and Thevenin equivalent method,the calculation method of steady-state loop closing current and surge current caused by loop closing operation in medium-voltage distribution network with distributed generation is studied.On these basis,the security conditions of loop closing operation are proposed.Aiming at the problem that the loop closing current cannot be accurately calculated due to the failure to obtain real-time data of each load point on the loop closing feeder and the random fluctuation of the output power of distributed generation,a method to calculate the probability distribution of loop closing current is proposed.Based on the probabilistic load flow theory,the output power of distributed generation and load are taken as random variables,then the probabilistic distribution model of each random variable can be established,and the probability distribution of loop closing current can be calculated with the cumulant method.On this basis,the security assessment method for interregional loop closing operation in medium-voltage distribution network with distributed generation is proposed.Based on the probability distribution of loop closing current,the security evaluation index of loop closing operation is established.Therefore,the security of loop closing operation can be quantitatively evaluated by the over-limit probability and over-limit degree of loop closing current from two perspectives.Finally,a local distribution network in a certain area of Shanghai is taken for case study,the effectiveness the security assessment method for loop closing operation proposed in this paper is verified through simulation and analysis.