Research On Calculation Method And Control Strategy Of Loop-closing Current In Smart Distribution Network

Loop-closing operation in distribution network is a conventional means to transfer power supply and ensure uninterrupted maintenance.However,loop-closing operation may produce excessive loop-closing current and impact the stability of distribution network.The operation mode in smart distribution network is more complex than that of the traditional distribution network.Thus the impact of loop-closing current is more serious and it is more difficult to determine the loop-closing conditions.Therefore,it is of great application value to estimate the loop-closing current and control corresponding accident hazard by rule and line with the use of high precision measuring devices,fast communication systems and regulation equipment.Based on the above reasons,this paper carries out a series of studies on the calculation and regulation of loop-closing current in smart distribution network.First of all,the loop-closing operations are classified,the cause of loop-closing current is explained and the loop-closing operations that produce larger loop-closing current are the focus.Considering the security problems that loop-closing operation brings,the influence of loopclosing current on relay protection is discussed.Moreover,real-time and high-precision data is of great support in safety condition analysis of the loop-closing operation in practical engineering,so the paper also detailedly introduces high-precision measuring device and fast communication system in smart distribution network.Based on the above,the safety conditions of various loop-closing operations are further analyzed,and the simulation model is established in Simulink to validate the rationality of the conclusions.Then,the calculation method of loop-closing current in smart distribution network based on PMU is proposed to determine whether the loop-closing operation meets the safety conditions.First,the method obtains the loop-closing path through node marking and depthfirst search,and obtains electric parameters by PMU.Equivalent treatment of distributed generation,transformer and branch line is carried out in two cases: calculating steady-state current and calculating surge current.And mathematical equations are established to obtain the mathematical expressions of the loop-closing steady-state current and the loop-closing surge current.Taking Shanghai Lingang smart distribution network demonstration area as the case and the model is built in Simulink for simulation verification.The results validate that the calculation error of the loop-closing surge current maximum is about 3.6%,and the calculation error of steady-state current amplitude is about 2.2%.The calculation result is of great accuracy and in line with the safety assessment principle of leaving a margin,which increases success rate of the judgment in permissible loop-closing operation.Finally,by analyzing the main factors affecting the loop-closing current,the loop-closing regulation means are determined,including adjusting the transformer ratio,static var compensator compensation and distributed generation reactive power output.Considering that other indexes will be affected when regulating the loop-closing current,the loop-closing comprehensive regulation model is established with the loop-closing current,voltage deviation and active power loss as the targets.In addition,an improved chaotic multiobjective immune particle swarm optimization algorithm is proposed to solve the regulation model.Compared with the traditional optimization algorithms,this algorithm has significantly improved the quality,distribution and convergence of the solutions.On the basis of the previous examples,the loop-closing network that does not meet the safety conditions is regulated.Compared with the targets values before regulation,the loop-closing current decreased by29.4%,the voltage deviation decreased by 40.2%,and the active network loss decreased by26.7%.The results of the case validate that the loop-closing comprehensive regulation strategy can create safe conditions for loop-closing operation and ensure the stability and economy of distribution network operation as far as possible.