Research On Protection Strategy Of Distribution Network Containing Microgrid

With the increasing demand for clean and sustainable energy in social production,distributed power and micro grid technology emerge as the times require.The two-way flow of fault power flow is caused by the incorporation of microgrid in some lines of distribution network.Due to the fixed action value and the lack of direction discrimination function,the phase current protection has misoperation and misoperation.The limited fault current supply capability of distributed generation also makes the microgrid line unsuitable to rely on the traditional protection mode of current amplitude setting.In view of the above problems,this paper proposes a comprehensive protection scheme.According to the impact of grid connected operation of micro grid on the system,the distribution network system including micro grid is divided into three areas:two-way power flow area,one-way power flow area and micro grid area.Analyze the fault characteristics of each area and configure corresponding protection.The distribution network side is divided into two-way power flow area and one-way power flow area.The two-way power flow area introduces the transmission network longitudinal protection principle,extracts the positive sequence impedance electrical quantity on both sides of the protected line to construct the longitudinal positive sequence impedance differential protection.At the same time,in order to solve the problem of low fault sensitivity in the criterion area,the longitudinal positive sequence impedance differential protection with braking characteristics is established by using the positive sequence impedance difference on both sides Protect.Considering the protection of the fault in the area where the line heavy load passes a large transition resistance and the occurrence of a three-phase metallic short circuit near the protection installation,the protection is improved accordingly and additional criteria are added.Extend phased current protection for one-way flow areas,avoiding full line dependence on communication.According to the topological structure of the line,the microgrid area is divided into the feeder connected by the bus and the tie line between the buses.The feeder connected by the bus determines the fault line by screening the amplitude of the maximum fault component positive sequence current of each feeder,and monitors the three sequence voltage of the bus as the start signal of the feeder protection connected by the bus.The current differential protection is provided for the two-way flow of power flow when the interconnection line between buses breaks down,and the cosine is introduced to improve the traditional current differential protection principle to balance the sensitivity and reliability requirements of the line protection.Through the time delay method,the protection of the feeder connected to the bus bar acts as a backup protection.The regional protection shall cooperate with each other to form a complete regional comprehensive protection scheme.Considering the requirements of distribution network side protection,micro grid side protection and national standards for parallel network switch,and cooperating with the protection on both sides in time,the micro grid operation mode switching strategy is formed,and then the process of parallel network protection is described.The change of the positive sequence power direction and the positive sequence current amplitude of the parallel node is measured to realize the fault direction discrimination function in the parallel node protection principle.When the three-phase metallic fault occurs near the parallel network,it will lead to the dead zone of the protection.Using the positive sequence voltage phase angle before the fault to construct the memory circuit to solve the problem.This paper briefly introduces the principle of islanding detection in the principle of parallel network protection,and explains the unplanned islanding detection index used in this paper.Finally,based on PSCAD simulation system,the protection strategy described in this paper is verified.Figure[37]table[25]reference[60].