Longitudinal Protection Technology Of Distribution Networks With Inverter-Interfaced Distributed Generators

With the gradual deterioration of the environment and the shortage of disposable energy such as fossil fuels,the development of the current power system has been severely restricted.The occurrence of multiple major blackout accidents also poses serious challenges to the security and reliability of the traditional power generation,transmission,and distribution modes.At the same time,the distributed generators(DG)represented by renewable energy,such as photovoltaic power generation,wind power generation and fuel cells,have been rapidly developed.A large number of distributed generators are connected to the distribution networks.On the one hand,they solve many of the inherent problems of the traditional power grids.On the other hand,they bring many adverse effects to the fault diagnosis of the distribution networks.The existing traditional directional components and relay protection strategies are difficult to meet their requirements.Thereforc,it is of great significance for the research of relay protection strategies and directional components suitable for intelligent distribution networks.Firstly,this thesis studies the grid-connected control mode and working principle of the inverter-interfaced distributed generator(ⅡDG),and proposes the PQ control mode of IIDG in the case of symmetrical faults and the negative sequence current suppression control strategy of double DQ in the case of asymmetric faults.In addition,the output characteristics of IIDG under different faults and different control methods are obtained.Through the analysis of the IIDG through △-Y0 isolated transformer grid connection,the suppression characteristics of the zero sequence fault component in fault output component are obtained.Through its combination with the zero-sequence equivalent network,the suppression coefficient of the zero-sequence fault component is finally obtained.Secondly,this thesis designs the fault directional components suitable for IIDG distribution networks.Based on the grid-connected location and number of distributed generators in the distribution networks,an equivalent impedance model is established.Based on the equivalent model of the power grid,the article studies how the distribution network under the influence of the high-permeability distributed generators can correctly determine the fault direction of the protection installation when the fault occurs at different locations,and designs the fault directional components based on the power frequency variation for the fault occurring in the side of the power grid and the fault directional components based on the difference characteristics of the sequence component in the IIDG grid-connected feeder.Finally,this thesis proposes a distributed regional longitudinal protection strategy applied for the distribution networks.Supported by a Multi-Agent system,the control system at the decision-making level of the system divides the associated domains online according to the topology of the distribution networks.At the same time,an Agent unit with current detection element and direction discriminant element is installed in each circuit breaker switch capable of cutting off short-circuit current.The protection area and the adjacency relation between each Agent unit are determined by the correlation domain divided by the control system.The whole system forms a mode in which the control system divides the protection area,WAN,and LAN online according to the topology of the distribution network to serve as information transmission channel,intelligent distribution terminal,and fast locating and isolating faults.