Fiber Longitudinal Differential Protection Device Design For Metro Medium-Voltage Ring Network

The medium-voltage ring network is the core of metro power-supply system, and it provides guarantee for the normal operation of locative and various types of electrical equipment within the station. Therefore the stable and reliable operation of medium-voltage ring network is very important. Current differential protection is the main protection in the protection configuration of medium-voltage ring network. Currently fiber longitudinal differential protection device used in metro medium-voltage ring network mainly depends on foreign products, so developing fiber longitudinal differential protection device with intellectual property right has a great social significance and broad market prospect.Firstly, the structure and operation of metro medium-voltage ring network was introduced in this paper. By analyzing failure mode of medium-voltage ring network, the protection scheme of medium-voltage ring network was discussed. The function of fiber longitudinal differential protection device was designed under the scheme, the principle of longitudinal differential protection was main analyzed, four conditions such as differential protection judgement, connection of fiber communication, communication protocol, Synchronized data-sampling, which can make longitudinal differential protection achieve were analyzed in this paper. Sampling data synchronization of protection devices on both sides of the line is the basis for longitudinal differential protection. By analyzing the advantages and disadvantages of traditional synchronized data-sampling method, a new method based on interpolation was proposed to synchronize data. When the protective device access to differential types of transformers, the synchronized data-sampling method was analyzed, the interpolation error was analyzed, and the accuracy of this method was verified in this paper. The types of protection principles and the operating conditions were analyzed, and protection logic diagram was given in this paper.To meet the demand of each function of fiber longitudinal differential protection device, the hardware of protective device was divided into AC plug, CPU plug, power plug and trip plug in this paper. The function is realized for each plug and the hardware devices are selected for the important components.Based on the hardware design of protective device, software design was realized by CCES platform in this paper. Analyzing the functions of protective device, the software was divided into four modules, such as data acquisition module, protection module, communicat-ion module, and display module, each module was programed in this paper.Finally, the functions of fiber longitudinal differential protection device such as data acquisition, packets sending and differential protection were tested and verified by building a test platform.