| This dissertation had discussion on the characteristic of HV/EHV transmission line and appeal for protection relays, had analysed behaviour and shortage about differential protection, moreover had brought forward improvement project. This dissertation is devoted in the study of composing a suit of digital current longitudinal differential protection for HV/EHV transmission line with excellent performance. The dissertation mainly concerns the following aspects: current phasor differential protection, the problem of synchronous sampling at all terminals, broken secondary circuit of current transformer, current transformer saturation etc.. Based on the research findings of these problems the scheme of a suit of digital current longitudinal differential protection has been proposed. This scheme has been applied on WXH-803/A digital transmission line protection of XJ GROUP CORPORATION. The feasibility and advantage of this scheme was verified by the results of dynamic modeling test in National centre for quality supervision & testing of relay and protective equipment and digital modeling test in north China Electric Power Research Institute.
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