| Along with the power system development such as power transmitted form west china to east china, power grid interlink project, ultra high voltage transmission, large numbers of EHV transmission line has been commissioned in china. In order to keep the stability of the power system, the higher demand of the characteristics of high voltage transmission line protection has been put forward. The dissertation mainly concerns about the study of how to research and development an EHV transmission line protection with high performance. The significant achievements are as follows: 1) In order to accurately measure the impedance, A new residual current compensation method on the current magnitude of each phase for Mho elements is presented in this paper. 2) Based on the electromagnetic transient model of the distributed transmission line, a new method based on the Bergeron model of the transmission line has been put forward. The influence of the distributed capacitance current has been considered in this new model. 3) How to get the correct phase selection result is still s difficult question for the backup protection of the transmission line protection. A new method for phase selection based on impedance in microprocessor based protective relay is presented in this paper. New scheme may be one of the solutions for the phase selection based on steady component.
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