Directional Current Differential Protection Based On Fault Components

Current differential protection is an main important principle of transmission line protection, which has a lot of advantages compared with other protection principle. Along with the development microcomputer and with the progress of fiber communication technique, research on current differential protection criterions is becoming more and more important than before. This paper concerns on this direction. To solve the problem that full current differential protection doesn't has enough sensitivity when grounding with big transition resistance, this paper proposes a new directional current differential protection criterion. Compares with other criterions, this one has great sensitivity to inner fault and great restraining ability for external fault and can endure large fault resistance.In this criterion an incremental component directional current differential protection is used as main protection of transmission line and an incremental component distance protection which has ability to endure fault resistance and has a clear direction is proposed to be back up protection. Main protection and backup protection use current fault components in the two circles following the happening fault and change to full current differential protection after the two circles. By doing this the criterion can ensure high operation reliability. Because main protection and back up protection is similar in principle, the criterion can be realized easily.In this paper the digital simulation of the proposed principle have been done on the 220kV and 500kV transmission line model and then comparison of the performance of fault component ratio-restraining differential protection was made .and zero sequence directional current differential protection. The results of simulation proved that this criterion has remarkable improvement in tolerance transition impedance and great restraining ability when short circuit outside the transmission line.Finally this paper introduces the hardware and software design of data acquisition &communication board which is a main part of microprocessor based differential protection device. The functions of this module include data acquisition, sample synchronization, data exchanging and communication. Serial communication controller Z85230 is used to receive and transmit data. Dual port RAM is used to exchange data and information between protection CPU and communication CPU. MAX125 transforms analog signals to digital signal and is used to acquire current and voltage value. Communications between two relay protection devices obey SDLC bit synchronize protocol. Optical fiber is used as transmit medium which has high transmit speed and has good performance in resisting disturbance。...