An energy differential relay for long transmission lines

The operating speed and sensitivity of the current differential protection must be lowered in order to deal with the problems caused by the capacitive currents of the long transmission line. To solve this problem, a new energy differential relay is put forward. The proposed scheme can distinguish between internal and external faults by comparing the energies of two methods. The first method is to calculate the energy flow in the line in a short time interval. The second method is to calculate the energy consumption of distributed elements on the transmission line with the assumption that there is no internal fault on the line. Special means are adopted: use of modal quantities of voltage and current; the instantaneous voltage and current are distributed linearly along the transmission line; the instantaneous voltage and current vary linearly during a sampling interval; the sampling interval is equal to the travel time of the protected line. Thus the energies can be calculated by using the sampled values at each end of the transmission line. It has been proven that the calculated energies of the two different methods are equal when there is no internal fault on the transmission line. The performance of the proposed method has been verified by EMTP simulation tests, dynamic simulation tests and the comparison with a competitive method.