Design and testing of a new microprocessor-based current differential relay for EHV teed feeders

The hardware design and laboratory testing of a new high-speed current differential protection scheme for application to EHV teed feeders is described. Special emphasis is placed on designing the relay comparator and its associated data acquisition units for inputting data into the comparator from a laboratory-based programmable transmission line. The protection algorithm, which includes all the necessary filtering and a sophisticated trip logic, has been implemented on a single digital signal processor. A comparison of the signal waveforms, in particular the differential and bias signals, has shown a very close correspondence between the signals attained from practical and theoretical models of the relay. Furthermore, an extensive series of tests carried out on the practical model using a programmable transmission line has shown that the performance attained from the hardware design, both for internal and external faults, is nearly consistent with that obtained from computer-aided design techniques