Investigation of impedance based protection technique for EHV 3-terminal double circuit lines

A digital impedance based protection scheme for a 3-terminal transmission line has been implemented on a digital signal processor board and tested on a physical model of a double-circuit 3-terminal line with a source at each end. One combined relay at each terminal of the double-circuit line, instead of two relays, is proposed. Each relay is fed by three voltage and six current signals. Real-time tests conducted on the physical model for various faults show that high fault resistance at the teed point, current infeed, remote-end infeeds, fault location, source impedance and far-end faults cause no problem in proper relay operation. Also, 100% of the lines are protected.