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

A digital impedance based protection scheme for a 3-terminal transmission l ine 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, instea d of two relays, is proposed. Each relay is fed by three voltage and six cu rrent signals. Realtime tests conducted on the physical model for various f aults show that high fault resistance at the teed point, current infeed, re mote-end infeeds, fault location, source impedance and far-end faults cause no problem in proper relay operation. Also, 100% of the lines are protecte d.