A NEW ACCELERATED TRIP SCHEME FOR SECOND-ZONE DISTANCE PROTECTION
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Application of distance protection without communication channel employs at least two zones. The first zone usually covers 70-90 percent of the protected transmission line and is set to trip without intentional delay. The second-zone is set to protect the remainder of the line along with an adequate margin. The operation of the second-zone distance protection is time delayed (normally 0.30.5 s) to coordinate with relays protecting other transmission lines and breaker-failure relays located at the remote-end bus. This time delay limits the application of distance protection in EHV (Extra High Voltage) transmission lines due to fast fault clearance requirements of EHV power systems. It is possible to use memory and logic capabilities of microprocessor-based relays and develop an accelerated trip scheme for second-zone distance protection. A literature survey reveals that there is at least one such scheme that has been proposed. However, this scheme has some limitations including that it is difficult to determine settings for this scheme. This thesis presents theoretical development, description and test results of a new accelerated trip technique for microprocessor-based distance protection of HV/EHV (High Voltage / Extra High Voltage) transmission lines. The technique is simple and provides relatively high-speed (0.06-0.07 s) operation of distance protection for second-zone faults on the protected line without using a communication channel. The technique has been extensively tested using EMTDC (an electromagnetic transient program) simulations under various power system operating and fault conditions. Simulation results show that selectivity and speed of the proposed scheme are satisfactory for use in distance relays protecting HV/EHV transmission lines.