Importance

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Abstract

Digital distance relaying of a three-phase electric power transmission line depends on accurate estimation of the postfault voltage and current phasors during the first postfault electrical cycle, when the line voltages and currents are corrupted by noise and transients. The digital relay herein responds to each postfault sample as it arrives and recursively electronically estimates the postfault phasors before the next sample arrives. The method involves a powerful state variable approach which for the voltages provides at least two state variables per voltage, and for the currents provides at least three state variables per current. Parallel processing of the samples equalizes computer burden, and an additional processor classifies the fault, computes faulted line resistance and reactance, relays the fault by tripping a circuit breaker, and documents the fault including its distance. Numerical results of part of the powerful estimation procedure which can be performed offline are prestored in the system, to permit the balance of the estimation procedure to occur in real-time, with slower inexpensive processing hardware. Relaying is expected within the first half of the first postfault electrical cycle for most Zone 1 faults.

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First Claim