THE EFFECT OF CONVENTIONAL INSTRUMENT （传统仪器的影响）.pdf

THE EFFECT OF CONVENTIONAL INSTRUMENT TRANSFORMER TRANSIENTS ON NUMERICAL RELAY ELEMENTS Demetrios Tziouvaras, Jeff Roberts, Gabriel Benmouyal, and Daqing Hou Schweitzer Engineering Laboratories Pullman, WA USA SUMMARY Current and voltage transducers provide instrument level signals to protective relays. Protective relay accuracy and performance are directly related to the steady state and transient performance of the instrument transformers. Protective relays are designed to operate in a shorter time than the time period of the transient disturbance during a system fault. Large instrument transformer transient errors may delay or prevent relay operation. In this paper, we describe the effect of conventional instrument transformer transients on numerical relay elements and discuss numerical relay designs that cope with conventional instrument transformer transients. The elements we examine include distance, line differential, and breaker failure. Current Transformer (CT) Transients CT output is impacted drastically when the CT operates in the nonlinear region of its excitation characteristic [1]. Operation in this region is initiated by: • Large asymmetrical primary fault currents with a decaying dc component. • Residual magnetism left in the core from an earlier asymmetrical fault, or field-testing, if the CT has not been demagnetized properly. • Large connected burden combined with high magnitudes of primary fault currents. When the CT saturates because of the dc component, it can do so in the first few cycles of the