My Mind, My Heart, My File

CT SATURATION CALCULATIONS – ARE THEY APPLICABLE IN THE MODERN WORLD? – PART I, THE QUESTION

Abstract - Previously, ANSI/IEEE relay current transformer (CT) sizing criteria was based on traditional symmetrical
calculations usually discussed by technical articles and manufacturers’ guidelines. In 1996, IEEE Standard C37.110-
1996 [1] introduced (1 + X/R) offset multiplying, current asymmetry and current distortion factors; officially changing
the CT sizing guideline. A critical concern is the performance of fast protective schemes (instantaneous or differential
elements) during severe saturation of low ratio CTs. Will the instantaneous element operate before the upstream breaker
relay trips? Will the differential element mis-operate for out-of zone faults? The use of electromagnetic and analog relay
technology does not assure selectivity. Modern microprocessor relays introduce additional uncertainty into the design/verification process with different sampling techniques and proprietary sensing/recognition/trip algorithms. This paper discusses the application of standard CT accuracy classes with modern ANSI/IEEE CT calculation methodology. This
paper is the first of a two-part series; Part II, the Findings provides analytical waveform analysis discussions to illustrate
the concepts conveyed in Part I.

Index Terms – Accuracy Class, Asymmetrical Current, CT Burden, CT Saturation, DC Offset, Digital Filter, and X/R ratio.

See detail : Curent Transformer (CT) Saturation Calculation