TITLE:
Research on the Reason for Transformer Differential Protection Mal-operation Caused by Sympathetic Inrush
AUTHORS:
Xiangfei Sun, Hongchun Shu, Jianping Zhou
KEYWORDS:
Sympathetic Inrush; CT Transient Saturation; Transformer; Differential Protection
JOURNAL NAME:
Energy and Power Engineering,
Vol.5 No.4B,
November
13,
2013
ABSTRACT: The paper is concerned with whether CT transient
saturation, which caused by sympathetic inrush, is the direct reason for
mal-operation of transformer differential relays. In order to analyze
transforming characteristics of saturated CT, the gain of each harmonic current
transforming from primary winding to secondary winding of transient saturation
CT is calculated. Based on the fact that series sympathetic inrush
is easier to lead differential relay mal-operation than parallel sympathetic
inrush, the effect of CT saturation on differential relayin the
case of both parallel sympathetic inrush and
series sympathetic inrush are investigated respectively. Theory
analysis and simulation show that higher harmonic will be transferred by
saturated CT more easily than lower harmonic, and then the second harmonic
proportion of CT secondary side is higher than that of CT primary side. Thus CT
transient saturation itself is not essential reason for differential relay
mal-operation. Parallel sympathetic inrush and series sympathetic inrush will
lead to different location CT saturated. Parallel sympathetic inrush will lead
to CT located system side saturated; second harmonic
ratio of differential current increase and dead angle is large. Series
sympathetic inrush will lead to CT located load side saturated; second
harmonic ratio of differential current decrease and dead angle is small.