Operation of ground sensor relays under conditions of partial CT saturation

Since the publication of the paper by C.W. Barnett, et. al. (see IEEE Trans. Power Deliv., vol.8, no.3, p.884-97, 1993), concerning low-ratio current transformer (CT) saturation at high fault current and resulting mis-operation or nonoperation of overcurrent relays, there has been a great deal of well-directed concern about the problem reported in that paper. However, some of that concern has also been directed at the application of ground sensor relays to low-ratio CTs in resistance grounded industrial power systems, where no operational difficulties have been reported. The authors conducted a series of tests to determine if there is any cause for concern. These tests documented both the CT secondary current wave shapes and the relay contact closure times for several CT-relay-fault current combinations. This paper reports the results of these tests and presents conclusions concerning the adequacy of present practice in this field