Sensitive Ground Relay Protection for Complex Distribution Circuits

Protection engineers always have desired to obtain a system of ground fault protection that will discriminate and actually trip out only the ground faulted circuit of a complex distribution network. This has been a difficult chore because the magnitude of the ground current often varies so greatly as to be out of the range of the normal protective relay settings. A system is proposed here that not only has proper discrimination, but also operates on very low amounts of ground current with the maximum ground current held to such a low value that practically no harm or damage is done by the ground fault. This development was encouraged by the necessity of joint use of poles by lines of communication circuits and distribution circuits. This method comprises the installation of a small bank of grounding transformers at each of the stations where the lines originate or terminate. With these small ground banks the maximum ground fault current is very small. Adequate relays and systems are used to isolate only the faulted sections of line. The installation of one 11-kv and one 16.5-kv loop will be described. These two systems have had several years of successful operation.