Evaluation of grounding and protection methods for a shipboard power system

摘要

Unscheduled interruptions on a shipboard power system can have serious consequences. Because a single phase to ground fault in ungrounded or high resistance grounded system does not affect the phase to phase voltages, it is possible to continue operating either system in a faulted condition. However, a second line to ground fault occurring on one of the other two phases, causes a double line to ground fault. This would require instantaneous tripping of breakers resulting in loss of service continuity. Hence rapid, secure, detection and location of the first ground fault is necessary to isolate the faulted circuit and maintain service continuity. Detecting a ground fault is easy and can be done with a zero sequence voltage relay. However locating the fault is very difficult because of the low values of ground fault currents. The paper discusses available methods in the industry to detect the direction to a ground fault. A theoretical approach for fault location using negative sequence currents is presented. In addition, a modified single ended algorithm for fault locating is introduced. The proposed algorithm is: simple, cost effective, easy to understand and implement, can be used on an existing or a new ungrounded and/or a high resistance grounded system and does not depend on communication channels. Limitations and key assumptions of the proposed algorithm are discussed.