Core balance ground fault protection of motors on a low-resistance grounded, medium-voltage system

The core balance current transformer is the basis for sensitive ground fault protection of motors. All phase conductors pass through the same opening in the current transformer so that it senses only zero sequence or ground current. Possible sources of nonfault relay current include current transformer error, harmonics, and unbalanced currents through impedances to ground, especially capacitances. The core balance connected ground fault relay is usually an instantaneous type, or a time-delay type of the short time or very inverse characteristic. The standards provide typical ranges of relay settings. They do not provide a procedure for determining the minimum settings that will not result in nuisance trips from nonfault currents nor the maximum settings that will provide adequate protection. This paper develops that procedure.