Applying CTs with digital ground relays

Abstract

Industrial power distribution system substation transformers and generator step-up transformers in power stations often use resistance-grounded wye secondary windings for medium-voltage power distribution. The purpose of this is to limit damage due to ground-fault currents, while providing sufficient fault current for the operation of ground-fault relaying. The relaying used to protect against ground faults in the system may not provide sufficient protection of the transformer winding against internal faults because the backup ground overcurrent relay in the transformer neutral-to-ground connection must be set to coordinate with downstream relays. In order to protect the winding itself, special relays are utilized. Ground differential protection can be provided by digital overcurrent relays in conjunction with auxiliary ratio matching transformers. Ground differential protection can also be provided in multifunction digital relays. Transformer protection relays may include this feature with one of the schemes used with component relays. If a feeder-protection relay is used on the secondary, in some cases, this may have a ground-directional feature that can be utilized for ground-differential protection.