Secure application of transformer differential relays for bus protection

The greatest challenge of bus differential protection is dealing with false differential current due to CT saturation. Percentage-restrained differential relays are often applied for bus differential protection applications. The percentage restraint principle provides security by requiring that the differential current exceed some percentage of the current flowing through the zone. There are few application guidelines for choosing a secure slope setting when applying this type of relay for bus protection. This paper reviews basic concepts of differential relays and current transformers and then describes a study that was performed to determine secure settings for a relay in a specific bus protection application. Results of the study and recommendations are included.