Reducing tripping times in medium voltage switchgear

Traditional coordination time intervals (CTI) between successive levels of protection in a main-tie-main (MTM) configuration have been about 0.25 to 0.3 seconds. When several layers of coordination must be accommodated, the backup clearing times can be excessively long. However, a variety of approaches may be used to reduce overall backup coordination times. When coordination time improves, there may also be significant impacts on total clearing time, arc flash incident energy, voltage regulation, motor dropouts and system integrity. This paper compares three approaches to reducing the maximum total clearing time in a typical MTM configuration including: Setting group change(s) with feedback from the tie breaker position; Use of high-speed relay-to-relay communications to quickly isolate fault locations; Use of an electronically triggered fault current limiter (ET-FCL) to limit available fault current.