An undervoltage relay to distinguish between blown potential transformer fuse and true bus undervoltage



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"False Tripping" of a bus incoming breaker because of a blown potential transformer fuse is a very common and frustrating problem for operators of generating stations and process plants. In generating stations, this results in undesirable shutdowns and time consuming sequential restartings. In process plants, it results in very costly process failures. This thesis presents an analysis of bus voltages during various types of faults causing true undervoltage on the bus. It also analyzes bus voltage during a blown-out potential transformer fuse. It includes an existing solution which distinguishes between the blown-fuse case and true undervoltage on the bus. To solve this problem a new undervoltage relay has been built, using a negative-sequence voltage circuit, a polar unit and a cycle timer. This relay distinguishes between true undervoltage and a blown potential transformer fuse. Two alternate solutions have also been provided. The first solution uses an undervoltage relay consisting of a composite current filter and a cycle timer. The alternate solution uses a combination of three undervoltage auxiliary relays an alarm relay and a timer.