Detection of and protection for internal low-current winding faults in overhead distribution transformers

Changing protection practices for overhead distribution transformers involves mounting the arrester on the transformer tank and using lightning-surge-tolerant fuses to provide protection. Concerns about protecting transformers against low-magnitude internal winding faults as a result of this practice are addressed in this paper. An analysis of a highpower test involving primary and secondary winding faults is presented. The magnitude, duration, and "signature" of the winding faults are determined and categorized. An analysis of the response of surge-tolerant fuses shows that they often will not detect low-current winding faults before they propagate to the available system fault current. Testing also demonstrated the performance of current-limiters to effectively reduce the available system fault current, thereby minimizing stress to the transformer. The use of current limiters is recommended with surge-tolerant fuses. The pressure rise in the tank for primary winding faults is mainly caused by low-fault current and, when protected with current limiters, the relief device easily vents the gases. The pressure rise for secondary winding faults is more rapid and can exceed the capability of the relief device to limit the overpressure.