Type and number of alocated protection for each device is related to importance of device. Also the need for transformer protection is strongly indicated when the average forced hours of downtime per transformer year is considered. The large value of 356 h average out-of-service time per transformer failure challenges the system engineer to properly protect the transformer and minimize any damage that could occur.
Overcurrent relays applied on the primary side of a transformer provide protection for transformer faults in the winding, and provide backup protection for the transformer for secondary-side faults. They provide limited protection for internal transformer faults because sensitive settings and fast operation are usually not possible. Insensitive settings result because the pickup value of phase-overcurrent relays must be high enough to take advantage of the overload capabilities of the transformer and be capable of withstanding energizing inrush currents. Fast operation is not possible because they must coordinate with load-side protection. Settings of phase-overcurrent relays on transformers involve a compromise between the requirements of operation and protection.
These settings may result in extensive damage to the transformer from an internal fault. If only overcurrent protection is applied to the high-voltage delta side of a delta-wye-grounded transformer, it can have a problem providing sensitive fault protection for the transformer. For low-voltage (wye-side) line-to-ground faults, the high-side line current is only 58% of the low-voltage per-unit fault current. When the wye is grounded through a resistor, the high-side fault current may be less than the maximum transformer load current.