The protection of the transformer is as important a part of the application as the rating values on the transformer. Entire texts are devoted to the subject of transformer protection.
When investigating a failure, one should collect all the protection-scheme application and confirm that the operation of any tripping function was correct.
Surge arrester protective level must be coordinated with the BIL of the transformer. Their purpose, to state what may seem obvious, is to protect the transformer from impulse voltages and high-frequency transients.
Surge arresters do not eliminate voltage transients. They clip the voltages to a level that the transformer insulation system is designed to tolerate. However, repeated impulse voltages can have a harmful effect on the transformer insulation.
Overcurrent devices must adequately protect the transformer from short circuits. Properly applied, the time–current characteristic of the device should coordinate with that of the transformer.
These characteristics are described in IEEE C57.109-1993, Guide for Liquid-Immersed Transformer Through-Fault Duration. Overcurrent devices may be as simple as power fuses or more complex overcurrent relays.
Modern overcurrent relays contain recording capability that may contain valuable information on the fault being investigated.
Differential relays, if applied, should be coordinated with the short-circuit current available, the transformer turns ratio and connection, and the current transformers employed in the differential scheme.
If differential relays have operated correctly, a fault occurred within the protected zone. One must determine if the protected zone includes only the transformer, or if other devices, such as buswork or circuit breakers, might have faulted.