Lesson 4

A 138/13.8 kV transformer in a 138 kV substation experienced a severe external fault on the low-voltage feeder system. The feeder protection should have cleared the fault, and transformer differential protection should have remained stable because the fault was outside the transformer zone. Instead, the 87T differential relay operated and tripped the transformer.

Inspection showed no evidence of internal transformer damage. The actual root cause was incorrect CT ratio compensation inside the transformer differential scheme after a relay replacement. During normal loading, the mismatch was not obvious. During the heavy through-fault, however, the error became large enough to create false differential current.

Was this a misoperation?

Yes.

The transformer differential protection tripped for an external fault, outside its intended secure zone behavior.

What the engineer should do next

The engineer must treat the event as a scheme configuration failure, not just a settings tweak. Differential protection depends on correct ratio, polarity, vector group compensation, and matching logic.

Corrective action/engineering remedy

• Verify all CT ratios, polarity, and phase connections on both transformer sides.

• Check transformer vector group compensation inside the relay.

• Confirm that the relay replacement process correctly migrated all compensation settings.

• Review differential slope and restraint settings for external through-fault stability.

• Test the scheme using balanced load, internal fault, and external fault simulations.

• Add a structured commissioning checklist for future relay replacement or firmware migration.

• Update documentation so future engineers can verify ratio and vector compensation more easily.

Final lesson

Differential protection is highly secure only when its compensation model is correct. The remedy is disciplined commissioning, validation, and migration control.