When a 20 MVA oil-filled transformer is tripped by the differential relay, which test is NOT typically used to determine the cause of the trip?

The correct choice highlights that frequency response analysis is not typically used to determine the cause of a trip event in the context of a differential relay activation for an oil-filled transformer.

In practice, when an oil-filled transformer trips due to differential protection, the focus is usually on understanding the immediate electrical and mechanical conditions that led to the trip. Total combustible gas analysis is crucial in diagnosing insulation failures, as gases are generated from thermal breakdowns, which are characteristic of problems in transformers. Transformer turns ratio testing helps ensure the transformer is functioning properly and can reveal problems with the windings, while winding resistance tests assess the integrity and connection of the windings.

Frequency response analysis, however, is generally used to identify issues such as core problems, mechanical displacement, or other structural physical changes rather than addressing immediate operational tripping events. Therefore, while it can provide valuable information regarding the overall health of a transformer, it is not the primary method used for troubleshooting trips caused by differential relays, making it the least relevant of the four tests listed for this specific situation.