The purpose of a Buchholz relay in transformer protection can be explained through its function and operational context:

1. Understanding Buchholz Relay: The Buchholz relay is a protective device used in oil-immersed transformers. It is placed between the main tank of the transformer and its conservator.
2. Operation: It detects the presence of gas produced by internal faults such as short circuits and arcing within the transformer oil. When a fault occurs, the generated gas accumulates in the Buchholz relay, causing it to activate.
3. Mechanism:
   • Minor faults generate small gas pockets, causing alarm signals.
   • More serious faults produce significant gas accumulation, tilting the float and triggering an electric contact to cut off the transformer from the circuit as a protection measure.
4. Conclusion: From the given options, the correct answer is:

   To detect internal faults such as short circuits and arcing in oil

   . This correctly describes how a Buchholz relay is used in detecting internal transformer faults, helping to prevent damage by providing an early warning system.

Other options are incorrect because:

• To detect overvoltage

  : This is typically handled by overvoltage relays or arrestors, not a Buchholz relay.

• To measure core flux

  : Core flux is usually monitored by other means within a transformer's design, not by the Buchholz relay.

• To measure temperature rise

  : Temperature is measured using thermometer-based devices or temperature sensors, not a Buchholz relay.