A buchholz relay is used for:?

The Correct answer Option A - It is protection of the Oil Cooled transformers against the internal fault as it is used to detect the internal fault in the Transformer winding when the fault starts between turn to turn or between the winding and Transformer core as the smoking starts inside of the Transformer and Buchholz relay start to sense that by giving  alarm indication and then Trip indication to make the Circuit Breaker in the OFF postion to isolate the Transformer. This type of the protection is depended on the Gas Presure to operate. It is more familiar in the Distribution Network, Transformer Capacity range between 500- 2000 KVA or more than till 5000KVA.

Buchholz relay is used to clear internal faults of transformer like inter-turn faults, winding faults, and core faults. Used in oil cooled transformer and installed between main tank and conservator. Provided when transformer rating is from 500KVA upto 750KVA. It is operated by gas pressure not by electromagnetic action. Its operating time is 0.1 secs.  

Protection against all internal faults. For overload,it will give alarm. In case of any short circuit, inter-turn fault, it will send trip signal to the circuit breaker.

OPTION C,IT CAN ACT AGAINST INTERNAL AND EXTERNAL FAULTS OF THE TRANSFORMER