The correct answer is: B. equal to and above 5 MVA.
Transformer differential protection is a type of transformer protection that uses the principle of current balance to detect faults within the transformer. It is typically used on transformers with ratings of 5 MVA or greater, as these transformers are more susceptible to internal faults.
The differential protection system consists of two current transformers (CTs), one on each side of the transformer. The CTs are connected in such a way that the current flowing through them is equal. If a fault occurs within the transformer, the current flowing through the CTs will no longer be equal. This imbalance in current will be detected by the differential protection system, which will then trip the transformer’s circuit breaker to isolate the fault.
Transformer differential protection is a very effective way to protect transformers from internal faults. It is typically used in conjunction with other types of transformer protection, such as Buchholz relay protection and overcurrent protection.
Here is a brief explanation of each option:
- Option A: above 30 kV. This is not the correct answer because transformer differential protection is typically used on transformers with ratings of 5 MVA or greater, not 30 kV or greater.
- Option B: equal to and above 5 MVA. This is the correct answer because transformer differential protection is typically used on transformers with ratings of 5 MVA or greater.
- Option C: equal to and above 25 MVA. This is not the correct answer because transformer differential protection is typically used on transformers with ratings of 5 MVA or greater, not 25 MVA or greater.
- Option D: none of the above. This is not the correct answer because transformer differential protection is typically used on transformers with ratings of 5 MVA or greater.