The correct answer is: D. 30 MVA upwards.
The forced oil and air circulation method is a type of transformer cooling system that uses fans to circulate oil and air through the transformer. This type of cooling system is typically used for transformers with capacities of 30 MVA or more.
The forced oil and air circulation method has several advantages over other types of cooling systems, including:
- It provides more efficient cooling than other types of cooling systems.
- It is less susceptible to overheating than other types of cooling systems.
- It is easier to maintain than other types of cooling systems.
However, the forced oil and air circulation method also has some disadvantages, including:
- It is more expensive than other types of cooling systems.
- It is more complex than other types of cooling systems.
- It requires more maintenance than other types of cooling systems.
Overall, the forced oil and air circulation method is a good choice for transformers with capacities of 30 MVA or more. It provides efficient cooling, is less susceptible to overheating, and is easier to maintain than other types of cooling systems. However, it is also more expensive and complex than other types of cooling systems.
The other options are incorrect because they do not represent the typical capacity range for transformers that use the forced oil and air circulation method.