Using small transformer for large generator

There are 12 transformers used in IEEE 39-bus system example, and every single of them is rated at 100 MW apparent power (default value) even though they are connected to generators having much larger apparent power rating. Why is that? Shouldn't we use a large transformer for a large generator?

Answer 1

[Sabin]

In the IEEE 39-bus system example, each transformer is assigned a default rating of 100 MVA primarily for modeling consistency and computational simplicity, rather than to reflect actual equipment ratings. This standardization allows for easier comparison of results and reduces complexity in power flow studies. In real-world systems, transformer ratings are indeed selected to match or exceed the generator ratings to handle the full output safely. However, in test systems like the IEEE 39-bus, the focus is on network behavior rather than precise equipment sizing, so using a uniform, smaller transformer rating is a modeling convenience rather than a representation of practical design, where larger generators would be paired with appropriately sized transformers.