I can’t see 10s or even 100ns rise times on a 400kV primary transformer. That is their issue. 50Hz transformers at the 250kV range are horrendously expensive. Can see a 400kV high frequency one being much cheaper.
In a LVA design you don’t need to worry about 1 or 2 switches per unit failing if designed right. Also they are solid state, so should have very long life times if rated correctly. Its often better to spend a lot up front to prevent downtime.
