EcoStruxure Transformer Expert is designed for oil filled transformers. There is no limitation on the size of the transformer. Suitability of the solution is usually defined by the economics of the situation of each individual transformer, rather than by technical limitations.
In general, we find that customers choose the transformers that are of most concern first (critical infrastructure, aged, wet, high temperature, changing load, etc). The solution is currently installed on transformers from 500 kVA to 290 MVA, however, customers are currently planning to install it on larger power transformers.