Is there any standard for safe distance between transformer and sub-station building if there is no partition wall? Say, transformer rating is 2MVA 6.6kV/0.4KV.
I have no experience with European requirements. But here in the US, the minimum distances are often driven by the insurance companies. Factory Mutual, in particular, has some very specific requirements.
The type of transformer (dry/liquid filled) and type of insulation (oil/less flammable) can also impact this.