The Low Voltage / VA ones I have dealt with [600 VAC and lower, 750 KVA and down] which were installed inside of a Designated "Electrical Room", normally would have supply air blowing in 24/7, and maybe a very small return air register / grill [if any!].
Some came directly from the cold air supply of HVAC systems, others were connected to like a "Dedicated Cold Air Supply For Equipment" Plenum, others just blew in outside air.
Don't recall seeing any that were negative pressure [return air only], probably due to the fact that the Buildings used ceiling space for return air [AKA Plenum Ceilings], and this would really "Suck" [no pun intended] for cooling transformers!
Now as for the PoCo's Transformer Vaults, they are normally Passive Cooled with noticeable vent stacks popping up from the vaults.
On one project in Westwood Village [on Wilshire Blvd. just down the street from Beverly Hills], the meters and MSB's were in the basement next to the PoCo's 34.5 KV Transformer Vault [separated by a locked explosion and fire resistant door]. This vault was only ventilated at the street, but did have a blower to move air.
So did the gear room, since it had a bunch of transformers inside. This area was positive pressure, but not ducted for return.
Scott S.E.T.
