A question about power transformer power rating on a diagram
Transformers in this rated power generally use oil as an internal cooling medium (which cools the windings and the core, inside the tank) and air as an external cooling medium (which effectively dissipates losses, on the surface of the radiator fins). Such transformers usually have fans that can be turned on to increase the cooling on the outside and, consequently, provide a slightly higher power.
In the case of your transformer, apparently, the transformer can supply 140MVA without any fan connected and approximately 10% more power (155MVA) with all fans turned on.