As a part of the one-line diagram, a power transformer is shown as below:

enter image description here

Here what I understand is that 400/132kV means 400V is the primary nominal voltage and 132kV is the secondary nominal voltage.

And MVA is the nominal apparent power. But what does 140/155MVA mean in this context? Are they minimum and maximum power? Why there are two numbers with a slash?

  • 2
    \$\begingroup\$ It's more likely a 400 kV to 132 kV step-down transformer. However, with no other context or provenance it might just as well be anything. \$\endgroup\$
    – Andy aka
    Commented Oct 6, 2020 at 8:59
  • \$\begingroup\$ Maybe nominal and surge MVA? What does the document you clipped the diagram from say about the nomenclature? \$\endgroup\$
    – Neil_UK
    Commented Oct 6, 2020 at 9:14
  • \$\begingroup\$ guess 140 is the effective power \$\endgroup\$
    – schnedan
    Commented Oct 6, 2020 at 13:08

1 Answer 1


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.


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