Description :
If someone is meaning no-load /fixed losses when energizing a transformer, then It can be said that no-load losses are eddy current loss & hysteresis loss of the core.
How much power is needed depends on the following factors :
Eddy loss depends on frequency, flux density, the volume of core, the thickness of laminations & the type of the core -whether NGO, CRGO, HRGO & grade of core (M4, M5, M6, MOH, etc.)
Similarly, Hysteresis loss also depends on frequency, flux density, the volume of core & the type of core material.
Voltage class (11 KV) & rating (2.5 MVA) can not say, how much power is needed to energize.
But we can guess the range – we know transformer efficiency is above 99%. If we consider efficiency 99.5 %, then the total loss ( no-load loss + load/Cu loss ) of a 2500KVA transformer = 2500*.005=12.5 Kw. Let’s consider no-load loss 20 % of total losses =2.5 Kw.