Transformer efficiency is one of the areas which may help in reducing energy cost, Transformers are one of the most energy efficient equipment and its efficiency can go upto 99%,if operated in the best efficiency area.
In transformers efficiency loss is subject to the loading of the transformer.
Total loss LT can be easily calculated by LT = LNL +LL*(%Load/100)2
Or Total Loss = No load Loss +Load loss X (KVA Load/KVA rated)2
As the load increases efficiency increases upto 50-60% loading and then it starts dropping as loading is increase. For better understanding let is check the efficiency graph of EEF1 and EFF2 Transformers.
Power factor considered as unity for graph plotting,
As in the graph it is clearly visible that at lower load say 10%, the efficiency is minimum, As the load increases to 30% it rapidly raising, 30-60% is the zone where transformers are operated at higher efficiency area. After 60% efficiency keeps on dropping till the full loading of transformer.
This graph shows where operating point should be for getting best efficiency of transformers. If multiple transformers are in use, we can balance load to for better overall efficiency gain.