No-Load Currents: Wet vs. Dry Transformers

has it to do with the construction of dry-type ones?? i mean their magnetisation need is reduced by using more high quality core materials?

It depends on the efficiency of the transformers assuming the kva capacity is the same. Oil filled is usually used for higher kva capacity or higher voltages. The transformer will have a constant watts loss across the core. This will not changes from no load to full load so there will be current draw on the primary side of the transformer at no load. The coils will also have a watts loss but this value will increase from no load to full load. The more power being channelled thru the transformer the greater the loss acrossed the coils. Typically oil filled transformer are more efficient than dry type.

No load current consist of mainly magnetising current plus other no load losses.To reduce these losses, (1) Laminated high grade,cold rolled,grain orientated silicon steel is used.The commercial and technical optimum thickness of each silicon plate is about 0.35 mm for for both oil immersed as well as cast resin distribution transformers (500KVA to 3000KVA) that comply to IEC 76 and 726.The flux density is around 1.65 Tesla.(2) stepped core design instead of square core is used for cylindrical distribution of magnetic field and save core and copper material.(3) Precise cutting and stacking core and joints to reduce air gap loss.

To answer your question,for the same power rating ( same MVA and voltage ratio ),the no load current can be the same.In practice,cast resin transformers are operating at higher core ( over 110 degree C )and winding temperatures (145--155 degree C ).The no load (loss) current is usually slightly higher than oil immersed transformer.

It is interesting to note that the total losses ( load loss and no load loss) of oil immersed transformers are higher than cast resin transformers of the same ratings.I compared these transformers manufactured by the same factory or different manufacturers.This can be used to justify the 60% higher cost for specifying cast resin transformers.

The largest cast resin power transformer I have seen is 25MVA,HV/HV windings.

Regards,

Khor

as far as I see this issue is very disputed one, since every source gives contradictory infos about this issue..