Saturday, 9 July 2016

Why Transformer Rated in kVA and not in kW?

Not only Transformers, even Alternators are rated in VA, kVA or MVA etc.

In this case the rating of a Transformer depends on the winding or oil temperature reaching the specified limiting value which is decided by the type of insulation minus a safety margin, something like 170 - 20 =150 °C.

Now the temperature rises because of the losses. The copper losses which vary with the load depend on I2 and small core loss which depends on supply voltage which is practically constant is also present. The loss will be same as long as current magnitude is same irrespective of what the power factor (pf) of the load current.

Just to clarify consider a single phase 110 V /220 V Transformer.

Let its ReqEquivalent Resistance be 3 ohm (referred to 220 V side).

Let it be rated 2.2 kVA. The rated current on 110 V side is 20 A and on 220 V side is 10 A.

First put a load on the 220 V side drawing 10 A at pf = 1.0.

The total loss will be = 3(102) = 300 W + core loss of, say 30 W.

The output will be = 220 (10) (1.0) = 2200 W = 2.2 kW.

The kVA = VI/1000 = 220(10)/1000= 2.2 kVA

Next put a purely inductive or capacitive load drawing the same 10 A but at a pf =0. 

Again the total losses will be = 3(102) = 300 W+ core loss of, say 30 W, same as before.

But the output will be =220 (10) (0) = 0 kW.

The kVA =VI/1000=220(10)/1000= 2.2 kVA

So it can be seen that with the same kVA but different kW the losses are same in both cases and the temperature rise will be identical in both cases and therefore when we design a Transformer the rating of Transformer must be given in KVA or VA or MVA.

Thank you!

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