Why are transformers rated in kVA, not kW?

Transformers are always rated in kVA, not kW. As the name suggests, a transformer simply transfers power from one circuit to another without changing the values of power and frequency. In other words, it can only increase or decrease the current and voltage values while keeping the power and frequency constant. General data are printed on the transformer nameplate for more detailed information,

Such as VA rating, single phase/three phase (power or distribution transformer), step up/down, connections, etc. Simply put, there are two types of losses in a transformer;

1. Copper loss

2. Iron loss or iron loss or insulation loss

Copper losses (I?R) depend on the current through the transformer windings, whereas iron losses or core losses or insulation losses depend on the voltage. That is, the total loss depends on the voltage (V) and current (I) expressed in volt-amperes (VA) and not on the load power factor (PF). This is why a transformer rating may be expressed in VA or kVA rather than W or kW.

Let’s explain in more detail why transformers are rated in VA and not kW?

When manufacturers design a transformer, they do not know what kind of load the transformer will be connected to, for example they are not sure of the exact application of the transformer in different scenarios. The load can be resistive (R), inductive (L), capacitive (C) or a mixed load (R, L and C). This means that there will be different power factors (PF) on the secondary (load) side of different types of connected loads, also depending on R, L and C. This way, the transformer's rating is expressed in volt-amperes (VA) rather than watts (W) in the case of a Transformer.

Let us use a solved example to clear the transformer rating in VA instead of W. As long as the magnitude of the current/voltage is the same, the losses in the transformer will remain the same. No matter what the power factor of the load current/voltage is.

example:

Assume that for a single-phase step-up transformer

Transformer rating in kVA = 11kVA

Primary voltage = 110V

Primary current = 100 A

Secondary voltage = 220V

Secondary current = 50 A.

Secondary equivalent resistance = 5Ω

Iron loss = 30W

In the first case, if we connect a resistive load to the secondary of the transformer at unity power factor Φ = 1, then the total losses of the transformer are copper losses + iron losses, that is: I?R + iron losses

Value: (50² x 5 ) + 30W = 12.53kW

i.e. the losses in primary and secondary transmission remain the same. (See also quadratic loss example below)

The transformer output will be: P = V x I x Cos?

Again, putting the minor value in (if we put the same value in the major value) P = 220 x 50 x 1 = 11kW.

Now transformer rating: kVA = VA ÷ 1000

KVA = (220 x 50) ÷ 1000 = 11 KVA.

Now, in the second case, connect a capacitive or inductive load to the secondary of the transformer with power factor Φ = 0.6. Likewise, the total loss of the transformer will be copper loss + iron loss, that is: I?R + iron loss

Value: (50² x 5 ) + 30W = 12.53kW

It therefore turns out that the primary and secondary losses are the same. But the output of the transformer will be: P = V x I x Cos Φ

Putting in the secondary value again (the same value if we put in the primary value) P = 220 x 50 x 0.6 = 6.6kW.

Now the rating of the transformer: kVA = VA ÷ 1000

KVA = 220 x 50 ÷ (1000) = 11 KVA

This means that an 11kVA transformer rating means it can handle 11kVA. It is our turn to convert 11kVA to 11kW (we can do it by increasing the power factor to 1 for a purely resistive load), which is unpredictable and even difficult to get as the power factor will be different in case of inductive and capacitive loads value.

From the above example, it is clear that the transformers have the same rating (11kVA) but different output powers (11kW and 6.6kW). This is due to the different power factor values after connecting different types of loads. This is very important to the transformer manufacturer. The losses in both cases are said to be unpredictable.

So these are the exact reasons why transformers are rated in kVA instead of kW.