Why transformer rated in KVA, Not in KW?

The transformer is rated in KVA and not in KW. The transformer supplies power to electrical loads like motors,heaters. The electrical loads have their own power factor.

The power factor at the transformer secondary and primary is equal to the combined power factor of all types of electrical loads connected to the transformer. Thus the transformer does not have fixed power factor and the power factor varies with types of load connected to it and also on percentage loading on transformer.

The transformer is a link between alternator and the load.The alternator supplies active and reactive power to the load. Similarly, the transformer is sort of power supply source which supplies the active kW and reactive kVAR to electrical loads. If transformer is rated in kW,the designer has to fixed the kVAR capacity of the transformer.However, kVAR depends on the types of loads connected to the transformer and it is not possible to fix the kVAR delivering capacity of the transformer.

In other words, if  transformer is rated on the basis of KW, the user has to operate the transformer at the designed power factor of the transformer. However, it is not pragmatic to operate the transformer at particular power factor or at particular kVAr because the load power factor varies and the system power factor changes with variation in power factor of the connected loads.

The transformer designer does not know at what power factor the customer will operate the transformer. The designer gives the choice to operate the transformer at whatever power factor by giving the rating of transformer in apparent power (KVA or MVA).

Apparent power(kVA) of the transformer is equal to the vector sum of  active power(kW) and reactive power(kVAr).

kVA^⃗= kW^⃗+ kVAr^⃗

kVA²= kW² + kVAr²

kVA =√(kW² +kVAr²)   --------(1)

Where,

kVA    - Apparent Power
kW     - Active Power
kVAr   - Reactive Power

From equation(1), if load is resistive , then power factor is unity and kVAr =0

kVA = kW    ( if load is resistive)

If power factor is low, the kVAR supplied by transformer will increase and correspondingly the active power delivery will gets decreased.The transformer will be underutilized if the power factor of the loads is low.

From above table, it is clear that the active power delivering capacity of the transformer gets lowered with lower power factor.

To fully utilize the transformer power factor should be high.The better the power factor,the lesser the kVAr demands by electrical system and thus the transformer active power delivering capacity can be increased.

The power factor can be improved by installation of capacitor banks to neutralize the reactive kVAr with capacitive kVAr.