Electrical power equipment is rated in terms of VA or kVA. When an electrical load has voltage V across it and current I through it, the power that appears to flow to it is a product of V and I that is VI.
However, if the load comprises of both resistance and reactance, this product is neither real power nor reactive power. In that case the V*I that appears to represent power is called apparent power. Apparent power is represented by symbol S and has units of volt-amperes (VA). Here are 5 reasons equipment is rated in VA.
Rating of Generators
Consider a generator rated at 440 V, 100 kVA. That implies that generator is capable of supplying 227 Amps (From Formula I = 100 kVA/440 V = 227 Amps). Now if generator supplies power to a purely resistive load having 100 kW rating @ 440 V, the generator will supply its rated current. (I = 100 kW/440 = 227 Amps).
Consider the figure below:
Here the generator is supplying a load with P = 100 kW same as witnessed before, but this time the load has Q = 160 kVAR. The apparent power now is 180 kVA (See formula above).
The generator now has to supply a current of I = 180 kVA/440 V = 409 A.
In second case, though the generator is supplying the same power as it was supplying to resistive load, however here the generator is now greatly overloaded, and as a result it will be damaged.
Rating of interconnecting wire
Just like previous case, the current rating of interconnecting wires is also impact and it should be considered.
Power losses in Transformer
A transformer has copper and core losses that depend on current and voltages.
- The copper losses or I2R losses of a transformer depend on the current
- The iron or core losses in a transformer depends upon the voltage of transformer
The total losses in a transformer therefore depend upon volt-ampere (VA) and not on the power factor of the load.
That is one of reasons, the transformer rating is expressed in kVA and not in kW.
Nature of loads being unknown at the time of manufacturing transformers
While manufacturing transformers, power engineers are unaware about kind of load that will be fed in future (It is never known exactly about nature of load and it can be inductive or capacitive or resistive). The power factor depends upon the load.
Practical loads are neither purely resistive, purely inductive (lagging) or purely capacitive (leading). Every load also comprises of some resistance plus inductance or capacitance.
Practical loads draw both active and reactive powers from the supply.
Temperature rise of the transformer
The temperature rise of a transformer depends on apparent power. It is directly proportional to the apparent power (kVA) which flows through it. This is also one of reasons that power rating of the transformer is given in kVA and not in kW.
So that was all about reasons: Why Power Equipment is rated in kVA and not in kW