The use of kVA (kilovolt-amperes) instead of kW (kilowatts) to rate transformers is primarily due to the distinction between real power (kW) and apparent power (kVA) in electrical systems. Transformers are devices that transfer electrical energy between circuits through electromagnetic induction. When rating transformers in terms of kVA, both real power and reactive power considerations are taken into account.
Real Power (kW): Real power is the actual power that performs work in an electrical system. It is the power that is converted into useful work such as mechanical energy, heat, or light. The real power component is crucial for understanding the energy consumption or delivery capability of a transformer.
Reactive Power (kVAR): Reactive power, on the other hand, does not perform any useful work but is necessary for maintaining voltage levels and ensuring the stability of the electrical system. Transformers inherently introduce reactive power due to the magnetizing current required for their operation.
The apparent power (kVA) is the vector sum of real power (kW) and reactive power (kVAR). Using kVA as the rating for transformers provides a more comprehensive measure of the total power flow capability of the transformer. This is important in systems with loads that introduce reactive power, such as motors, which require both real and reactive power for their operation.
In summary, the use of kVA instead of kW in rating transformers acknowledges the importance of both real and reactive power in electrical systems. It provides a more complete understanding of the transformer's capacity to handle the total power, including the power that doesn't contribute directly to useful work but is essential for the overall stability and efficiency of the electrical system.