Why Transformers Are Rated in kVA

The term kVA stands for kilovolt-amperes and refers to the apparent power rating of a transformer. To understand why transformers are measured in kVA, let’s break down the concept:

• kV stands for kilovolts, a unit of voltage.
• A stands for amperes, a unit of current.

The product of voltage and current in a circuit is the apparent power, measured in volt-amperes (VA). When it comes to large power systems, the unit of kilovolt-amperes (kVA) is used to denote the power capacity of a transformer, where 1 kVA = 1,000 VA.

Unlike real power, which is the actual usable power in a system (measured in kilowatts or kW), apparent power takes into account both the real power and the reactive power (due to inductance and capacitance in the system). Since transformers handle both voltage and current without taking into account the power factor (which determines the real power), they are rated in kVA.

The reason transformers are rated in kVA and not kilowatts (kW) is that transformers are passive devices. They do not consume power; they simply transfer it from one circuit to another. The real power consumed in a system depends on the load connected to the transformer, which in turn depends on the power factor of that load. Since a transformer does not have a fixed power factor, its rating is specified in kVA, which represents the maximum apparent power it can handle without considering the power factor.

In other words, kVA takes into account both the voltage and current while ignoring the phase difference (power factor) between them, which is essential for devices like transformers that do not consume power directly but only transfer it.

When selecting a transformer, the kVA rating is used to ensure the transformer can handle the required electrical load. The kVA rating provides a measure of the transformer’s capacity to deliver the necessary voltage and current to supply power to electrical systems.

For example, if a transformer is rated at 100 kVA, it means the transformer can handle up to 100,000 volt-amperes of apparent power. If the transformer needs to supply 100 kW of real power, the required kVA rating will be affected by the power factor of the load.

In summary, a transformer is called kVA because its rating is based on the amount of apparent power (measured in kilovolt-amperes) it can handle, taking into account both the voltage and the current in the system. Since transformers do not consume power but transfer it, they are rated by kVA rather than kW, which is the unit for real power. This allows for the correct selection and operation of transformers in power systems, ensuring they meet the needs of both residential and industrial applications efficiently.