kVA ≠ kW: A Power Concept Misunderstood by 90% of People

There is an 1000 kVA transformer with an existing load of approximately 200 kW. If we want to add about 600 kW of new load, can this transformer withstand it?

This question involves two units: kVA and kW. To answer it correctly, we must understand the relationship and difference between these two units.

 

● Kilovolt-Ampere (kVA)

Definition: A unit used to express the capacity of electrical equipment (such as transformers, motors, etc.).In an AC circuit:Voltage (kV) × Current (A) = Apparent Power (kVA).The capacity of a transformer is rated in kVA.

● Kilowatt (kW)

Definition: A unit of power equal to 1000 watts.kW is commonly used to express the power of generators, engines, motors, tools, machines, electric heaters, and radio/TV transmitters.

kVA is the unit of apparent power.

kW is the unit of active (real) power.

In addition to apparent power and active power, there is reactive power, measured in kvar (kilovar).

 

What are the differences between these three types of power?

1.Apparent power is the total power supplied by the power source.

2.Active power is the power actually consumed by the load, converted into other forms of energy.For example, the electricity bill you pay is based on active energy.

3.Reactive power refers to power that is not actually consumed but temporarily stored.Equipment with capacitors or inductors continuously charges and discharges without real energy consumption; this is called reactive power.

 

Power Factor

To determine how much active power a power source can deliver, we introduce the power factor (PF), defined as the ratio of active power (P) to apparent power (S), denoted as cosφ.

Example:

A 1000 kVA transformer:

At a power factor cosφ = 0.6, it can output 600 kW of active power.

At a power factor cosφ = 0.9, it can output 900 kW of active power.

Assuming electricity costs 1 RMB per kWh:

At PF = 0.6: the transformer generates 600 RMB/hour of economic benefit.

At PF = 0.9: the transformer generates 900 RMB/hour of economic benefit.

Important note:Transformer capacity is rated in kVA, while load power is in kW.The relationship is:Active Power (kW) = Apparent Power (kVA) × Power Factor (cosφ)

A 1000 kVA transformer can only deliver 1000 kW at a perfect power factor of 1, which is almost impossible in practice.

In engineering design, a load rate of about 90% is economical and reasonable.Power supply companies typically require the power factor to be above 0.9; penalties apply if it is too low.The power factor also cannot exceed 1, as this would raise system voltage and disrupt operation.

Conclusion

The 1000 kVA transformer originally supplies 200 kW.After adding 600 kW, the total active load is 800 kW.

As long as the power factor is raised to the required value,the transformer can operate safely and continuously under this load.