Jul 24, 20215 min read

What is kVA in generators? this answer will AMAZE you

Updated: Feb 25, 2022

The above picture shows the nameplate of a 304kW generator. Here, the kVA [that is 380kVA] rating of the generator is also marked.

kV rating is one of the most important parameters of a generator, it helps to avoid any overloading due to the poor power factor. But it’s hard to understand the real meaning of kVA, here in this blog I will try to explain to you the importance of the kVA rating in generators.

Before diving into the briefing of kVA rating of generator you must understand the basics of electrical power.

Real Power, Reactive power and Apparent power

Due to reactance load, an AC power system poses three types of electrical power that is

real power/active power (kW)
reactive power (kVAr)
and apparent power/complex power (kVA)

Real power: It is also known as true power. It is the actual power a load is consumed to do useful work. Real power is symbolized by "kW" and pronounced as kilo-Watt.

Reactive Power: It is also known as imaginary or non-real power. It is the power that is throwback by the load, towards the power source due to reactance in load. It is best described as unused power which flows between load and power source. Reactive power is symbolized by "kVAr".

Apparent Power: It is also known as complex power. It is described as the combination of active and reactive power. And it is also described as RMS of voltage and current. Apparent power is symbolized by "kVA".

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What is kVA in generators?

kVA in the generator defines the right capacity of its electrical alternator and it is directly proportional to alternator current, regardless of the relative proportions of engine power and reactive power. The performance of the electrical alternator is limited to apparent power. And this value helps to protect the generator from overloading.

Brief about the kVA rating of the generator.

The generating capacity of a generator is generally referred to in units of real power output, such as kilowatts or megawatts. This is appropriate in that the decisive constraint on how much power can be generated is the ability of the engine to deliver the mechanical power.

In the generator, the electric generator will be sized to handle any amount of real power the engine can deliver. However, generator performance limits are still important in the operational context, because they also apply to reactive power, which is independent of the engine power.

Indeed, the appropriate measure of capacity for the electric generator as such is not in terms of real power, but rather in units of apparent power, kilovolt-ampere (kVA) or megavolt-ampere (MVA).

Thus, a combination of real and reactive power [that is equal to apparent power] must be considered to determine whether the generator is operating within its range or is in danger of becoming overloaded.

“Overloading” for a generator primarily means overheating due to high current, though some mechanical factors may also be relevant. Excessive temperature will cause the insulating material on the generator windings to deteriorate and thus lead to an internal fault or short circuit.

Different rates of thermal expansion between the winding conductors and the core at excessive temperatures can also cause insulation damage through movement and abrasion. Depending on the particular operating condition, “hot spots” may develop on different components, which is problematic because the temperature cannot readily be measured everywhere inside the generator.

Possible sources of mechanical damage under excessive loading include rotor vibration due to imperfect balance, vibration due to fluctuating electromagnetic forces on the components, and loss of alignment between the engine and generator shafts due to thermal expansion or distortion of the generator frame.

Any of these types of damage are irreversible in that the generator will not recover after the load is reduced. Therefore, rather than waiting for signs of distress under high loads, generators are operated within limits specified by the manufacturer that will allow for some margin of safety to assure the integrity of the equipment.

To a first approximation, these limits are indicated by the generator rating in kVA or MVA. Since apparent power [kVA] is directly proportional to current, regardless of the relative proportions of real and reactive power, this is synonymous with a limit on the current in the armature or stator windings.

Apparent power is important in the context of a generator. Actually, the crucial quantity concerning thermal capacity limits is only the current. In practice, though, the current is often inconvenient to specify.

Since the operating voltage of a generator is usually quite constant, apparent power [kVA] is a fair way of indicating the current. The point is that apparent power is a much better measure of the current than real power because it does not depend on the power factor. Thus, utility equipment (like generator) ratings are typically given in kVA or MVA.

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What is reactance load?

Reactance is the property of an electrical device (like a motor) to influence the relative timing of an alternating voltage and current.

What is the Power factor?

It can also be defined as the ratio of real power (kW) absorbed by the load to the a pparent power flowing in the circuit. The power factor is represented by cosϕ.

For residential load, the value of the power factor varies from 0.8 to 0.9 whereas for industry the same is can vary from 0.6 to 0.8 due to high reactance. That’s the reason industries required power factor correction equipment to limit the power factor.

Difference between ‘kW’ and ‘kVA’ in the generator?

Kilowatt is correspondent to the average power consumed by the load connected with the generator. It is also called real power, active power or true power, and is measured in watts as well as Kilowatt. Whereas one Kilowatt is equal to 1000 watts.

1 kW = 1000 Watts

Also, the Apparent power corresponds to the power flowing in the generator circuit. And it is directly proportional to the ‘kW’ rating of the generator.

kVA ∝ kW

kVA = kW/cosϕ

How to convert kW to kVA?

To convert kW to kVA all you need to do is divide kW by power factor. For residential load, the average power factor is 0.85 whereas for the industrial load it is 0.8 without power factor correction equipment and it is 0.9 with power factor correction equipment.

kVA = kW/cosϕ