Understanding Diesel Generator Set Specifications

Diesel generators are used to provide back-up power to buildings in the event of power outages and other emergencies. They are also used as a permanent power source in areas not covered by the electricity grid, such as remote mining facilities and oil fields. However, it is important to have a clear understanding of the technical specifications of diesel generators before purchasing them.

When a diesel generator set is operated under the conditions for which it is designed, it has a higher efficiency and a longer service life. However, it is important to understand the difference between kilowatts (kW), kilovolt-amperes (kVA) and power factor (PF).

• Kilowatts (kW) is used to measure the actual electricity provided by the generator, which is used directly by the appliances and equipment in the building.

• Kilovolt-amperes (kVA) measures the apparent power. This includes the active power (kW) and also the reactive power (kVAR) consumed by equipment such as motors and transformers. Reactive power is not consumed, but circulated between the power supply and the load.

• The power factor is the ratio of active power to apparent power. If the building consumes 900 kW and 1000 kVA, the power factor is 0.90 or 90%.

The diesel generator nameplate has ratings in kW, kVA and PF. However, the operating conditions are determined by the connected load and not by the generator. To ensure that your building is equipped with the right generator set, the best advice is to have the set sized by a professional electrical engineer.

What limits the output of a generator?

The maximum kilowatt output of a generator is determined by the diesel engine driving it. For example, consider a generator with 95% efficiency driven by a 1000 hp diesel engine.

1000 hp is equivalent to 745.7 kW, which is the shaft power supplied to the generator.

With an efficiency of 95%, the maximum output power is 708.4 kW

On the other hand, the maximum kVA depends on the rated voltage and current of the generator. The generator set can be overloaded in two ways.

If the load connected to the generator exceeds the rated kilowatt, it overloads the engine.

On the other hand, if the load exceeds the rated kVA, it will overload the generator windings.

It is important to bear this in mind, as the generator may be overloaded in kVA even if the load kVA is below the rating.

Consider a generator with the following specifications: 1000 kW, 1250 kVA, 80% PF, 480 V and 1503 A. This generator can operate at a power factor of over 80% as long as the kW and kVA ratings are not exceeded.

If the building consumes 1000 kW and 1100 kVA, the power factor will increase to 91% without exceeding the generator set capacity.

On the other hand, if the generator is running at 1100 kW and 1250 kVA, the power factor increases to 88% only, but the diesel engine is overloaded.

Diesel generators can also be overloaded at kVA only. If the unit is run at 950 kW and 1300 kVA (73% PF), the windings are overloaded even if the diesel engine is not overloaded.

In conclusion, diesel generators can exceed their rated power factor without any problems, as long as the kW and kVA remain below their rated values. It is not recommended to go below the rated PF, as the generator operates at a lower efficiency. Finally, exceeding either the kW rating or the kVA rating can damage the equipment.

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