Diesel generators are used to provide backup power for buildings during blackouts and other emergencies. They are also used as permanent sources of electricity in places without grid coverage, such as remote mining facilities and oil fields. However, the technical specifications of diesel generators must be clearly understood before purchasing them.
When a diesel generator set operates under the conditions for which it was designed, it has greater 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) are used to measure the real electrical energy supplied by the generator, which is used directly by the building's appliances and equipment.
- Kilovolt-amperes (kVA) measure apparent power. This includes real power (kW) but also reactive power (kVAR) consumed by devices such as motors and transformers. Reactive power is not consumed and instead circulates between the power source and the load.
- Power factor is the ratio between real and apparent power. If a building consumes 900 kW and 1000 kVA, the power factor is 0.90 or 90%.
The nameplates of diesel generators have nominal values for kW, kVA and PF. However, operating conditions are determined by the connected load and not by the generator. To ensure your building has the correct generator set, the best recommendation is to have the unit sized by professional electrical engineers.
Does your building have the right diesel generator?
What limits a generator's output?
The maximum output in kilowatts of a generator is determined by the diesel engine that drives it. As an example, consider an electric generator with 95% efficiency powered by a 1,000 HP diesel engine:
- 1000 hp is equivalent to 745.7 kW, and this is the shaft power supplied to the generator.
- At 95% efficiency, maximum power is 708.4 kW
On the other hand, the maximum kilovolt-amperes depend on the rated voltage and current of the generator. There are two ways the generator set can be overloaded:
- If the load connected to the generator exceeds the rated kW, it will overload the engine.
- On the other hand, if the load exceeds the rated kVA, it will overload the generator windings.
Keeping this in mind is very important, as a generator can be overloaded by kilovolt-amperes even if the load kilowatts are below the rated value.
Consider a generator with the following specifications: 1000 kW, 1250 kVA, 80% PF, 480 V, and 1503 A. This generator can operate at power factor above 80% as long as the kW and kVA ratings are not exceeded.
- If a building consumes 1,000 kW and 1,100 kVA the power factor increases to 91%, but the generator set capacity is not being exceeded.
- On the other hand, if the generator operates at 1100 kW and 1250 kVA the power factor only increases to 88%, but the diesel engine is overloaded.
- A diesel generator can also be overloaded by just kVA. If the unit operates at 950 kW and 1300 kVA (73% PF), the windings are overloaded even if the diesel engine is not.
In short, a diesel generator can exceed its rated power factor without any problems as long as kW and kVA remain below their rated values. It is not recommended to go below the nominal PF, as the generator operates with lower efficiency. Finally, exceeding the kW or kVA rating may damage the unit.
How Leading and Lagging Power Factor Affect a Diesel Generator
If you connect just electrical resistance to a generator and measure voltage and current, their AC waveforms will match when displayed on a digital meter. Both signals alternate between positive and negative values, but cross 0V and 0A at the same time. In other words, voltage and current are “in phase”:
In this case, the load has a power factor of 1.0 or 100%. However, most devices found in buildings have a power factor other than 100%, which means their voltage and current are offset relative to each other:
If the AC voltage peak is ahead of the current peak, the load will have a lagging power factor. Loads with this behavior are called inductive, and include electric motors and transformers. The following graph displays voltage and current for an inductive load:
On the other hand, if the current is ahead of the voltage, the load will have a leading power factor. Loads with this behavior are called capacity, and include batteries, capacitor banks and some electronic equipment. The following graph displays voltage and current for a capacitive load:
Most buildings have more inductive loads than capacitive loads. This means that the overall power factor is typically lagging and diesel generator sets are designed for this type of load. However, owners should be careful if a building has many capacitive loads, as generator voltage becomes unstable with advanced power factor. This will trigger automatic protections, disconnecting the unit from the building.
In places like New York City, building codes establish strict requirements for emergency power systems. To ensure your building has an adequate generator set that meets codes, the best recommendation is to consult the experts.
