Diesel generators are commonly used to provide backup power for buildings during blackouts and other emergencies. They are also used as permanent power sources in places with no grid coverage, such as remote mining facilities and oil fields. However, the technical specifications of diesel generators often cause confusion, and they can be misused as a result.
When a diesel genset operates under the conditions for which it was designed, it has a higher efficiency and a longer service life. However, the first step is understanding the relationship between kilowatts (kW), kilovolt-amperes (kVA) and power factor (PF) in generators.
Kilowatts (kW) are used to measure the real electric power provided by the generator, which is used by the appliances and equipment in the building.
Kilovolt-amperes (kVA) measure the apparent power. This includes real power (kW), but also reactive power (kVAR) drawn by devices like motors and transformers. Reactive power is not consumed, and instead it cycles between the power source and the load.
The power factor is the ratio between real power and apparent power. If a building consumes 900 kW and 1000 kVA, the power factor is 0.90 or 90%.
The nameplates of diesel gensets have rated values for kW, kVA and PF. However, the operating conditions are determined by the connected load, not by the generator. To make sure your building has the right genset, the best recommendation is having the unit sized by professional electrical engineers.
Does your building have a suitable backup generator?
What Limits the Output of a Generator?
The maximum kW output of a generator is limited by the diesel engine that drives it. For example, consider a generator with 95% efficiency that is driven by a 1000-hp diesel engine:
1000 hp is equivalent to 746 kW, and this is the shaft power provided to the generator.
With 95% efficiency, the maximum output is 708 kW
On the other hand, the maximum kVA depend on the rated voltage and current of the generator. Exceeding the kW rating overloads the diesel engine, and exceeding the kVA rating overloads the generator windings, even if the kW output stays below the rated value.
Assume a generator has the following specifications: 1000 kW, 1250 kVA, 80% PF, 480 V and 1503 A. This generator can run at power factor above 80%, as long as the kW and kVA ratings are not exceeded. Consider the three following scenarios:
If a building consumes 1000 kW and 1100 kVA, the power factor increases to 91%, but the genset 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 being overloaded.
A generator can also be overloaded only in kVA and not kW. If the unit runs at 950 kW and 1300 kVA (73% PF), the windings are overloaded even if the diesel engine is not.
To summarize, a diesel generator can exceed its rated power factor with no problem, as long as the kW and kVA stay below their rated values. Going below the rated PF is not recommended, since the generator operates less efficiently. Exceeding either the kW rating or the kVA rating can damage the unit.
How Leading and Lagging Power Factor Affect Generators
If you connect only electric resistance to a generator, and you measure voltage and current, their waveforms will match when displayed on a digital meter. In this case, the load has a power factor of 1.0 or 100%. However, most devices found in buildings have a power factor below 1.0, which means their voltage and current signals don’t match.
If the voltage is ahead of the current, the load has a lagging power factor. Loads with this behavior are called inductive, and they include electric motors and transformers.
On the other hand, if the current is ahead of the voltage, the load has a leading power factor. Loads with this behavior are called capacitive, and they include batteries, capacitor banks, and some electronic equipment.
Most buildings have more inductive loads than capacitive loads. This means the overall power factor is normally lagging, and diesel gensets are designed for this type of load. However, owners must be careful if a building has many capacitive loads, since the generator voltage becomes unstable with leading power factor. This will trigger automatic protections, disconnecting the unit from the building.
In places like New York City, building codes establish stringent requirements for emergency power systems. To make sure your building has a suitable genset that meets codes, the best recommendation is asking the experts.
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