Generator Size and Operation Restrictions

There are inherent limits to the active and reactive power that can be delivered when choosing a generator. Normally, generators are sized for a specific kW at a power factor of 0.8. However, in order to fully comprehend the operation, the generator reactive power curve must be examined.

There are inherent limits to the active and reactive power that can be delivered when choosing a generator. The images show typical generator operation limits to demonstrate this.

On the x-axis of the smaller image is active power P, and on the y-axis is reactive power Q. Green represents what would be considered normal stable operating areas for a generator. The operation becomes more unstable as the generator operating point, measured in terms of active and reactive power, moves outside of this region.

Normally, generators are sized for a specific kW at a power factor of 0.8. However, in order to fully comprehend the operation, the generator reactive power curve must be examined. The image below depicts information similar to the first, but with more detail on where various limits apply. Operation in the green zone is relatively safe, whereas operation in the red zone should be avoided. Operation within the yellow zone is possible, subject to a thorough analysis.

Typical generator reactivate power curve

As shown, generator operation for various levels of reactive power, both leading and lagging. It can be seen that a stable operating region exists within a lag power factor range of 1 to 0.8. As we leave this region, we may encounter difficulties. Leading power factors, in particular, can cause unstable operation. Low lag power factors can cause rotor overheating.

Generator Dimensions

Generator sizing can be done either by hand or with software. For all but the most basic systems, it is best to use the manufacturer’s software to select a generator of sufficient size.

While software is frequently used, a thorough understanding of the variables that influence the size of a generator will help to ensure that the size chosen is appropriate. The following are the primary considerations:

Generator loading – the total connected load has a direct impact on the ultimate size of any generator. This is most likely where most people begin when sizing a generator.

Starting a motor causes a large current inrush and subsequent generator voltage drops. Any generator must be sized so that the voltage drop allows the motor to accelerate the load. During this time, there is also some short-term heating on the generator.

As previously discussed, power factor has a direct impact on generator operation.

Voltage and frequency – Generator sizing must be done at the application voltage and frequency.

Allowable voltage and frequency dips – generators, unlike the grid, are not infinite (large) sources. Voltage and frequency variations will occur, and the generator must be sized to keep these within allowable limits.

Cyclic loading refers to loads that cycle and place varying demands on a generator. Including these in the sizing model will help to ensure that generators are not overbuilt.

Non-linear loads (UPS, VFD, etc.) generate high frequency harmonics, which can overheat the generator. Furthermore, if lightly loaded, these can result in leading power factors.

Other factors to consider include the type of fuel used and the ambient temperature. The final generator size will be influenced by load shedding schemes and future growth plans.

In addition to being undersized, generators can also be oversized. Generators can operate at loads of less than 30{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe}.

During the sizing process, standby and prime generator sets will be distinguished. Standby generators typically have a limited number of hours of operation and no overload capability. Prime sets and have periodic overload capability and run continuously at the rated load.

Rating Categories

Standby generators are used to provide emergency power. They have a running time limit and no overload capability.

Prime – generators are used to provide continuous power at variable load. Typically, they would also be able to deliver a 10{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} overload for one hour. ,

Continuous generators, like prime generators, are used to provide continuous power. These are typically used to supply constant (non-varying) loads up to a fixed limit and lack overload capability.