Generator Sizing & Operation Limits 

By on

When selecting a generator, there are inherent limits on the active and reactive power which can be delivered.  Generators are normally sized for a certain kW at a power factor of 0.8.  However, to understand the operation fully, it is necessary to look at the generator reactive power curve. 

Generator Operation

Generator Operating Curve
Generator Operation Area

When selecting a generator, there are inherent limits on the active and reactive power which can be delivered.  To illustrate this, the images show typical generator operation limits.

The smaller image depicts active power P, on the x-axis and reactive power Q, on the y-axis.  What would be considered the normal stable operating areas for a generator is shown in green.  As the generator operating point, measured in terms of active and reactive power , moves out of this region, operation become more unstable.

Generators are normally sized for a certain kW at a power factor of 0.8.  However, to understand the operation fully, it is necessary to look at the generator reactive power curve.  The image below shows similar information as the first image, but with more detail on where various limits apply.  Operation within the green area is pretty safe while operation in the red should be avoided.  Operation within the yellow is possible, subject to a detailed analysis being carried out..

 Generator Reactive Power Curve
Typical generator reactivate power curve

Generator operation for various levels of reactive power, both leading and lagging as shown. It can be seen that a stable operating region lies within a region of lagging power factor from 1 to 0.8.  As we move out of this region, we can start experiencing issues.   In particular leading power factors can result in unstable operation.  Low lagging power factors can result in rotor overheating.

Sizing of Generators

Sizing of generators can be carried out either by hand or using software.  For anything but the simplest systems, it is recommended to use manufacturers software so that a generator of adequate size is selected.

While software is often used, a good understanding of the variables which affect the size of a generator, will better ensure that the selected size of appropriate.  The key considerations are:

  • Loading of the generator - the total connected load has a direct bearing on the ultimate size of any generator.  This is probably where most people start in sizing a generator. 
  • Motor starting - starting of motors results in large current inrush and subsequent generator voltage drops.  Any generator needs to be sized such that the level of voltage drop still enables the motor to accelerate the load.  There is also short term heating on the generator during this period.  
  • Power factor - has a direct effect on the operation of the generator as discussed earlier.
  • Voltage and frequency -  sizing of generators needs to be carried out at the application voltage and frequency.  
  • Allowable voltage & frequency dips - unlike the grid, generators are not infinite (large) sources.  There will be voltage and frequency variations, and the generator needs to be sized to ensure these are below allowable limits.
  • Cyclic loading - loads which cycle, place varying demands on a generator.  Factoring these into the sizing model will help ensure that generators are not oversized. 
  • Non-linear loads - non-linear loads (UPS, VFD, etc.) generate high frequency harmonics, which can heat up the generator.  In addition, if lightly loaded, these can cause leading power factors. 
  • Other Considerations - in addition to the above; the type of fuel used, ambient temperature. Load shedding schemes and provision for future growth will all affect the final selected generator size.

In addition to being undersized, it is also possible to oversize generators.  At loads below 30%, generators can

During sizing a distinction will also be made between standby and prime generator sets.  Standby rated generators are typically based on a limited number of hours operation and have no overload facility.  Prime sets and run continuously at the rated load and do have periodic overload capability.

Types of Rating

  • Standby - these generators are used for supplying emergency power.  They have a limitation on running hours and no overload capability.
  • Prime - generators are used to continuously supply permanent power at variable load.  Typically they would also have the ability to deliver a 10% overload for one hour.,
  • Continuous - similar to prime, continuous generators are used to supply permanent power.  These would tend to be used to supply constant (non-varying) loads up to a fixed limit and do not have overload capabilities.


Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



How to Check a Circuit is Dead

If you want to check a circuit is dead (not live), you should always use the three point method. First check a known live circuit, then check the dead...

UPS Battery Sizing

Various techniques exist to enable the correct selection of batteries for UPS applications.  The procedure described below is one of the more common. ...

Fault Calculation - Symmetrical Components

For unbalance conditions the calculation of fault currents is more complex. One method of dealing with this is symmetrical components. Using symmetrical...

UPS - Uninterruptible Power Supply

A UPS is an uninterruptible power supply.  It is a device which maintains a continuous supply of electrical power, even in the event of failure of the...

How to Write an Electrical Note

Electrical notes are a collaborative collection of electrical engineering information and educational material. Any registered user can add content. ...

Capacitor Theory

Capacitors are widely used in electrical engineering for functions such as energy storage, power factor correction, voltage compensation and many others...

ABB Technical Guides - Motor Operation

ABB has produced a range of technical guides that offer concise explanations of the major technologies and technical issues in low voltage AC drives. ...

Maxwell's Equations - Gauss's Electric Field Law

Gauss's Electrical law defines the relation between charge ("Positive" & "Negative") and electric field.  The law was initially formulated by Carl Friedrich...

myElectrical - Cable Sizing Tool Upgrade

Our IEE cable sizing was wrote a few years ago and had become rough around the edges. I thought it was time to give the tool a service. Unfortunately when...

Mobile Phones (Brick to Implant)

The mobile phone was born in 1973. They were the size of a brick and weighed a couple of kg, making them difficult to fit into your pocket. At a few thousand...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note