What information is involved in the selection of generators?

Sizing a diesel generator involves calculating the following load factors:

Continuous operating loads, derating factors, transient performance, non-linear loads at any given time

1. Total continuous load (operating kVA)

Calculate the total kVA of all motors and machinery that will be running simultaneously

Calculate the total kVA of all heating and air conditioning units that will be running simultaneously

Calculates the total kVA of all lights that will be running simultaneously

Operating kVA (rkVA) is equal to the sum of the maximum possible load requirements at one time.

2. Allow any derating

Common derating elements are:

Altitude/environment/temperature/dust/humidity/power factor

Each diesel generator has its own derating characteristics for each project, so its technical data must be referred to. If more than one derate is applied, the multiplier becomes cumulative.

When derating is applied, the continuous rating of the generator is reduced by a multiplier.

3. Transient load performance

When the motor starts, its initial kVA (starting kVA) is greater than its running kVA. This is due to the initial inrush current required to start the motor. The skVA can be several times the rkVA, causing a larger initial load demand and causing a terminal voltage sag.

In general, a voltage dip greater than 25% will cause the generator to stall/shut down and thus become an important factor to consider. Generators have specific starting voltage characteristics that must be considered.

Various startup methods are used in the application to reduce skVA. Each has its own performance characteristics and will provide their starting impact. Technical data can be obtained from the manufacturer.

Motors coming online at different stages, calculations based on the highest possible demand scenario. When there are multiple motors starting at the same time, the total skVA is the sum of the skVA of each motor.

4. Nonlinear load

A non-linear load (NLL) is defined as an electrical device that draws a non-sinusoidal current from a power source (diesel generator), generating harmonics in the current waveform, which in turn distort the voltage waveform.

The current distortion (ID) is a function of the number of pulses. The level of distortion can lead to instability in the load control system and generator excitation system.

In order to limit voltage distortion (VD), the alternator needs to have a sub-transient reactance that allows this level (lower is better).

There are several guidelines/charts to enter ID% and acceptable VD% and provide X”d not to be exceeded.

The selected diesel generator must have an alternator that operates within the limits of X”d. For applications with a high proportion of NLL to the total load, it may be necessary to use an oversized alternator.

Diesel generator size

Once the maximum kVA for each load factor has been determined, the diesel generator will be sized for the highest demand, thus meeting the remaining load factor requirements.