When you’re looking for an industrial generator, selecting the appropriate size is crucial. Thanks to the availability of powerful software to handle calculations and predict performance, industrial generator sizing has never been easier.
However, there are still many parameters to consider. Most software allows for setting tolerances to specific circumstances and preferences. Ultimately, the application dictates where you can be flexible and where you have to be strict in your sizing.
- Generator Power Output
The power output of a generator is the first parameter most people consider. How much power do you need to generate onsite? While it may seem straightforward (I need 100 kilowatts (kW) of power, so I will buy a 100kW generator), it’s important to do a deeper dive into the systems the generator will be supporting in order to account for all power needs.
A good practice is to select a generator that can support all the loads of the system with about 80% power output, which allows for some expansion of the loads being supported and ensures there will always be enough power.
- Generator Voltage Dip
Adding loads onto the system causes voltage dip. As the generator is running and a new demand for power gets dropped on the block, the voltage momentarily dips. The extent of this dip is significant—the generator can handle a dip of 25% and recover in seconds, but a dip of 30% or more can cause damage to the alternator. For this reason, voltage dip tolerance should be set to 30%.
The most significant cause of voltage dip is motor load. Electronic motors tend to have a massive surge of current needed to start the motor, causing high demand on the generator. Mitigate this demand by reducing the voltage starting method on the motor itself. A variable frequency drive (VFD) or soft-start motor can accomplish this by slowly ramping up the motor demand, so the surge is not instantaneous or as large.
If changing the motor starting method is out of the question, stepping the motor loads is another way to reduce the voltage dip on a system. Staggering the start of the largest motors allows the generator to take on the loads one by one and have a steadier voltage output.
- Frequency Dip
Frequency dip is a similar parameter for generator sizing. While voltage dip is a reaction by the alternator in response to adding loads, frequency dip is caused by the engine’s reaction to adding loads. The frequency of the engine spinning (typically 60 Hz) dips slightly and recovers.
The tolerance for frequency dips needs to be tighter than that of the voltage dip, as generators are designed to run at one frequency continuously. The higher end of the allowable frequency dip is about 15%, but ideally it should be even lower. If a generator has a 15.5%-16% frequency dip when using sizing software, it is not ideal, but the system will still likely work. Ideally, the recommendation is to never go above 15%.
- Total Harmonic Distortion (THD)
Total Harmonic Distortion (THD) is an indicator of how “clean” electricity is. The lower the THD, the cleaner the electricity. THD can increase when reduced voltage starting methods are used for motors, for instance.
This is not a major problem for the generator itself, but could be for the loads the generator is powering. Any sensitive equipment receiving this power could be damaged by THD, so it needs to be tightly controlled. Commonly, less than 10% is the metric for THD, but 10%-15% is still adequate as long as there is no sensitive equipment downstream.
How to Determine the Correct Generator Size
Usually, the tolerances for these parameters are set before sizing. Many people assume that the tighter the tolerance, the better the equipment and generator will be. In some regards, that is true. Having less voltage dip or THD in the system is a good thing. However, overdoing it with the limits on these parameters can cause a larger than necessary generator size. If you kept both the frequency dip and THD at 5%, the alternator would need to be much larger to handle the same loads at a tighter tolerance than if set to 10% or 15%. Understanding what is acceptable and what is unnecessary for these tolerances clears the way to proper sizing and selection.
In addition to these parameters, stepping and load control/management can be an effective way to utilize a smaller generator instead of a larger one. There are various methods of load shedding or load management, but they share the same goal: parsing the loads to reduce the instantaneous demand on the generator.
Consider this analogy: you are walking with a backpack on, and someone is going to put 100 lbs. of weight into your bag. Load management is adding weight 10 lbs. at a time. Without load management, 100 lbs. would be dropped into your backpack all at once. Which method is more taxing on your legs?
Load management can be based on the current demand, i.e., when there is too much current draw on the generator, the lowest priority load is disabled. It can also be based on time, so loads come on gradually in a predetermined order. These techniques can be a great tool to reduce inrush current and reduce the size of the generator required.
Use The KOHLER Generator Sizing Tool
When using the KOHLER Generator Sizing Software, using steps for the loads simulates that these loads will not come on at the exact same time. If you have two pumps with large inrush, but know or have some method of load control to ensure they are not going to be started at the same time, sizing them in two separate steps will give the sizing software a more accurate depiction of how the generator will respond to the loads.
Knowing the parameters and tolerances are useful in designing the appropriate generator system for any application. Additionally, awareness of the impacts of different load types and sizing techniques to adjust the system performance leads to finer control and better understanding of efficient design, from both the electrical loads and the generator set.