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Learn how harmonics interact with generators increasing voltage distortion to ensure loads operate correctly on the generator or utility source.
If you have harmonics on your power system and you’re using a backup generator, there are things you need to think about. There are key points you need to recognize to make sure you understand how harmonics are going to interact with generators. The generator’s impedance Xd double prime is typically about 18% to 20%.
If you compare that percentage to a utility transformer or a transfer switch that would be on your other source, that impedance would usually be about 5% or 6%. So, your generator has about three to four times as much impedance as your utility transformer. Because the voltage distortion is directly proportional to what that impedance is, you’re going to have more voltage distortion running on your generator. That’s the fact of this simple Ohm's law equation—voltage distortion equals current distortion times harmonic impedance. So, Zh is either Xd double prime for the generator or the impedance of the transformer.
Traditionally, people used to oversize a generator in order to “live” with harmonics. For example, they would go from a 1000 kVA generator with 18% impedance to a 2000 kVA generator, essentially creating a 9% impedance instead of a 18% impedance on a 1000 kVA basis. So instead of going from a 5% or 6% impedance and 18% on the utility, they went to about 9%—an increase to about 1.5 times instead of 3%.
Voltage distortion can get significantly worse for the same exact load based on the utility source versus the generator source. For example, field measurements taken at a manufacturing facility showed about 2.3% distortion running on the facility’s utility source and the exact same load caused almost 6% THD on the voltage.
When you have a generator, because that system impedance is very high, you can have excessive voltage notching as well. On a waveform, voltage notching issues typically show that where the voltage crosses through 0 multiple times, there is multiple 0 crossings.
As you look at generators and apply them on systems that have more time on occurrence, you’re going to have voltage distortion. How much depends on how big the generator is compared to your system source impedance.
