What is the Ohms Law explanation for harmonics?

Where do harmonics come from and what are the sources of harmonic currents?

It’s easy to know where harmonics come from if we understand Ohm’s Law. Ohm’s Law states that voltage equals current times impedance. If we take that one step further and say voltage distortion equals current distortion times harmonic impedance, we can get an idea about the percentage of harmonics we have on the power system and the types of problems we may have. 

We can tell the difference between linear and nonlinear loads by looking at waveforms. Clean, sinusoidal waves indicate linear loads—the voltage and the current look the same. With nonlinear loads, the voltage and the current do not look the same because the current distortion coming from the loads is affecting the voltage, therefore, producing harmonics.

Say for instance a facility has only linear loads with across-the-line motors and decides to add variable frequency drives (VFDs) to make the system more efficient. But, adding those VFDs has also added current distortion coming from those loads. The harmonic loads produce current and push it back up through the system. The more current distortion you add, the more you’re going to distort the voltage. Voltage distortion is more likely to cause misoperation—current distortion is likely to result in heating and other similar issues. 

Instead of running on the utility system, which is coming through a transformer with 5% or 6% impedance, we switch over to a generator for emergency backup. If we do that, the source impedance is going to change. That’s our harmonic impedance, looking back into the power system. 

If a generator has 18% impedance, that percentage is at least three times what it is on a normal source. So, what happens is, we get a lot more voltage distortion from the same amount of harmonic current. We have three times the amount of voltage distortion based on the current distortion that’s the same if the harmonic impedance increases by three times. 

It is important to know harmonic currents come from the loads, which in turn causes harmonic voltage distortion. The typical loads that produce harmonic currents include rectifiers, VFDs, LED lighting and computer power supplies. 

Harmonic currents can raise the voltage distortion based on adding more of that same current or increasing system impedance, as in the case of a generator. 

When we look closely at power systems, we could have voltage issues for harmonics, current harmonic issues or harmonic impedance issues. It really depends on your system to really understand what the problems might be. As we think about harmonics, we need to consider Ohm’s Law and that harmonics come from the current and cause voltage distortion depending on the impedance.