What if I had 4000% Ithd on my neutral conductor?

Help! What if I have 4000% distortion on my neutral?

A particular facility had an electrical fire a few years back. They conducted measurements to make sure they were not on track for that happening again, but got really nervous when they found 4000% current distortion on the neutral. 

Actually, it can be easy to reach 4000% current distortion because of all the loads on the power system. You can have transformers, busway, lights, variable frequency drives (VFDs)—all of these are harmonic sources. As you add these loads, they are going to push the current out and back into the system. Current, by nature, has to flow in a loop. It has to go out and come back. Oftentimes people think that harmonic currents go out in the phases and back on ground, but they don’t. The currents go out on phase A, and they come back on phase B and phase C—they go out on phase B, and they go back on phase A and phase C, and so forth. That’s what happens on three-phase loads.

On single-phase loads, the current goes out, for example, a light, and it comes back on the neutral. If the loads are balanced, the phase currents and the harmonic currents for the most part are going to add up to 0 because they’re all out of phase by 120 degrees.

If you then take a look at the harmonic spectrum, the fundamental is the 1st harmonic. There is a 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th and so on. The fundamental can be a positive or negative sequence harmonics, with the current flowing in the opposite direction. Zero sequence harmonics add up. So, you have A plus B plus C. For example, if you had 10 amps of 3rd harmonic on each phase, you would have 30 amps on the neutral. That is why we oversize neutrals. If there are 30 amps on the neutral and all the other currents balance out, in other words, the 60 Hz current balances out, you can see how easily it is to get 3000% or 4000% distortion. 

Say for instance you have 30 amps of 3rd harmonic current and 1 amp of 60 Hz current. If you multiply that by 100%, you end up with 3000% distortion. So, all you need is 40 amps of 3rd harmonic current to end up with 4000% distortion on the neutral. 

In the case of the facility concerned about the 4000% distortion on the neutral, the facility was using a 3 MVA transformer and everything—all the bus work and cable—was running at 4000 amp capacity, 40 amps of harmonic current on the neutral. There is no concern whatsoever with 1% of the current capacity flowing on that system.

You can have very, very high percent distortion levels on the neutral. It’s mostly going to be 3rd harmonics or multiples of the 3rd. You only need to be concerned if the actual amps of harmonic current are more significant than the capacity of the system. This could happen on 120/208 volt systems. 

Sometimes the neutral current can be almost double the phase current. That’s where you need to really be concerned about percent distortion and how much that’s affecting your system. Those are the things you need to think about in terms of distortion, especially harmonic currents on the neutral and the 3rd harmonic.