Current Harmonic Distortion - Power System

Whenever the current in the power system is no longer sinusoidal, we say it is distorted, or that there is harmonic distortion. Harmonic distortion is problematic in that most of the power system's operation is based on, and dependent on, the assumption of sine waves.

This sine wave mentality has led to a number of commonly held beliefs that are now jeopardized by harmonics. Two are important for this discussion. The first belief is that to find out the amount of current or voltage in the system, simply get a cheap voltmeter or ammeter. If we are concerned about "excessive" currents, we must measure to find out how much current there is.

The second belief si that a fairly balanced panel will have little neutral current.

Current harmonic distortion changes both of these beliefs.

First, the actual technology used in most hand-held meters today assumes the signal being measured is sinusoidal. The meter reads the average value of the signal, assumes it is a sine wave, and adjusts it to display the value in rms. This is known as an average detecting, RMS calibrated meter, and is by far the most common type of meter used.

When harmonics are present, the adjustment used by these meters is wrong. The actual measurements may be as much as 50% in error.

Because of this, harmonically distorted signals must be measured with a true-RMS meter. These meters cost more, but they give accurate readings.

The second belief, that there is no neutral current in a perfectly balanced panel, is based on the fact that three identical sine waves, each 120 electrical degrees apart from each other, will offset, or cancel out. Thus, three balanced loads should have little or no neutral current.

However, when harmonics are present we don't have sine waves, at least not solely at 60 Hz. The distorted waveform can be broken down into a set of sine waves. The basic component, called the fundamental, is a sine wave at 60 Hz. The second harmonic is a sine wave at 120 Hz. The third is a sine wave at 180 Hz, etc.

When distorted currents share a common neutral, most of these higher frequency sine waves cancel out just like what we expect from the 60 Hz sine waves. However, some harmonics don't cancel. In fact, they add in the neutral. These harmonics are called zero sequence harmonics, and they are the reason that high neutral currents exist, even though the loads may be perfectly balanced.

One very clear sign is to view the neutral current waveform. If high currents are the result of zero sequence harmonics, then the predominant frequency of the neutral current will be 180 Hz, not 60 Hz.

Currents as high as 200% of the phase conductors have been seen in the field. This large level of current can easily burn up the neutral creating an open neutral environment with very serious consequences.

If high neutral current due to distorted current is the culprit, then the first step is to eliminate shared neutrals wherever possible. This mainly refers to branch circuits. Where this can't be done, such as on a three-phase wye panel, try oversizing the neutral wire so it won't overheat.

If these efforts don't work, then the next step is to reduce the distortion. This can be done through three methods. First, use a passive filter to reduce the current from one or two specific harmonics.

An active filter, the second method, reduces all the harmonic currents. It is more costly and complex to use, but it works better than passive filters.

The third method is to use transformers. Delta-wye transformers reduce certain harmonics, particularly zero sequence harmonics. Zig-zag transformers can also reduce zero sequence harmonics, but without changing the system type between delta and wye.