Harmonic Distortion Symptoms - Distribution Equipment

Components of power distribution systems conduct current and are therefore sensitive to current distortion. This distortion forces us to re-evaluate many of our normal concepts regarding electricity, especially with respect to the power system.

First and foremost, current and voltage distortion must be measured with a true RMS device. If it doesn't say true RMS, it is probably an averaging-type meter that can provide seriously inaccurate data.

Second, we must change our concept of transformer loading. When a transformer conducts distorted current it generates more heat per amp than if the current were sinusoidal. This means that transformers will overheat even if they are not fully loaded electrically. Transformer derating and the use of K-rated transformers should be considered.

Third, common wisdom says that if a three-phase wye panel is balanced there will be no current in the neutral. When current harmonics are present, some of the harmonics just don't cancel in the neutral, thus leading to high current readings even when the panel is balanced. Currents as high as 200% of the phase conductors are possible.

Lastly, harmonic currents can cause breakers and fuses to operate incorrectly. Even though currents do not exceed their limits, breakers will trip. Often this is because the current level is measured with an averaging meter. The meter may indicate only 15 amps, while there are actually more than 27 amps. The breaker is functioning correctly, the meter is not.

There are also times when high inrush currents from electronic loads will trip a breaker. If breakers trip, determine whether there was a non-linear load turned on at the same time.