Moving Harmonics out of Harm's Way

By BARBARA HORWITZ, Associate Editor -- Consulting-Specifying Engineer, 10/1/2000

Are harmonics refusing to jibe with the harmony of a facility's power flow?

In the old days of predominantly linear loads, harmonics were a natural occurrence that didn't cause problems because with normally balanced, three-phase AC systems that have nondistorted waveforms, the harmonic currents got cancelled out.

However, along with advances in technology has come the proliferation of nonlinear loads that do not draw sinusoidal current, and therefore, are not as kind to harmonics.

Common harmonics-producing equipment includes:

• Personal computers.

• Variable-frequency drives (VFDs).

• Uninterruptible power supplies.

• Fluorescent and high-intensity-discharge lighting ballasts.

• Switch-mode power supplies.

• Rectifiers.

When harmonics are introduced into the power flow going to and from this equipment, the currents create voltage drops. For example, third harmonics or triplen harmonics, usually produced by ballasts or switch-mode power supplies, cause overheating and voltage problems which negatively affect transformers and other components of the electrical system. Yet another flavor of harmonics come in fifth and seventh waves, which create electromagnetic fields in motors and generators that run counter to the equipments' rotational direction.

Solution oriented

A number of approaches can be utilized to deal with harmonics. Going back to the source, the first line of defense can be to install a transient-voltage surge suppressor (TVSS) at the main distribution switchboard. This serves to protect the entire electrical system by suppressing large voltage surges. Utilizing TVSS units at the distribution switchboard for essential loads can be helpful as well.

Another idea is to use more copper in order to absorb the additional heat created by harmonics. These additions can come in the form of oversized neutrals for panelboards, feeders, branch circuitry and K-factor-rated transformers. While K-rated transformers are a popular option to rectify the overheating issue, the units do not alleviate the voltage distortion issue. As a result, this approach needs to be evaluated on a case-by-case basis, and not used as a one-size-fits-all type of solution.

For harmonics-causing electronic ballasts, one solution is specifying low-total-harmonic-distortion (THD) ballasts for all fluorescent lighting, ideally at less than 10 percent THD.

When dealing with harmonics caused by variable-frequency drives, a solution can be to add isolation transformers that work to isolate the sine waves heading to and from the VFDs.

By utilizing some of these approaches as well as understanding the source of the harmonics problem, engineers will hopefully find that the facilities they design have power quality that is singing a happier tune.