Energy Smart Services Tech Brief
Harmonic distortion: Problems and solutions

Like surfers, most electrical devices are looking for the perfect wave. For alternating current, perfection is defined by a sinusoidal (or sine) wave in which electrical voltage changes smoothly from positive polarity to negative and back again 60 times per second. Unfortunately, modern equipment is having a negative effect on the quality of this perfect wave. A variety of solid state devices, including desktop computers and other microprocessor-based devices, create high levels of harmonic distortion. (See Harmonics Strike a Sour Note below for more details).

Harmonic Problems

Harmonic distortion may or may not create a problem for your facility. You may have harmonics present, but experience no adverse effects. However, as harmonic levels increase, the likelihood of experiencing problems also increases. Typical problems include:

  • malfunctioning of microprocessor-based equipment.
  • overheating in neutral conductors. transformers, or induction motors.
  • deterioration or failure of power factor correction capacitors.
  • erratic operation of breakers and relays.
  • pronounced magnetic fields near transformers and switchgear.

To make matters worse, harmonics can sometimes be transmitted from one facility back through the utility's equipment to neighboring businesses, especially if they share a common transformer. This means harmonics generated in your facility can stress utility equipment or cause problems in your neighbor's facility and vice versa. Electric utilities have recognized this problem and are adopting standards, like the Institute of Electrical and Electronics Engineers (IEEE) Standard 519 which defines allowable harmonic distortion at customer service entrances. This standard is designed to protect both businesses and utilities.

Solutions

There are a number of ways to deal with harmonics, but not all solutions are appropriate for a given problem. The first step in solving a harmonics problem is to carefully examine your power system and loads to define the nature, source and manifestation of the problem. EWEB's Power Quality Team can help identify harmonic distortion in your facility.

Treat Symptoms

In some cases, it's best to simply treat the symptoms. If your only problem is neutral conductor overheating, you can increase neutral conductor size. For transformer overheating, you can install special K-rated transformers designed to tolerate harmonics. You can redistribute or relocate harmonic producing loads around your facility to balance harmonics and produce a more sinusoidal waveform. A "zigzag" transformer uses phase shifting to accomplish much the same thing.

Treat Sources

Another solution involves reducing the level of harmonics produced by equipment. Impedance may be added by installing line reactors at harmonic sources. Tuned filters may be installed to eliminate specific harmonic frequencies. Both have a long track record, are reasonably priced and work effectively. This approach must be carefully implemented to avoid creating other problems, such as harmonic resonance.

Six-pulse rectified power supplies like those found in many variable frequency drives, may be replaced with twelve or higher pulse rectifiers. This solution is not likely to be cost effective unless done when the equipment is purchased.

A new class of harmonics mitigation devices injects a mirror-image waveform of the harmonic portions of the distorted waveform. By canceling out the harmonics, the waveform returns to its 60 Hertz base. This type of device – called an active harmonic filter – is based on variable speed drive technology. Active harmonic filters are relatively new and rather costly, but offer several advantages. They are inherently current limiting, have no resonance problems, are "intelligent" and adaptable, and can be configured to either correct the full spectrum of harmonics or to target specific harmonics. Although this technology is new, it has important advantages and should be watched carefully.

It is worth stressing that the particular solution for your facility must be the result of careful analysis and isolation of the problem. No harmonics mitigation strategy should be employed without first assessing the situation.

New Equipment

An ideal time to consider harmonics mitigation strategies is during the design of new facilities or at the time of equipment purchases. Harmonics producing equipment can be identified and mitigation devices installed at the equipment. Transformers and neutral conductors can be specified properly. Some variable speed drive manufacturers now offer harmonics correcting components as standard features of their drives and others offer them as factory installed options. Be sure to ask your drive representative about harmonics correction when specifying a new variable speed drive.

Help from EWEB

Harmonics are not a problem for everyone. Most facilities probably have some level of harmonic distortion, so the mere presence of harmonics does not warrant concern. However, you should be concerned when you see the problems described above. Another time to give harmonics some thought is when purchasing new equipment that is known to produce harmonics.

EWEB has information on harmonics problems and solutions. We can help you diagnose harmonics problems using our array of metering devices and troubleshooting skills. Call our Power Quality Hotline at 541-685-7676.

Harmonic History

For most of the twentieth century, the predominant use of electricity for business and industry was to power motors, lights and heating devices. These uses have little effect on the 60 Hertz (cycles per second) sine waveform of the electricity delivered to them from their utility. They are called linear loads, because the current (amperage) rises and falls in proportion to the voltage wave.

A few industries like steel mills and aluminum smelters used electricity to power arc furnaces, which distorted the waveform, because the current flow was not directly proportional to the voltage. These loads are called non-linear loads.

Non-linear loads cause waveforms that are multiples of the normal 60 Hertz sine wave to be superimposed on the base waveform. These multiples are called harmonics. For example, the second harmonic is a 120 Hertz waveform (2 times 60 Hertz), the third is a 180 Hertz waveform, and so on. The combination of the sine wave with all the harmonics creates a new, non-sinusoidal wave of entirely different shape. The change to the wave is called harmonic distortion.

In the last 20 years, there has been an explosion of microprocessor based equipment which are also non-linear loads. Examples include computer systems, variable frequency drives, AC/DC converters, electronic ballasts, X-ray machines, MRI equipment and uninterruptible power supplies.

What was once a problem for a very limited number of heavy industries, is now a concern for some smaller business, too.



Harmonics Strike a Sour Note
Harmonic Waveform

For most people, the word "harmonics" brings to mind something musical. If you could look at a plucked guitar string in slow motion, you would see it vibrates in several ways. First, it vibrates end to end, anchored at the head of the guitar and the bridge. This is called the fundamental. The string also vibrates as if anchored at the bridge and in the middle of the string. This vibration on top of the fundamental vibration is called the second harmonic. The frequency of the second harmonic is two times the fundamental. The frequency of the third harmonic is three times the fundamental, etc. Harmonics are superimposed on the fundamental to produce the sound we hear.

The translation to electricity is almost direct. Electricity is produced and delivered in its fundamental form as a 60 cycles per second (Hertz) sine wave. Once inside your business, certain types of equipment can superimpose harmonics on the basic sine wave. Harmonics are multiples of the 60 Hertz wave. For example, the second harmonic is at 120 Hertz, the third is at 180 Hertz, etc.

Because harmonics are superimposed on the fundamental waveform, the frequency of the electricity no longer follows a smooth sine wave. Most electrical equipment expects to see a smooth frequency and distortions created by harmonics can cause a variety of problems.