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 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
- 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.
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.
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.
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.
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.
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.
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
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
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.