Harmonics/Harmonic Distortion

What are Harmonics / Harmonics Distortion?

There has been much discussion and interest in recent years about the subject of power quality. In the past, power received from the power grid and used by consumers was described as ‘clean power’, meaning that it had a pure sinusoidal wave form.

More frequently today, electricity consumers are finding that they have to deal with the problem of ‘dirty power’. Dirty power is a slang term used to describe a variety of voltage and current contaminations (harmonics) on the pure sinusoidal waveform. Harmonics can come in the form of short-term transients or steady-state, continuous distortions that degrade the usefulness of power within a business facility or residential dwellings.

What can cause Harmonics (Harmonic Distortion)?

The sources of dirty power can be external to a business (as might be the case if an adjacent industrial plant is contaminating the power grid with harmonics, or the source can reside within the business itself.

Harmonic distortion is a specific type of dirty power that is usually associated with industries increased use of adjustable speed drives, power supplies, solar inverters and other devices that use solid-state switching. However, harmonic distortion can be generated by a variety of nonlinear electrical devices existing within business and residential power circuits. Harmonic distortions can radiate out from a source into the power grid causing problems for other power consumers. Harmonic distortion can cause increased power consumption and serious operating problems in certain plant environments, it is important that plant engineers or facilities managers understand the fundamentals of harmonic distortion, know how to recognize the symptoms of this problem, and know that harmonic distortions can be filtered out of power circuits once they are identified.

Measuring Harmonics / Harmonic Distortion

Harmonics can be measured as individual orders (frequency), or as a combined total harmonic disturbance in an alternating current electrical circuit. These can be measured as point-in-time readings using a range of portable power analysers. However, a business manager needs to know the bigger picture of a whole 24 hour working day or, in fact a whole business cycle. Sophisticated permanent power quality logging over weeks rather than days will show when harmonics are most prevalent, whether it is high during critical peak demand periods, or otherwise in off peak periods. This will help to identify which electrical devices in a business contribute most to harmonics, or if the source is external.

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Analysing Harmonics / Harmonic Distortion Reports

Harmonics data extracted from a power quality logging exercise can reveal the offending frequencies of harmonics causing problems within a business facility. Other telltale signs of the presence of harmonics is elevated power circuit voltages. It is fairly predictable which order of harmonics will be associated with different types of electrical devices. The power quality professional will include detailed harmonics information in a comprehensive power quality report and recommend solutions to reduce harmonic distortion to acceptable levels.

Solutions for Harmonics / Harmonic Distortion

When harmonics suppression is required in the presence of nonlinear loads, or the amount of harmonic distortion must be reduced to solve power quality problems (or avoid penalties), the most common, reliable and lowest cost solution is often the use of passive harmonic filters. These filters are designed to allow the lower frequency electricity (50hz) to pass unimpeded, while stripping off the higher frequency harmonics (150hz to 2,500hz) and dumping them to earth.

More complex situations where harmonics vary due to highly active industrial processes may cost justify more complex ‘active’ harmonic filtering to clean up a dirty power situation.

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Additional Information on Harmonics / Harmonic Distortion

Harmonics typically seen on a power system can be subdivided into two distinct categories by the nature of the problems they create and the remedies they usually require. They are:

  1. Those harmonic currents that are the dominant harmonic orders created by three-phase nonlinear loads. These are the 5th, 7th, 11th, 13th, and higher order odd harmonics that are not multiples of three.
  1. Those harmonics created primarily by single-phase nonlinear loads. These are3rd order harmonics and higher multiples of three. These are sometimes referred to as triplen or zero-sequence harmonics and are usually accompanied by some 5th, 7th, and other higher order harmonics.

Harmonics are a steady-state phenomenon and should not be confused with short-term phenomena that last less than a few cycles. Transient electrical disturbances, overvoltage surges, and under voltage sags in the supplied voltage are not harmonics. Some of these short-term disturbances in voltage or current can be mitigated too with suppressors, transformers and line reactors.