What is Total Harmonic Distortion?

June 08, 2017

Imagine a tone being played by two musical instruments -  let’s say middle C (261.63 Hz). While the pitch played on a piano may be the same as on an electric guitar, its timbre is very different. The sound wave generated from a piano or a simple synthesizer is relatively simple and clean, but the sound wave coming from an electric guitar is often very distorted for effect and contains natural harmonic overtones.

Electrical power currents can also become distorted – just not usually on purpose. The presence of harmonic distortion in a power load can present issues that are more serious than what your taste in music might be.

Sound wave sine curves of a synthesizer and an electric guitar playing the same note.




Total Harmonic Distortion defined

Total harmonic distortion (THD) is the cumulative degree of distortion within an electrical current compared to the ideal. Most household electrical systems draw linear loads. On a linear current sine curve, the peaks and troughs are smooth, even, and sinusoidal. Some distortion can take effect in residential circuits but not enough to cause significant efficiency issues.

Comparison of linear and non-linear power current sine curves.

However, many industrial and commercial applications use non-linear loads. The way generators, converters, and power supplies draw power introduces a degree of harmonic distortion (the result of combining multiple waves of varying frequency) that can seriously affect a power system’s efficiency. These kinds of loads are inductive and have a low power factor: the amount of non-working power within a given load is a large proportion of total apparent power.

THD and your power bill

Total harmonic distortion is inversely proportional to power factor. If a given load has a higher power factor, its THD factor will be lower and the system will be more efficient. Fortunately, most power utilities adhere to standards that require supply voltage to have a relatively low THD factor; the power entering your facility is relatively linear.

Efficiency issues arise when devices draw inductive loads from the source. Devices that use inductive loads like generators, electric motors, and ballasts draw more power to operate correctly because their power factor is relatively low. This is probably the biggest factor that contributes to wasted energy and increased electric bills.

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Other tolls of high harmonic distortion

Over time, the harmonic distortion can negatively affect electrical equipment and lead to degradation of individual components. Increased current leads to excessive heat and interference with over-the-wire communications. Both of which, if left unchecked, can cause long term damage to not just electrical equipment but entire electric systems as well.

Total Harmonic Distortion and Power Metering

Because of this, there is an increased need to monitor individual devices and areas within a commercial facility in order to improve overall efficiency. Power meters are able to measure many parameters, such as reactive energy, apparent power factor, and line frequency among many others – giving facility managers the information they need to make necessary and appropriate countermeasures.

CLICK HERE to learn more about submetering and who could benefit from its implementation.

Topics: Energy Management, HVAC/R