Total Harmonic Distortion – what happened to my perfect wave?

Total Harmonic Distortion (THD) is at its most basic a measure of how much your load is distorting the perfect waveform of the power provided by your utility. Many Packet Power power meters can now report harmonic distortion on both current and voltage. But is it information you need to know?

THD is always present in current and voltage but too much distortion can cause problems. Understanding THD is the first step in ensuring it isn’t creating problems in your facility.

Total Harmonic Distortion overview

Total Harmonic Distortion (THD) is a measurement of how much the voltage or current waveform is “distorted” or changed from its conventional sine wave shape. Power comes from the utility in the form of a clean sine wave. As it goes through different types of loads, the voltage and current get utilized at different rates causing distortion to be reflected back from the load onto the system.

Distortion mainly occurs in multiples of the carrier frequency (50 or 60 Hz) which are referred to as harmonics. For example, the 3rd harmonic on a 60 Hz line would be 180 Hz, the 7th would be 420 Hz. THD is the cumulative percentage of distortion for all harmonic orders relative to the total power. Distortion is measured separately for the current (THDI) and voltage (THDV).

Packet Power - THD image

What causes harmonic distortion

Just about all non-linear loads create harmonics. Examples of these types of loads include non-incandescent lighting, computers, uninterruptible power supplies, telecom equipment, copy machines, battery chargers, and devices with a solid state AC to DC power converter. Distortions in current cause distortions in voltage.

How much THD is too much?

Harmonics are innocuous at lower levels but very high levels may cause unwanted effects such as heating on the lines or disturbances in some equipment. While there is no firm limit in the US, IEEE 519 recommends that general systems like computers and related equipment have no more than 5% total harmonic voltage distortion with the largest single harmonic being no more than 3% of the fundamental voltage.

Look for an upcoming post on how data center managers can use THD information.

Contact if you’d like more information on a low cost solution for getting the data needed to mitigate potential high cost problems.


Alan Katz
Packet Power Article