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Harmonic Distortion (THD) Measurements

Measuring Distortion in the Power Line Voltage Waveform

Twice I have measured the total harmonic distortion (THD) of the voltage delivered to my home.  The first time was for 8 days in late April 2018 and again for 14 days in late January 2019.  Results were very approximately the same each time with average THD of 3.1% (1187 measurements) and 3.4% (1936 measurements).  A notable difference was that during the second (2019) experiment, THDs as high as 4.1% were measured at times.  Below is a description of the first experiment.

THD During April 2018

During the last week of April 2018, I measured the total harmonic distortion (THD) of the power line voltage waveform delivered to my home by Hydro One in Ontario.  Measurements began the afternoon of 22 April and continued until the morning of 1 May, although I used only the measurements taken during April, for a total of 8 days.  Measurements were made with an AEMC Instruments Power Quality Logger, model PQL-120.  A total of 1187 measurements were made, one every 10 minutes. 

History of THD distortion.

Based on the 1187 separate measurements, the average total harmonic distortion during the 8-day period was 3.08%.  As you can see from the charts, values ranged from 2.6% to 3.7%, but were most often in the range of 2.9–3.3%. 

Histogram of THD measurements.

Interestingly, the variation or spread in the amount of distortion was generally less during the nighttime hours (2300-0600 hours) and greatest during the day and evening hours (0600-2300), as shown below. 

THD at time of day.

Specified Limits of THD

For delivered voltages of less than 400 volts, like your house voltage, Hydro One specifies a THD limit of 11% for short-term and 8% for long-term variations.  Distortion at the times of my own measurements was well within Hydro One's stated limits.