Impact of assessment period on voltage THD measurements

This paper examines how voltage total harmonic distortion (VTHD) indices obtained from medium voltage sites calculated over different assessment periods vary across a year. The aim of the study is to determine how much variation is present in VTHD indices calculated using a one week assessment period and, if the variation is large, attempt to determine an assessment period which reduces the variability to an acceptable level. The study findings indicate that there is significant variation in indices calculated using the commonly used weekly assessment period. It is clear that monitoring for one random week at any given time during the year is not acceptable for compliance evaluation as the error between the minimum weekly value and the maximum weekly value across the year can be very large. Moving to a two week or four week assessment period reduces the variation, however a quarterly assessment interval is required to reduce variation to a level that may be acceptable.