When analysing voltage harmonics, the Total Harmonic Distortion (THD) measurement is a good method to use. THD is the sum of all the harmonics referenced to the fundamental value. The voltage fundamental value is typically always present. However, the same cannot be said of current. When loads turn off, current drops and can drop to near zero. When this occurs, misleading THD values can be seen on the current channels. For example, if there is 1A of harmonics and 100A of fundamental, then the THD will be 1%. However, if there is 1A of harmonic noise and the current drops to 0.5A, the THD can be 200%, which can be misleading. IEEE519 recommends using Total Demand Distortion (TDD) when measuring current.
TDD will reference the total current harmonics to the maximum average current recorded during the test interval, which indicates the reference value will always be valid. Some utilities have their own reference value they use throughout the grid. The Megger PQ PC software allows them to enter this value to use as a reference when viewing TDD.