Time-varying harmonics of single-phase nonlinear appliances

The paper deals with the time-variant behavior of harmonic currents produced by single-phase nonlinear (NL) appliances. The harmonic currents have been monitored in a broad field measurement activity that has concerned several samples of the more widespread appliance types in a power demand range up to hundreds of Watts. For the measurement activity a custom system allowing the continuous harmonic monitoring of voltage and current signals has been used. The results obtained show that for almost all the investigated appliances a not negligible variation of harmonic currents with time can be found. The possible causes of the observed behavior are discussed in detail. Finally, the utility of specific statistical and graphical tools for approaching the problem is discussed.