Estimating the interharmonic content in the power supply system

The international IEC 61000-4-7 standard provides guidance in harmonic/interharmonic measuring. Although it considers interharmonic measuring as a special case it does not give a systematic method for their evaluation. Basically it assumes that voltage and current signals are periodic with a frequency of 50 Hz, composed only by harmonic components. That is one of the reasons for the new amendment proposal to the IEC 61000-4-7 standard. This amendment sets out the process for measuring harmonics and interharmonics in voltage and current signals in the electric power supply as well as for testing equipment pieces accordance with emission limits given standards as IEC 61000-3-2. This papers presents the study of the spectral estimation for current signals proposed by the revision of IEC 61000-4-7 standard. Two very different real signals are analysed in order to show situations where the spectral estimation method proposed provides right results and others where the results may be erroneous. It also includes the study of analytically synthesised signals which clarify the results of real signal analysis.