INTER-HARMONIC DETECTION AND ESTIMATION IN LOW-FREQUENCY CONDUCTED ELECTROMAGNETIC INTERFERENCE MEASUREMENTS

摘要

The IEC 1000-4-7 standard is currently mandated to fix both procedures and instruments for evaluating the level of harmonics and inter-harmonics in electromagnetic interference measurements for power supply systems and connected equipment. Prescriptions and recommendations related to inter-harmonics seem to be not well detailed; this is particularly true for those showing a non-stationary behaviour. As a matter of fact, only a digital signal-processing approach, based on the Discrete Fourier Transform, is strongly recommended; little or nothing is said both about the sampling rate to be adopted and the time window to be observed. The paper aims at designing suitable measurement procedures based on advanced digital signal-processing algorithms in order to overcome the aforementioned limits. In particular, to allow non-stationary inter-harmonics to be monitored and properly analysed, a new approach, based on the use of Time-Frequency Representations, improved by a recently introduced reassignment technique, is proposed. After a detailed description of the proposed approach, its efficiency and suitability is assessed by means of tests on numerical signals as well as on actual signals directly acquired on the field.