Black-Box Model of a Single-Phase Industrial Variable Frequency Drive

When an electrical device is power quality compliant, it means that its efficiency and coexistence within an energy system satisfies the standards that cover the harmonic range up to order of 40 (2/3 kHz), or in the electromagnetic compatibility (EMC) frequency (frequency higher than 150 kHz). Low-frequency (LF) EMC range, which covers the frequency range from 2kHz to 150kHz, is a grey area in standards for energy systems and the measurement of emissions in this range. Line impedance stabilization networks (LISNs) are the devices used to measure the EMC of devices. LISN standards cover only part of the LF EMC range (usually above 9kHz), thus making this range especially interesting for research. In this paper, measurements were made on a variable frequency drive (VFD) systems in the low-frequency electromagnetic compatibility range, where the switching frequency of the converter varied in steps from 5kHz to 15kHz, a common range for VFD and photo-voltaic inverters switching frequency, which corresponds to the LF EMC range. This research focuses firstly on common mode (CM) current measurements for a single-phase system where a VFD driving a motor without load is connected to two one-phase LISNs, where the active and passive part of the converter were computed to derive its black-box model. Because the results are different for each frequency, the authors found it convenient to derive a black-box model for 10kHz frequency, as it is also in the range of LISN standards.