Measurement of network harmonic impedance in presence of electronic equipment

Harmonic levels and their propagation in the power system are mainly determined by the network harmonic impedance. It is also an essential parameter for the calculation of harmonic current emission limits. Different methods for measuring the network harmonic impedance have been developed in the last decades, but all of them assume that the impedance is constant within a cycle at fundamental frequency, which is true in case of passive elements only. As nowadays most of the equipment in low voltage grids contains power electronics including rectifier circuits, network harmonic impedance will vary within a half cycle of power frequency. As more and more equipment operates with switching frequencies of several ten kHz, knowledge about the network harmonic impedance in the frequency range up to 150 kHz is also of significant importance. Based on a review of existing measurement methods, the paper presents an extended measurement method, which is able to address both, above mentioned issues. The application of the method is illustrated by two example measurements in different low voltage grids.