High-frequency noise in power grids, neutral and protective earth

Sammanfattning: This licentiate thesis deals with the high-frequency distortion, 1 kHz and higher, introduced into the power grid by the numbers of power converters in use today. These devices can be a source of electromagnetic interference and equipment damage despite the fact that the equipment complies product-standards testing. The thesis discusses upcoming sources of emission, uncertainties in emission level and changing circumstances, with focus on the impact on the neutral conductor and the protective earth. The principles of three-phase power transmission and distribution are presented together with a description of small-signal distribution in order to show the similarity between the two, especially addressing conductive and radiated "signal losses". The response in neutral and protective earth to high-frequency harmonics and fundamental-frequency harmonics is studied separately using a combination of measurements and Matlab simulations and by using symmetrical components. This thesis also includes a description of voltage filters especially designed for measurement of high-frequency harmonic voltage as well as a description of the measurement of high-frequency harmonics in various buildings. The simulations confirmed the cancellations of symmetrical fundamental frequency current and the summations of the zero-sequence harmonics in the neutral wire. The high-frequency harmonic current increased when the number of loads symmetrically increased in the three phases, and there was almost no cancellation. Adding the effect of increasing impedance in wires resulting from increasing frequency, one origin of conductive high- frequency leakage current in the grid is shown.