Loss factor and permittivity measurement at medium-frequency high voltage with an HVDC offset

Medium frequency (MF) transformers are key components of power electronic topologies of modern energy supply systems. The insulation of MF transformers is exposed to the low frequency AC or DC grid voltage and at the same time to rectangular medium frequency converter switches featuring high amplitude and steepness. The high harmonic content of such voltage can cause increased dielectric losses in the insulation, leading to local hotspots [1]. Studying dielectric properties of insulating paper materials to be utilized in MF transformers by applying mixed voltages is of great interest. Such mixed voltage variations of MF and superimposed DC or low frequency AC components are shown in Figure 1. Combination of MF AC and DC voltage components is considered in this work. Both components are in kV-range, so that the dielectric parameters are derived from measured u-i-signals. By using mixed MF AC voltage with a DC offset following questions are treated. Firstly, the loss factor and relative permittivity are measured at high MF voltage amplitudes that yields values for realistic field strengths. Secondly, the impact of DC voltage on tand and er can be studied. Thirdly, other effects, e.g. change of material size due to high DC field can be observed by means of tand and er.