Investigation of Charge Carrier Transport Processes in Transformer Oil-Pressboard Barrier Systems by Simultaneous Kerr-effect and Polarization Current Measurements

The main focus of this paper is on the study of polarization and conduction processes in the oil and oil-impregnated pressboard insulation system of a converter transformer in a High Voltage Direct Current (“HVDC”) transmission system. In this paper, measurements on HVDC transformer insulation system mock-ups by using the electro-optic Kerr-effect and polarization current analysis are presented and discussed. The measurements show that the switch-on behavior is essentially determined by the capacitances of the materials. A comparison with the measurement results of bare oil gaps reveal that the intrinsic charge carriers of the oil dominate the conduction processes only at the first few hundred seconds. Then a transition takes place where the field migrates into the solid insulation material, which determines the behavior for long-term DC stress. This shows that both, the initial and the steady state behavior of the investigated arrangements can essentially be modelled by a simplified equivalent circuit diagram consisting of
$R$
's and
$C$
's. But these models are not able to represent the transient behavior of the insulation system as a comparison between the measurement and simulation results confirm. It follows that the simplified RC model is not completely invalid, contrary to common opinions, but has a limited validity in the investigated range.