HVDC transformers are one of the main components for long distance transmission, for which many investigations have been done all over the world in order to increase their efficiency and reliability. Regarding their insulation, oil immersed pressboards are widely used, representing one of the most important elements playing a role in transformer behaviors. Under DC operation, space charges accumulating between the oil and pressboard have been reported as the influential cause possibly giving rise to unexpected service failures under the transient voltage from the grid. However, it might be possible to presume that the accumulation of space charge could be facilitated inside the multilayered pressboard causing insulation breakdown in a different way. In addition, other parameters could be potential issues to investigate: non homogeneous temperature distribution along the oil circulation path and the DC field intensity. The former could explain the conductivity variation causing the non-uniform DC field distribution, while the latter plays a role in the variation of conductivity. With these in mind, three parameters have mainly been put under experimental investigation. For this purpose, a DC voltage is applied to the oil immersed pressboard, and then the polarization and leakage current are measured and analyzed. The mobility of the space charge is calculated from the former as a function of the number of layers, while the conductivity is obtained from the latter.
