High voltage DC power transmission technology has attracted considerable attention and will play an important role in the UK future transmission system. This project investigates the performance of the oil-paper insulation system used in HVDC transformers under a variety of electric stress conditions. It attempts to determine the effects of oil resistivity and other insulation conditions parameters on the capability of the insulation to withstand the electric stresses seen within HVDC transformers particularly during polarity reversal or other changes in stress. The pulsed electro acoustic (PEA) technique is used to measure the charge accumulation in oil-paper insulation system under different stresses and oil conditions, especially the polarity reversal. Oil conductivity will play an important role in the polarity reversal test of HVDC transformers, therefore, it is important to understand the characteristics of oil under dc conditions. The project intends to measure oil conductivity over a range of electric fields and temperatures. In addition, oil conductivity provides an important diagnostic measure for the insulation of a transformer.