High voltage direct current (HVDC) power transmission has received significant attraction recently due to the interests in off-shore wind farms and supergrid. HVDC transformers are an integral part of the HVDC transmission system and are one of the most expensive components. The insulation system in the HVDC transformers experiences both AC and DC voltages and its characteristics under such conditions need to be fully understood to ensure its reliable operation. One of the major issues related to HVDC transformers is the electric field distribution due to easy formation of space charge under dc voltage. The present project intends to address the electric field distribution and its relationship with temperature and polarity reversal using numeric modelling. Space charge data for oil/paper insulation will be experimentally obtained using the pulsed electroacoustic technique and they will be incorporated into the model. The outcomes of the project will benefit both electricity operator and transformer manufacturers in terms of reliability and polarity reversal tests.