Transformer oil, or insulating oil, is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant. As the oil is in a direct contact with electrodes and paper insulation and experiences high temperatures, certain additives have to be included to stabilise its properties. On the other hand, key information about transformer insulation system can be extracted from the oil. It has been claimed that transformer oil contains about 70% of diagnostic information. However, due to lack of understanding and limited research, traditional oil test program utilizes only some of diagnostic parameters.
DC conductivity measurements have provided an important diagnostic measure for many solid insulation systems such as power cables. However, it receives little attention as far as transformers are concerned. The UK government has introduced initiatives and proposals to ensure more of our energy comes from renewable energy. Offshore wind farms play a key role in the UK energy policy and the number of wind farms in the UK will be steadily increasing. The energy transfer from the offshore wind farms to the NGC transmission systems is likely to be based on dc links. A high voltage dc converter transformer is an essential part of the dc link. The behaviour of insulation system in a high voltage dc converter transformer needs to be investigated as the insulation experiences a dc voltage component in addition to ac voltage. In the light of increasing interest in high voltage dc converter transformers, the study of oil conductivity becomes an ever urgent issue for oil producers, transformer manufacturers and power transmission and distribution utilities. Initial literature survey indicates that the conductivity of a liquid dielectric may span several orders in magnitude depending on the status of the liquid and its value is very sensitive to the dc field applied, temperature, moisture, contamination. Oil is particularly responsible for functional serviceability of the dielectric system in transformers. On the one hand, condition of oil can be a decisive factor, which determines the life span of the transformer. On the other hand, it also contains abundant diagnostic information which can be potentially used for transformer life management.