RESEARCH ON OIL-PAPER INSULATION CHARACTERISTICS UNDER NONUNIFORM ELECTRIC FIELD

Converter transformers are key equipment in High Voltage Direct Current(HVDC) power system, while its insulation is an important issue that restricts its safety and reliability. Oil-paper insulation is the most widely used as insulating materials for converter transformers. The increase of local field strength is one of the common fault states in converter transformers, which will cause the dielectric properties of oil-paper insulation to change and cause insulation failure. In order to clarify the relationship between the change of the dielectric properties of oil-paper insulation and the breakdown performance under the condition of increased local field strength, the electric (corona discharge) and thermal (130℃) aging system for oil-paper insulation was built in this paper, and analysed the change of oil-paper insulation performance under different electric field distributions. In addition, the relationship between the breakdown strength of oil-paper insulation after aging and the change of characteristic quantities such as dielectric constant is discussed from the aspect of mechanism. It is found that under the operating state of nonuniform electric field distribution, the dielectric constant and loss factor of the insulating paper will fluctuate significantly in the low frequency range where the local field strength is high. At the same time, the insulating paper quickly cracks,Causing the breakdown performance of the insulating paper to rapidly decrease. The research results in this paper are helpful for revealing the breakdown mechanism of oil-paper insulation and for the safe operation of converter transformers under working conditions.