The moisture in SF₆ insulated CTs considering current and change of ambient temperature

SF₆ insulated current transformers (CTs) are widely used in high-voltage power systems due to their favorable insulating properties. Field experience shows that the high moisture content in an SF₆-insulated CT is an important factor responsible for its insulation faults. Furthermore, the moisture content in an SF₆-insulated system is generally dependent on the ambient temperature. Thus, in this paper, the temperature distribution and moisture content of an SF₆-insulated CT are studied by considering current flowing through it and a sharp change in the ambient temperature. It is found that the temperature distribution of an SF ₆-insulated CT in operation is extremely uneven, and the measured moisture content becomes higher than the initial moisture content. The relative humidity in the lower part of the CT might reach a dangerous level when the ambient temperature changes rapidly. The mechanism of moisture condensation is proposed to explain the experimental results. Besides, the possible preventive measures for insulation failure are also discussed.