Potential Distribution in the Bushing Core of a Solid-Gas Insulated High Voltage Bushing in the Very Low Frequency Region Under the Effect of a Temperature Gradient

In the field of HVDC bushings, temperature gradients inside the bushing core have more impact on the electric field distribution compared to AC. Furthermore, more experiences are existing in the DC case for the oil-insulation system than for solid-gas. The aim is to study the potential distribution in the bushing core of a solid-gas insulated high voltage bushing at very low frequencies (5 mHz to 100 mHz) under the effect of a temperature gradient. Previous investigations suggest that, after a DC voltage is applied, it takes dozens of hours before a true DC field condition is reached due to the end of all polarization processes in the insulation material. An investigation at very low frequencies is therefore reasonable to determine the temperature behavior of the capacitive-resistive transition. As test object, a single-sided 180 kV bushing core is used (only one end of the core is graded). Five metal foils of the capacitive grading were carried out of the ungraded side of the core, so potential measurements on this foils could be undertaken, and furthermore, the distortion of the electric field by the metal foils is lower compared to a double-sided grading. The test arrangement also allows measurements under different insulation gases with different pressures. Air is used as insulation gas since the test voltages are lower than or equal 30 kV. AC measurements are performed via capacitive sensor. In case of DC, the sensor evaluates the initial magnitude of the discharging process of a charged capacity. The distortion of the field distribution within the bushing core, caused by the influence of the measuring device has no significant impact, because its additional capacitive load is less than 80 pF. Since it is of deterministic nature, its influence can be taken into account. To perform a temperature gradient, the primary conductor is heated until the temperature gradient reached a constant value, and then set under voltage. At the one hand, the temperature and voltage distribution in the bushing core is measured, and on the other hand it is verified with our investigations of the material properties.