Time dependence of spacer charging in SF<inf>6</inf> under DC stress

Abstract

As gas insulated DC transmission systems unite the advantages of space saving installation and big transmission capacity, this technology will become essential soon. The main problem in operation with respect to insulation seems to be identified today, which is charge accumulation on the surface of the insulators, so called spacers. These trapped charges can cause a reduction of dielectric strength and can lead to a breakdown, especially at polarity reversals and overvoltages. For experiments, a full encapsulated test setup was constructed. Test voltages up to 700 kV DC and a pressure up to 8 bar SF6 are possible. The main component of this setup is a 3 axis fully automated robotic handle system for moving the probe over the test samples inside the encapsulation. So the surface charge can be measured at all positions at the sample surface. The time dependence of the charging mechanism till the saturated state is of big interest. Thus the experiments presented in this paper deal with the influence of the measurement itself on the charging process.

Micro discharges in HVDC gas insulated systems

“…Further, it has to be mentioned that in case of a slightly misaligned sample also the measuring gap between sensor and sample is not always 3 mm. The influence of spacing between sample and sensor was investigated in [34] in detail. According to these findings, this effect has small impact on our present measurements.…”

“…The test setup was described in detail in [10,34]. During the charging of the sample, the measurement sensor is in its parking position away from the test sample surface on the right-hand side in Figure 2.…”

“…This is a high precision electrostatic voltmeter using the principle of a vibrating Kelvin probe. The sensor is mounted on the horizontal axis of the automatic handling system and is positioned with 3 mm distance to the surface of the test sample when measuring [34]. The Kelvin probe has a sensitivity level of 1 V and a step of 1 kV can be followed within 3 ms. Fast measurement procedures are possible and the impact of the measurements on the surface charge distribution is small [34].…”

“…A resolution of 5 mm² can be achieved placing the sensor 3 mm away from the sample surface. The overall measurement repeat accuracy of the setup with another electrode configuration is ~10% of the measured value [34]. The probe was calibrated before every measurement by setting it exactly to zero when measuring the potential of the grounded frame on the sample upper side.…”

Accuracy of surface potential measurements of HVDC spacers

“…The used test set up was already explained in detail in recent publications [4,5]. The required DC voltage is generated by a Greinacher cascade.…”