Dielectric Coatings for High Voltage Gas Insulated Switchgear

Abstract

In the 1960’s, gas insulated switchgear (GIS), gas insulated busbar (GIB) and other gas insulated substation components were developed to overcome several disadvantages of air insulated substations. Gas insulated substations, although more expensive than the air insulated counterparts, are almost insusceptible to atmospheric conditions such as precipitation, pollution and ice formation. Moreover, the surface area of gas insulated substations is significantly smaller than that of air insulated substations at the same voltage level and power rating. The direct influence of gas insulated substations on the environment is also small compared to air insulated substations because of the lack of corona and thus audible noise and radio frequency interference, especially in wet weather conditions.
From the above mentioned advantages it seems that gas insulated substations are advantageous with respect to air insulated substations. However, the main disadvantage, next to the construction costs, is the fact that almost all GIS is filled with sulphur hexafluoride (SF6) as an insulation gas, which has a very high global warming potential of roughly 23,000 times that of CO2. The global warming potential of SF6 has resulted in strict governmental regulations on the usage and storage of SF6. Therefore, the urge from the industry to replace SF6 with a more environmentally friendly insulation gas has become very strong over the past years.
Unfortunately, most readily available and environmentally friendly insulation gases have a relatively low electrical breakdown strength, which would require the operating pressure or the dimensions of GIS to be significantly increased. Increasing the operating pressure or the dimensions of GIS would be unfeasible. Therefore, the main challenge in the field of GIS is to improve the electrical breakdown strength of GIS without increasing the size or raise the operating pressure above the current design limits.
The investigation into the improvement of the breakdown strength of GIS has taken two main paths. Firstly, ongoing research is being conducted to develop a new replacement gas which has a breakdown strength comparable to that of SF6. This research has recently led to several replacement candidates. Secondly, research has shown that the breakdown strength of GIS can also be improved by the introduction of a dielectric coating layer on the electrodes inside GIS. This thesis focusses on the improvement of the breakdown strength of GIS with the application of a coating layer.
In this thesis the lightning impulse breakdown voltage of gas-coating insulation systems is evaluated with the use of lightning impulse breakdown tests on a rod-plane electrode configuration. These tests include a wide variety of coating materials with a wide range of material properties and are mainly conducted in dry air as an insulating gas. Next to the breakdown tests, a range of material characterisation experiments are performed to obtain more information on the coating material structure and to find a relation between the coating material parameters and the breakdown strength of the gas-coating insulation system. The material characterisation experiments include dielectric spectroscopy, surface roughness measurements, conduction current measurements, electrical breakdown tests and optical microscopy...