The Behaviour of Water Drops on Insulating Surfaces Stressed by Electric Field

The insulating materials of high voltage equipment are stressed in service over years by several environmental factors. One ageing factor is the humidity, which in combination with the electrical stress causes changes of the conditions on the insulating surface. Polymeric insulators are widely used because of their high contamination resistance and high tensile strength-to-weight ratio, while constituting an unattractive target for vandals. Especially with silicone rubber housings, polymeric insulators exhibit a high surface hydrophobicity. Water drops will be formed on the polymer surface when the insulator is exposed to rain and moisture, and hence the conductive contamination dissolved with water is discontinuous. Hydrophobicity is transferred to pollution layers and after loss of hydrophobicity under severe environmental stress, such as extreme continuous exposure to moisture, it will be recovered in a relatively short time. The aim of this work is to analyze the early ageing conditions of an electrically stressed silicone rubber surface. The influence of water drops under electric field stress on the partial discharge inception phenomena shall provide the focal point of the investigation. Around the water drops the electric field is intensified. The water drop will be deformed and elongated along the direction of the electric field lines. The deformations will cause local electric field intensifications; the critical point will be the triple point between the water drop, air and the insulating material. This will be a starting point for partial discharges, finally leading to material deterioration. The water drop movement on the insulating surface is observed and recorded using a high speed camera and a mirror system. The mirror system simultaneously records three different plane views of the water drop. In this way, a three dimensional recording is possible, while providing three dimensional information about the water drop movement. The recordings revealed that the water drops forms sharp edges at the triple point and elongate in the direction of the electric field lines. The partial discharge activity is analyzed using a standard partial discharge measurement system. As the discharges taking place at one water drop are marginal, the partial discharge system is not sensitive enough to detect the ignition of the discharge. Therefore, a photomultiplier module was used to detect the inception of the discharge activity on the surface. The first light emission of the partial discharges is amplified and converted into an electrical signal. It was revealed that the photomulti-plier module detects the first partial discharge at a voltage value which is up to 13 % smaller than the value measured using the standard partial discharge system. Using a UV camera it was observed that discharges take place on the surface starting from the TP of the drop towards electrodes, exactly where the sharp edges are present. During experiments the following parameters varied: water drop volume, water drop conductivity and ambient conditions. The effects of different parameters on the partial discharge inception conditions are analyzed. The inception electric field strength decreases by increasing the water drop volume. Small water drops exhibit a higher contact angle than larger water drops. The contact angle increase by decreasing the water drop volume. The ambient conditions also play an important role in determining the inception electric field strength at water drops and it is a parameter which changes very often in normal duty, especially for outdoor insulation. At high humidity and in the presence of the electric field, the oxygen molecules hydrate with the water molecules from moist air, forming large negative ions. As the production of large ions increases exponentially by increasing the humidity, the probability of an avalanche starting is higher at higher humidity than at lower ambient humidity. Hence, the inception electric field strength decreases by increasing the absolute humidity. Two different decay rates were measured. The humidity correction factors presented in this work can be used to predict exactly the inception electric field at the water drops on an insulating surface at a certain absolute humidity value. The practical importance is that the ambient conditions, i.e. absolute humidity, influence PD activities reversely compared with the general breakdown electric field strength. The ambient conditions should always be measured when PD measurements are carried out. Long-term observations and measurements on PD activity will only be of any relevance if the results are obtained under the same humidity conditions or if they are corrected with respect to the humidity level. The water drop deformations recorded with the high speed camera were modelled using a drawing tool, where the shape and volume of the drop are reproduced very accurately. The electrostatic simulations are carried out using the modells and the inception voltage values from the experiments. The simulation results show electric field intensification in the region of the TP. In order to verify the simulation results, an analytical calculation was carried out for an undeformed water drop having a contact angle of 120°. For the deformed water drop the theoretical calculation of the electric field distribution at the water drop is no longer possible. Therefore, the theoretical consideration revealed that criteria to verify the simulation results must be found. Two criteria are developed, one global and one local. If the simulation results fulfil both criteria, the values can be considered to evaluate the electric field intensifications in the region of water drop. The first criterion, named “integral criterion” is a global validation criterion, e.g. is valid for the whole path along the sample. The interesting point is near the water drop, where the electric field is intensified using a local validation criterion. As the TP is a singularity point, e.g. values of the electric field strength tend towards infinity, the values in the TP cannot be considered in the evaluation of the electric field intensification. The analysis of the electric field curves along the surface at different mesh settings revealed that up to a distance of 0,1 mm from the TP, the curves corresponding to the fine meshes converge and this point is named convergence point (CP). The CP is the point where the electric field intensifications are considered. The electric field intensifications analyzed at the CP show high electric field intensification in the presence of the water drop on the surface. The variation of the electric field strength with the contact angle and with the contour of the drop is shown by measurements and demonstrated by simulations. For deformed drops, the electric field intensifications are higher and can reach values eight times higher than the applied electric field. The tangential component of the electric field is the affected component by the local field intensifications. Taking into account the magnitude of the local electric field intensification, the deformation of the water drops on the insulating surface must be considered by electrical resistance of the polymeric insulating surfaces. The absolute value of the electric field is to be considered in order to compare the experimental measurements with the simulation results. For the deformed water drops the absolute values of the electric field are comparable with the inception electric field strength in air. The ageing model of a polymeric material was upgraded in the region of the early ageing phase. The water drop deformations, their implication on the local electric field intensification and the influence of the ambient conditions on the inception electric field are included in the ageing model for polymeric materials.