Humidity dependence of bulk and surface resistivity of polymers for HVDC applications
Typ
Examensarbete för masterexamen
Program
Publicerad
2022
Författare
Valcarce Alba, Diego
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Abstract
Polymers are a key component of HVDC insulation systems. When insulation is exposed to a DC field, electrical charges tend to accumulate on polymeric surfaces that leads to modifications on the local
electric field. Surface and bulk resistivities of polymers are fundamental material characteristics and their behavior under various operating conditions is crucial when it comes to predicting surface charging phenomena and designing an optimal and reliable insulation.
In the present work, dependencies of bulk and surface resistivities of three different silicon rubber materials used in HVDC applications on temperature and humidity of surrounding air were studied experimentally.
The measurements were conducted using 1 mm thick samples installed in a three-electrode arrangement energised with the test voltage of 1 kV. The test setup was mounted inside a sealed oven, where
the humidity was controlled using salt solutions. The measurements were performed at three levels of relative humidity (20-30%, 60% and 90%) and temperatures 30 °C and 60 °C.
The obtained results provided actual magnitudes of the measured characteristics, which are important for practical use of the polymer. It was found that both bulk and surface resistivities of all studied
materials decrease with increasing temperature as well as air humidity. However, the character of these variations was distinctive for each material. Physical reasons for the observed effects are scrutinised.
Furthermore, the sources of experimental errors and uncertainties in the measured data were analysed and improvements for the measurement procedure were suggested. Thus, it was noticed that at high
humidity levels, water condensation caused formation of a water film bridging the gap between the measuring and guard electrodes in the tree-electrode system and methods to avoid this phenomenon were
i explored. Also, it was noticed that a steady state of the measured current was not being fully reached even in four hour long experiments and longer measurement time has been recommended to obtain
more accurate data.
Beskrivning
Ämne/nyckelord
HVDC insulation , electric conductivity