Experimental Investigations on Water Droplets on Polymeric Insulating Surfaces under the Impact of High Electric Fields

Non-Ceramic Insulators, NCIs, are widely replacing traditional porcelain or glass insulators, due to their low weight, high contamination resistance and hydrophobic surfaces. However, aging of the NCI's surface can gradually decrease the surface hydrophobicity, which leads to corona and even surface flashovers. Deformation of a water droplet affects the E-field distribution present on the insulating surface, making it difficult to estimate the droplet induced electric field enhancement on the insulator surface or to predict the partial discharge inception electric field strength. PD inception E-field strength of water droplets on hydrophobic surfaces under variation of a large set of parameters is measured, analyzed and discussed in this study. A systematic investigation on the behavior of water droplets at AC, DC and combined AC-DC stress is analyzed and compared, particularly in deformations and PD characteristics point of views. The major parts of these investigations have also been used to validate the dynamic E-field simulation package recently developed by the Computational Electromagnetics Laboratory (TEMF) of TU Darmstadt. A preliminary good agreement between simulations and experiments has been obtained. However, further cooperation is still needed to complete the validation of new software package. The maximum allowable E-field strength along a polymeric insulating surface of wettability class WC 1 for a single water droplet on a at surface has been estimated experimentally to be between 6 and 7.5 kV/cm (peak). The PD inception E-field strength was reduced to about 4.5 kV/cm (peak) for more water droplets on the surface like random spray and dew droplets. In spite of the benefits of polymeric insulators rather traditional glass and porcelain insulators, they are still in practice a relative new technology. The behavior of water droplets on the surface of polymeric insulators have been shown to be a key parameter to facilitate understandings of long-term and aging behavior of this type of insulators