Effect of Thermal Ageing on Electrical and Mechanical Properties of Medium Voltage Cable Joint Components

Abstract

Medium voltage cable joints are expected to function for 30-40 years after successful installation. To optimize and improve the design of these joints, it is essential to find out the changes that the components inside a cable joint go through. In this thesis, the effects of accelerated thermal ageing on electrical and mechanical properties of medium voltage cable joints have been investigated. For this research, three cable joint components were selected, that are being used and developed at Lovink Enertech. These components are silicone grease (insulation used at cable insulation-deflector interface), Lovisil (main insulation liquid filled inside the joint) and deflectors (geometric field grading to distribute electric field). These components were thermally aged and then tested, with the aim of observing the extent of change in physical and electrical properties.
Silicone grease was tested for its breakdown strength, under normal and tangential voltage application. Normal breakdown strength of unaged grease was measured at different temperatures, and of aged samples was performed at 50 °C, as that is the average operating temperature inside a cable joint. Tangential breakdown strength test was performed on XLPE and unaged silicone grease interface, at room temperature. It was observed that the breakdown strength under normal electric field of silicone grease reduced with ageing and with operating temperature. Also, the tangential breakdown strength is approximately half of the normal breakdown strength. For the deflectors, experiments to determine hardness of the material, young’s modulus, conductivity and weight of silicone grease absorbed into the deflector, were performed to characterize and compare the material properties at different stages of ageing. It was observed that silicone grease got absorbed into the defector at all ageing temperatures, as the weight of the deflectors increased. Hardness of the inner surface also increased with ageing. Whereas there was not a considerable change in young’s modulus and conductivity of deflector samples. Lovisil was thermally aged and tested to observe changes in its breakdown strength and influence of crosslinker polymer on the same. The measurements were made using Baur vessel, sphere-sphere electrode configuration and under two voltage applications: constant ramp and step voltage. Lovisil got darker with ageing and a decrease in the breakdown value was observed. Measuring breakdown strength under constant ramp voltage gave more reliable and repeatable results in comparison to step voltage. It was also noted that crosslinker enhances the dielectric properties of the Lovisil and is responsible for its curing and discoloration.
Three joints of 12/20(24) kV were prepared and aged to see if silicone grease helps in avoiding degradation at the cable insulation-deflector interface. It was observed that not applying silicone grease at the interface led to a degradation, whereas applying it did not cause any damage even after overstressing the joints. These tests also validated the newly developed silicone grease as an insulation at the interface.