54 / 2018-08-20 16:36:40

Conductivity of HVDC Cable Insulation Materials: Case Study between XLPE Nanocomposite and Polymer Filled XLPE

conductivity,nanomaterials,polyolefin,temperature denpendence

In extruded HVDC power cable systems, electric field distribution in insulation layer mainly relies on DC conductivity of insulation material, so it is vital for the design of HVDC power cable systems. However, DC conductivity of insulation material is affected greatly not only by electric field strength, but also by temperature. Temperature difference between inner part and outer part of insulation layer for extruded HVDC power cable may result in the inversion of electric field, leading to breakdown of power cable systems. Therefore insulation materials with stable DC conductivity from room temperature (25℃) to working temperature (70℃ or 90℃) is of great importance for extruded HVDC power cable systems. In this paper, DC conductivity of nano Al2O3/XLPE composite and a novel polyolefin material under different temperature including 30℃, 50℃, 70℃ and 90℃ was compared. About 250µm plate samples of nano Al2O3/XLPE composite and a novel polyolefin material were made based on the same LDPE. But the main difference is that the filler used in this novel polyolefin material is an amorphous polymer with a special function group, whose glass transition temperature is ~102℃. First of all, composite of nano Al2O3 and LDPE or blend of polymer and LDPE was made respectively. And then ~2wt% dicumyl peroxide was added to both of them by absorbing at 70℃ for 24h in an oven. The same crosslinking process was completed by a press vulcanizer. Before DC conductivity test, all the samples were put into an oven to remove crosslinking by-products and thermal stress. A three-electrode system with the measuring electrode of 25mm was used to investigate DC conductivity, and an oven was used to provide stable temperature environment. A DC electric field of 20kV/mm was applied to samples, meanwhile, DC current was measured by Keithley 6517B and recorded by a computer for 1 hour. Median of current value for the last minute was used to calculate DC conductivity and average of three samples was taken. It was found that the novel polyolefin material has lower DC conductivity than nano composite during the temperature range. With the increase of temperature, difference in DC conductivity between these materials decreases. Eventually at 90℃, both these materials are in the same order of magnitude, about 10E-12S/m, but DC conductivity of this novel polyolefin material is still lower than that of nano Al2O3/XLPE composite. Temperature coefficient of these two materials under 20kV/mm were calculated, and it was pointed out that the temperature coefficient of this novel polyolefin material is lower than that of nano Al2O3/XLPE composite. Morphology observation by optical microscope with materials etched in potassium permanganate concentrated sulfuric acid solution shows that the filler of this novel polyolefin material disperses in the base material XLPE with a scale of several micrometers. And introduce of special function group does have a giant effect on DC conductivity behavior. Thus, it was believed that structure difference in these materials leads to the variation in DC conductivity, however, detailed charge transportation characteristics in this novel material, especially under different temperature, still need further study.