Experimental Investigation of Dielectric Breakdown Strength and Thermal Stability of Micro-Arc Oxidation Ceramic Coating on Aluminum Winding

Abstract

A ceramic coating grown on the surface of aluminum wires by a micro-arc oxidation (MAO) process has high electrical breakdown strength and thermal stability. However, the aluminum wires with ceramic coating by MAO have been not used in transformer windings previously. The objective of this paper is to determine the electrical breakdown strength and thermal stability of the ceramic coating on aluminum wires by MAO for its potential usage in transformer windings. In this paper, electrical breakdown strength of aluminum windings processed by MAO was tested by a manually controlled up-down method. Thermal stability was evaluated through a Differential Scanning Calorimeter DSC822e and a thermogravimetric Analyzer TGA/SDTA851. The experimental results indicated thickness of the coating varied between 20 and 30 μm, which was measured by SEM. Electrical breakdown strength of the ceramic coating in the air was between 22.9 and 47.8 kV/mm, which was at the same level as that of epoxy resin (20 kV/mm) and NOMEX paper (18 kV/mm ~ 40 kV/mm). In the insulation oil, electrical breakdown strength of the ceramic coating was between 76.7 kV/mm and 115 kV/mm. The electrical breakdown strength of the ceramic coating in the air was recoverable because of a rapid formation of new oxide layer. Thermal properties of the ceramic coating on aluminum windings by MAO kept stable under the temperature from 25 degrees Celsius to 550 degrees Celsius, which was significantly better than those of traditional transformer winding insulation materials such as epoxy resin (25 degrees Celsius ~ 180 degrees Celsius) and NOMEX paper (25 degrees Celsius ~ 220 degrees Celsius). Therefore, the aluminum windings with ceramic coating based on MAO showed a potential as dry-type transformer windings.