Using refractive index to monitor oil quality in high voltage transformers

Insuring reliable operation of high voltage electrical equipment, such as transformers and cables, is of great importance to the power industry. This is done by monitoring the equipment. A large portion of this monitoring includes analyzing the quality of the insulating oils and observing various compounds formed in the oils during aging. Most often, transformer monitoring includes routine oil sampling and analysis, which has proven to be very effective at diagnosing faults and determining the insulation condition. Many techniques have been demonstrated for the purpose of online monitoring, and various commercial products are available. However, utility companies are still looking for more cost effective methods to monitor their equipment between sampling intervals. The work presented here was performed in order to investigate the use of refractive index for monitoring insulating oils. The refractive indices of various oil samples obtained from the field were measured and differences were observed. Accelerated aging experiments were conducted in a laboratory and increases in the refractive indices of these artificially aged oils were observed. Experiments were conducted to determine what by-products would contribute to this increased refractive index by investigating the effects of individual groups on the refractive index change. These groups included aromatic compounds, polar compounds, furans, acid, and fault gases. We observe that the formation of furans, acids, and fault gases cannot be detected using refractive index for the concentrations typically found in the field. We conclude that changes in the refractive index of an oil can be used as an indicator of the oil’s aging and its break down and the formation of aromatic and polar compounds.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat