5. ULUSLARARASI MARDİN ARTUKLU BİLİMSEL ARAŞTIRMALAR KONGRESİ, Mardin, Turkey, 15 - 16 January 2021, pp.62-63
The dielectric strength, or breakdown voltage, of transformer oil measures its ability to support non-defective electrical voltage. Since these oils must insulate under high electrical potential, a significant reduction in dielectric strength means that the oil will no longer perform. A decrease in dielectric strength may be due to the presence of contaminants such as water, conductive particles, oil degradation by-products, residues, and cellulose paper breakdown. Dielectric strength can be determined relatively simply by applying an increasing proportion of AC voltage between two electrodes immersed in transformer oil. It is important to detect failure in transformer oil early, as it helps to further assess the health of the transformer and prevent further deterioration of the transformer. Determining the refractive index changes is an important method to monitor the deterioration of transformer oil. Oil deterioration is directly related to the refractive index of transformer oil. The density of the luminous flux used in the refractive index measurement changes with the change in the refractive index of the oil. It shows that an old oil has a higher refractive index than a new oil. As the refractive index of the oil increases, the breaking stress decreases. Therefore, the refractive index change will increase or decrease with the level of oil contamination, which indicates that the oil is new or old.
In this study, the relationship between refractive index values and diffraction voltage values of oil samples taken from different high power transformers at different times was investigated. Previously, the refractive index values of the samples were measured with an IR280 refractometer. The refractive indices of the oils varied between 1.4699-1.4820. Then, voltage diffraction tests were performed on the samples taken with the OTS 60 AF device. The tests started with the process of heating the oil and reducing the moisture in it and then the voltage values were increased starting from 0 V up to 60 KV. While the breakdown voltage of 1.4699 oil was obtained as 60 KV, breakdown voltage of 1.4820 was determined as 26.5 KV. It was seen from the results obtained that oils with a low refractive index break at higher voltages and become conductive. The refractive indices of the samples whose refractive indices were measured before the voltage diffraction test were also measured after the test. Since the high temperature created by each voltage diffraction test in the oil causes deterioration in the chemical structure of the oil, a slight increase was observed in the refractive index values after the test. This clearly reveals the deterioration of the oil and the change in its chemical structure. The higher the value of the refractive index of the transformer oil, the lower the light power losses in the oil sample.