Determining the color index of transformer insulating oil using UV-Vis spectroscopy

The changes in color of the transformer oil have been used as measuring tools in determining the condition, performance and health of the transformer. Currently, the determination of color index is done by using a color comparator based on American Society for Testing and Materials (ASTM) D 1500 standard. This technique requires the tester or observer to compare the samples with two ASTM color references. Color is subjective and each individual could report a different color index. It could cause an error of 0.5 color index since the index difference between two consecutive ASTM color references is 0.5. Therefore, this work proposes the determination of color index of insulating oil using ultraviolet to visible (UV-Vis) Spectroscopy to eliminate any human handling error and improve the accuracy of the color index readings. Absorbance spectral responses of oils with different color index were used to correlate the color index with the absorbance spectral response of oil. Results show that the bandwidth of the absorbance spectral response is proportional to the color index of the oil. Modeled equation was developed to relate the bandwidth and color index of the oil. The equation was tested and verified by measuring the optical absorbance of another batch of transformer oil samples and inputting the bandwidths to obtain the color indices.