Determining the inhibitor content of transformer insulating oil using UV-Vis spectroscopy

Monitoring and inspection of inhibitor content in transformer insulating oil has been a routine test for power utilities. Although new uninhibited transformer insulating oil contains naturally occurring inhibitors, these inhibitors could deplete over time during operation. Once the inhibitor depleted completely, the oil would start to deteriorate at a faster rate and eventually could lead to faults in transformer. The common method of determining the weight percentage of inhibitor in transformer oil is through Fourier Transform Infrared (FTIR) spectroscopy. However, this method is laboratory-based, which means oil sampling at site is necessary and the oil samples need to be transported to the laboratory. The necessity for oil sampling, coupled with the high cost of FTIR equipment have led to high maintenance cost. Therefore, this work investigated the possibility of using single wavelength or waveband optical detection for the determination of weight percentage of inhibitor in transformer oil using ultraviolet-visible-near infrared (UV-VIS-NIR) spectroscopy. Result of the work shows that the samples with inhibitor produced an absorbance peak at 1403nm. The peak absorbance of the spectral response is proportional to the weight percentage of inhibitor content that was measured using the conventional method, which is IEC 60666. An equation was derived to model the peak absorbance and weight percent of inhibitor content (%IC) of the oil and it was verified with additional oil sample with a known weight percent of Inhibitor content.