A New Method for Detecting Trace Methanol in Insulating Oil Based on Terahertz Spectroscopy

Methanol is a unique aging product of transformer oil paper insulation cellulose. Headspace gas chromatography is mainly used for the measurement of methanol in the insulating oil currently, and the testing process is cumbersome, time consuming and not capable for online monitoring. It is of great significance to develop an efficient and accurate method suitable for on-site detection of methanol content in oil. This paper explores a testing method of methanol in the insulating oil by terahertz spectroscopy. By analyzing the fingerprint spectrum of insulating oil with different methanol content at 0.1-2.0 THz, it is found that the characteristic peak-to-peak value around 1.2 THz and 1.7 THz is closely related to the methanol concentration in the insulating oil. Moreover, the collective vibration caused by the association relationship of trace methanol in insulating oil is revealed by molecular simulation, which leads to the appearance of the characteristic peaks. Based on the relationship between the characteristic peak and the methanol concentration in the oil, this paper initially achieved the detection accuracy of 0.15 ppm trace methanol in the new insulating oil. The research results of this paper reveal the association relationship of trace methanol in insulating oil and propose an important new method, which is expected to be applied to online monitoring of methanol and other degradation by-products in the insulating oil of oil-paper insulation system.