Oxidation Stability of Insulating Liquids by Rapid Small Scale Oxidation Test

Abstract

Insulation materials are an important component of transformers; the life of the electrical equipment depends on the quality of solid and liquid insulating materials used. Mineral insulating oils, synthetic esters and natural esters are different types of liquid insulating materials. During prolonged service in electrical apparatus these oils undergo deterioration, mainly due to continuous oxidation. Hence, the long-term performance of insulating oils depends on the stability to oxidation. This behaviour is usually termed as resistance to the oxidation process or oxidation stability. Oxidation stability test as per test method IEC 61125 and ASTM D2112 are familiar methods to test the oils. According to IEC 61125 (Method C) the insulating oil, is conditioned at 120°C, with copper catalyst and by bubbling zero air over a duration of 48 hours for natural esters, 164 hours for uninhibited oils and synthetic esters, 500 hours for Inhibited oils. Rotating Pressure Vessel Oxidation Test method (RPVOT) as per ASTM D2112 requires copper catalyst and oxygen pressure of 620 kpa is allowed to oxidize the oil sample at 140°C. Zero air bubbling method assesses the oil based on measurement of dissipation factor, acidity and and sludge development for mineral oil and synthetic esters and dissipation factor, acidity and viscosity for natural esters. RPVOT method measures the induction period. Although the IEC 61125 is conventional, it is time consuming and the RPVOT curve for natural esters cannot be distinguished due to the poor stability characteristics of natural esters. This paper presents the results carried out to measure oxidation stability by using a new method named as Rapid Small Scale Oxidation Test (RSSOT) as per ASTM D7545, using petroxy equipment to measure the induction period. Oxidation stability test has been carried out on mineral insulating oils, synthetic esters and natural esters. The results are compared to assess the suitability of the rapid small scale method for evaluating the stability of insulation oils. It is found that the rapid small scale method is suitable to measure the oxidation stability and the results are comparable to the conventional methods. Further, natural ester and synthetic ester was analyzed both with copper catalyst and without catalyst. It was found that presence of copper catalyst accelerates the rate of oxidation of both synthetic ester and natural ester. RSSOT has the advantage of being a rapid method, simple and easy to use, and where only a small quantity of sample is needed, thereby reducing the risks to safety and waste sample disposal problems.