Influence of Micro-oxygen on SF6 Decomposition Characteristics Under Partial Discharge

Abstract: Due to the influence of O2 on final products and volume fraction of SF6 decomposed components, the content of O2 should be taken into account when diagnosing the fault of gas-insulated equipment through detecting volume fraction and variation law of components. Therefore, using the constructed PD decomposition test device, SF6 and O2 mix gas with different O2 content is tested under the same test condition. Besides, gas chromatography technology is adopted  to study the influence of O2 on SF6 decomposed components under PD. Result indications are as follows: whether O2 is injected or not, CF4, CO2, SOF2 and SO2F2 produce; an increase in O2 will restrain the production of CF4 while cannot affect the CO2; the increase in O2 can facilitate the generation of SOF2 and SO2F2 ; the effect of SOF2 is stronger than that of SO2F2; within the same discharge time, with the increasing of O2 volume fraction, volume fraction of SOF2 and SO2F2 reduce gradually and tends to be stable in the end; regardless of O2 injected, the ratio gradually decreases and finally becomes steady along with extension of discharge time.   

Key words: partial discharge, SF6, oxygen, decomposed component, gas chromatography, content analysis   

1. Introduction

When SF6 with good electro-negativity serves as an insulating medium for high voltage equipment, the insulating strength can be improved greatly. Hence, it is extensively applied in the external insulation. The SF6 chemical property is stable and it is not easy to decompose. However, when insulation defect causes partial discharge (PD) in the equipment, SF6 will decompose and generate low fluorides such as SF2, SF3 and SF4. If there is no other impurities, these low fluorides would reform into SF6 and do not affect the performance of equipment. It is inevitable that there exists less impurities such as air or water in the SF6 due to all kinds of reasons. The impurities react with decomposed items and more complex products generate, such as SOF2, SO2F2, SOF4, SO2, CF4, CO2, HF and H2S. Among all these products, some will remain in the SF6 for a long time, reducing the electro-negativity, while others dissolve in water and turn into acidic materials, which corrode internal components and damage inner solid insulation. The industry standard - Quality Standard for Sulphur Hexafluoride of Transformer in Service (DL/T 941-2005) stipulates that air (N2 and O2) quality for 110kV and above running equipment shall be less than 0.2%.