SF6 decomposition of typical CT defect models

Detection of decomposition products is a newly-emerging method for the condition diagnosis of SF6-insulated equipment. Based on the two grading structures that are commonly adopted in SF6-insulated current transformers (CT), three models consisting of the needle-to-plane, the ball-to-plane, and the plane-to-plane were established. Gas decomposition was investigated in a 90 L discharge chamber under operating conditions. Products were analyzed with gas chromatography (GC) and X-ray photoelectron spectroscopy (XPS). Typical gas decomposition products are CF4, SO2F2, CS2, SO2, and S2OF10. For the needle-to-plane with a gap of 30 mm, near-linear growths with time are observed in the concentrations of SO2F2, SO2, and S2OF10 when the applied voltage is set as 33 kV. Addition of the adsorbent suppresses the concentrations of all three products, especially SO2. Besides, CF4 and CS2 are also found to be the characteristic components of partial discharge involving polyester films. And their concentrations are higher when the electric field lines are parallel to the surface of the polyester. As for the polyester films, the atomic ratio of C decreases, the atomic ratio of S increases, and the atomic ratio of F increases significantly as the deterioration becomes more serious. Moreover, the concentration ratio of S2OF10/SO2F2 in the needle-to-plane is mostly within the range of 0- 2, whether with the addition of adsorbent or not. And the ratio is much higher (2-10) when polyester is involved in the discharge process. It could be used to diagnose the insulation condition of CT effectively.