Method of Determining Compression Set for GIS Seal Ring (O-ring) Based on Directional Section Diameter

Gas-insulated substation (GIS) is rapidly popularized in the construction of large hydropower projects and urban highvoltage power grids. Pressured insulating gas sulfur hexafluoride (SF6) in the GIS is sealed by the rubber O-rings, namely GIS seal ring, and the lifetime prediction method for GIS seal ring has attracted attention of experts and scholars. Compression set of O-ring is one of the most important indices to determine the lifetime while the key is to accurately measure the cross-section diameter. In this paper, compression set for standard cylindrical specimen was reviewed, which was proposed for testing the vulcanized rubber material. The rubber thickness gauge was an instrument specially invented for testing dimension of rubber specimens. Then the characteristic of the cross-section of used GIS seal ring was analyzed. The cross-section has good axis symmetry, but the inclination of the axis challenges the measurement. To solve the problem, test method of determining compression set for O-rings based on the directional section diameter (DSD) was proposed. Given a direction k, the k-DSD is defined by the thickness of specimen measured in direction k, denotes $\boldsymbol{d}_{\mathbf{k}}$ . The $\boldsymbol{d}_{\mathbf{v}}$ and $\boldsymbol{d}_{\mathbf{p}}$ are two typical DSDs, that $\boldsymbol{d}_{\mathbf{v}}$ described thickness in vertical direction when placing the O-ring specimens on a flat plate in natural state, while $\boldsymbol{d}_{\mathbf{p}}$ is the shortest DSD. Finally, this paper conducted experiments on measuring $\boldsymbol{d}_{2}, \boldsymbol{d}_{\mathbf{v}}, \boldsymbol{d}_{\mathbf{p}}$ of new and used 252kV GIS seal ring samples by rubber thickness gauge and Vernier caliper. Statistical analysis of the data implies better precision and accuracy when measuring $\boldsymbol{d}_{\mathbf{p}}$ by a rubber thickness gauge. This paper provides reference for the lifetime prediction method for GIS seal ring.