Effect of Defects on Electric Field Distribution in Optical Fiber Insulator by FEM Analysis

Optical fiber current transformer (OFCT) is widely used in flexible DC transmission to improve the ability of information perception of power grid, and optical fiber insulator is the key component of OFCT to provide insulation and protection. However, discharges in optical fiber insulator seriously affect the stable operation of the power system. Here, the effect of defects including bubbles and cavities on the electric field distribution in insulator is explored to figure out the inner discharge mechanism. A ± 400 kV optical fiber insulator 3D simulation model is constructed in COMSOL Multiphysics, and differences in the electric field distribution in the insulator are compared and analyzed by finite element method under different defects. The results show that the bubbles do not have a significant effect on the electric field distribution. However, when there is a cavity, the inherent electric field distortion on the fiber sheath due to the mismatched potential caused by the semi-conductive fiber coating layer will extend into the cavity. The electric field strength at the interface between the cavity and the fiber sheath is up to 6 kV/mm, which exceeds the breakdown strength of the air. Therefore, the existence of the cavity causes the air discharge on the surface of the optical fiber sheath, which can make damage to the optical fiber sheath and leads to insulation failure.