Study On Corona Ring Design Of Composite Insulator For Ultra High Voltage DC Transmission Lines

Due to the geometry configure of composite insulator, the two metal electrodes bring high electric field area near the ends of the insulator. With the increase of the potential levers and the length of insulator, the electric field becomes higher and higher near the ends. High electric field will cause partial discharge on the insulation material and the surface of the metal electrodes, and bring premature aging of the insulator. So the high electric field is a dangerous enemy to the safe operation of the power systems. The first±800kV ultra-high-voltage DC transmission line will be built between Yunnan and Guangdong in China, the potential lever is higher and the uniformity of the potential distribution is worse, so the electric field is much higher than the existing lines. So how to improve the electric field distribution is an important question on the production of the UHV equipment and the safe operation of the power systems.Based on the existing calculation methods of electric field, the paper presents a new calculation method for unbounded field problem concluding complex parts by introducing the domain-decomposition and iterative processes depending on finite element method and the asymptotic boundary condition. The tower-conductor-insulator system in UHV transmission lines is decomposed to two area by the new calculation method. One is the concerned domain with finer meshes composed by the composite insulator and area nearby, the other is the unbounded space domain with coarser meshes composed by the tower, conductor and area outside. The concerned domain and the unbounded space domain are calculated separately by the iterative process on the common domain until the result become stable. This method uses less time to calculation the complex field model and has more accuracy on the concerned domain. It also makes the asymptotic boundary condition to get more accuracy by a further boundary without too much computer time. The calculation method is validated by the compare of experimental result and computed result.The paper builds a 3D computed model of 800kV composite insulator with tower,conductors and shielding wires based on the domain-decomposition and iterative method. The distribution of the potential and electric field distribution on the surface of the composite insulator is studied. The effect of the corona ring for improving the electric field distribution on the composite insulator is studied by changing the parameters of the corona ring, and the proper parameters of the ring semi-diameter, semi-diameter of the ring tube, and the position of the ring in its vertical plane are investigated on the consideration of controlling electric field that cannot exceed the corona inception level. To improve the high electric field on the two ends of composite insulator on±800kV UHV DC transmission line, the paper presents a new configuration ring consisted of a big ring and a small ring on each end of the insulator. The position of the small ring is between the ordinary big ring and the end fitting of the insulator, and it could improve the high electric field near the end of the insulator. An optimization size of the rings is calculated based on a mass of computations and studies. It also offers some help for external insulation design of the UHV transmission lines.