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Compared to bushings with lower voltage levels, the ±1100 kV converter valve‐side bushing has thicker radial insulation and longer axial insulation resulting in a more uneven temperature distribution and more severe electric field distortion, especially under high‐ambient‐temperature and high‐current conditions. Improving the distribution of the electro‐thermal coupled field of the bushing is critical for the safe operation of converter transformers. This study discusses the distribution of electro‐thermal coupled field of a typical bushing and proposes a new method to evaluate the conductivity of multiple capacitor layers under inhomogeneous axial temperatures. The effects of four bushing designs are investigated as per the adjustment of the capacitor screen structure, the replacement of the current‐carrying structure, and the modification of resin‐impregnated paper material. The results show that merely adjusting the capacitor screens is not feasible due to size limitations. By decreasing heat accumulation, adopting a single‐tube structure and increasing the thermal conductivity of resin‐impregnated paper material can reduce electric field distortion to a certain extent. Unlike the other three methods, hierarchically controlling the material resistance effectively balances the internal and external electric fields. Simulation results demonstrate the feasibility of these methods to improve the distribution of the electro‐thermal coupled field in the bushing. [ABSTRACT FROM AUTHOR]

The iso-margin method based on the classical analytical equations is widely used in the development of extra high voltage (EHV) bushing. Therefore, its effectiveness and feasibility have been fully validated. However, with the theoretical formula, it is difficult to evaluate the influence of stray capacitance and temperature on the internal electric stress of the ultra high voltage (UHV) bushing. It?s necessary to further improve the traditional iso-margin method. Firstly, the basic principle of iso-margin method has been briefly described. After that, the E-field distribution of condenser bushing was investigated on the Finite Element Method (FEM) computing platform. Then, the mathematical model of the advanced equal margin design method was established. Furthermore, this paper presents the program flow of optimization which combines the simulation of electro-thermal coupling process and the particle swarm optimization (PSO) algorithm. The proposed methodology was applied to the design of the prototype of the UHV resin impregnated paper (RIP) oil-SF6 bushing, which realizes the uniform axial E-field distribution of the bushing condenser. Meanwhile, the partial discharge margin between adjacent foils can be equal. Moreover, the hot-spot temperature is lower than the operation limiting temperature of the RIP material. The bushing condenser was fabricated according to the optimal structure design. The prototype of bushing has passed through all the type tests. In this paper, the FEM electro-thermal coupling simulation and the advanced equal margin design method were applied to the inner insulation structure optimization of the UHV RIP oil-SF6 bushing. Meanwhile, the proposed methodology provides some theoretical guidelines for the future research and development of other types of bushing on the UHV level. [ABSTRACT FROM AUTHOR]

Increased moisture content within an oil‐paper insulation system can significantly reduce its life expectancy, and for oil impregnated paper condenser type bushings, it is the most common cause of failure. Distinction between moisture contents using traditional power frequency tests of dissipation factor and capacitance can be difficult, particularly at ambient temperatures. Dielectric frequency response is becoming an established technique to measure the dissipation factor and capacitance of a bushing, but to date an accurate model to evaluate the condenser moisture content has not been presented using this technique. In this study, a dielectric frequency response bushing model is proposed and finite element method software is used to simulate the variation in dissipation factor and capacitance of a bushing with varying moisture content, as a function of frequency and temperature. The modelled results are compared with measurements reported in the literature and from the authors' own field measurements, where a good agreement is demonstrated. It is shown that the distinction in dissipation factor between moisture contents at frequencies <0.1 Hz is in excess of ten times greater than at 50 Hz. The proposed model can be used by practitioners to evaluate moisture content within the condenser insulation and distinguish between moisture contents ranging from 0.2 to 4.0%. [ABSTRACT FROM AUTHOR]