Abstract
The electric field distribution at the end of a large hydro-generator is highly nonuniform and prone to corona discharge, which damages the main insulation and significantly reduces the service life of the hydro-generator. In order to reduce the thickness of the main insulation and the physical size of a large hydro-generator, it is necessary to understand the distribution of the electric field at the end of its stator bar. In this paper, the stator bar at the end of a large generator is simulated using the finite element method to determine the distribution of the potential, electric field, and loss at the rated voltage, as well as to elucidate the differences between the linear corona protection, two-segment nonlinear corona protection, and three-segment nonlinear corona protection structures. The influences of the arc angle, length of each corona protection layer, intrinsic resistivity of the corona protection material, and nonlinear coefficient are also analyzed. The results manifest that the angle of the stator bar should be 22.5°, the difference in resistivity between the two adjacent corona protection coatings should not exceed two orders of magnitude, and the resistivity of the medium resistivity layer should be nearly 106 Ω·m or 107 Ω·m, for an optimal design of the corona protection structure.
Highlights
In large hydro-generators, the area of the partial discharge (PD), sometimes known as corona, is mostly concentrated at the end of the stator winding/bar [1,2]
In the stator core slot area of the bar, and for a few centimeters outside of it, the coating is usually a graphite-loaded paint or tape that is referred to as semi-conductive coating. It is called the outer corona protection (OCP), which prevents the build-up of any voltage between the surface of the stator bar and the stator core, and prevents surface PD
In the 1970s, the resistance capacitance link model was widely applied in analyses of the corona protection structure at the end of the stator bar to rapidly find a suitable solution without considering the state of the space electric field
Summary
In large hydro-generators, the area of the partial discharge (PD), sometimes known as corona, is mostly concentrated at the end of the stator winding/bar [1,2]. In the 1970s, the resistance capacitance link model was widely applied in analyses of the corona protection structure at the end of the stator bar to rapidly find a suitable solution without considering the state of the space electric field. In 2014, Zhang et al [16] applied the finite element and multi field modeling methods to calculate the electric field and loss density distribution of the corona protection layer at the end of the stator bar in a large generator. Sun [17] improved the traditional finite element analysis method, set the electric field control equation in the corona protection coating boundary, effectively combined the resistance-capacitance chain method and the finite element method, and addressed the error in the process of transforming surface resistivity to volume resistivity This method is efficient and accurate and satisfactorily analyzes the electric field at the end of the stator bar.
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