The effect of aging of surface non-metallic coatings on the ampacity of medium voltage rectangular bus bars

Abstract

There are two major types of non-metallic coatings for bus bars: oil-and epoxy-based coatings. The oil-based coatings have a limited lifespan of a few years, while the epoxy-based ones have a lifespan of over ten years. In contrast to the lifespans of indoor substations and components in them, the lifespans of coatings on bus bars are significantly shorter. Accordingly, it is obvious that the surface coating aging can affect the ampacity of rectangular or other shaped bus bars. The extent to which this effect can reduce the ampacity is analysed in this paper. An indoor substation, in which medium voltage (MV) rectangular bus bars of the same type can be found inside and outside the substation building, is considered as a case study. In order to simulate the aging process of the surface coating, it is assumed that the surface radiation properties (thermal emissivity and solar absorptivity) gradually change in the following manner: a new paint/coating layer, a layer of dirt over oxidised paint/coating, and a layer consisting of heavily-oxidised paint/coating, heavily-oxidised metal and dirt under both indoor and outdoor conditions. The results obtained for indoor and outdoor conditions are compared against one another in order to quantify the effect of solar irradiation. In addition, each non-metallic layer over the bus bars is considered as additional thermal insulation. The aging process is analysed analytically, and then validated numerically by finite element method (FEM) in COMSOL. Finally, the discussion yielded some important conclusions relevant for practice.