Monitoring of power transformer bushings in high voltage substations

Power transformer bushings are among the least reliable components in high voltage systems, therefore the knowledge of their wear condition is very important for a proper network management. The aim of this work is to study the limits of applicability of an on-line monitoring method for transformer bushings previously presented by the authors. The technique allows to continuously estimate the variation of the bushing capacitance and its dissipation factor, without requiring to disconnect the bushing itself from the network. The method is based on a high frequency impedance measurement at the bushing tap. It is clear that its value is not only related to the bushing, but it also depends on the other substation devices. For this reason, it is vital to discriminate the variations of the parameters due to the bushing from those due to the rest of the components. In order to understand these dependencies and to evaluate the applicability of the monitoring method, a high-frequency equivalent model of the substation is required. In this work, a typical substation has been modeled considering a frequency around 1 MHz. Through proper simulations, the applicability of the method has been investigated, in particular the impact of the high voltage transmission lines has been studied.