On-line analysis of power transformer bushings

The approach to the maintenance of devices and systems has strongly changed in the last years, thanks to the availability of more and more sophisticated technologies. In particular, when the servicing involves very critical processes, a new interesting strategy is represented by the Condition Based Maintenance (CBM). It is based on the continuous observation of some physical parameters which permits to infer the status of the system and thus to detect the need to service it. In this paper a novel monitoring method is proposed. It allows the application of CBM techniques to one of the least reliable components in HV and MV systems: the transformer bushing. Starting from some theoretical and practical considerations, it will be shown that it is possible to measure the capacitance and the power factor of transformer bushing, without putting the plant out of order. It is known that these two parameters are particularly useful to schedule the maintenance activity. This goal can be achieved by superimposing a high frequency sinusoidal voltage between the bushing tap and the ground. Some considerations prove that having chosen a proper frequency, the consequent current is much more sensitive to the bushing capacitance than to the other impedances. Finally the architecture of a measurement system is also presented.