Diagnosing the insulation condition of dry type transformers using a multiple sensor partial-discharge-localization technique

In this paper, an electrical partial discharge (PD) measurement technique for a dry type power transformer is presented. The method has been tested in the laboratory as well as on a transformer in a test bay using an automated partial discharge evaluation based on the comparison of quasi-simultaneously detected PD patterns at different decoupling points. Therefore a multiple sensor system is placed on the surface of the transformer coil, thus PD signals can be measured at various sites along the winding before they are processed and compared. The described technique enables, besides a localization of the PD source, a determination of the apparent charge, which can be more accurate than conventional narrowband PD measurements via a quadripole in parallel. A discussion of the results leads therefore to a proposal about improvements concerning the calibration for actual PD routine tests on dry type transformers.