Status assessment of transformer windings via on-line stray inductance measurement

Power transformer windings must be able to withstand the electrodynamic stresses caused by external short-circuits. One technique for detecting permanent deformations in winding geometry (and any related conductor insulation damage) due to these effects consists in using the transformer stray inductance as a diagnostic sensor. Should the variations of this inductance be more than 5 or 6 percent (compared to the rated value), the transformer is considered faulty. Measurements of stray inductance are normally performed by means of a specially devised Maxwell bridge, but only in out-of-service-condition. A voltamperometric method has recently been suggested for on-line monitoring this inductance with precision at least of the same order via an intelligent system, thus permitting the real-time diagnosis, without service interruptions, of the state of the transformer windings and immediate shutdown when necessary. The method employs a data acquisition system with high-speed simultaneous sampling on two or more channels, depending on the transformer phases. Further positive tests were performed on a 250 MVA three-phase autotransformer. This paper presents an ultimate improvement of the method that, by means of a special procedure, allows to obtain automatically a correct adjustment of the system without the need to set the transformer in no-lend condition. So, the proposed technique proves suitable for inclusion in a new and more general on-line control procedure for power transformer protection and service management