A new on-line method based on leakage flux analysis for the early detection and location of insulating failures in power transformers

Power transformers are among the most expensive pieces of equipment used in electrical systems. For this reason, major research effort has focused on detecting failures of their insulating systems prior to unexpected machine outage. Although several industrial methods exist for the on-line and off-line monitoring of power transformers, all of them are expensive and complex, and require the use of specific electronic instrumentation. For these reasons, in this paper the on-line analysis of transformer leakage flux is presented as an alternative and efficient procedure for assessing machine integrity and detecting the presence of insulating failures during their earliest stages. A 12 kVA 400 V/400 V power transformer was specifically manufactured for the study. A finite element model of the machine was designed to obtain the transient distribution of leakage flux lines in the machine's transversal section under normal operating conditions and when shorted turns are intentionally produced. Very cheap and simple sensors, based on air core coils, were built in order to measure the leakage flux of the transformer, and nondestructive tests were also applied to the machine in order to analyse voltages induced in the coils before and after the failure. The obtained results revealed the possibility of detecting very early stages of failure, as well as of locating the position of the shorted turn in the transformer windings.