Detection of insulation faults on disc-type winding transformers by means of leakage flux analysis

Power transformers figure amongst the most costly pieces of equipment used in electrical systems. Major research effort has therefore 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, this paper will present on-line analysis of transformer leakage flux as an efficient alternative procedure for assessing machine integrity and detecting the presence of insulating failures during their earliest stages. A 12kVA 400V/400V 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, simple sensors, based on air core coils, were built in order to measure the leakage flux of the transformer, and non-destructive tests were also applied to the machine in order to analyse pre- and post-failure voltages induced in the coils. Results point to the ability to detect very early stages of failure, as well as of locating the position of the shorted turn in the transformer windings.