Introduction
Power transformers are key elements in the electrical grid. If a power transformer fails, the financial impact of the outage time is in most cases considerably larger than the damage on the transformer itself. Due to the fact that the transformer fleets are getting older, the condition assessment and the appropriate maintenance strategy of power transformers is getting more and more important.
To assess the condition of a transformer, various electrical measurement methods can be used. But a large number of diagnostic measurements are affected by residual magnetism. Therefore, it is difficult to analyse a reliable condition assessment of transformers. Residual magnetism can also have an impact on the inrush current. A too high inrush current can reduce the lifetime of a transformer due to the mechanical forces on the insulating paper. It is therefore recommended to demagnetise the transformer before performing diagnostic measurements and re-energising it.