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Frequently Asked Questions (FAQ)

2. Poor oil dielectric

a low value of the dielectric strength of the oil Ud (kV/ 2.5 mm), measured in accordance with e.g. IEC 60 156 , can be caused by:

Moreover - please never forget that the
Ud-value strongly and inevitably varies with the transformer temperature.

The solution of the Ud-problem

If properly recognized, the problem of particles in the oil can be mostly solved by the on-line filtration of the oil inventory of the transformer under normal operational conditions - See VS-06.

The problem of the low Ud-value of oil induced by the high relative humidity is a lot more difficult, because the relative humidity of the oil is generally the function of two variables:

It means, for example that for the same water content in the oil, we get considerably higher Ud-value by the higher temperature - for more detailed description of the relation between the Ud-value and the relative humidity. See \ News \ The dielectric strenght of transformer oil.

Moreover, the insulant system of the transformer consists of the oil and the cellulose , and the water content in the oil is therefore the function of another two variables:

At low transformer temperatures the water remains in the celllulose, the relative humidity of the oil is low and therefore we always get acceptable Ud-values. On the contrary, the increase of the transformer temperature always induces strong migration of the water from the cellulose in the oil, the oil humidity grows and the Ud-value strongly and inevitably sinks.

To avoid human errors in the prediction of the safety operational limits of the transformer ARS-Altmann released the software pack TRACONAL. The safety limits of a transformer are there determined there by so called Temperature Loading Curve (TLC) - See TRACONAL procedure D, which predicts (for the known water content in the cellulose), Ud-values for the whole range of operational temperatures of a transformer and this way enables to determine the critical operational area of the transformer.

For a more detailed description and the understanding of the physics behind the Ud-value and the TLC - See \ News \ The dielectric strenght of transformer oil


The solution of the poor dielectric of a transformer related to the low dielectric strength of the oil: