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Frequently Asked Questions (FAQ)

3. High water content in the oil

Please never forget
the water content in transformer oil always strongly varies with the transformer temperature
a moisture diagnostic based on water-in-oil only
judge a „cold“ transformer (erroneously) as „dry“ and a „hot“ transformer (erroneously again) as „wet“ !!!

any proper evaluation of the water contamination of a transformer should be described by a temperature invariant value - it means by the water content in the cellulose ,
single readings of the water content in the oil regardless of
a transformer temperature is, from the point of proper diagnostics, mostly misleading !

Moreover, the high water content in the oil can be caused by measuring errors.

On-line measuring methods:

Off-line measuring of the water content in the cellulose e.g. by the RVM or PDC method, are extremely time-consuming (transformer has to be long-term disconnected ) and has , with the exception of the FDC method, a low precision and repeatability.

Generally, a high water content in the cellulose implicates either a poor maintenance or design errors:

The recommended solution of the problem:

1) proper measuring and evaluation:

All mentioned measuring (virtual) errors can be very effectively suppressed by the application of the new SIMMS method because :

  • the measuring procedure a priori excludes any oil contamination – the connection lines are evacuated at first and the oil is never exposed to the atmosphere during the measuring procedure.
  • reading shows the water content in the bulk of liquid insulation - oil is continuously forced from the transformer through the SIMMS and returns to the transformer
  • the SIMMS is during the reading hermetically connected to the transformer - the air entry while negative pressure in the transformer is therefore impossible
  • the measuring distortion due acids is excluded – the capacity sonde reads only the real humidity of the oil , the reading is therefore independent from the actual content of acids in the oil - for the experimental verification of the last statement See \News\ The Oil-Moisture Diagnostic problem of aged transformers.
  • The evaluation of equlibrium conditions is based on long-term measuring of the oil humidity and the two temperatures of the transformer and its corresponding data set is in-situ evaluated by the lap-top.

Any proper conclusion
about the water contamination of a given transformer
must be based
on the averaged value of the water content in the cellulose !

2) The transformer dehydration

See FAQ – What kind of transformer dehydration should be used ?

3) The preventative maintenance


Under normal operational conditions only an effective hermetization of the transformer quarantees the long-term suppression of external and internal water sources in a transformer.

The reason is simple - only the effective long-term suppression of the entry of O2 and H2O into a protected oil-cellulose system effectively retards the accumulation of the water in the cellulose and stops the internal production of the water in the transformer.