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Sorted by date: 2004

  • 2004

    • English The Oil-Moisture Diagnostic Problem of Aged Transformers

      (Papers) 2004-04-26 13:18:40 by Altmann

      Summary: The present indirect diagnostic method of taking an oil sample for the estimation of water contamination of power transformers is based on the parallel measurement of two values - water content in the oil by the Karl Fisher method C w,KF (ppm) and the average temperature of the transformer oil cellulose system T(C). This method can sometimes lead to inaccurate, contradictory and quite curious diagnostic results. An example of a diagnostic discrepancy, is when the aged oil in a transformer is replaced by new oil. Following the oil change, the C w,KF value is always higher in the aged oil compared to the new oil at the same transformer temperature. This discrepancy is hard to accept and understand. Let's say, we have a transformer with 10,000 kg oil and 1,000 kg of cellulose. Before and after the oil change the transformer is operated at the same temperature (T= 50C). The water in oil results are (C w,KF) 30 ppm in the aged oil and 20 ppm in the new oil. According to a Nielsen Equilibrium chart ([L2] - Fabre-Pichon Diagram), before the oil change the water content in the cellulose (Cp) is 4.3 %, or 43 kg (1000 x 0.043). However using the same chart, the new oil value gives a water content of 3.2 % or 32 kg of water ? With the aged oil a maximum of (10,000 x 0.000030) 0.3 kg of water will be in the oil, and 0.2kg for the new oil. Why is there a variance of 11kg of water, and how can the water disappear just by changing the oil ?

    • English Impact of Vacuum Treatment on DGA

      (Papers) 2004-04-26 13:14:39 by Altmann

      Summary: State of the art for the diagnosis of undesired changes in the Insulating Systems of oil-immersed transformers is DGA - Dissolved Gas in oil Analysis [L1]. By the detecting variations of the individual gases over longer periods, it is possible to estimate the type, the size and in certain cases the localisation of the damage. The introduction of new vacuum treatment methods for transformers has raised worries weather results of DGA would not be deteriorated or changed in an unwanted manner