Absorption indeks of insulation at end of service life – Part II: Old-aged transformers equipped with silica gel air breather

Oil and AI It is well known that oil quality has a significant effect on the insulation resistance and on AI and PI values. The...

byVitaly Gurin


Oil and AI

It is well known that oil quality has a significant effect on the insulation resistance and on AI and PI values. The better the quality of the oil, the higher the insulation resistance, and more reliable the transformer. But modern transformer oils with low conductivity (including Russian GK oil) have AI and PI close to 1 (see the last paragraph of Annex 3). This means that for insulation of the transformer as a whole, the AI and PI values will be lower when filling with a higher quality oil than when filling with a lower quality oil. To quantify this reduction in the AI, the following experiment was performed at ZTZ. The Russian old TKp oil (obtained from the oil by acid-base treatment) and GK oil (obtained by the hydrocracking method) were alternately poured into a three-phase three-winding transformer. The comparison of the AI at a voltage of 2.5 kV is shown in Table 4. As follows from this table, GK oil increases the insulation resistance by approximately an order of magnitude, and on average halves the AI value.

 

Measured data indicate that old-aged transformers with a high moisture content have the AI value significantly dependent on the temperature.

 

It is noteworthy that when oil is filled with TKp, the AIs in the schemes LV – (HV + MV + GND) (1.96 and 2.06 at different temperatures) are less than in the MV – (HV + LV + GND) schemes (3.52 and 3.95). And on the contrary, when pouring the GK oil, AI in the LV – (HV + MV + GND) scheme is larger than in the MV – (HV + LV + GND) scheme (1.43 and 1.24, respectively). This can be explained as follows. It is known that when changing the oil in a solid insulation of a transformer, up to 10 % of the old oil remains. The solid insulation near the LV winding of our transformer has fewer oil channels, and TKp oil remaining in it is older than in the oil in the solid insulation near the MV winding, where there are fewer oil channels.

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