Reduction in Transformer's dissolved gases

[adeschamps]

Hello everyone.

We have a new 70MVA (34.5/1.0kV)transformer suppliying an EAF (Electric Arc Furnace). We took 2 oil samples within 4 days and I don't really understand the results:

GAS-----Feb-08-------Feb-12
H2-------3.79---------2.36
O2-------4,636--------2,492
N2-------14,704------8,123
CH4-------1.30--------0.75
CO--------4.41--------2.19
CO2-------83.05-------27.02
C2H4------0.13---------0.0
C2H6-------0.0--------0.13
C2H2-------0.19-------0.0
TGCD-------9.82-------5.43

None of the values is dangerous but I don't know how did the ammount of gases went down. We didn't do any degassing.
This got me thinking if the DGAs were perfomed correctly.
Is there any way a tranformer could lower it's dissolved gases by itself in such short time?

Thank you.

 

[ScottyUK]

Differences in sampling technique, difference in time between sample being taken until analysis, different temperature at time of sample....

 

[prc]

The variations noted is within normal measurement band. If the sampling is not proper, values can differ.

 

[itsmoked]

You mention 'new'. The first sample wasn't a cold or just energized transformer was it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marmite]

The solubilities of the fault gases in mineral oil as well as their temperature dependence are also important factors for consideration in fault gas analysis. There are two orders of magnitude difference between the least soluble (hydrogen) and the most soluble (acetylene) gas. The majority of gases that are indicative of faults are also those that are in general the more soluble in the oil. When rates of gas generation are being followed it is important to take into account the solubilities of these gases as a function of the oil temperature. Over a temperature range of 0 to 80C some gases increase in solubility up to 79% while others decrease their solubility up to 66%.
Regards
Marmite