Please be a bit clearer with your 1100.10 giga ohm.
If you mean 1100 Giga ohms or 1.1 Tera ohms, that is an excellent reading.
A transformer can be tested for its insulation resistance at the HT side, LT side and between HT and LT windings.
Generally these values depend upon the age, overloading, poorly ventilated transformer room.
Most 11 kV distribution transformers in New Zealand are tested at 2.5 kV dc at the primary and values of 500 Mega ohms to high 100's of Giga ohms can be measured. (100 kVA to 2 MVA)
You didn't say how heavily the transformer is overloaded in the last 2 or 3 years.
Has their any load survey been done on it?
Overload WILL destroy a transformer in a reasonable short time.
Transformer oil will gradually deteriorate and parts of varnish from the windings will discolour the oil and the acidity level will go up.
Deterioration of the paper between winding layers will occur too and add to the process.
If an arc occurs between windings and no Buchholtz* protection is in place the transformer will burn out.
The best practice is to get an oil sample taken from the bottom tap and analysed by an other oil lab and await their results.
I have done a fair amount of oil samples on 10 and 20 MVA 33/11kV substation transformers.
When taking oil samples it is very important to work clean !! First drain about 1/2 litre of oil from TX before taking sample. It should be done via a clean plastic hose into a clean type of glass seringe with T connector to only allowe oil and NO air in the sample. The air will add to possible poor readings. Also a 1/2 litre bottle sample is taken for analysis.
* Buchholtz protection is generally used in substation transformers and will pick up gasses caused by arcing in the TX. If these are small amounts they will be collected in a small reservoir up to a certain level 500 mL, then a trip will occur. If a large arc or explosion happens the Buchholtz relay will act immidiately and trip the primary and secondary of the TX to avoid backfeeding.
