low IR of Power transformer

[emadshaaban1]

hello for all
Our step up power Transformer 11kv\20kv 35M va was defected due to low winding Insulation resistance
Now we are swapping a spare transformer to be fixed , but while testing the insulation resistance of 20kv calble it gives:
L1 24 G-ohm
L2 31 G-ohm
L3 1.6 G-ohm
Does it mean cable is not ok?

 

[waross]

The surface near the test connection point may be dirty or damp.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DTR2011]

In my experience an oil filled transformer generally has IR values in very high Meg/Gig or Terra Ohms. Was
this the only test conducted to condemn the transformer? Generally something like a Power Factor / Dissipation Factor / Tan Delta
test would be conducted to confirm poor IR readings. I am assuming Oil Filled as it was mentioned 35 MVA. If this is a dry transformer, that is another story.

 

[waross]

but while testing the insulation resistance of 20kv calble it gives:

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Do you have the original cable test results? Are they good?

Surface contamination would be my first guess. If it's a heatshrink or coldshrink type the first easy thing to try is gently wipe the termination with a clean rag soaked in alcohol. Wipe from the lug toward the sheath. Re-test.

After that you may need to consider re-termination. Depending on joint type, a good jointer may be able to strip the existing joint without damaging the cable so you don't lose length.

 

[emadshaaban1]

Thank every body for your help
We've check the cable lugs but there are no sighn of any damage .
Does the cable of 1.6G ohm for 20kv is acceptable or not?

 

[Parchie]

@emadshaaban1,
20kV cables I have I tested and I got 2 Gohms!

AFAIK, insulation testing (Megger) is just one of the tests avaliable. The best way of knowing is for you to refer to the cable manufacturer/vendor. Please call them to be sure.

 

[ScottyUK]

It's not the actual value of the cable IR test which is of concern, but the spread of values.

 

[prc]

No one should replace a transformer only because of poor IR value of transformer. Many years back we were getting transformer oils with poor resistivity as the oil was made by acid cleaning that was not removing polar contents from oil. Those days IR of less than 10 meghoms were common for even for large HV transformers. Today, oil is made by hydrocracking which removes all polar components (oil color almost like water not mild yellow as earlier) These oils have very high resistivity and transformer IR runs above 20,000 meghoms.It does not mean transformer cellulose insulation have become better or drier !

 

[emadshaaban1]

dear (prc);
they reason of replacing transformer is because of Differential protection trip and the insulation value was 200 kohm,
but now when we disconnect the defected transformer the disconnected cable insulation value was :
L1 24 G-ohm
L2 31 G-ohm
L3 1.6 G-ohm

before swapping the new transformer we are wondering about the statues of 20kv cable