CORE BALANCE READINGS OF A TRANSFORMER

[appunni]

Sir,
One of our testing team tested a transformer of which some name plate details of the transformer is as given below:
16/20MVA, 66/33KV, 3phase, imp.volgae:10.11% at working tap(13).
When core balance test was conducted with a 230V single phas supply on 66KV side, the following values were obtained.
applied voltage measured voltages

Van=236.3 V Vbn=211.6V;Vcn=23.4V
Vbn=234.8V Van=144.1V;Vcn=88.1V
Vcn=238.2V Van=37.6V;Vbn=199.7V
When the test was conducted from 33KV side the following values were obtained.
Van=235.9V Vbn=209.5V;Vcn=25.35V
Vbn=234.5V Van=146.5V;Vcn=86.9V
Vcn=238.2V Van=39.1V;Vbn=197.1V
We suspect that there is some fault in the transformer. I request for your valuable remarks about this and about whether any further tests/checkings are to be done in this regard. I request to explain the cause of the abnormality in the readings.
with regards,
appunni

 

[xabproject]

Dear Appunni,
1. Have you compared the values with Factory Test report to suspect that something is wrong (with your observations) OR is this a factory test itself? If it is a factory test just treat it as a bench mark if all other tests are okay.
2. Prima facie, nothing seems to be wrong with the readings.
However, can you check with the value of excitaion current that was flowing in each limb/phase when each limb/phase was excited with the 230V supply? The central limb/phase value should have a characteristic relationship with extreme limb/phase values.
If you have discrepant observations in current values also, kindly furnish the values for further investigation.
Hope this will be useful to yourselves.

 

[edison123]

appunni,

when you test phases individually, the voltage applied to middle phase sould divide almost equally between the extreme phases. your results seem to indicate core dislocation resulting in gaps at the lamination joints. to rule out interturn problems, i suggest surge comparison test on all three hv & lv phases. if it is ok, then you may have to inspect the core physically.

 

[gmazumder]

Is the transformer a new one or an old one, has the transformer been shifetd from some location to this location and how do the results compare with the factory tests results. What all tests have been conducted on this transformer and what is the sequence of the tests carried out.
The observations of the valued members are correct but i have commissioned transformers with far worse values on the center limb in magnetic balance. The need is to check the magnetising current values at low voltage from both the sides and also perform the single phase low voltage short circuit test on the transformer.

best wishes,
gmazumder

 

[jbartos]

Question: Have the transformer tests and measurements adhered to any industry standards?