The winding resistance Measurement of the Transformer most impotent test because
1. To find out the week part and break down in the winding
2. To find out the Bolt tightness and connection shoes gripping
Normally winding resistance measured between
1. Phase to Phase in Delta & Star connection
RY , YB, BR
2. Phase to neutral in Star connection
RN, YN, BN
This site test result will be compared with factory test result and the error should be less than ≤ 5%
Issue in site Test Result
Transformer Rating
67 MVA ,
115/13.8 kV
Dyn1 three winding Transformer
During site test the LV side winding resistance as below
As per site test result R phase winding resistance is high.
The result was sent to factory and factory suggested to clean the surface and meassure value again with 10 DC volt.
The Transformer winding resistance again measured with Omicron CPC 100 .
The inject current ratings of the kit are 2A, 6A, 400A.
So 6A was injected for measurement and the results are same.
Measured value 7.577 mΩ at 75 ⁰c and FAT value 6.874 mΩ at 75 ⁰c error value is -10% and it should be ≤ -5%. ( Test report attached )
Minimum current is enough for measure the resistance. 2A current was injected and result was same.
But minimum current will take more time to stable the reading (resistance value).
Problem Analysis and Rectification
Factory testing engineers came to site.
After complete all rectification process winding resistance was measured and result found was with in the limit. 5.533m ohms
1. To find out the week part and break down in the winding
2. To find out the Bolt tightness and connection shoes gripping
Normally winding resistance measured between
1. Phase to Phase in Delta & Star connection
RY , YB, BR
2. Phase to neutral in Star connection
RN, YN, BN
This site test result will be compared with factory test result and the error should be less than ≤ 5%
Issue in site Test Result
Transformer Rating
67 MVA ,
115/13.8 kV
Dyn1 three winding Transformer
During site test the LV side winding resistance as below
| PHASE | SITE RESULT AT 30⁰ C | SITE RESULT AT 75⁰ | FAT RESULT AT 75⁰ C | ERROR |
| RN | 6.599 mΩ | 7.577 mΩ | 6.872 mΩ | ≥ 10% |
| YN | 5.788 mΩ | 6.646 mΩ | 6.674 mΩ | ≤ 5% |
| BN | 5.752 mΩ | 6.604 mΩ | 6.6322 mΩ | ≤ 5% |
| RY | 12.18 mΩ | 13.99 mΩ | 13.36 mΩ | ≤ 5% |
| YB | 11.38 mΩ | 13.07 mΩ | 13.125 mΩ | ≥ 10% |
| BR | 12.19 mΩ | 13.99 mΩ | 13.323 mΩ | ≤ 5% |
As per site test result R phase winding resistance is high.
The result was sent to factory and factory suggested to clean the surface and meassure value again with 10 DC volt.
The Transformer winding resistance again measured with Omicron CPC 100 .
The inject current ratings of the kit are 2A, 6A, 400A.
So 6A was injected for measurement and the results are same.
Measured value 7.577 mΩ at 75 ⁰c and FAT value 6.874 mΩ at 75 ⁰c error value is -10% and it should be ≤ -5%. ( Test report attached )
Manufacturer suggested 10A dc but in equipment there is no option
for that value.
The winding resistance will not vary with injected current.
Because winding resistance is very low in terms of mΩ.
The winding resistance will not vary with injected current.
Because winding resistance is very low in terms of mΩ.
Minimum current is enough for measure the resistance. 2A current was injected and result was same.
But minimum current will take more time to stable the reading (resistance value).
Problem Analysis and Rectification
Factory testing engineers came to site.
Factory Action -1
Factory Action - 2
After complete all rectification process winding resistance was measured and result found was with in the limit. 5.533m ohms
