Abnormal Tan Delta Value of new 160MVA Auto Transformer during Pre-commissioning check. After release from the factory and after receipt at site, the Tan Delta value as obtained were of very abnormal.
Observation
On receipt of the auto transformer, assembly was done at site.
The oil filtration and other commissioning works were also done.
The LV tests like Voltage ratio, magnetic Balance, Excitation current and Loading stability tests were done and found in order.
The winding resistance, Turn’s ratio tests etc.. were also done at site and results were found in order.
Action Analysis
Connections | Mode | KV Applied | mA | Watt loss | Tan Delta | Capacitance in pF |
CH+CHL | GND | 10 | 40.78 | 2.163 | 0.533 | 13031 |
CH | GAR | 10 | 19.88 | 13.01 | 6.582 | 6334 |
CHL | UST | 10 | 20.86 | 8.933 | 4.276 | 7335 |
CL+CHL | GND | 10 | 69.40 | 27.56 | 3.985 | 22124 |
CL | GAR | 10 | 48.96 | 27.61 | 5.685 | 15570 |
While doing tan-delta tests, the results became irregular and abnormal.
So, suspecting problems in the testing kit, other test kits were brought for comparison of the test results and found with similar kind of irregular and abnormal value.
The results obtained by different kits are shown in the table no-1 below.
The results as obtained conclude about some issue in the transformer, not in the KITs.
But some problems in the transformers are not being accepted as no other test results are supporting to this tan delta abnormal value.
So experts were referred about such abnormality.
On visit the site and in the presence of experts and engineers from OEM, the test results were also taken for the tan delta kit. This time also the results were obtained of similar value as before.
This time it was suspected on the earthing of the transformers and especially on the CORE-BODY-YOKE connections.
The commissioning team was asked to produce the CORE-BODY-YOKE IR measurement value that has been taken during pre-installation condition.
The team explained that this value had not been taken during installation time.
The same has not even been asked by the testing team or the OEM requirement.
Now, the TOP COVER BOX was opened for the access of this link connection between CORE-BODY-YOKE.
It was found with looseness of the all the links on this box.
The required links were tightened properly and then the tan delta measurement was taken and found with the following results as mentioned in table-3.
Analysis of the results
Connections | Mode | KV Applied | mA | Watt loss | Tan Delta | Capacitance in pF |
CH+CHL | GND | 10 | 41.06 | 1.188 | 0.290 | 13080 |
CH | GAR | 10 | 20.95 | 0.788 | 0.372 | 6674 |
CHL | UST | 10 | 20.12 | 0.416 | 0.207 | 6411 |
CL+CHL | GND | 10 | 70.89 | 2.399 | 0.340 | 22661 |
CL | GAR | 10 | 51.03 | 1.961 | 0.384 | 16251 |
Due to looseness of the links between the CORE-BODY-YOKE, during the application of the HV AC supply of 10 KV to the specimen, it was found with contribution of Earthing for the measurement got changed.
The mode selected as GAR, GND and UST are not being properly maintained during the testing methods, so results as supposed to be obtained in the TAN Delta value are not normal as obtained before the measurement practices.
But after tightening the required modes got satisfied of producing the correct value as obtained in table-3.
Recommendations
Actual cause, as obtained reg looseness of CORE-BODY-YOKE was analysed, rectified and result found OK .
This is always recommended for keeping practice for measurement of CORE-BODY-YOKE, IR value before taking any installation of Transformer at site. The value should be above minimum of 25 Mohm. This value changes and kept as reference for different type of manufacturers