At 220/66kV substation, for 220/66 KV 100MVA Transformer, it was found with oil leakage from the Tan-delta point of incomer 220 KV CT (Current Transformer). Then the oil level was checked and found with low level. On filling the oil, the IR value was measured from Primary point to Ground, but found with ZERO-ohm resistance
Action Taken
- The Oil leakage on CT was found at the Tan-Delta point.
- As suspected that this had been resulted during Tan-Delta measurement by the testing wing and loosened during that point.
- So, the tightness was attended for arresting of the oil leakage.
- At the same time, oil level was also checked and found with less than that of the limit. So, required level was filled and tested with the Primary to Earth megger value.
- The result was found with ZERO value.
- ZERO resistance of the running CT was suspected with the process of taking Insulation resistance measurement.
- So, the grid personnel were inquired about the measurement process.
- The actual mistake point was revealed on inquiring that the grid personnel had taken the measurement, keeping Transformer winding in the system, without removing of the NEUTRAL EARTH.
- On knowing the mistake, the NEUTRAL EARTH of transformer was removed and IR value taken thereafter.
- This time the actual megger value came above the allowable limit.
11. The Transformer was charged along with this CT and stood OK
Technical Action
On first instance and hurriedly without thinking about the transformer neutral earth link in the system, the grid operator took the insulation resistance measurement by the available motorized 5KV IR tester. ( Refer fig).
Because of transformer neutral earth link in the system, the IR measurement came ZERO.
NOTE: This Study seems to be very silly, but sometimes operator forget to remove the Earthing Point during IR measurement. Not only from transformer, sometimes Grid operators do the mistake of removal of EARTH LINK from the PT system.