power transformer megger issues
power transformer megger issues
(OP)
I am having an issue with megger values between frame and transformer tank.the values are coming zero.the rating of the transformer is 7.5 MVA,132/11 kv.
The transformer is in nitrogen filled condition.
Please tell me the reasons and the after effects due to this.
The transformer is in nitrogen filled condition.
Please tell me the reasons and the after effects due to this.






RE: power transformer megger issues
Possibly the frame and the tank are bonded together and your reading is indicating that the bond is properly installed. A ductor may be a more appropriate instrument for this test.
Bill
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"Why not the best?"
Jimmy Carter
RE: power transformer megger issues
- The frame and tank are deliberately internally bonded
- You haven't disconnected an external bond somewhere
- There has been a breakdown of the frame-tank insulation.
If it's an actual breakdown, you can have issues with circulating current (this is the reason for bonding at a single point only), and it can pollute your transformer DGA results.
Cheers,
mgtrp
RE: power transformer megger issues
RE: power transformer megger issues
Please explain the circulating current effects in small and large transformers.my doubt is is it depends up on MVA or Voltage class?..
RE: power transformer megger issues
RE: power transformer megger issues
This is so that you may intentionally remove the intentional ground so you can use a megger to check for inadvertent ground paths, which, as mentioned above, can result in circulating currents which cause core heating and corresponding problems in the transformer.
Finding that you have these inadvertent paths is cause of the transformer to be removed from service for remediation. I have heard of people using welding machines or other high-current sources to attempt to 'burn' the inadvertent ground away, but seriously, the work needs to be done properly and usually involves some serious work, including draining and de-tanking.
old field guy
RE: power transformer megger issues
RE: power transformer megger issues
2)The circulating current from double grounding of core frame occurs in large transformers. It depends not on kV rating, but kVA rating (to be exact kVA x% impedance).The leakage flux from windings impinge on metal frame, inducing voltages that can cause a circulating current, heating the metal. This current will be zero at no-load and maximum at full load.
3) Even in large transformers, such double grounding may not cause problem always. Nearly 25 years back, I came across such a grounding in a 600 MVA GSU bank. Customer noticed it during annual maintenance shut down. Since I was almost sure that the insulation breakdown was at tank bottom where the core frame is anchored to tank( where leakage flux cannot reach) I did not suggest rectification ( that required costly transport to factory).The unit is still going strong, but I was not that lucky all the time.
RE: power transformer megger issues
But can u please explain why the leakage flux effect is not in the bottom point(the bottom) as you explained.