power transformer megger issues 2

[vipineng]

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.

 

[waross]

What do you mean by the frame?
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
--------------------
"Why not the best?"
Jimmy Carter

 

[mgtrp]

Presumably it means that:
- 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

 

[prc]

Since it is a 7.5 MVA unit,normally it will not give any problem. Circulating currents can happen in large units with high leakage flux, leading to thermal gases +acetylene in DGA. Some times it can even melt metal at joints in current paths.

 

[vipineng]

@Prc,
Please explain the circulating current effects in small and large transformers.my doubt is is it depends up on MVA or Voltage class?..

 

[spects]

First we need to know what you mean between frame and tank. Most of the time the tank is bolted to the frame and therefore you would get zero IR . Your IR of your windings to Transformer Tank should be within manufacture spec above 100meg ohms.

 

[oldfieldguy]

If by 'frame' you mean the core of the transformer, you should find the core grounded to the tank. On many transformers this ground connection is made to be accessible by opening manholes at the top fo the transformer.

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

 

[vipineng]

By frame,I meant the core clamp.normally the core-transformer body,core-core clamp,core clamp -transformer body is seperated.by insulation to avoid circulating currents.so there will be IR values.is there any difference in small rating(in terms of MVA) and large rating transformers in terms of circulating currents and its effects???...

 

[prc]

1) Many manufacturers do not provide isolation between core frame and tank for transformers with this range of MVA ratings.( up to 50MVA)
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.

 

[vipineng]

Thanks @prc

But can u please explain why the leakage flux effect is not in the bottom point(the bottom) as you explained.