Transformer overheating (external) - Circulating Current? 5

[KnicksJets]

We have a GSU transformer that has been running for over a year now. There are two support bars holding a conservator tank over the main tank. These bars are between phase A and C (two bars are right above phase B) low voltage bushing box.
We recently noticed overheating in the bolts in these support bars, temperature as high as 320 deg F. We were told it could be because of the circulating current.

Any input? If it's true, can anybody explain the phenomenon?

 

[prc]

The shorting plate short the currents in three phases of outer case of IPB,making it in to zero.When it is broken, the current will try to reach the currents in other phases by moving around.It is not correct to use stainless steel bolts to avoid heating.with stainless steel bolts heating will be more as resistivity of stainless steel is more than mild steel.

 

[KnicksJets]

Gentlemen thanks again for the valuable input.
I will definitely post the update after we take any action. Our outage is on March 2009 and we are hoping we won’t run into any problem until then. We are simply taking thermal image of the hot spots on weekly basis for now.

Scotty, PRC is right, there are three separate LV box. The picture you saw was for the ‘A’ phase LV bushing box. The red material is a neoprine gasket providing insulation between the flanges. Please refer to the attachment where you will see ‘B’ and ‘C’ LV bushing box and one of the SS steel support for the conservator tank. Thanks to all the insights, and would like to hear more from you on the subject.

Wagross, an excellent find. This looks very very similar to our problem. We installed this transformer last outage, during which we had to manipulate the isophase bus box at the transformer end and the shorting plates. We re-weld the plates together w/o any problem. Please refer to the attachment that shows the plates location. My question is why didn’t they weld the original shorting plate, instead of making a new temporary shorting plate and welding them? Is that what they did?

PRC, great insight. So the shorting plates are to short the currents induced by the isophase conductor. I had contacted an isophase company and their suggestion was to use SS steel bolt (which you and the example given suggest won’t work), fiberglass washer and phenolic tubing. Most of the suggestions I am getting matches yours and Scotty’s. I am not familiar with frp (fiberglass reinforced polymers) tubes. How and where would I use it (in the hole where the bolts screw in?). The washers should be fiber glass or insulating material also I’d assume. Should this be enough or I need the copper cables as well? Copper path in the lower two bolts (one cable connecting lower two “hot” bolts) is enough or you recommend cables connecting all the bolts (or close to the bolts).

Thanks again for the inputs. Will post the result when the work is done.

 

[waross]

A comment on the washers and tubing for the bolt. The tubing goes on the shank of the bolt. The washers go under the head and under the nut, and between the iron supports. They will most likely be backed by steel washers.
The purpose of stainless steel is to reduce magnetic flux in the bolt and so reduce the self heating of the bolt. If you were to hang a steel bolt in free air beside the overheating bolt and it were to get hot, switch to stainless or silicon-bronze. If a bolt in free air does not heat, there may be no need to change, particularly after it is insulated.
Your bolt assembly stack would be
1> head of the bolt
2> steel washer
3> insulating washer
4> steel or iron structural member
5> insulating washer
6> steel or iron structural member
7> insulating washer
8> steel washer
9> nut
The reason is to interrupt any current passing through the bolt. The insulating tube is placed on the shank of the bolt to keep it from contacting the sides of the holes in the structural iron.
If repairs to the shorting plate correct the problem this may be moot.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

pdshah,explanation by waross might have cleared your query. The copper cable need not be under the bolts.It may not work if the pieces are made of steel and painted.That is why I suggesested to weld a stainless steel pad near to bolts and use it for connection.If you use cables,bolt isolation will not be required.Of course you can provide both.

 

[ScottyUK]

Hi Pdshah,

So the main bonds are between the boxes. I assume the support we are discussing is the light coloured item passing between the central and right phases, partially obscuring the sling point? Presumably there is also a similar support just out of shot? If that is the case then the column is in an area with little or no flux cancellation because it is outside of the main bond between the LV boxes and there is virtually no image current in this area. I think you probably have a shorted turn created by the supports plus the conservator tank and main tank and what you are seeing is a circulating current problem. You're getting some good advice above to try to create an insulated break in the current path: the aim should be to insert an insulated pad somewhere in the current path and open the conductive loop forming the shorted turn, taking care that the bolts do not inadvertently bridge the isolating pad. In my opinion the design is inherently flawed - the support column should not be in that region in the first place.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

Scotty;
I'm wondering if insulating the bolts may just cause the current to divert through the conservator tank. We may end up treating a symptom rather than the root cause.
Looking at the bottom of the vertical support in the last picture posted, what are your comments on attempting to insert an insulating material under the bottom of the support member and insulating the bolts. This looks like a shorted turn issue and insulating the support could open-circuit the shorted turn.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Bill,

Exactly, I am almost certain this is a shorted turn problem. The problem isn't the bolts themselves, it's the closed loop of conductive components (supports, conservator, transformer main tank) which are linked by the leakage flux. Current has to circulate as a result of the flux linkage. The solution I was trying to describe was to establish a full electrical isolation somewhere in the loop to prevent circulating currents; I agree that insulating pads combined with insulated bolts is the way to go.





----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[prc]

We cannot stop the circulating currents in such a large GT.What we can give is more area for current to flow so that temp after heating is within permissible limits.That is how such problems are tresolved.When you insulate the bolts,current will move through the vertical supports and close on conservator.I dont think it is a shorted turn issue as Scotty is considering.

More than insulating the bolt,I will prefer the bypass connection as any damage to insulation will again create heating.

 

[KnicksJets]

Gentlemen
Thanks for the valuable inputs. Here is the update on the plan.
While I can see the reason behind welding a stainless steel pad underneath, at this time we have decided to just isolate the bolts by installing insulating spacers.
This is the plan suggested by one of the expert vendor:

- Enlarge the holes on the bracing.
- Put a fiberglass spacer between the bracing and the conservator tank mount
- Insert a piece of phenolic tubing in the drilled holes
- Replace the existing hardware with 316 stainless steel (use fiberglass washers on both sides of the bolts to isolate them from the braces)

Let me know if you find any issues with it. We are planning to take the action around 2nd week of March. I will post the result when we get back online around April.

 

[ScottyUK]

Sounds pretty much what I suggested on 18th December. No, I don't see any problem with the proposal other than the inconvenience involved.

Let us know how it develops.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Quick question gents; What kind of potentials are expected with these sorts of induced currents and what are these insulators are expected to withstand? A couple of volts? Hundreds?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

High currents and very low voltages.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

So insulation stand-off of these 'isolators' is not an issue at all?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Could easily get into a few tens of volts if the flux linkage is high as it appears to be in this case. It's very dependent on installation geometry because of the flux passing through air rather than being confined to a magnetic core: there are several powerful magnetic fields interacting with a large ferrous structure, so modelling it would be 'interesting' to say the least. Normally mechanical strength will outweigh any insulation considerations.


----------------------------------

If we learn from our mistakes I'm getting a great education!