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Transformer overheating (external) - Circulating Current?
5

Transformer overheating (external) - Circulating Current?

Transformer overheating (external) - Circulating Current?

(OP)
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?

RE: Transformer overheating (external) - Circulating Current?

Unintentional circulating currents occur when a metallic structure near current carrying conductors can act like a single winding of a transformer.  This can put voltage potentials on that metallic structure or induce currents large enough to heat them.

Perhaps something has changed to exacerbate the problem with your transformer.  Unbalanced loading or higher loading?  Or oil that may have conducted the heat away is no longer touching the bolts, etc?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer overheating (external) - Circulating Current?

I assume the LV connection is via isolated phase bus. IPB has near-perfect magnetic field cancellation over the majority of its length, but at discontinuities such as Tee's and bushing enclosures the IPB usually has a significant residual magnetic field which is not cancelled by the image currents in the outer casing. The residual magnetic field will create eddy current heating. Check or mbad or missing connections to the IPB structure around the bushing housing.
  

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

RE: Transformer overheating (external) - Circulating Current?

Hey Scotty.

Question:
You could get induction heating which would be the shorted transformer situation.

And eddy current heating which would be a different mechanism.


Is this correct?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer overheating (external) - Circulating Current?

Yeah, a shorted turn would certainly be possible but they're usually fairly easy to spot because anything large enough not to melt is going to be a reasonable size. The IPB casing is grounded at one, and only one, point with the rest of the duct being supported on pad insulators. Bridging the pad insulator can have 'interesting' effects on whatever forms the short.

Scaffold platforms erected near the IPB duct ends can get unexpected currents induced in them too. Smoking scaff tubes is the usual symptom. Obviously permanent structures would also be affected but design review should stop them even being built in that location.


BTW, the last sentence of my post above should read "Check for bad or missing connections to the IPB structure around the bushing housing."
  

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

RE: Transformer overheating (external) - Circulating Current?

Meant to say that a broken or high resistance bonding conductor will cause poor image current cancellation, and these type of failures are the most common type of problem with IPB. Normally the main bonds are welded channel or box section spanning between the casings and these don't cause problems, so specifically look at the flexible conductors between the main duct and the bushing enclosure. If they are laminated aluminium foil, which is fairly typical, then corrosion can be troublesome.
  

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

RE: Transformer overheating (external) - Circulating Current?

I think that as the conservator and the support legs encircle only 2 of 3 [or 4] outgoing bars, the sum of currents between the legs is not 0 and there is a high magnetic field produced by this difference current. This close magnetic field produces, in turn, a circulating current which may heat the bolts. You have to deploy one of the legs in order to encircle all 3 or 4 bars.

RE: Transformer overheating (external) - Circulating Current?

2
One or both of the supports may be changed to aluminum. An insulator may be installed in one of the supports.
As I visualize the installation, you have magnetic encirclement of only part of the magnetic circuit. In power wiring we expect heating problems to get severe when the vector sum of the encircled currents exceeds about 200 amps. Stated another way, put all your cables through the same hole in the steel.
When single conductor cables enter individual holes in a steel panel you will have issues. It just takes a hacksaw cut to join the holes to interrupt the magnetic encirclement and avoid the heating. I believe, from the description of the problem, that this may be the same effect.

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

RE: Transformer overheating (external) - Circulating Current?

You have not mentioned about the GSU rating,type of bus connection on LV side ie isolated bus duct or open bus bars.Better to contact transformer manufacturer who can give solutions. In such cases, the solution is to give ample contact area for circulating currents so that heating will be reduced.In this case bolts are carrying current more than  their cross sectional area will permit. One way is insulate bolts so that current wuil not go through them and by pass current  through short copper wire connections.Experienced transformer experts will give solutions to you after studying the lay out etc.

RE: Transformer overheating (external) - Circulating Current?

Pdshah,

Why don't you post a picture showing the support bars of your transformer for everbody's use?

RE: Transformer overheating (external) - Circulating Current?

(OP)
First of all thanks all for the input. I still have few questions, while I am attaching few pictures and answering your queires:

Attachment 1: Conceptual drawing of the heated bolts.


PRC: The GSU rating is 607 MVA 22KV on LV side and 345 KV on HV side. LV is connected to the isophase bus with rating 23 KV and 16,000 amps each phase.
Are you suggesting we connect a copper wire across the bolts (the lower two bolts in the X bar)?

Waross: Would appreciate if you clarify your suggestion.

7anoter4: The support legs are between Phase A and C. The legs are left and right to Phase B.

ScottyUK: Thanks for the explanations. Your response make sense and goes along with my recently developed theory. The LV bushing box is connected to the Isophase bus box by bolts and insulated by neoprene gasket. The tanks is leaking oil (from the top of the tank with neoprene gasket in the manway cover) suggesting the gasket might not be doings its job. Are we saying that the Isophase bus box should be completely isolated from the LV bushing box?

Itsmoked: Thanks to your response. Loading has been same. We have more oil leak, sure, but this part of the structure (Support rods) were not being cooled anyway. It looks completely discolored.

Anybody sees any other potential problem than the bolts melting away and causing the support to collapse (we don't see it happening as there are plenty other supports to the conservator tank)?
 

RE: Transformer overheating (external) - Circulating Current?

Pdshah,

If you look at the IPB main bar casings you will see massive cross bars between the outer casings of the phases just before they reach the LV bushing box. They aren't just for mechanical bracing, they carry the full image currents. Once you are beyond these main bonding bars the image current cancellation is poor or non-existant and external fields are high. The bushing box is normally connected to the main structure of the IPB at a single point to maintain its connection with earth, but not to provide a path for image currents to flow. Some designs provide a supplementary bond in the form of a heavy copper or aluminium bar between the bushing boxes although I am unconvinced how much this contributes to external field reduction. If it is fitted you may wish to investigate the soundness of the joints. The external magnetic field in the region beyond the main bonds will be very high and any closed loop formed by supporting steelwork and the like will experience large circulating currents. From the sketch I expect that is what is happening.

Bill's earlier suggestion of an insulated pad may have some merit.
  

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

RE: Transformer overheating (external) - Circulating Current?

(OP)
I am attaching the picture of my LV Bushing box to the Isophase bushing box.
ScottyUK, I don't see the crossbar you refer to in our isophase box.
There was one suggestion that the isolation between the LV Bushing box and the isophase box (the neoprene gasket) might not be adequate.

RE: Transformer overheating (external) - Circulating Current?

That's a big 'un.

I can see the flexible connections which extend the IPB casing onto the bushing box (just above the date), so I'm guessing that they are using one large LV bushing box and relying on the box itself to provide the interconnection between the three IPBs. If I'm wrong please describe how and where the three IPB casings are interconnected.

I can see a lot of red sealant - Hermetite? - which may be contributing to your problem if it is causing a high resistance path between IPB casings. I'd expect something like Biccon or Alcoa compound if that is supposed to be a current-carrying joint. With a 16kA image current you don't need much resistance before it becomes a problem.
  

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

RE: Transformer overheating (external) - Circulating Current?

I think  your problem is the heating of bolts marked in red.
As Scotty raised, normally common (for 3 phases) LV bushing box is used in such application.But it seems in your case they are separate.Then the problems of circulating current will be high.LV bushing turret is usually of  aluminium,In case it is of non-magnetic stainless steel,leakage flux coming out of box ix more.This flux is impinging the two vertical supports of conservator.The induced voltage in these will be different.When they are connected by the X stiffeners,they will carry enormous current ( I have seen hundreds of amperes)This current will pass through bolts as painted surface will not allow current flow.Heating will be directly proportional to the load current in LV.

I have seen this type of heating in many large GSUs.Some times small air equaliser pipes may get melted.Then increase the pipe size to give more area for current flow.

Easiest way  to solve the issue, is to stop this current flow.This you can do by insulating bolts on one side.( using frp tubes and washers)
Another way is weld small stainless steel pads on the X bars and vertical supports near the bolts and then use at least 150 mm2 copper  cable with socket to connect between pads.This way you can by-pass current through the bolts.

Will you come back with the results for the benefit of all?

RE: Transformer overheating (external) - Circulating Current?

Hi prc,

Am I mis-interpreting your post or are you suggesting interrupting the current flow at the flanges where the red sealant is evident? If that box forms the main interphase bond at the transformer end then the whole IPB will not behave as it is designed to because image current will not flow in the casing and the whole IPB will have a massive external field. If the joint is designed to carry current then the joint should be made using the techniques you would employ for a busbar joint, because that is essentially what you are jointing.
  

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

RE: Transformer overheating (external) - Circulating Current?

Scotty, I was not referring to flanges,but just the contact area of the X cross bars used to connect the vertical supports of the conservator.I believe isophaase box is closing the currents of isolated buses or there is  separate conducting jumpers between the outer case of the buses.Even if bus duct current is getting cancelled, I think external flux may be substantial to create potential difference on the vertical support.

RE: Transformer overheating (external) - Circulating Current?

Agreed!
  

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

RE: Transformer overheating (external) - Circulating Current?

Magnetic encirclement;
The primary cause of heating in the vicinity of large currents is stray flux. At lower current levels, cutting the magnetic path will introduce an air gap in the magnetic circuit that will dramatically reduce the flux density and the resulting heating. However with extremely high currents the magnetic fields may be strong enough to cause heating without magnetic encirclement.
I will defer to Scotty on this one. He has more expertise in this field than I.
However, i did some searching on Iso-Phase bus to brush up on the principles and found a case history of an almost identical problem and the resolution of the problem.
http://www.reliableplant.com/ArticleSendtoFriend.aspx?articleid=7947
I hope this helps.

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

RE: Transformer overheating (external) - Circulating Current?

Bill,

That's a great real-world example of how important the bonding bars are and why the joints on them are so important. Thanks for posting it.

The guys involved probably thought it was mechanical bracing or similar and not a fundamental part of the IPB.
  

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

RE: Transformer overheating (external) - Circulating Current?

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.

RE: Transformer overheating (external) - Circulating Current?

(OP)
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.
 

RE: Transformer overheating (external) - Circulating Current?

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

RE: Transformer overheating (external) - Circulating Current?

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.

RE: Transformer overheating (external) - Circulating Current?

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.
  

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

RE: Transformer overheating (external) - Circulating Current?

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

RE: Transformer overheating (external) - Circulating Current?

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.
 
 
 
  

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

RE: Transformer overheating (external) - Circulating Current?

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.

RE: Transformer overheating (external) - Circulating Current?

(OP)
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.
 

RE: Transformer overheating (external) - Circulating Current?

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.
  

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

RE: Transformer overheating (external) - Circulating Current?

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

RE: Transformer overheating (external) - Circulating Current?

High currents and very low voltages.  

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

RE: Transformer overheating (external) - Circulating Current?

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

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer overheating (external) - Circulating Current?

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!
 

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