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Brief removal of cooling on generator stepup transformer ok?
6

Brief removal of cooling on generator stepup transformer ok?

Brief removal of cooling on generator stepup transformer ok?

(OP)
Generator stepup transformer 25kv/345kv Shell Form 650MVA FOA rating.  No FOA or FA rating.

We are studying POSSIBLE options to address identified thermal hotspots in the power supply to the fans/pumps.    Someone had proposed removal of all cooling power for a brief interval to allow repairs.  During this period the transformer would be fully loaded and have no pumps or fans operating.

We are trying to determine what duration if any would it be safe to operate like this.

My concern is that with oil flow greatly reduced (only small amount of natural circulation), we have no ability to determine actual winding temperature.  (delta between oil temperature and winding hot-spot will be much higher than it would be with flow).

What do you think? Has anyone ever done it?

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RE: Brief removal of cooling on generator stepup transformer ok?

Get out your attorneys.
 
If a failure occurs during this "exposed" period the finger pointing will be intense.

I would agree with the concern about what's really happening in the core with non-mechanical circulation.

RE: Brief removal of cooling on generator stepup transformer ok?

Hello Pete

I've been in a similar situation.  We (2 linemen/substation ops and 2 power plant electricians) constructed a temporary service out to a point near the control cabinet of the trans in question.  "Phasing" was done to assure everything would mate up and a sketch detailing temporary connections was made.  The linemen and electricians worked out the plan to parallel temporary connections to keep the fans and pumps on, and the connections were made.  When the temp power breakers were closed, the main breaker in the trans cabinet was opened and the repair work began. (Actually, we pulled some new AC power conductors to the control cabinet from inside the plant and replaced the old main breaker in the control cab as well.)  The temporary connections were ugly to say the least, but they worked fine for the duration of the cable pull, and no problems were experienced.  Do you have linemen/sub ops and electricians available?  If you havn't already done so, I'd involve a couple of these chaps and see what suggestions you might glean from them.
At any rate, I'd not remove pumps and fans for ANY length of time on a unit of such size and loaded to capacity.  Given the fact the units' rating is with FOA only, I'm sure that when the pumps/fans went off, the temp rise would be large and fast. Now, I realize that I'm not in your shoes, but it seems to me you'd find your time much better spent in devising a method for temporary feed of the fans/pumps.  Anyhow, good luck with your project.

RE: Brief removal of cooling on generator stepup transformer ok?

Don't even think about this! If something goes wrong and you can't restore the cooling system components in, I would think less than a minute, you would be forced to trip the unit off line. The ramifications of this would be tough on the career.

Transformer oil is not the greatest conductor of heat and with out the forced circulation component of the cooling system you will be relying on the oil's ability to conduct heat. I would not count on natural circulation helping you out much in a transformer that was designed for FOA operation only. Too risky for me. Find another solution.




RE: Brief removal of cooling on generator stepup transformer ok?

How severe are these thermal hot spots ? Can you get along till next available shut down by external cooling of these supply hot spots ?

RE: Brief removal of cooling on generator stepup transformer ok?

It is a very risky buissness pls be careful,or you propose a shut down.  

RE: Brief removal of cooling on generator stepup transformer ok?

answer : Dont Do It !

this sort of work style causes bad press for engineers - its cutting corners and the implications of this are wide - power outage and as it has been said before almost career suicide.

Id be wary of undertaking this work.

Rugged

RE: Brief removal of cooling on generator stepup transformer ok?

The fact that the transformer does not have an ONAN rating suggests that the design may be using oil directors to cool hot spots in the transformer where convection is unable to provide adequate cooling. Using oil directors is not an uncommon situation on really big transformers. In Europe this would be classed as an ODAN or ODAF design. If your transformer is of this type it will quickly get in to trouble at full load, although the bulk oil temperature would probably indicate that everything is ok for a reasonable period. The first you know of trouble could be when a gassing fault develops at one of the hotspots. Be very wary of this - highlight the risks to the station manager in writing and let him make the call. I think he would shy away from the risk.

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

One day my ship will come in.
But with my luck, I'll be at the airport!

RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
Thx.   I think most folks at our plant are now convinced it would be a bad idea to intentionally enter this condition.

There still remains a persistent question which arises during discussion of various repair and coping strategies -  “How long do we have to restore cooling IF we lose all cooling while operating at 100% load”.   This scenario might arise during transfer to an alternate supply or if we leave the connection as-is and it burns open.

Interestingly enough, we do have some experience in this.  Over a decade ago, the entire cooling on this transformer was intentially removed for 15 minutes during 50% load conditions.   People involved at that time (not me) felt there would be no danger as long as they monitored the temperautres. The oil temperature was recorded as increasing by 3C after 15 minutes  (no mention of what happened when they turned the cooling back on).  There are some comments written in some documentation indicating that the Engineers at the time thought this was indication that the transformer could have operated much longer in this condition.  I don’t agree with that because again they are sensing oil temperature from the top of the tank (or computed winding temperature based on oil at the same point) and they are not sensing actual winding temperature.  I believe the difference between oil and actual winding can be much higher than normal when flow is secured so monitoring any parameter based on oil temperature gives only a false sense of securtiy.  On the other hand, it is interesting to note that the transformer did not suffer any adverse consequences from the 15 minutes at no flow during 50% power based on review of oil samples, Doble testing, and subsequent internal inspections for other reasons.

For kicks, I did a quick calculation of temperature rise on the transformer on the assumptions
1 – no heat is dissipated
2 – entire transformer heats uniformly (bad assumption).

Q = 1,226,000 watts  = heat generation at full load based on factory test data of similar transformer.

Msolid = 887,000 lbm = mass copper and steel.
Csolid = 0.1 BTU/lbm/degreeF  = approx specific heat capacity of steel and copper.


M_oil = 102,000 lbm = mass of oil
C_oil = 0.5 BTU/lbm/degreeF

Total heat capacity – Msolid*Csolid + M_oil*C_oil =132,675 BTU/degreeF = 252,000 J/degreeC

HeatupRate = Q / TotalHeatCapactity = 0.00487 degrees C/sec = 0.3 degrees C per minute

For a 15 minute period at full load this would give 0.3*15 = 5C.  At 50% load we might expect somwhere slightly above ¼ of this.  Seems consistent with observed 2C rise in oil temperature at 50% load.

But once again the huge caution on this excercize is that the winding temperature itself likely rises much faster than the average temperature.  

===================================================
So in conclusion, everyone agrees we won’t intentionally enter this condition, but I still would be interested to hear comments on how long we might have before damage.  With the understanding that this is clearly outside of the transformer design, can I tell people that a 5 minutes loss of power if it should occur probably wouldn’t damage the transformer?

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RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
On the subject of natural convection – I note there is a basic difference of construction of coolers between transformers which have some rating and those such as generator stepups that have only an FOA rating.

The transformers with OA ratingare radiator style with surfaces arranged for vertical air flow in the coolers. This facilitates a natural convection air cooling as air rises through the cooler.   The FO0-only generator stepup transformer have coolers arranged for horizontal air flow which is not facilitated by natural convection.   Any air which tends to rise does not efficiently cool the tubes because it doesn’t go between the horizontal fins on the tubes in the cooler.

The above feature discusses natural circulation air flow.  There may also be features of the OA and FA style transformers that facilitate natural convection oil flow which are absent in FOA-only generator stepup transformer, but those are not immediately apparent to me (any thoughts?)

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RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
The shell form GSU actually has a pretty good height difference between the coolers above and the core below which should facilitate natural convection of the oil.  Not that this gives any license to operate the tranformer outside of design, but I'm just interestd in hearing comments to help understand/predict the performance of the transformer under the postulated loss of cooling.

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RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
I wonder if there is any difference for pumps used in applications with natural oil circulation capability (such as FA/FOA) and those withou (FOA-only).  i.e. some characteristic of those used in the FA/FOA transformer that allows low flow resistance with pump in the off-state  As far as I can tell from looking at some transformers there is not.  

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RE: Brief removal of cooling on generator stepup transformer ok?

Electricpete asked:

 “How long do we have to restore cooling IF we lose all cooling while operating at 100% load”?

If you lost all pumps and fans on the unit in question and tripped the unit at the same time I believe in the minutes and hours that followed the trip you would experience insulation degradation induced solely from residual heat radiating from the iron core and copper windings.

I would suggest a solution along the lines Subtech mention.
Set-up a temporary motor control center near the GSU. Run temporary cables for each fan and pump motor. Tie the feeds from the temporary MCC into the existing motor circuits, do it right at the motors if you have to. One by one switch all pumps and fan motors over to the temporary MCC as well as any other devices feed from the questionable power source. Once all load is removed from the existing supply you are home free.



RE: Brief removal of cooling on generator stepup transformer ok?

You can perform the loss of life calculations from the IEEE standard to determine what the potential damage would be for a certain load for a period of time.  
What about installing temporary fans, to allow the repairs to the cooling system?

RE: Brief removal of cooling on generator stepup transformer ok?

From what I know of shell form transformers it is suicide to run them without the oil pumps running. The oil does not circulate well using natural convection. We are very carefull never to run our shell forms without at least one pump running. I would say this is true of this unit also since its rating is FOA and no FA rating.

RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
"If you lost all pumps and fans on the unit in question and tripped the unit at the same time I believe in the minutes and hours that followed the trip you would experience insulation degradation induced solely from residual heat radiating from the iron core and copper windings."

That doesn't sound right to me.  We have experienced events at station which simultaneiousl took out transformer and never any damage.

What could possibly generate heat after the power is removed?

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RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
Correction
"We have experienced events at our station which simultaneiously took out the transformer and cooling and never had any damage"

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RE: Brief removal of cooling on generator stepup transformer ok?

What could possibly generate heat after the power is removed?

Obviously, no additional heat will be generated within the transformer once it is de-energized. However, I submit that, for example, the inner core of this shell form power transformer is operating at a higher temperature than the surface of the core. Upon transformer de-energization and subsequent removal of oil flow through the transformer the surface temperature of the core will rise. Portions of the insulation system close to the core, in particular, the high-density kraft paper tubes lining the legs of the core are going to be exposed to higher temperatures than they were when the transformer was energized and fully loaded with oil flow through these areas. Hence my statement: "If you lost all pumps and fans on the unit in question and tripped the unit at the same time I believe in the minutes and hours that followed the trip you would experience insulation degradation induced solely from residual heat radiating from the iron core and copper windings."

One example of this that comes to mind is a water-cooled transformer associated with a large VFD located in an electrical room. A power failure knocks out power to the transformer and the water circulation pumps. 30 minutes after the power went off a smoke detector went off in the electrical room. Upon investigation it was found that a plate type heat exchanger running the full length of the top of the transformer core was hot enough to have it’s paint starting to bubble and smoke, setting off the smoke alarm. The heat exchanger was never this hot with the transformer under full load and the cooling system functional.

I will stand by my opinion that if the cooling system fails on a 650 MVA transformer with an FOA class cooling strategy operating at full load and you de-energize the unit at the same time as the cooling system failure you will expose portions of the insulation system to temperatures that will be detrimental to the life of the unit.




RE: Brief removal of cooling on generator stepup transformer ok?

Electripete:

The trasnformer manufacturing company that I work for would allow a maximum half our of operating at full load after losing all the cooling system (fans and pumps). This applies only to a transformer without ONAN rating. The unit in this case is designed to constantly have oil moving and fans running to achieve the required Winding Rise, typically 65 Celsius oir 80 Celsius for the Hot Spot Winding Rise.
One thing that might help is if you do this procedure during a cold weather. Even when you have a high winding rise, the actual winding temperature might be below the maximum allowable value and you can keep the unit running for longer time.

RE: Brief removal of cooling on generator stepup transformer ok?

If you ever take this unit out of service you may want to consider adding some backup equipment as it is obvious that this item is or major importance.

Id look to add some addittional circulation pumps and also some spare cooling fans that can be brought in should you ever lose the power or control to the unit. I would also back this up with another source of supply.....

rugged

RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
Additional fans is easy.

There is no practical way to rig up additional pumps while the transformer is running since that is intrusive on the oil system.  But perhaps as someone else suggested wire alternate power to some of existing pumps.

At this point as I said before I am most interested in just understanding the answer to the question - how long do we have before damage occurs IF we encounter a situation where power is inadvertantly lost.

GamezBeCJ - what type of transformer was that? Do you know roughly the MVA rating and voltage?  Shell or core form?

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RE: Brief removal of cooling on generator stepup transformer ok?

How long have you got - Id say looking at it and the design of the transformer you would not have long - if the oil flow stops you get very quick localised heating - same as if you have a blockage, the area would develop a hot spot leading to oil gassing and breakdown - probably take out the protection and drop the load. best case. worst case would be catastrophic failure.

http://www.powerlineman.com/downloads/download.html

go have a look at the video file called

Transformer Failure.mpeg

Now that is something that you really really dont want to provoke - as I said power xformers need to keep what they were designed to operate with working, if not then you fall down very very rapidly.

Rugged

RE: Brief removal of cooling on generator stepup transformer ok?

From  the discussions ,it is a large transformer with directed cooling capacity only.It means oil is directed in to winding and guided.It is better you consult the manufacturer (as he knows all the weak spots in it )and he will be able to give a number to you.Normally this is checked like this .Take the full load winding hot spot temperature.Then assume that the entire winding copper loss (heat )is absorbed by the copper .Then find the time for copper temp to reach 140 C from the starting temperature ie full load condition.(usually less than 98 C )
Usually this figure is 10- 20 minutes.Be cautious ,as when winding temp goes over 98 C ageing rate goes up and chances of gas bubble formation and consequent failure is more esp if there is lot of moisture in paper .(You can ascertain this by checking water content in oil when trf is under  max hot condition ) If the trf is old and water content is pretty high ,limit hot spot temp to 120-125C .You can also save the situation by undertaking the operation when ambient temp is as low as possible and load is as low as possible.

RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
Thx PRC.  That sounds like a parallel analysis to what I suggested above, except with a more conservative assumption that heat stays in the copper, rather than heating up the entire transformer mass uniformly.   IF we could neglect heat transferred to the environment, then the actual response probably lies between the two.

My transformer outline drawing indicates that the weight of the “core and coils” is 752,000 pounds. What fraction would be copper?

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RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
Now that I think about it, a better way to say it (rather than saying the actual response lies between the two IF we ignore heat transfer) is that your approach is conservative.   I like that approach.  Still need to know what fraction core and coils is copper.

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RE: Brief removal of cooling on generator stepup transformer ok?

electricpete,sorry ,I will not be able to guide you to correct number for copper as I have no experience with shell type transformers.But your assumption of heat distributing to entire inside mass inside transformer is not correct as the time constant for such heating is more than 2-3 hours while we are considering a time frame of 10-15 minutes .So the conservative calculation will be safe.Of course a finer calculation can be seen in the IEEE/ANSI loading guide on transformers.

RE: Brief removal of cooling on generator stepup transformer ok?

Eletripete:

Sorry it took so long. Usually in America you will se transformers with that kind of cooling (OFAF or ODAF) starting from 100 MVA up to 500 MVA. The half hour I mentioned is a little bit conservative, since this is considering that the top oil rise is at is rated value (say 65 celsius). If the unit would loose cooling while partially loaded, you would have some more time before you start causing excesive aging to the insulation system. I would say that at the most you would have about 3 hours.
Regards.

RE: Brief removal of cooling on generator stepup transformer ok?

One more thing. As it has been sugested before. Contacting the manufacturer is a very good idea. He has all the calculations and models needed to predict the cooling capacity of the tank alone, and would give you a much more accurate estimate.

RE: Brief removal of cooling on generator stepup transformer ok?

The Russian power station engineers removed the cooling for a short time at Chernobyl. Look what happened to them!

RE: Brief removal of cooling on generator stepup transformer ok?

Can't ask them can we...

RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
for the record I am only interested in responses that have some technical basis related to transformers

opinions and analogies are worthless to me

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RE: Brief removal of cooling on generator stepup transformer ok?

there are on-line devices that can check for degradation of insulation (release of aromatics, gases, CO2, etc)
these devices (there are quite a few in the market, i would refrain from mentioning any brands) can be installed while the unit is in operation.

after obtaining a baseline... the instrumentation can be used to monitor the oil for any symptom of decomposition of the oil and/or the insulation.

in any case... removing the pumps/fans from service with the unit in operation would require by-passing some protections and trips... in Europe this is against the law, and voids any insurance, warranty and creates criminal liabilities...

in the us. i do not know... but at least i would check the insurance policy... if there is a conscious decission to bypass protections... it may also amount to criminal liabilities... even if nothing happens, the plant logbook will have to log the by-passing of the protections... so it will be subject to scrutiny by the cognizant authority and create a liability... something like "reckless driving".

if the risk of not doing the repairs on-line is high... the risk of doing the repairs without disclosing the situation to all parties is even greater in $$$
if push comes to shove... i would either schedule an outage or... bring all parties and explain the situation and justify the technical case for by-passing protections... this may include:
cognizant authority
neighbours representative
labor unions
customers
owners
fire marshall
OSHA
etc.

HTH

saludos.
a.

RE: Brief removal of cooling on generator stepup transformer ok?

(OP)
We have a Hydran monitor installed on this unit.  I don't have much confidence that the response from that unit in the absence of oil flow would be very fast.  Also if hydrogen is not generated, the sensitivity of that device is fairly low (lower response to CO and CO2 than to H2).  Surely if we got an increase in Hydran ppm indication that would be incentive to do something immediately but I wouldn't count on the device as a means to guarantee we're operating within a safe region any more than I would count on the temperature indicators (as discussed above delta between winding hotspot and top oil would be much higher during low oil flow).

I have never seen a generator stepup transformer with a trip upon loss of cooling.  Have you abeltio?  

Although not conclusive, that fact would seem to argue that the protective relay folks think the transformer can withstand momentary loss of cooling.

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RE: Brief removal of cooling on generator stepup transformer ok?

what i have seen is that the cooling is controlled by temperature switches, which in turn send signals to the dcs...
there are two levels alarm, and normal shutdown... cannot recall the settings.
and of course the buchholz relay.

in one sad incident... the buchholz actuated, the operators thought it was a relay glitch...  bypassed the protection and had a very hot copper soup in their hands in less than 10 min
they are still waiting for a new transformer.

with this, i mean that if there is a fault... it escalates to catastrophic failure very quickly.

hth

saludos.
a.

RE: Brief removal of cooling on generator stepup transformer ok?

Electripete:

Do you have to service all pumps and fans at once?, can you do one by one?.
You could de-energize one device at a time, try to reduce the load on the transformer to the minimum possible and keep an eye on winding temperature indicator.
If you do this during winter time, it is even better.

RE: Brief removal of cooling on generator stepup transformer ok?

electricpete,none of the hydran will be of any help to you in your plan of action.The failure mode ,if it happens will be sudden ,probably you will get no time .However the only item that  will give some indication will be  gas operated  relay .(Do you have one ?)This will give an alarm or better watch for gas during the period of operation ,if you have brought the gas sampling device at ground level.

You need not by pass any protection device.Winding temperature indication will not go up as some of us suspect.It is a slave operated by winding current.Winding current is not increasing during cooler switch of mode .Oil temp will not go up as time of cooler shut down is much less than oil heating time constant.

If you can give me moisture content in oil with trf oil temp (at the time of sampling ) -take a sample when oil temp is max -I can help you in finding the water content in paper .If it is less than 2% ,we can safely reach a hot spot of 130-140 C.From test report pl find out the hot spot temp  rise reached .Then you can calculate the time by assuming a copper wt.It will not be far out compared to a  core type trf.Pl find the copper losss at fuul load from test report
Pete,I did not understand your problem fully.What is meant by "hot spots in cooler power supply "

RE: Brief removal of cooling on generator stepup transformer ok?

Pete,
//Additional fans is easy.

There is no practical way to rig up additional pumps while the transformer is running since that is intrusive on the oil system.  But perhaps as someone else suggested wire alternate power to some of existing pumps\\.

Eventhough you not interested,
the pumps is to circulate oil, so if you have mobile oil purification plant that can do online, I think you can do oil circulation with purification plant with the same flow/gpm specs. you can do this with spare pumps for your xfmr from your ware house. just be sure you remove the air from temporary hose or pipes.
I donot know if your xfmr have valves connection for online purification.
This if you decide do not want to loose the pump duirng transistion of supply to temporary supply.


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