Brief removal of cooling on generator stepup transformer ok? 6

[electricpete]

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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[itsmoked]

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.

 

[subtech]

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.

 

[ARC12]

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.

 

[edison123]

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

 

[a430]

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

 

[ruggedscot]

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

 

[ScottyUK]

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!

 

[electricpete]

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.

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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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[electricpete]

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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[electricpete]

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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[electricpete]

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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[ARC12]

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.

 

[Dumbo2929]

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?

 

[BEPC13]

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.

 

[electricpete]

"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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[electricpete]

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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[ARC12]

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.

 

[GamezBeCJ]

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.

 

[ruggedscot]

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