Operating Large Power Transformers in Very Cold Weather 7

[freethase]

Hi All,

I'm researching how various power plants operate their large power transformers in extremely cold weather. Particularly interested in transformers that are rated in the hundreds of MW and 1000 MW range.

With extremely cold weather, issues arise with oil temperatures becoming too low. This is especially true with OFAF types. I'm looking for:

[li]specific operating procedures[/li]
[li]when and how are cooling fans switched off (what oil or winding temps?)[/li]
[li]how are transformer temperatures monitored?[/li]
[li]user experiences[/li]
[li]transformer user manuals where the issue of very cold operating temps is addressed[/li]
[li]etc. (any insight into the topic is appreciated)[/li]

Thanks!

 

[itsmoked]

Watching from the sidelines here, I'm completely baffled as to why bar heaters aren't bolted all over these breakers and there isn't 50kW of inline heaters in the oil circuits of these big transformers. Draining water and worrying about dropping circuits seems way over the top from some (simple) axillary heating.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

You're preaching to the choir, fella...

CR

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[prc]

When large transformers are to be operated at sub zero ambient temperatures,follow the instructions from the manufacturer as the guidance depends on the design and type of accessories used. However the following general precautions may be relevant.The mentioned IEEE paper is the only guidelines that I know in the public domain on the subject.Standard guidelines for energization at cold temperatures ( as per IEEE C57.12.00.,below -20C is considered as unusual service) is given vide clause 3.8.3of IEEE std C57.93-2007 Installation and maintenance guide for transformers.

1) The most important part is the energisation at cold temperature.It shall be always on no-load and shall be maintained for at least 2 hours before putting load in stages of say 20 % for 30 minutes. The oil used shall have a pour point at least 10C lower than the lowest cold ambient temperature. IEC 60296 on Transformer oil (ed3.0 2003 &ed4.0 2012)has specified 4 grades o oil with LCEST (lowest cold start energising temperature)of-5, -20,-30 &-40 C with viscosity of 1800 m2/s at that temperature.For temperatures below this external heating will be necessary.

2) Avoid OLTC operations at such low temperatures untill the heating picks up.See the guidelines from tap changer manufacturer.

3)The major issue is formation of ice ( from the moisture separated out from oil) that may float over oil which can cause flash over across live parts. To avoid this, low density oil is preferred for such use. Density of ice varies between 0.88-0.92 while oil density varies 0.82-0.91 depending on the type and grade.

4) Static electrification will not be an issue at low temperatures as oil velocity will be very low at such temperatures.For more details on this phenomena(which was first reported from Japan in early 1970's) refer to CIGRE Brochure No170 of 2000 "Static Electrification' .Can be purchased from

http://www.e-cigre.org

or can be downloaded free for members.This is an issue only for transformers with directed cooling where oil is forced over the windings thorough insulation parts. Oil when moves fast over insulation, oil acquires positive charges and insulation acquires negative charges.This can cause flash overs or PD which can lead to major dielectric failures.

 

[squeeky]

Very interesting. So glad I'm in Africa. So you want a transformer? Is that with or without ice?

 

[crshears]

Without ice, thanks, since the cost of repairs equals impressed voltage x current x labour cost in dollars per hour 'cubed'

 

[prc]

D R weig, can you explain how to get those old GE reviews that you showed. We have to purchase from Google? How to do this?

 

[DRWeig]

Hi prc,

Google books is pretty cool. Normally I buy from Amazon, but this particular GE Review (volume 17) doesn't seem to be available. So I can add it to my Google library and read it on the PC / Tablet whenever I want. If it were available for sale, you'd find that every third page or so is blank on screen.

Also, if you click the "Get this book in print" button, you'll see other vendors who may or may not have exactly what you're looking for. Finally on that list is "Find in libraries" which is really cool. I'm in Atlanta area -- it says there's a copy at Georgia Tech and a copy at Emory U.

In sum, you don't have to purchase unless the book is for sale at one of the listed vendors.

I hope that makes sense...

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[FPelec]

Hello freethase,
we have some HV substations in the Alps; minumum winter temperatures are between -10 °C and -20 °C.
When using OFAF transformers we generally stop fans when oil temperature falls below 55°C; we also stop oil pumps when oil temperature falls below 15°C.
best regards,
fpelec


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[prc]

DR Weig, please bear with my computer illiteracy.
"So I can add it to my Google library and read it on the PC / Tablet whenever I want." Can you explain a bit more as I failed to get at it.

 

[freethase]

FPelec, what kind of transformers are these? What's the typical kVA? 15 C seems really low for oil temperature.

 

[FPelec]

Freethase,
we generally have distribution power transformers (132/20 kV, 16 to 63 MVA) and interconnection autotransfomers (400 kV, 250 and 400 MVA). The latter have OFAF cooling.
Yes, 15°C is a very low temperature for oil, but consider that in case of reduced load (15% of rated power during the night) there are almost only no-load losses.
Those are typically 75 kW for a 250 MVA autotranfsormer. Their cooling system is designed for a top oil - air delta temperature of 65°, considering a 8% overload, and one fourth of the coolers out of service.
This means that the no-load losses alone will lead to about 10°C delta T, if fans and oil pumps are still turned on.
By turning off fans (and thus switching to OFAN cooling), maybe the delta T will reach 20°, but when ambient temperature is -20°C, this means that oil temperature would be still around 0°C, which could cause some troubles.
Turning off also the oil pumps is an extreme countermeasure of OFAF cooling, but service experience shows that in this way we can keep oil temperature significantly above troublesome values using no-load losses only.



Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[crshears]

Hi FPelec,

Within our utility the transformer oil temperature controls are quite routinely configured to first shut down all fans on falling temperature [OFAN], then shut down all pumps on a further decrease in temperature [ONAN]. I'd have to query the database to confirm this, but for the most part this is our standard approach, and it is not considered at all extreme.

I know that on certain of our transformers some of the oil pumps run continuously due to mfr's recommendation; we also have others that must be unloaded if all oil pumps fail [no self-colled rating], and only a few quite old ones [thankfully, very few] that must promptly be removed from potential on loss of all oil pumps.

Carl

 

[FPelec]

Hello Carl,
our autotransformers do not have an ONAN rating, but only an OFAF one.
From the constructive point of view, when the fans are turned off, the coolers effectiveness drops significantly (see attached photo). When the (centrifugal) oil pumps are turned off, the oil flow between the tank and the coolers is almost fully blocked.
That is the reason why I think that shutting down oil pumps is an "extreme" countermeasure. Note that in normal operation, i.e. oil temperature is above 15°C, if more than one oil pump is lost the transformer is removed from service.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[crshears]

Hi there FPelec, different graces in different places, I expect...

For sure we prefer our oil temps to stay above 15°C as well, which to my understanding is precisely why our oil pumps routinely shut down; when low transformer loadings are combined with low ambient temperatures [like -30°C], oil shrinkage can be a very serious problem.

As noted above, we do an immediate field dispatch for inspection upon receipt of a low oil level alarm; this is generated by low conservator oil level. In severely cold weather it sometimes occurs that before field personnel arrive at the station the oil has shrunk so much that the oil level gets drawn all the way down the piping between the main transformer and the conservator, at which point a main tank gas accumulation alarm is received...in which case if we follow our standing instructions means that the transformer must be taken off potential and a 'gas' sample drawn and analyzed; meanwhile the oil shrinks even more and...you get the picture; we really don't ever want things to descend to that level, pun intended, if at all possible.

'Cooler's effectiveness drops significantly'? Perfect!

CR