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Operating Large Power Transformers in Very Cold Weather
7

Operating Large Power Transformers in Very Cold Weather

Operating Large Power Transformers in Very Cold Weather

(OP)
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:

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


  • Thanks!

    RE: Operating Large Power Transformers in Very Cold Weather

    I would hope you know that it dosen't get that cold in much of world. And if the plant is operating, the transformers won't get cold.
    If it gets cold enough to freeze the cooling fans, then we probally don't need them anyway.

    RE: Operating Large Power Transformers in Very Cold Weather

    (OP)
    But it does get cold enough to bring oil temps below what they should be. For example, we have procedures in place to stop some of the cooling fans if oil temps fall below 43 C. If this happens AND if winding temperatures indicate < 67.5 C we then stop "automatic" operation of fans (as they should not all be running when it's that cold).

    I realize these procedures vary from transformer to transformer, but I'm interested in how others deal with this and I'm having a hard time finding reference material online.

    RE: Operating Large Power Transformers in Very Cold Weather

    What is your ambient temperature?

    RE: Operating Large Power Transformers in Very Cold Weather

    (OP)
    Lately we've had nights where ambient temp drops to around -7 F and -20 F.

    I'm finding a lot of information on operating procedures for high temps but there doesn't seem to be much in the way of cold temps.

    RE: Operating Large Power Transformers in Very Cold Weather

    In the UK where we get moderately cold winters - we get down to -10°C fairly regularly, but rarely down to -20°C - I'm not aware of any particular problems with cooler banks, nor of any special efforts to limit the capability of the coolers in cold weather. The additional cooling from a low ambient is usually beneficial on a GSU transformer where the top oil temperature is often high. At the very least it saves running hours on the fans and/or pumps if the weather is cold.

    RE: Operating Large Power Transformers in Very Cold Weather

    Typically operators like cold temps for their transformers... Conversly, air blast breakers are scary things to operate when the temp gets too cold.

    If you have a transformer that has been out of service in the cold and you want to bring it online there will be some amount of time (24hrs?) that you want the unit on pot (off-load) to warm the bank up, before you put it fully in service.

    RE: Operating Large Power Transformers in Very Cold Weather

    (OP)
    The main issue is that when the oil temps fall below 40 C, there's the potential for static electrification, which can completely ruin a transformer.

    I can see how this wouldn't be an issue for most of the world, but for areas where it gets really cold, there have to be some documented procedures in place to keep the oil within safe limits.

    RE: Operating Large Power Transformers in Very Cold Weather

    Marks, you still have airblast breakers? Most companies have switched to gas.

    We do not have static problems with transformers. However we use arresters for lightning, and most power transformwers have a low tempeture that the fans turn off (it's automatic from the factory that way). We also ground the case of the transformer, and the windings are connected to some voltage, with a ground reference (except the few delta connections). And we do have some high isocronic levels, and we don't see static problems. But as I said almost everything has a ground reference.

    RE: Operating Large Power Transformers in Very Cold Weather

    Here's a thought from GE, not sure if it's applicable: Old GE Book

    Best to you,

    Goober Dave

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

    RE: Operating Large Power Transformers in Very Cold Weather

    And here's a nice IEEE Paper.

    Best to you,

    Goober Dave

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

    RE: Operating Large Power Transformers in Very Cold Weather

    Interesting to see the problems those with more extreme climates than ours have to deal with. Thanks. smile

    RE: Operating Large Power Transformers in Very Cold Weather

    (OP)
    Appreciate the responses everyone. This is probably going to take a little more digging on my part :). DRWeig, the second one was a pretty interesting read--thanks for that.

    RE: Operating Large Power Transformers in Very Cold Weather

    Cooling and Heating schems of a 18/24/30MVA designed to operate in -65 to 45 Celsius (ambient temperature)

    Our XFMERs here usually have Fans/Heaters running continuously and some with a thermostat as the oil needs to be a certain temp for it to operate. Hope it helps.

    Cheers

    RE: Operating Large Power Transformers in Very Cold Weather

    cranky108:

    Yep. Quite a few 500kV air blast breakers still in service. We have to man the sites when the temp gets too cold. Slowly switching to SF6...

    RE: Operating Large Power Transformers in Very Cold Weather

    Quote (The main issue is that when the oil temps fall below 40 C, there's the potential for static electrification, which can completely ruin a transformer.)


    Can anyone provide more information on this comment from freethase?

    RE: Operating Large Power Transformers in Very Cold Weather

    2
    Hi there freethase, we have to be more PC than your utility, so we are obliged to staff our air blast breaker stations in cold weather ;o)

    And SF6 breakers don't handle the cold at all well; in our jurisdiction it seems nigh impossible during periods of sustained cold to provide enough heat to prevent gas liquefaction...overnight we once had all of the SF6 breakers in a switchyard trip themselves out of service one at a time on low SF6 density, interrupting a number of radial ckts and the customer load that was on them, and we were powerless to do anything about it until the daylight hours when the sun warmed the breakers, the alarms and trips cleared, and we were able to return the breakers to service.

    But I digress.

    Pretty well all of our transformers, some as high as 750 MVA FCR, typically have all fans shut down by the time the oil temp drops to ~ +40 C.

    Most of our stepdown transformers are equipped with under load tap changers for output voltage regulation. One sometimes very unwelcome protective feature on these banks is that the ULTC's lock out if the oil temp gets too cold - - which is understandable, since you wouldn't want taps moving when they're submerged in insulating oil that's halfway to being treacle.

    We have specific instructions to follow during very cold weather; a transformer low oil temperature alarm is one thing, but a lightly loaded bank may annunciate with low conservator oil level due to shrinkage, which means rousing some poor soul out of bed in the wee hours to confirm the bank hasn't developed an oil leak.

    Some of us old guys will deliberately skew the tap positions on paralleled transformers to deliberately produce circulating currents just for the warming effect; often enough this is enough to clear the low level alarm.

    Anecdote:

    At a large hydraulic generating station where I worked we once had an 86 MVA single-phase bank of 135 kV L-N primary develop a fault during a particularly severe and sustained period of cold weather. Maintenance crews came in and swapped it out for our spare bank. When it came time to place the spare forced oil, water cooled transformer in service, our supervisor obstinately insisted that the cooling water had to be in service on the bank since an instruction stipulated that this type of transformer could not be placed in service without cooling water.

    Us lowly operators protested that since the oil in the transformer was at about -20 deg C, the first thing that would happen was that the water side of the heat exchangers would freeze solid since the oil would cool the water, not the other way around, and that it would be better to leave the cooling water out of service until the oil temperature was on the plus side of the freezing point of water...but we were overruled, and being subject to discipline if we did not comply, we did as directed.

    Result? Both HXRs pronmptly froze solid; we knew this was the case because return cooling water from the HXRs only flowed into the tundishes inside the plant for about five minutes. Since maintenence personnel were still on site, we had them remove the HXRs from service one at a time and melt the ice out of the water boxes with a steam cleaner [there was no need to de-ice the tubes, because without any cooling water in service that transformer's share of the 250 MW of generation being stepped up to system voltage soon enough melted the ice in the tubes] ...but this took time, and the oil temperature in the transformer had risen to between 75 and 80 deg C before we managed to get both HXRs back in service.

    We operators figured it was our supervisor who should have been disciplined...

    A sage gentleman acquaintance of mine told me of an axiom he had once heard early in life and had applied often during his thirty years as a police officer. The axiom was: "Instructions are written for the guidance of the wise...and for the blind obedience of all others."

    CR

    RE: Operating Large Power Transformers in Very Cold Weather

    (OP)
    crshears, thanks... that's exactly what I was wondering. So, you guys would turn all fans off before oil temps hit 40 C. Your comment about how they'd "skew the tap positions," on parallel transformers, that's really interesting. The kinda stuff that only comes from experience, I take it? (heck, is that safe?).


    JaPaBo-- so, "static electrification" happens (and has happened quite a bit) when a potential builds up across the oil (for example, the oil nearest the coil builds up a positive charge and the oil near the casing builds up a negative). When the potential difference becomes too great, a path for current can be created. It can be affected or caused by a variety of things, including contaminants, oil pump speed, temperature, and other things that affect the dielectric breakdown of the oil. I've come across a few papers on it on the web describing how it happens. But basically, worst case scenario, it can cause a short between phases.

    RE: Operating Large Power Transformers in Very Cold Weather

    Hi, freethase, I can't speak to static electrification of oil, never having heard of it; but I do wonder, can this happen quickly enough @ 50 or 60 Hz to cause such severe damage? The only scenario in which I can conceive of this happening would be if an electrically 'floating,' viz., ungrounded internal component were to build up a standing DC electrostatic charge...then again, I had best leave discussions of this topic to those who know something about it.

    As to skewing tapchangers for the heating effect: in my view it isn't intrinsically unsafe, per se, but it is a calculated risk. It's not a good idea when there are high winds about, or ice accretion is ongoing, or when other circumstances prevail that could interrupt one of the two supplies in a dual element spot network configuration. Deliberately putting a large spread on the taps does after all force large amounts of reactive power to flow into that bus from one HV circuit and out through the windings of the other transformer into the companion HV circuit, and if a contingency occurs while such a spread exists there could be a minute or two of over- or under-voltage on the LT bus or busses until the operator can re-adjust the taps.

    But if it's a calm, clear night with temperatures getting down into the -25 dec C range and a low transformer oil annunciation is received, it can be surprisingly effective to create some artificial circulating current in the transformer windings and generate just that little extra bit of warmth needed to clear the alarm.

    CR

    RE: Operating Large Power Transformers in Very Cold Weather

    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

    RE: Operating Large Power Transformers in Very Cold Weather

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

    CR

    RE: Operating Large Power Transformers in Very Cold Weather

    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 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.

    RE: Operating Large Power Transformers in Very Cold Weather

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

    RE: Operating Large Power Transformers in Very Cold Weather

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

    RE: Operating Large Power Transformers in Very Cold Weather

    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?

    RE: Operating Large Power Transformers in Very Cold Weather

    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

    RE: Operating Large Power Transformers in Very Cold Weather

    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.

    RE: Operating Large Power Transformers in Very Cold Weather

    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.

    RE: Operating Large Power Transformers in Very Cold Weather

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

    RE: Operating Large Power Transformers in Very Cold Weather

    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.

    RE: Operating Large Power Transformers in Very Cold Weather

    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

    RE: Operating Large Power Transformers in Very Cold Weather

    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.

    RE: Operating Large Power Transformers in Very Cold Weather

    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

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