Electrical loads and fan assisted transformers 2

[buddy91082]

Question using fan assisted transformers:

Can the fans be used under normal conditions - let's say average demand load is above transformer OA rating and kicks on fans - is there a time limit the fans can run for increased kVA rating? Can you size your average deamdn load, not peak load, on fan assisted rating?

Thanks.
b

 

[cvance]

The rated output of a transformer may be increased with the addition of cooling stages in the form of fan packages, forced oil, forced water, as well as a combination of two or more of any of these. In the case where you have an additional fan package your transformer nameplate would read ONAN/ONAF (OA/FA). The associated kVA rating would be something like 1000/1250, meaning the base rating of the transformer is 1000 kVA and with the additional fan package engaged you would get 1250 kVA maximum output. These maximum outputs would each be at the same rated temperature for both the base and ONAF rating (i.e. 65 deg. C). The top (highest) rating is also good for continuous operation. Remember, when performing any impedance calculations the per unit or % impedance for your given transformer is in reference to your base rating. Note: this nomenclature if for oil immersed transformers, dry type transformers would be the same but nomenclature would be AN/AF.

 

[mikekilroy]

we sometimes, due to conditions such as max physical size limitations, have to put 100-500kva xfmrs into Nema 12 style enclosures too small to allow proper ventilation. We have put fan assisted vents in with thermostat control to extend life of the fans. But we also gave customer thermostat outputs from the xfmr to monitor - in case the fans stopped working.

 

[Gragas]

Depends on the application really...

As a general design standard? No, I am not a big fan. (no pun intended) Depending on the type of plant and the overall system design, believing that you will not be adding to that power feed over the life of a plant is not a smart idea.

Also, the design layout of the system comes into play heavily. If its just a single feed to a remote area or similar, my above caution applies.

Other items that come into play possibly: how critical is the feed? Does it serve additional loading if there was some kind of outage? Is it every a possibility that could happen added later on? Are you stocking spare transformers or using "off the shelf" designs? How long can you take an outage for?

Again, it all depends a bit. I typically run into fan ratings only in specific cases during an outage of some kind with a transformer supplying more than the norm. Otherwise, the fans are for CYA or I do not include them and only design "provisions for future fan cooling".

 

[rcw retired EE]

On a retrofit where you are adding fans to a transformer to boost output, remember to check the conductor sizes.

In my experience, large transformers (>50 MVA) are usually applied to use the force cooled ratings for "normal" operation to minimize the capital cost. We see many specifications that require the transformer to be sized to handle the continuous peak design load with one stage of cooling out of service. That buys a 33% margin on the transformer design since the typical large ONAN/ONAF1/ONAF2 ANSI transformer has ratings of 100%/133%/166% for the self-cooled/first stage/second stage cooling.

 

[dpc]

Just to be picky, if the transformer nameplate indicates provisions for future fans and provides a fan-cooled rating, then adding fans with appropriate automatic controls is not a problem at all. However, if no fan-cooled rating is given on the nameplate, you'll have to consult with the manufacturer. You can't just stick a fan on and assume the rating is increased.

 

[buddy91082]

Thanks for the replies. So in summary, if a transformer has an upper and lower kva rating, the upper rating achieved through fans, the upper rating can operate continuously with the fans.

Any good reading material out there on this subject?

Thanks.
B

 

[Marmite]

I wouldn't jump to that conclusion. What does the transformer specification say? The OFAF rating will be based on a specified ambient temperature and potentially also on a typical load profile assumed to pre-exist prior to the period of running with fans.
Do you really want to force a load shed if you lose a fan?
If only life were as simple as cheap transformer plus cheap fans is equivalent to more expensive transformer with no fans.

Regards
Marmite

 

[buddy91082]

Marmite,just looking for design guidelines. The transfomer does not exist yet. We are are going thorugh a load analysis and looking into using a transfomrer with fans or without. I want to try and understand how to use the fan rating, if it can be used for continuous use, and any other best practices.

Thanks.
b

 

[prc]

In case the rating is less than say 30 MVA it is better to go for only ONAN cooling ie without fans. Advantages are saving in auxiliary loss, less maintenance liability from fans. The cost increase from extra radiators will be compensated by the savings from the cost of fans plus cost of control cubicle.In IEC world,when ONAN/ONAF is specified, the ONAN rating compared to ONAF is usually 60,70 or 80% depending on the nature of peak loading and the fans will be switched on during peak load period,either automatically or manually.In US,ONAF rating will be 120 % or 133 % 160 %of ONAN rating. ONAN/ONAF rating is used up to 315 MVA 3 phase /500 MVA single phase ratings.

 

[bacon4life]

IEEE C57.91 describes how loading affects the lifetime of a transformer. The IEEE transformer specifications are based on continuous loading, however this guide helps you convert a load and ambient temperature profile into an equivalent constant load.

If you load the transformer to the ONAF1 rating on an ONAN/ONAF1/ONAF2 rated transformer, theoretically the oil temperatures would be at the nameplate 65 degree rise. By turning both sets of fans on, you would reduce the temperature rise and significantly extend the life of the insulation. On our transformers with thermostatically controlled fans, the fans come on at well under the applicable MVA rating.

Some utilities leave the fans on continuously during the hottest months of the year.

Fans can definitely be a maintenance headache, so be sure you get ones that will last longer than the transformer. Be sure to include a switch to alternate which set comes on first in order to equalize the wear. Also, having an individual breakers for each fan costs more, but will allow for the rest of the group to stay on when you do have the inevitable fan failure.

When writing the transformer specification, the cost for losses also plays into how much cooling is required. In a recent tender, our fairly high evaluated costs for both load and no load losses lead to vendors submitting natural cooling options instead of forced cooling options. The low losses mean less excess heat to dissipate.

 

[dpc]

If the nsmeplate has a forced cooled rating (FA or ONAF), then the transformer can operate continuously at that rating as long as the fans are operational. The other responders have provided a lot of additional useful information, but the bottom line is that the transformer can run long-term at the forced cooled rating.