Electrical loads and fan assisted transformers
Electrical loads and fan assisted transformers
(OP)
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
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






RE: Electrical loads and fan assisted transformers
RE: Electrical loads and fan assisted transformers
RE: Electrical loads and fan assisted transformers
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".
RE: Electrical loads and fan assisted transformers
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.
RE: Electrical loads and fan assisted transformers
RE: Electrical loads and fan assisted transformers
Any good reading material out there on this subject?
Thanks.
B
RE: Electrical loads and fan assisted transformers
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
RE: Electrical loads and fan assisted transformers
Thanks.
b
RE: Electrical loads and fan assisted transformers
RE: Electrical loads and fan assisted transformers
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.
RE: Electrical loads and fan assisted transformers