Adding oil pumps to a ONAF transformer
Adding oil pumps to a ONAF transformer
(OP)
The proposal of adding oil pumps to a bank of 1 phase ONAF GSU transformers (100 MVA bank, 220/13,8 kV, conservator type) is under discussion. The reason is not to increase the power capacity of the transformers (as the capacity of the generators connected to the bank will not change), only to lower the hot-spot temperature with the goal of extending the transformer life and decreasing the failure probability. The transformers are new.
The OEM of the transformer has been contacted and the price for the job is not a deal breaker. The OEM has proposed in-line pumps with axial impeller and states that even a failure in the pumps will not interrupt the normal oil flow by convection, so the ONAF rating is not in jeopardy. Our concern is the probability of a pump failure blocking the cooling system flow from the respective cooler (each 1 phase unit has 4 coolers), e.g. in the case of a bearing failure, can anyone comment on the possibility?
Any other concerns we should have?
The OEM of the transformer has been contacted and the price for the job is not a deal breaker. The OEM has proposed in-line pumps with axial impeller and states that even a failure in the pumps will not interrupt the normal oil flow by convection, so the ONAF rating is not in jeopardy. Our concern is the probability of a pump failure blocking the cooling system flow from the respective cooler (each 1 phase unit has 4 coolers), e.g. in the case of a bearing failure, can anyone comment on the possibility?
Any other concerns we should have?






RE: Adding oil pumps to a ONAF transformer
Make sure you include decent quality isolation valves. One day you'll need them and that won't be the day you want to find out that they aren't up to the job. Have the pipe shop make you a spool piece to replace the pump and get the cooler back in operation under natural circulation if you don't carry a spare pump on the shelf.
RE: Adding oil pumps to a ONAF transformer
Scotty has some very good suggestions.
RE: Adding oil pumps to a ONAF transformer
Scotty: The isolation valves seem to be solid. The idea of an spool piece is a very good one, thank you!
Rcwilson: You have just added to my worries: metal debris from a failed pump, the impellers and motor body are cast aluminum (Al Si10 MG DIN 1725). I don't know which material is used for the ball bearings. I would be happy to learn more about what maintenance you suggest (although I'm sure that the OEM will have a suggestion on the topic).
RE: Adding oil pumps to a ONAF transformer
We never pulled that pump out of the transformer cooling pipe, because it required draining the whole transformer since the isolation valves did not work as Scotty warned. It was a 50 MVA, 13.8 kV voltage regulating transformer feeding an aluminum potline operating at 95-100% load 24/7. We did tap changer maintenance about every 12-18 months based on # of operations, switching the potline to the standby regulator during maintenance.
Root cause was assigned to the pump failure. But a competing theory was that my crew had improperly topped off the main tank using a "dirty oil" hose that was only for draining the nasty oil from the tap changer tank. That may have dumped some metal shavings from the worn out tap changer mechanism into the main tank. There wasn't much left of the transformer. It is possible that when the lid was sent high in the air the explosion also ruined the pump, making it look like the source of the metal particles. It was fortunate that the spare regulator could be switched into service quickly and another plant had a surplus unit that worked as a replacement. Since production got back on line and replacement cost wasn't large, the root cause investigation was minimal. (We would not get away with that in a modern well run facility).
Other 50 MVA regulating units had more modern pumps that ran quietly 2-3 years continuously. The vacuum tap changers on those regulators allowed much longer maintenance intervals. We checked amp draw and vibration readings during operation, like any other pump. I don't recall any pump issues.
RE: Adding oil pumps to a ONAF transformer
A better soution is to add more fans, or fans with higher air flow or change the direction( from horizontal mounting to bottom of radiators) OEM can help you in this.
RE: Adding oil pumps to a ONAF transformer
RE: Adding oil pumps to a ONAF transformer
Bill
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"Why not the best?"
Jimmy Carter
RE: Adding oil pumps to a ONAF transformer
http://www.epri.com/abstracts/Pages/ProductAbstrac...
RE: Adding oil pumps to a ONAF transformer
The major issue is converting ONAN/ONAF unit in to OFAF has limited effect in reducing the hot spot in winding as the oil inside the winding will move just as in case of ONAN case,but with ODAF oil will flow over the winding surfaces removing heat quickly and reducing hot spots.