I load bank generators. I use a 2 megawatt 480 volt bank. I want to use it with generators with 4160 volt output. I have test 12,400 volt, 2 megawatt generators that have 2500 kva transformers in place. I do a 5 hour test at 25% 1hour, 50% 1 hour, 75% 1hour. and 100% 2hours, and the transformer shows no sign of warming up. What I see is it possible to use a smaller transformer. Derate for duty cycle. I am tring to get the weight down for moving around. What will work???
Transformers Loads
- Thread starter gen51
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iggy2
Senior Member
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- NEw England
See the exceptions under scope of 450 - transformers used for testing are not covered by the NEC.
Having said that, yes, you could undoudtedly use a smaller transformer, given that the loading is less than 100% for 3 of 5 hours. Perhaps an 80% rating would work? Type of transformer and ambient temp also matters a great deal. Oil filled are probably tougher for overload than dry type; cast coil more so.
But is a 2 MVA transformer much simpler to move than a 2.5 MVA?
Having said that, yes, you could undoudtedly use a smaller transformer, given that the loading is less than 100% for 3 of 5 hours. Perhaps an 80% rating would work? Type of transformer and ambient temp also matters a great deal. Oil filled are probably tougher for overload than dry type; cast coil more so.
But is a 2 MVA transformer much simpler to move than a 2.5 MVA?
mike_kilroy
Senior Member
- Location
- United States
i deduce from ur post that u do not yet have a xfmr, only comparing the heat of the 2.5mva one on the 2mva generator and thus think smaller is ok for ur 2.0mva load test. if this is correct, then right out of the gate u can use a 2.0mva xfmr since that is 100% rated.
so question, if above is correct assumption from ur post, seems how much smaller than 2mva can u go?
I would say there are 2 limits: 1) temp rise, & 2) current carrying capacity of wire (gauge) in xfmr.
for 1) I would ask the xfmr mfgr u get quotes from to tell u their thermal time constant. You know then how it will heat - in 1 time constant it will get to 67% of max temp, 3 it wil get to 98%. I think u will find the thermal time constant is about 8 hours so u should be able to downsize quite a bit.
so much so that 2) will be an issue; the smaller xfmr may not be built with heavy enough wire for your current draw at 2mva.... If so I would ask in quote for larger wire size if required.
U can offload both questions to the xfmr mfgr in your request for quote - let them tell u how small u can go! free advice we give when asked for a xfmr quote all the time. also, this offloads the responcibility of it working to them (us)
Also since size & weight is issue for u, I would ask for an autoxfmr to make both smaller (and cheaper) - since a 4000v->400v auto will be 1/10th the kva equiv size of the iso model
so question, if above is correct assumption from ur post, seems how much smaller than 2mva can u go?
I would say there are 2 limits: 1) temp rise, & 2) current carrying capacity of wire (gauge) in xfmr.
for 1) I would ask the xfmr mfgr u get quotes from to tell u their thermal time constant. You know then how it will heat - in 1 time constant it will get to 67% of max temp, 3 it wil get to 98%. I think u will find the thermal time constant is about 8 hours so u should be able to downsize quite a bit.
so much so that 2) will be an issue; the smaller xfmr may not be built with heavy enough wire for your current draw at 2mva.... If so I would ask in quote for larger wire size if required.
U can offload both questions to the xfmr mfgr in your request for quote - let them tell u how small u can go! free advice we give when asked for a xfmr quote all the time. also, this offloads the responcibility of it working to them (us)
Also since size & weight is issue for u, I would ask for an autoxfmr to make both smaller (and cheaper) - since a 4000v->400v auto will be 1/10th the kva equiv size of the iso model
Last edited:
MSU
Member
- Location
- Hattiesburg, MS
Rated KVA is a continuous rating. The transformer also has a short term rating, which is dependant upon the temperature of the transformer hotspot prior to overloading. Basically, as long as the hotspot does not exceed manufacturers stated maximum, you can do as you wish. A damage curve on the transformer will head you in the correct direction. It will relate I^2 T to transformer damage.
Best guess is that you would need a 2000KVA. Next standard rating down is 1500, and the load profile which you indicate would produce considerable heating into the transformer prior to the 2Hr @100% shown as the last step.
Another consideration, if liquid filled, is to specify a 1500KVA with a 55/65DegC rise, which yields a continuous rating of 1500 x 1.125 or a 12 1/2% increase. That may add enough weight to negate the load capacity increase.
You're probably stuck at 2000KVA.
Best guess is that you would need a 2000KVA. Next standard rating down is 1500, and the load profile which you indicate would produce considerable heating into the transformer prior to the 2Hr @100% shown as the last step.
Another consideration, if liquid filled, is to specify a 1500KVA with a 55/65DegC rise, which yields a continuous rating of 1500 x 1.125 or a 12 1/2% increase. That may add enough weight to negate the load capacity increase.
You're probably stuck at 2000KVA.
mike_kilroy
Senior Member
- Location
- United States
math 101....
Kva_rms=2000kva*[(1*.25^2+1*.50^2+1*.75^2+2*1^2)/5hr)]^.5=2000kva*.75=1500kva right out of the gate without any concern other than if this 5 hour cycle will exceed the thermal time constant of the transformer - putting this cycle into a 1.5mva xfmr... with a thermal time contant of 8-12 hours. NO problem.
Now, if you want to let xfmr mfgr work for their order, let them go the next step and figure how much smaller than 1500kva they can go with this 5 hour cycle before reaching max xfmr temp.... w/o doing the math my guess is 750kva.
but at least u can see a 1500kva unit will handle your job easy. no need to go 2500 or 100% load 2000kva either. make it an autoxfmr and cut the size/weight to 1/5 again.... get quotes from the custom xfmr mfgrs and be sure.
Kva_rms=2000kva*[(1*.25^2+1*.50^2+1*.75^2+2*1^2)/5hr)]^.5=2000kva*.75=1500kva right out of the gate without any concern other than if this 5 hour cycle will exceed the thermal time constant of the transformer - putting this cycle into a 1.5mva xfmr... with a thermal time contant of 8-12 hours. NO problem.
Now, if you want to let xfmr mfgr work for their order, let them go the next step and figure how much smaller than 1500kva they can go with this 5 hour cycle before reaching max xfmr temp.... w/o doing the math my guess is 750kva.
but at least u can see a 1500kva unit will handle your job easy. no need to go 2500 or 100% load 2000kva either. make it an autoxfmr and cut the size/weight to 1/5 again.... get quotes from the custom xfmr mfgrs and be sure.
mike_kilroy
Senior Member
- Location
- United States
I have to apologize for using the wrong equation to figure heating value of the power put into a transformer; rms is meaningless for POWER; the heating value of a changing load is the average power, not the rms. So the heating equivelant of this cycle is KVA=2000*((1*.25+1*.5+1*.75+2*1)/5) = 2000*.7= 1400 kva
So only 1400kva rated xfmr is required if this cycle was repeated forever. since it is not and is much shorter than the thermal time constant of this size xfmr, a smaller yet xfmr can be used.
So only 1400kva rated xfmr is required if this cycle was repeated forever. since it is not and is much shorter than the thermal time constant of this size xfmr, a smaller yet xfmr can be used.
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