Parallel Transformers and Strange Power Factor
Parallel Transformers and Strange Power Factor
(OP)
I have two 75KVA transformers connected to a 3 phase 480V bus energized from a generator. The transformers are intended to back up each other to supply a sub panel in a delta to Y configuration to provide 208/120VAC 3 phase. The breakers of the load side of the transformers are interlocked so that only one can supply the sub panel, but both can be energized by the bus at the same time. The generator can monitor the system's power factor. When one transformer is connected to the bus and supplying the sub panel I get a reasonable power factor of 0.73. When the second transformer is connected to the bus (but no load connected), the generator measures a power factor of approximately 0.50. Is this typical for paralleled transformers or should I be concerned? Can someone explain why I would have such a large difference in power factor simply by energizing the second transformer? Thank you.






RE: Parallel Transformers and Strange Power Factor
xnuke
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RE: Parallel Transformers and Strange Power Factor
RE: Parallel Transformers and Strange Power Factor
I expect the loaded transformer does not have much applied load or else the reactive current of the unloaded transformer would not effect the power factor so much. I also expect the generator doesn't have much other 480V load applied either.
Being that you are using a generator and the transformer is 75kVA, I'm doubting the power factor really matters that much.
RE: Parallel Transformers and Strange Power Factor
By the way, the transformers are connected to a common supply. They are not paralleled unless the secondaries are also connected together.
If transformers are paralleled and if they are not on the same tap, the result will be a large circulating, reactive current and the PF will drop. This is not the case here.
Bill
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"Why not the best?"
Jimmy Carter
RE: Parallel Transformers and Strange Power Factor
the actual load[kW] is very low and then a tolerable difference between no-load currents it could create such a difference .
If the load is elevated -let’s say more than 30%-then the second transformer magnetic core is saturated-they could be primary winding short-circuited turns.
RE: Parallel Transformers and Strange Power Factor
"When the second transformer is connected to the bus (but no load connected),"
Two unloaded transformers. I would expect the PF of either one or both to be around 10%
If both transformers are supplying the load, what is the load in either kW or KVA.
One transformer: Load VARs plus transformer VARs.
Two transformers: Load VARsplus 2 x transformer VARs.
Bill
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"Why not the best?"
Jimmy Carter
RE: Parallel Transformers and Strange Power Factor
LionelHutz; there was about 10-15A on the generator. If I'm using a generator and a transformer, why would the power factor not matter that much?
Waross; thank you for pointing out they are not paralleled, my terminology was inaccurate- you can tell I'm new to this. Only one is loaded. I don't have a method for determining the kW or kVA on the loaded transformer, I've been relying on the genset's monitoring of the full bus.
RE: Parallel Transformers and Strange Power Factor
When the second transformer was connected then a no-load current of 6% [normal] of rated it could be added [90*6/100=5.4 A].Since the power factor of the new no-load current is very low [0.1] we may consider this new current as pure reactive.
Then the total current will be 10*[0.73+jsqrt(1-.73^2)]+j5.4=7.3+j(6.8+5.4)=7.3+j12.2=14.2 A[absolute].
pf=7.3/14.2=0.51