Parallel Transformers and Strange Power Factor 1

[CuriousElectricalGuy]

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.

 

[xnuke]

Sounds like the inductive excitation current of the two transformers is dropping your power factor. This is probably happening since there is no load on the second transformer, and it is only drawing excitation current. By the way, I don't think 0.73 a "reasonable power factor" for continuous operation. Consult your generator documentation for power factor limits.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CuriousElectricalGuy]

Thanks xnuke. If the only loads connected to the transformer were florescent lights and some motors, would a 0.73 power factor be considered reasonable?

 

[LionelHutz]

It could be normal. How much load is on the loaded transformer? How much other 480V load is on the generator?

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.

 

[waross]

KVA is determined by both kW and KVAR. PF will drop if more KVARs are required. The KVA of an unloaded transformer is almost entirely KVARs. If you increase the KVARs without increasing the load, the PF will drop.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

In my opinion, if the both transformers are identical and in good conditions then this difference in reactive power could be explained in this way:
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.

 

[waross]

You state:
"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
--------------------
"Why not the best?"
Jimmy Carter

 

[CuriousElectricalGuy]

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.

 

[7anoter4]

As waross said, if the measured current on generator was 10 A when a single transformer was connected and the power factor was 0.73 then the active power P=sqrt(3)*10*480*0.73=6069 W and the reactive power Q=P*TAN(ACOS(0.73))=5682 kVAr
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