Transformers in Parallel

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Paul76

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Hi guys,
I pretty new in this area and I really need a help. I have two transformers,
T1- 1000VA, 5,94%, 20/0,4 kV/kV and
T2 - 1600VA, 6,14%, 20/0,4 kV/kV.
Both transformers are DYn5.
I can't put them in parallel, because, I get next measure (measure with load secundary)

A1-A2 is 230V
A1-B2 is 460V
A1-C2 is 230V

B1-B2 is 230V
B1-A2 is 230V
B1-C2 is 460V

C1-C2 is 230V
C1-A2 is 460V
C1-B2 is 230V

where A1,B1,C1 are phases of T1 and A2,B2,C2 are phases of T2.
 
With transformers of different impedance you are going to have some circulating current. Your impedances are relatively close, but not exact, and therefore there will be some current.

What you should find, if all is right, is transformer LV windings should be 400V L-L, and 230V L-E, and it should not matter if you measure A1-A2 or A1-B2.

If wired and phased properly, with them being operated in parallel the A1-B1 measurement should read practically zero since there is no reference.

You say both are Dyn5, which is a step in the right direction, however, are the HV windings fed from the same source, or is another transformer connected ahead of it somehow? Are the internal winding connections accessible? If so, check to make sure they have not be inadvertantly mis-configured.

Also, phases could be rolled on the HV side; e.g. L1 to H2, L2 to H3, and L3 to H1, (or some other combination) or on the LV side as well.

I would disconnect everything, ring out the conductors, label everything, draw a diagram of what you have and what you need, then re-connect. if there is an intermediate transformer somewhere on the 20KV side, then you won't be able to parallel without swapping out one transformer, or buying another intermediate transformer.
 
What is the base voltage you are measuring the difference in voltage? Are you measuring the 230 V between A1 and A2 on a 4 kv base, or is that measurement on some smaller scale, like a 480 V base? That will probably tell you if you are dealing with a phase shift on the high side of the transformer, or a difference in source impedance.

If it is 230 V on a 4 kv base, there is probably a phase shift somewhere on the high side of the 20kv-4 kv transformers that you are trying to parallel. Are the 20 kv sources fed from the same transformer secondary? Do you have phasing sticks to check and see if the 2 sets of 20 kv lines are in phase?


If it is on a 480 V base (or somewhere near that magnitude), the difference in voltage is realtively small (230 V on a 4 kv system), then I think the difference in voltage say between A1 and A2 is caused by the fact that the source impedances of the 2 systems are different enough to cause a voltage drop in one system that is not in the other.



Kingpb, why would A1-B1 read zero volts? Shouldn't you get Line to line voltage?
 
Wirenut,

I think that kingpb's statement about A1-B1 giving zero volts was from a difference in notation:

I think that the OP was using A1, B1, C1 for the secondary output of one transformer, and A2, B2, C2 for the secondary of the second. kingpb's comment reads as though he is using A1, A2, A3 for the first transformer and B1, B2, B3 for the second transformer.

It looks like the OP is using European notation, where the 'comma' is used like a decimal point. So 0,4kV means 400V.

Just guessing from the numbers, I think that one of the transformers is actually DYn11; the output voltages look like two wye secondaries shifted 60 degrees apart.

-Jon
 
winnie said:
Wirenut,

I think that kingpb's statement about A1-B1 giving zero volts was from a difference in notation:

I think that the OP was using A1, B1, C1 for the secondary output of one transformer, and A2, B2, C2 for the secondary of the second. kingpb's comment reads as though he is using A1, A2, A3 for the first transformer and B1, B2, B3 for the second transformer.

It looks like the OP is using European notation, where the 'comma' is used like a decimal point. So 0,4kV means 400V.

Just guessing from the numbers, I think that one of the transformers is actually DYn11; the output voltages look like two wye secondaries shifted 60 degrees apart.

-Jon
Click to expand...

D'oh! I see it now. Well in that case the phase shift is the likely culprit either from a difference in internal winding configurations or from different sources having phase shift(s) occuring upstream.
 
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