Two sets of 3 phase transformers 2

[nmodhi]

Hello all,

I'm trying to figure out what would happen when one phase is blown or removed if two sets of three phase transformers were all connected to the same load. The transformers are all 750 KVA & impedence of 5.3%, secondary cables are of equal length. Any ideas?

 

[rbulsara]

Not sure what you mean by two 3phase xfmrs connected to the same load or how they are connected. What happens is also dependent on to what it is that you are talking about and where? What are you particularly interested in?

Transformers themselves would not care. 3 phase motors won't like it too much. Most will stop running, some may keep running with single phasing, some would get damaged. Single phase load on the lost phase would not work, other single phase loads on sound phases may keep on working. Depending upon the protective set up, the device feeding the transformer may just trip.

Rafiq Bulsara

http://www.srengineersct.com

 

[nmodhi]

Thanks for the quick reply

I attached a diagram of what Im dealing with. Im just wondering if one of these transformers were to fail would it effect the distribution and how? I can't seem to figure it out. I believe you're correct though, the device feeding the transformer may just trip, but would it trip after a certain period of time after installation?

 

[nmodhi]

I dont think the file attached, I hope this works

 

[wareagle]

nmodhi
I could not get your drawing. Something is wrong. I too am having trouble understanding the transformers connected to the same load.

 

[magoo2]

Sounds like
a) 2 paralleled transformers or
b) at least 2 feeding a common low side bus but not paralleled on the primary.

 

[jghrist]

I also cannot get your picture. It sounds like you have two paralleled delta-wye transformers and are worried about a fuse blowing on one of them leaving one phase open. If this happened on one transformer, two secondary voltages would become 1/2 normal voltage. With the secondary energized by the intact transformer, however, the voltage on the open phase would be held to normal voltage. I'm not sure what would happen to the load.

 

[nmodhi]

It's actually a two paralleled wye-wye connection, i'm working on a drawing to post, I apologize for the delay but jghirst what would happen if they were just wye-wye and not delta-wye with the same reasoning?

 

[serottaboy]

If they are two paralleled wye-wye transformers and one phase is lost on one of the transformers, then the capacity of the one phase will be halved and the capacity of the other two phases will still be full rated. Assume A-phase fuse blows on one transformer. A-phase capacity on that transformer will be 0kVA. B- and C-phase capacity will remain 250kVA. The other transformer will remain 250kVA per phase. If your A-phase load is less than 250kVA, everthing stays happy. If it is greater, then the 2nd transformer (A-phase) will become overloaded and overcurrent protective devices may begin acting (depending on load and device setting). It won't really matter if your loads are delta or wye, but it would make a difference if your transformers are wye-delta, delta-wye, or delta-delta.

If they were delta-wye, then it would depend on what failed. If it was one phase within the transformer, the transformer would act as an open-delta primary and the transformer capacity would be 57.7% of original rating. If it was a phase conductor, then you would have a much more complex situation: voltages from secondary would keep primary currents flowing in transformer, but only one phase of the delta would have full source power (B-C phase primary). A-phase bushing would still have potential and there current would still flow in A-B and A-C primary windings--they would just be a load sourced from the secondary, however.

If your transformers are three-phase and one phase of the transformer had a fault, it would probably be only a matter of time before conditions inside the transformer (combustion products in oil, etc) would give your trouble on the rest of the transformer.