Hi everyone,

I have been working on an ongoing set of coursework related to delta tertiary windings in wye-wye transformers. So far I have successfully answered:

• Why third harmonics are a problem in wye-wye connected transformers
• Why the delta-connected tertiary provides third-harmonic balancing

I am stuck on the final question relating to a small fundamental component of current circulating in the delta tertiary. Whilst I can see why given, say, a 50Hz input the 150Hz current would flow in the delta, I cannot see why a 50Hz current would flow.


Any pointers in the right direction would be a great help.

 

Perhaps it relates to the presence of zero sequence currents.

 

OK, so the third harmonic is effectively a zero phase sequence current, right? If you take the third harmonics and phase shift them the same amount as you phase shift the fundamental, they all end up in phase at three times the frequency (correct me here if I am wrong). How does the third harmonic waveform being a zero sequence current then explain a 50Hz component? I have researched zero sequence briefly but nothing I have read suggests that three 150Hz in-phase waves would create a 50Hz component.

 

Any unbalanced 3-phase current case can be resolved into three sub-sets at 50Hz. Nothing to do with the third harmonic.
The three 50 Hz (3-phase) subsets are positive sequence, negative sequence and zero sequence. This will be apparent in unbalanced 3-wire conditions - particularly under fault conditions.

 

I'm doing exactly the same piece of coursework. The other sections seemed to be relatively well documented in journals and books but I'm really struggling to find anything that mentions this small 50hz component circulating in the tertiary delta aside from the information we were provided.

Someone on my course mentioned this may be due to lack of electrical isolation between the phase windings, another mentioned it could be attributed to the way the transformers are wound (A and C are concentric windings, B is a sandwich winding).

t_n_k, I'm struggling to understand what you're getting at here.

Any additional help would be appreciated!!!

 

The OP was apparently asking in which circumstance one might find an example of common in-phase circulating current at 50Hz (system frequency) in a delta tertiary winding.

I cited the case of fundamental frequency zero sequence currents that can flow, say in the case of power system faults.

Are you familiar with the analysis of power system faults using symmetrical components?

I guess it would be possible (by way of example) to draw or simulate a system diagram with say a single phase ground fault on a 3-phase transmission line connecting a generator and load via a Y-Y-Δ(tertiary) transformer. One would presumably "see" zero sequence fundamental current in the tertiary Δ winding(s).

As to the other examples you cited - I haven't thought about those cases.