synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
The transformer configuration can affect whether inverters can detect and shut down under an open phase condition from the utility.
With a delta primary, an open phase would cause the transformer to be driven with a single-phase L-L voltage across the delta winding that remains fully connected. The two other windings would be in series with each other, and each would share about 1/2 of the L-L voltage that's across the transformer terminals that are still driven. I think it's likely that inverters could reliably shut down under these conditions.
With a wye-grounded primary, an open phase going to the primary would leave two of the three windings still being driven by their respective L-N voltages. And so they would continue to induce magnetic flux through their associated leg of the transformer core with a 120 degree phase difference between them. But what happens in the leg that's no longer driven depends on the type of core construction used (e.g., 3-leg or 5-leg).
3-legs works fine under normal use because the sum of the fluxes going through each of the three legs stays close to zero with balanced 3-phase excitation voltages. In other words, there is no "zero sequence" flux. All of the flux passing through each leg returns through the other two legs. However, there is no adequate return path for zero sequence flux like there is through the extra legs of a 5-leg core. And so what happens in a 3-leg core under an open phase condition is that a voltage very close to the proper magnitude and phase shift is "regenerated" on the wye winding that's disconnected (and thereby maintains a net condition of no zero sequence flux). As a result the corresponding winding on the secondary also would have a regenerated voltage close to what it would normally be. But there would be little such voltage with a 5-leg core because the extra legs can pass any zero sequence flux. See the paper at the link below for more details.
The bottom line is that delta-wye and 5-leg Yg-Yg configurations should not prevent inverters from detecting an open phase condition, but there could be an issue with 3-leg Yg-Yg
With a delta primary, an open phase would cause the transformer to be driven with a single-phase L-L voltage across the delta winding that remains fully connected. The two other windings would be in series with each other, and each would share about 1/2 of the L-L voltage that's across the transformer terminals that are still driven. I think it's likely that inverters could reliably shut down under these conditions.
With a wye-grounded primary, an open phase going to the primary would leave two of the three windings still being driven by their respective L-N voltages. And so they would continue to induce magnetic flux through their associated leg of the transformer core with a 120 degree phase difference between them. But what happens in the leg that's no longer driven depends on the type of core construction used (e.g., 3-leg or 5-leg).
3-legs works fine under normal use because the sum of the fluxes going through each of the three legs stays close to zero with balanced 3-phase excitation voltages. In other words, there is no "zero sequence" flux. All of the flux passing through each leg returns through the other two legs. However, there is no adequate return path for zero sequence flux like there is through the extra legs of a 5-leg core. And so what happens in a 3-leg core under an open phase condition is that a voltage very close to the proper magnitude and phase shift is "regenerated" on the wye winding that's disconnected (and thereby maintains a net condition of no zero sequence flux). As a result the corresponding winding on the secondary also would have a regenerated voltage close to what it would normally be. But there would be little such voltage with a 5-leg core because the extra legs can pass any zero sequence flux. See the paper at the link below for more details.
The bottom line is that delta-wye and 5-leg Yg-Yg configurations should not prevent inverters from detecting an open phase condition, but there could be an issue with 3-leg Yg-Yg
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But your question was "he has asked me for an explanation of why a delta primary is necessary." and I think the answer is pretty clear that it is not necessary. Everything else is just people talking about minutia.