three-phase loads and sources...

[PG1995]

Hi,

The following is written in a textbook about three-phase circuits.

"It is appropriate to mention here that a balanced delta-connected load is more common than a balanced wye-connected load. This is due to the ease with which loads may be added or removed from each phase of a delta-connected load. This is very difficult with a wye-connected load because the neutral may not be accessible. On the other hand, delta-connected sources are not common in practice because of the circulating current that will result in the delta-mesh if the three-phase voltages are slightly unbalanced."

Question 1:
Why is it very difficult with a wye-connected load? The text says that the primary reason is that the neutral may not be accessible.

Please have a look here.

Let me try to answer it. Case 1. Let's assume that the source is wye-connected. I believe that if a neutral is missing in a wye-connected load then removing a load from any of the phases will disrupt the functioning of loads on other two phases. Case 2. If a neutral is present then at least the loads on two phases could function okay utilizing the power from two sources. Do you think think that in general terms my answer is okay?

Let's again assume that the source is wye-connected. Case 3. I believe that if a load is delta-connected removing a load from any of the phases will not disrupt the functioning of loads on other two phases but I think that the load won't be able to get the same power because it would only interact with two of the sources. Is my answer okay?

Question 2:
Another point made is that delta-connected sources are not common in practice because of the circulating current that will result in the delta-mesh if the three-phase voltages are slightly unbalanced.

Let me try to answer it. Please have a look here. One could incorrectly think that in a delta-connected source, the current could flow between the windings because all the windings are interconnected. But as it is obviously when we look at the interference of individual voltage waveforms that the current cannot flow between the windings because overall voltage between the windings cancel out. But if there is a slight voltage difference between the windings voltage then that current flow would occur between the windings, and perhaps this circulating current would be called delta-mesh. Do I make any sense?

Thank you.

 

[Diver300]

I think the textbook is trying to say that the delta connected load can easily handle differences in the three loads without changing the voltages on the other loads. With a Y load, where no neutral is available, if the loads are unequal, the voltages will be as well.

In a delta source, there is the obvious problem of no neutral, so the voltages to ground won't be controlled. On the circulating currents, it is very easy for the three voltages to not cancel out. The third harmonics of the three phase voltages will all be in phase and add up. On a generator all sorts of mechanical tolerances could lead to voltage or phase angle differences. On a transformer, the coupling can differ between phases. Most three-phase transformers that I have seen have all three bobbins in a row, so the middle one will have a slightly different characteristic from the others.

I would think that the lack of a neutral and the circulating third harmonics are good enough reasons to never make a delta source for a normal power system. However I think some automotive alternators are delta connected.