Current flow in a 3-Phase Wye system

[mull982]

I've always understood that in a 3-phase 3-wire delta system current was balanced by all three legs in the system. For instance i current flowing down phase A would return to the source on both phases B and C. So any current that appears on one leg would return on the other two and thus be balanced. This is the principal of operation of a zero sequence CT.

What about a 3-wire wye system? Does a current flowing down phase A return via phases B and C, or does it return via the nuetral point? I've been confused about this wye configuration. I thought there was a possibilty of the current returing via the nuetral point and if they were all balanced then there would be no current flow and if unbalbanced there would be flow.

So my question is for a three phase load on a 3-wire wye system do the currents return to the source via the other two phases?

 

[charlie b]

First, even in a delta 3-wire system, there is no expectation that the currents are balanced. Yes, current leaving on Phase A will return via Phases B and C. But the amount of the A that goes into B need not be the same as the amount going into C. It all depends on the loads that you connect to each phase.

In a 4-wire WYE configuration, if the loads are balanced, then all the current leaving via Phase A will return (equally) on Phases B and C. But if the loads are not balanced, then some of the Phase A current will return via the neutral wire.

 

[mull982]

First, even in a delta 3-wire system, there is no expectation that the currents are balanced. Yes, current leaving on Phase A will return via Phases B and C. But the amount of the A that goes into B need not be the same as the amount going into C. It all depends on the loads that you connect to each phase.

In a 4-wire WYE configuration, if the loads are balanced, then all the current leaving via Phase A will return (equally) on Phases B and C. But if the loads are not balanced, then some of the Phase A current will return via the neutral wire.

So if I have a wye secondary transformer, and the only load I have on it is a 3-phase motor I'm assuming that the phases will be balanced and all of the current will flow down one phase and return on the others. If one of these phases were to become unbalanced then you are saying that I would have current flowing in the nuetral? My system is only a wye 3-wire system with the nuetral being resistance grounded. What happens in this case?

I thought nuetral current only came into play when single phase loads were using a nuetral? If I have all three phase loads or L-L single phase loads, can I still have current flowing in the nuetral? I thought that even with unbalanced 3-phase loads the current still returned on the other legs Without a nuetral wire where does this current flow?

In a delta system you are saying that current does return on the other two legs but may not be in equal amounts depending on the loads. Again using transformer with a delta secondary and only 1 3-phase motor I'm asuming that the current are balanced and returning equal on each leg.

Are you also saying that if I have a motor drawing 100A on each leg of a delta xfmr but have a L-L single phase load on two of the legs then the 100 wont split up 50-50 between the legs because of the L-L single phase load?

I know my questions may be confusing but I appreciate the help in trying to figure this out.

 

[charlie b]

So if I have a wye secondary transformer, and the only load I have on it is a 3-phase motor I'm assuming that the phases will be balanced and all of the current will flow down one phase and return on the others.

True.

If one of these phases were to become unbalanced then you are saying that I would have current flowing in the neutral? My system is only a wye 3-wire system with the neutral being resistance grounded. What happens in this case?

If the neutral wire is not connected to any load, if it is connected to the center point of the transformer and to no other point, then it obviously cannot carry current. Any imbalance in the phases will affect how much current flows in each phase, in much the same way it does in a delta system.

I thought neutral current only came into play when single phase loads were using a neutral? If I have all three phase loads or L-L single phase loads, can I still have current flowing in the neutral?

Again, if no load has a connection to the neutral, then the neutral will not be carrying any current.

In a delta system you are saying that current does return on the other two legs but may not be in equal amounts depending on the loads.

Correct.

Again using transformer with a delta secondary and only 1 3-phase motor I'm assuming that the current are balanced and returning equal on each leg.

True, unless of course there is something wrong with the motor.

Are you also saying that if I have a motor drawing 100A on each leg of a delta xfmr but have a L-L single phase load on two of the legs then the 100 wont split up 50-50 between the legs because of the L-L single phase load?

No. The current associated with the motor will remain balanced amongst the three phases. It is the current associated with the L-L load that is split amongst the two affected phases.

 

[gar]

080804-1132 EST

mull982:

Are you also saying that if I have a motor drawing 100A on each leg of a delta xfmr but have a L-L single phase load on two of the legs then the 100 wont split up 50-50 between the legs because of the L-L single phase load?

Try to view your question this way. First, assume there is zero internal impedance in the 3 phase source. Thus, any load does not change the line-to-line voltages, or a line-to-neutral.

Connect the motor to the 3 phase source. Under load this causes 100 A in each line. Now remove the motor and connect your single phase load between two lines. This produces X amps of current in each of those lines to which the single phase load is connected, and zero amps in the third line.

Next connect the motor and the single phase load to the 3 phase source. The odd line will have 100 A, and the other two lines will each have the vector sum of 100 and X as their current.

This is true whether it is a delta or Y.

Now move the single phase load from line-to-line to line-to-neutral. Two of the three phase lines now have 100 A and the one with the single phase load has the vector sum of 100 and the single phase current, and the neutral has the single phase current. Obviously this is a Y with a zero impedance neutral based on my earlier assumption.

.

 

[mull982]

Ok thanks for the responses, its starting to become clear.

One more question. Lets say I did have a wye 4-wire system so therefore did have a nuetral wire and my loads were all 3-phase loads or L-L single phase loads. Would any of these unbalanced currents cause current to flow in the nuetral, or does current only flow in the nuetral when loads are connected L-N?

Therefore even if I had a nuetral and had all 3-phase and L-L loads then I would not expect to see any amount of current in the nuetral even if phases were unbalanced?

 

[winnie]

Would any of these unbalanced currents cause current to flow in the nuetral, or does current only flow in the nuetral when loads are connected L-N?

Therefore even if I had a nuetral and had all 3-phase and L-L loads then I would not expect to see any amount of current in the nuetral even if phases were unbalanced?

Exactly. No matter how unbalanced the loads are, if there is nothing connected to neutral, there will be no current flow on the neutral.

However: if you are talking about a large resistance grounded industrial system, you may have 'stealth' connections between line and neutral, for example you may have significant _capacitive_ current flow returning via bonded metal through to the neutral.

-Jon

 

[gar]

080804-1243 EST

mull982:

I believe charlie already answered that question. If nothing is connected to the neutral, then no current flows in the neutral.

If you have Y loads or single phase loads connected to the neutral, then you may have a neutral current, but then that does depend upon balance in a very broad meaning of balance.

.

 

[George Stolz]

The way I think about it is this:

Half of the current that leaves the A phase wants to go to B, and the other half wants to go to C.
Half of the current that leaves the B phase wants to go to A, and the other half wants to go to C.
Half of the current that leaves the C phase wants to go to A, and the other half wants to go to B.

Now focus on just the current leaving the A phase.
View attachment 2065

In order for half of the current to flow to B, and half to C, the resistances must be even on those two paths. If they're not, then the unbalanced current is forced to take the detour to it's desired location, by way of the neutral that connects the two phases at the transformer.

Does that make any sense? Probably not, it's how my mind works.

 

[mull982]

The way I think about it is this:

Now focus on just the current leaving the A phase.
View attachment 2065

In order for half of the current to flow to B, and half to C, the resistances must be even on those two paths. If they're not, then the unbalanced current is forced to take the detour to it's desired location, by way of the neutral that connects the two phases at the transformer.

Does that make any sense? Probably not, it's how my mind works.

It makes sense except for this last statement. You mention that if unbalanced then the current takes a detour by way of the nuetral that connects the two phases. However as others have said, if there is nothing connect to the nuetral then there will be no current flow in the nuetral.

 

[charlie]

I've always understood that in a 3-phase 3-wire delta system current was balanced by all three legs in the system. . .

What do you think would happen if I had a 480 volt, ungrounded, 3w, delta system and installed a 480-124/240 volt, single phase transformer for office and other loads? If I connected to two phases, the transformer bank would now be unbalanced.

I am not sure that this is confusing or helping this conversation. :smile:

 

[Smart $]

It makes sense except for this last statement. You mention that if unbalanced then the current takes a detour by way of the nuetral that connects the two phases. However as others have said, if there is nothing connect to the nuetral then there will be no current flow in the nuetral.

For Line to Line loads, the detour is by way of the neutral node, or connection point, in the transformer or transformer bank. There is no load connection to and no current on a neutral [load] conductor.

PS: I believe George's approach to understanding a wye system may help some newbies, but is ultimately confusing for a complete understanding.

 

[mull982]

What do you think would happen if I had a 480 volt, ungrounded, 3w, delta system and installed a 480-124/240 volt, single phase transformer for office and other loads? If I connected to two phases, the transformer bank would now be unbalanced.

I am not sure that this is confusing or helping this conversation. :smile:

Yes this makes sense. The transformer bank would be unbalanced however the current would only flow between the two phases that the transformer was connected to.

 

[rattus]

Yes this makes sense. The transformer bank would be unbalanced however the current would only flow between the two phases that the transformer was connected to.

True, but all three xfrmrs contribute to that current.

 

[mull982]

True, but all three xfrmrs contribute to that current.

I do not follow you here.

I'm assuming the three xfmrs you are referring to is the 480V ungrounded delta xfmr?

 

[rattus]

I do not follow you here.

I'm assuming the three xfmrs you are referring to is the 480V ungrounded delta xfmr?

Yes. We would see reflected load currents in all three primary windings.

 

[gar]

080805-1330

mull982:

In my post numbered 5 I separated the 3 phase and single phase loads to help you see what was the distribution of currents.

Now consider a balanced delta with secondaries AB, BC, CA. Separate secondary AB at points A and B and leave the other two secondaries connected in series.

If there was no circulating current thru the original closed delta, and this would be the ideal case, then the voltage from secondary AB must be exactly the same as the voltage from the series combination of BC and CA. This also means the voltages must be exactly in phase.

With the separated arrangement connect your single phase load across secondary AB. You will measure some amount of current thru the load. The load voltage and current will be slightly affected by the internal impedance of the transformer.

Disconnect the load from AB and connect it to the series combination of BC with CA. The resulting load current and voltage will be virtually the same as from the AB secondary.

When all three secondaries are connected together the current supplied from AB will be approximately 1/2 of the load current assuming equal internal impedances in each transformer secondary. 1/2 of the energy comes from AB and 1/2 from the other two and 1/4 from each of the others.

.