When Do Zero Sequence Currents Flow?

[EmagSamurai]

I've always understood that zero sequence currents flow in delta windings or in the neutral of a wye connected device. However, I've recently been revisiting the subject, and I'm having some second thoughts.

My current intuition is that if no neutral is present in a wye wound motor, then no zero sequence currents will flow because there is no path for them. Now I'm wondering if a delta wound motor will behave the same way if there is no short to ground? That is, will zero sequence currents flow in a delta wound motor if there is no fault condition? Transformers are a bit different, but at the moment it's motors that are giving me heart burn.

 

[Bugman1400]

I've always understood that zero sequence currents flow in delta windings or in the neutral of a wye connected device. However, I've recently been revisiting the subject, and I'm having some second thoughts.

My current intuition is that if no neutral is present in a wye wound motor, then no zero sequence currents will flow because there is no path for them. Now I'm wondering if a delta wound motor will behave the same way if there is no short to ground? That is, will zero sequence currents flow in a delta wound motor if there is no fault condition? Transformers are a bit different, but at the moment it's motors that are giving me heart burn.

Zero sequence current always has to have a ground path somewhere. It either has to be at the load or at the source. If neither exists then there can be no zero sequence current....only positive and negative. For example, an ungrounded source (delta or un-grd wye) feeding an ungrounded connected load (motor or whatever) cannot have zero sequence current for an imbalance or ground fault.

 

[EmagSamurai]

Thanks!

 

[LMAO]

Are ground fault or phase imbalance the only possible reasons for Zero Sequence Current?

 

[EmagSamurai]

There may be others that I'm not considering, but the light finally went on for me when I thought about applying KCL to the circuit. A positive sequence current, by definition, is a current that is in phase with the same magnitude on each of the phases. For a wye connected motor, these zero sequence currents all try to enter the center connection in phase. Unless there is a neutral, there is no where for them to go, so they are "blocked." With a neutral, these in-phase currents now have somewhere to go, which is why the neutral current is three times the zero sequence current. Similar for a delta winding. the zero-sequence current tries to enter each terminal of the delta (i.e., all three terminals are at the same potential with respect to the zero sequence), but there is no where for them to go unless a ground is present (i.e., a potential difference).

That's at least my tenuous grasp on it at the moment. Hopefully someone else will chime in if I'm way off base here.

EDIT: You might be asking a different question than I answered. I believe some lighting balasts, some rectifiers, and 4 pulse VFDs cause 3rd order (zero-sequence) harmonics as well.

 

[GoldDigger]

There may be others that I'm not considering, but the light finally went on for me when I thought about applying KCL to the circuit. A positive sequence current, by definition, is a current that is in phase with the same magnitude on each of the phases. For a wye connected motor, these zero sequence currents all try to enter the center connection in phase. Unless there is a neutral, there is no where for them to go, so they are "blocked." With a neutral, these in-phase currents now have somewhere to go, which is why the neutral current is three times the zero sequence current. Similar for a delta winding. the zero-sequence current tries to enter each terminal of the delta (i.e., all three terminals are at the same potential with respect to the zero sequence), but there is no where for them to go unless a ground is present (i.e., a potential difference).

That's at least my tenuous grasp on it at the moment. Hopefully someone else will chime in if I'm way off base here.

EDIT: You might be asking a different question than I answered. I believe some lighting balasts, some rectifiers, and 4 pulse VFDs cause 3rd order (zero-sequence) harmonics as well.

From your description it sounds like you are mixing the concepts of zero sequence voltage (trying to produce a zero sequence current) with zero sequence current (there will not be any as long as there is no path for it.)

 

[EmagSamurai]

I'm trying to find a clear way to explain a situation where without a neutral (or ground path), no zero (fixed typed) sequence current will flow. However, if you had the ability to connect the neutral (no other changes to the system), then the zero sequence current would immediately begin to flow.

Also, is it not the case that a zero sequence (edit: fixed typo) voltage would be present, but the lack of a neutral "looks" like an infinite zero sequence (edit: fixed typo) impedance?

Again, this is a topic that was only glossed over in my college power class, so I'm trying to get a handle on it now.

EDIT: Original post I stated positive sequence in several places when I intended zero sequence.

 

[Ingenieur]

There may be others that I'm not considering, but the light finally went on for me when I thought about applying KCL to the circuit. A positive sequence current, by definition, is a current that is in phase with the same magnitude on each of the phases. For a wye connected motor, these zero sequence currents all try to enter the center connection in phase. Unless there is a neutral, there is no where for them to go, so they are "blocked." With a neutral, these in-phase currents now have somewhere to go, which is why the neutral current is three times the zero sequence current. Similar for a delta winding. the zero-sequence current tries to enter each terminal of the delta (i.e., all three terminals are at the same potential with respect to the zero sequence), but there is no where for them to go unless a ground is present (i.e., a potential difference).

That's at least my tenuous grasp on it at the moment. Hopefully someone else will chime in if I'm way off base here.

EDIT: You might be asking a different question than I answered. I believe some lighting balasts, some rectifiers, and 4 pulse VFDs cause 3rd order (zero-sequence) harmonics as well.

A pos seq is not in phase or the same magnitude
0, 120, -120
pos seq is the phase rotation, say abc
neg seq is acb
zero seq are in phase
ideally
pos only is nml unfaulted operation
zero a ph-gnd fault
neg ph-ph fault
Although in reality you have small neg and zero seq components during nml operation

find a text on symmetrical components
it's a linear transformation (and back) from the phase to seq domain

 

[EmagSamurai]

A pos seq is not in phase or the same magnitude
0, 120, -120
pos seq is the phase rotation, say abc
neg seq is acb
zero seq are in phase
ideally
pos only is nml unfaulted operation
zero a ph-gnd fault
neg ph-ph fault
Although in reality you have small neg and zero seq components during nml operation

find a text on symmetrical components
it's a linear transformation (and back) from the phase to seq domain

Sorry. I fixed my typos. I intended to type zero sequence.

 

[Ingenieur]

Here are some abc to 012 conversions
note if no ground no zero
transformers first

 

[Ingenieur]

Next
transmission lines

 

[EmagSamurai]

Nice! Thanks for taking the time to do that. I need to look at these a bit to understand what's going on with them.

 

[Bugman1400]

Just a point of correction....pos seq current does not always mean an unfaulted circuit. In fact, a bolted 3PH fault (no imbalance in the fault...all phase faulted equally) will be all positive seq current.