Non-linear loads and neutral current 4

[Mbrooke]

If all none linear loads are connected phase-to-phase, will a wye grounded wye grounded transformer still pass neutral current? And to what degree?

 

[cuky2000]

If the member should be agree, below is a summary and suggestion to consider in this post:

1) The sum of balance fundamental currents will cancel on the neutral. Agree
2) For grounded-wye/grounded-wye: if there is a neutral current on the secondary, there will be neutral current on the primary Agree

3) Assesment of the Impact of triplen harmonics generated from non-linear load line-to-line connected:

3.1) Neutral line conductor up to the transformer: No third harmonic current flow for line-to-line connection. Agree
3.2) Delta Transformer: if there is 3rd harmonic produce circulating current. On hold for later discussion
3.3) Wye-Grounded Transformer: Conduction on transformer neutral to ground LV & HV: Divided Opinions.

QUESTION REFRACED: Does a balance non-linear load will produce current to flow into the ground from the transformer neutral point into the ground in a wye-grounded (earthed) transformer? Your thoughts.

 

[jghrist]

Cuky,

The discussion to this point did not include a delta connected transformer.

I agree that balanced third harmonic currents in the wye grounded transformer windings will add together in the transformer secondary neutral and that the transformer primary neutral will also have a corresponding current. The problem with your analysis is that there is no third harmonic current in transformer windings if the load is all connected phase-to-phase.

QUESTION REFRACED: Does a balance non-linear load will produce current to flow into the ground from the transformer neutral point into the ground in a wye-grounded (earthed) transformer? Your thoughts.

ANSWER: No, not if the load is connected phase-to-phase.

 

[stevenal]

Cuky2000,

I said nothing of a delta connected transformer, I spoke of delta connected loads. Connected balanced phase to phase, that is. I purposely used the word delta, since I think it may help illustrate how triplen current circulates in the load. While jghrist goes for the math, I prefer the graphical intuitive approach.

Your last link does not work either.

Refraced? Not in my vocabulary, but the answer is: Yes if the load is connected wye, no if the load is connected delta.

 

[cuky2000]

Jhgrist & Stevenal: I do concur with both of you that current will flow to the ground if the balanced non-lineal load is connected wye-grounded. Otherwise, not current will be flow into the ground at the transformer load.

 

[che12345]

Dear All,

I am confused after reading the learned opinions.
I wish to know:
a) with a delta linear or nonlinear, balanced or unbalanced load connected between the three phases, the system will work fine. There is no neutral for a delta connected load. Where is the current coming from that is flowing through the earth to the earthed neutral point of the transformer secondary?
b)with a start linear or nonlinear, balanced or unbalanced load connected between the three phases, the system will work fine without having a neutral connection at the load neutral point i.e. neutral is floating. The Y transformer secondary neutral is not connected to the load. Where is the current coming from that is flowing through the earth to the earthed neutral point of the transformer secondary?
Please advise

Che Kuan Yau (Singapore)

 

[jghrist]

che12345,

Your raise an interesting point. I think we have all finally concluded that in case a) with delta load, there will be no current flowing from the transformer neutral to earth.

In case b) with a star load and neutral floating, the load currents in the phases have to sum to zero. This is basic Kirchhoff's current law. The sum of all currents flowing into the neutral point is zero. Can the phase loads be balanced and still have a third harmonic component? I think not. If the fundamental components are all the same magnitude and 120 degrees apart, then they will sum to zero. No problem. If there was a balanced third harmonic component, the three currents would all have the same magnitude and the same phase angle, and would not sum to zero.

All of the load currents sum to zero based on Kirchhoff's current law. The winding currents in the transformer are the same as the load currents. Therefore, the winding currents also sum to zero and there is no current flowing from the transformer neutral to earth.

 

[waross]

@warross: thats what I think, but it won't cause neutral current on the LV, just the primary MV.

Looking at it again, I agree with you.
A non linear load connected from A to be will cause triplen currents in both A and B secondary windings but there will be no involvement with the neutral.
The triplen currents will be reflected in the primary windings and will be seen on the primary neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear All,

I arrived at the following conclusion:
a) for a delta linear or nonlinear, balanced or unbalanced load connected between three phases; there is no neutral at the load. There shall be no current flowing through the earth to the earthed neutral of the transformer secondary.
b) for a star linear or nonlinear, balanced or unbalanced load connected between three phases with the load star point (neither earthed non-connected to the transformer neutral) i.e. floating in the air. There shall be no current flowing through the earth to the earthed neutral of the transformer secondary.

Your learned advice would be much appreciated if anyone disagrees.

Che Kuan Yau (Singapore)

 

[Mbrooke]

@Waross, thats my bet. Not that its final however, my cogs are still turning trying to reach an answer.

 

[jghrist]

I arrived at the following conclusion:
a) ...no current flowing through the earth to the earthed neutral ...
b) ...no current flowing through the earth to the earthed neutral ...

I agree.

 

[jghrist]

Looking at it again, I agree with you.
A non linear load connected from A to be will cause triplen currents in both A and B secondary windings but there will be no involvement with the neutral.

I disagree. Triplen loads from A to B will be cancelled by triplen loads from C to A, leaving none in line A to the transformer. Triplen loads from A to B will be cancelled by triplen loads from B to C, leaving none in line B to the transformer.

 

[stevenal]

Another way to do Kirchhoff:
Draw a region around the secondary winding and load together. The sum of the current entering the region is zero. Since the only path into the region is through the neutral ground connection, the current through the connection is zero.

And now we have the idea that triplen currents in the load can bypass the secondary winding and appear on the primary winding? On what theory? Transformer windings only change the magnitude from one side to the other. Nothing can be added on one side without it influencing the other.

 

[protoslash]

so most agreed on no neutral current in response to OP's original questions.

awaiting prof cuky's comments

 

[Mbrooke]

Where did Cuky get that diagram though?

 

[che12345]

Dear All,

No rocket science. Simple logic. I repeat:
a) irrespective of what type of delta connected load, with neither of the corners nor any tapping in the winding is earthed, i.e. floating in the air. How can there be any current flowing through the earth to the earthed neutral transformer secondary?
b) irrespective of what type of star connected load, where the load star point is NOT earthed, i.e. floating in the air. How can there be any current flowing through the earth to the earthed neutral transformer secondary?

Che Kuan Yau (Singapore)

 

[itsmoked]

che12345; Am I missing something? What I see is everyone agreeing with that. Why are you asking again?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The triplen currents may cancel in the secondary windings but cancelled does not mean that they don't exist.
Triplen currents in the secondary windings will be reflected in the primary windings.
The triplen currents will not cancel in the primary windings but will flow back to the source on the neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

The triplen currents may cancel in the secondary windings but cancelled does not mean that they don't exist.

They don't cancel in the secondary windings. They cancel in the phase-to-phase connected load before they get into the secondary windings. There are no zero-sequence currents in the lines to a phase-to-phase connected load. If there were, restricted earth fault protection would not work, as stevenal has pointed out.

 

[cuky2000]

I thought I did comment earlier. However, I took the freedom to elaborate a bit more presenting supplementary info and a graphic with three typical cases of harmonics generated by a balanced load as follow:
a) For circuits (CASES 1 & 2), there is not ac line currents or triplen harmonics flowing in the neutral and ground since there is no neutral metallic conductor between the load and the transformer.
b) For CASE # 3, consisting of Wye load with a metallic link in the neutrals, current will flow including the triplen harmonics that will be injected into the ground at the transformer.
c) The triplen harmonics are the only components contributing to the neutral current.
d) Triplen harmonics (of orders 3h) correspond to a zero sequence with the three phasors superposed with each other.
e) Grounded-wye windings on both sides of the transformer allow balanced triplens harmonic current to flow from the LV to the HV side of the transformer.

Mbrooke, Not sure if I understood well your question but I developed the diagram using personal experience in the subject, supported by interpretation (occasionally wrong) of technical sources and Standards. The diagram was built with readily available tools such as MS Office, MathCad and low label graphical editors. Here is another example:
====>>>>>

 

[Mbrooke]

Ok, I understand now. Which leads us to ask- is there any primary neutral current? I've been racking my mind and I can't seem to get my brain to spit out an answer.