Zero sequence current in a system neutral

[mull982]

If there is unbalanced current flowing the the neutral of a 3-phase, 4-wire system is this current considered zero sequence current?

I know that in a 3-phase, 3-wire, system there is no zero sequence current unless there is a ground fault with ground current flowing. Even if the 3 phases are unbalanced the vector sum of the three phase currents will always equal zero in an unfaulted condition, even if unbalanced, and therefore there is no zero sequence present unless there is a ground fault.

So for unbalanced current flowing on the neutral of a 3-phase system, is this current considered zero sequence current?

 

[skeshesh]

For y-connected loads, the neutral current will the sum of line currents therefore the neutral current due to the unbalance is 3 times the zero sequence value. Google symmetrical components and spend some time solving a few problems. You seem to have a good understanding of three phase systems and good grip on math from your previous posts so it shouldn't take too long to get a hang of it.

 

[mull982]

For y-connected loads, the neutral current will the sum of line currents therefore the neutral current due to the unbalance is 3 times the zero sequence value. Google symmetrical components and spend some time solving a few problems. You seem to have a good understanding of three phase systems and good grip on math from your previous posts so it shouldn't take too long to get a hang of it.

Following up on what you said about unbalance being 3x zero sequence current this is how I derive and view what you are saying.

We know the symmetrical equations to be:

Ia = Ia(0) + Ia(1) + Ia(2)
Ib = Ia(0) + a^2Ia(1) + aIa(2)
Ic = Ia(0) + aIa(1) + a^2Ia(2)

So if we know that neutral currents results from the sum of all three phase currents we add together the (3) equations above. When adding these up we would see that the a(1) and a(2) terms would cancel so all we would be left with would be the 3 I0 currents or 3I0. Is this correct for deriving that neutral current will be 3 times I0.

So with that said we can say that neutral current is indeed considered zero sequence current. Is that correct?

 

[billsnuff]

found this thread on zero sequence:http://www.eng-tips.com/viewthread.cfm?qid=19366&page=516

and this:

Reference Protective Relaying Principles and Applications by J. Lewis Blackburn.
Considering a three-phase system, symmetrical components (positive sequence, negative sequence, and zero sequence) allow one to analyze power system operation during unbalanced conditions such as those caused by faults between phases and/or ground, open phases, unbalanced impedances, and so on. The positive sequence set consists of the balanced three-phase currents and line-to-neutral voltages supplied by the system generator. They are always equal in magnitude and phase displaced by 120 degrees rotating at the system frequency with a phase sequence of normally a, b, c. The sequence currents or sequence voltages always exist in three's, never alone or in pairs.
The negative sequence set is also balanced with three equal magnitude quantities at 120 degrees apart but with the phase rotation or sequence reversed, or a, c, b. (If the positive sequence is a, c, b as in some power systems, then negative sequence will be a, b, c.) For the negative sequence set, again the sequence currents or sequence voltages always exist in three's, never alone or in pairs.
The members of the zero-sequence set of rotating phasors are always equal in magnitude and always in phase. Once again, if zero sequence currents or zero sequence voltages exist, they must exist in all three phases, never alone or in one phase.

 

[skeshesh]

...When adding these up we would see that the a(1) and a(2) terms would cancel...

So with that said we can say that neutral current is indeed considered zero sequence current. Is that correct?

I think you meant Ia(1) and Ia(2) terms cancel out as "a" is a complex operative, so assuming that yes you're right.

I think stating neutral current is considered zero sequence current is not a good way to put it - I'd say neutral current will equal three times the zero sequence current when using symmetrical components.

 

[mivey]

Wow mull, they sure gave you a lot of answer. The answer to your simple question is: yes.

 

[skeshesh]

Wow mull, they sure gave you a lot of answer. The answer to your simple question is: yes.

I don't think so; the original question:
"So for unbalanced current flowing on the neutral of a 3-phase system, is this current considered zero sequence current?"

The answer is no. Neutral current is not considered zero sequence current. Neutral current equals 3 times the zero sequence current. Neutral current and zero sequence current while related are not the same thing.

 

[mivey]

I don't think so; the original question:
"So for unbalanced current flowing on the neutral of a 3-phase system, is this current considered zero sequence current?"

The answer is no. Neutral current is not considered zero sequence current. Neutral current equals 3 times the zero sequence current. Neutral current and zero sequence current while related are not the same thing.

You are complicating the question beyond what was asked. He was not asking for a current quantity but a current type. My answer is correct as given.

 

[mull982]

O.K. so it sounds like with current flowing on the system neutral there is zero sequence flowing in the system, and the quantity of the neutral current is 3 times the zero sequence? Is this correct?

Now in a 3-phase 3-wire system there is no zero sequence current even for unbalanced conditions. Isn't this correct?

 

[skeshesh]

You are complicating the question beyond what was asked. He was not asking for a current quantity but a current type. My answer is correct as given.

I'm giving an accurate enough answer as I thought necessary so the OP does not simply think that neutral current and zero sequence are the same thing. I don't exactly know what you mean by a "current type". In any case I respect your opinion as I've read your posts frequently and they have been of quality.

Mull,
Yes thats right. It seems like you got the hang of it.

 

[mivey]

I'm giving an accurate enough answer as I thought necessary so the OP does not simply think that neutral current and zero sequence are the same thing. I don't exactly know what you mean by a "current type". In any case I respect your opinion as I've read your posts frequently and they have been of quality.

In other words, it is common language to say "the zero sequence current flowing in the neutral" or "the zero sequence current flowing in the ground". It is also common to refer to ground currents and neutral currents as "zero sequence currents". It just speaks to the nature of the current as it relates to sequence components. When talking about the magnitude of I0, you will find other terms in the context like "equals", etc. or specifically saying something like "THE zero sequence current is...".

I was pretty sure after mull's second post that he was talking about the sequence component type of the current in the neutral, not the magnitude.

Thanks for the positive comment.

O.K. so it sounds like with current flowing on the system neutral there is zero sequence flowing in the system, and the quantity of the neutral current is 3 times the zero sequence? Is this correct?

Now in a 3-phase 3-wire system there is no zero sequence current even for unbalanced conditions. Isn't this correct?

As skeshesh noted, it appears you've got it. In the unbalanced 3-wire case the positive and negative currents are the only ones there.

There is a cool toy here to get visual feedback on different configurations:
Power Quality Teaching Toy

But don't forget that a grounded source feeding a 3-wire distribution system can still be capacitively coupled to ground out on the system and thus have a "hidden" 4th wire. This would not happen with an ungrounded source as there is no path back at the source.

 

[mull982]

Thanks for the help guys. I now think I grasp the concept.

So in the neutral then there is only zero sequence present (3x I0) and not any positive or negative sequence componenets right?

Now we looked at the equations earlier and said that the Ia(1) and Ia(2) terms all canceled out leaving us with just the I0 terms. We saw this with the origonal equation sets assuming that the magnitudes were all 1. What if though the magnitudes for Ia(1) and Ia(2) were not all 1 but were something different to represent the unbalanced case? Would they still cancel?

Also isn't there positive and negative sequences present in a L-G fault along with the zero sequence since the L-G fault model involves the positive, negative, and zero sequence impedances all in series?

Thanks for the discussion and the help.

 

[mivey]

Thanks for the help guys. I now think I grasp the concept.

So in the neutral then there is only zero sequence present (3x I0) and not any positive or negative sequence components right?

Correct. It may or may not be 3I0, depending on what is flowing in the ground conductor, earth, etc.

Now we looked at the equations earlier and said that the Ia(1) and Ia(2) terms all canceled out leaving us with just the I0 terms. We saw this with the original equation sets assuming that the magnitudes were all 1. What if though the magnitudes for Ia(1) and Ia(2) were not all 1 but were something different to represent the unbalanced case? Would they still cancel?

They could. Look at the toy I linked. You can set the positive current to 1, the negative current to 0.5, and the zero current to zero.

Also isn't there positive and negative sequences present in a L-G fault along with the zero sequence since the L-G fault model involves the positive, negative, and zero sequence impedances all in series?

yes

Add: but keep in mind that the impedance networks are different. Zero sequence current will only flow in the zero sequence impedances, the positive currents in the positive impedances, and the negative currents in the negative impedances. It is not quite the same as a single current flowing through a set of series resistors.

It might be easier to picture the 0,1,2 components much like x,y,z components in a 3-D coordinate system. They must combine to give the space coordinates but have their own separate axis.

The currents have their own impedance network, much like a higher-order harmonic current would produce a different impedance for a frequency dependent component than would be seen for the fundamental current. The circuit can be separately calculated for each harmonic current. The results can be combined to get th net effect we see in the real world.

 

[skeshesh]

Thanks for the help guys. I now think I grasp the concept.

So in the neutral then there is only zero sequence present (3x I0) and not any positive or negative sequence componenets right?

Now we looked at the equations earlier and said that the Ia(1) and Ia(2) terms all canceled out leaving us with just the I0 terms. We saw this with the origonal equation sets assuming that the magnitudes were all 1. What if though the magnitudes for Ia(1) and Ia(2) were not all 1 but were something different to represent the unbalanced case? Would they still cancel?

Also isn't there positive and negative sequences present in a L-G fault along with the zero sequence since the L-G fault model involves the positive, negative, and zero sequence impedances all in series?

Thanks for the discussion and the help.

The tool provided by Mivey is great fun - playing around with various scenarios that you can think of will give you a better understanding.

You're right about calculation of a L-G fault. In addition you may have a fault impedance. When calculating a fault of any sort you're looking at the equivalent impedance (or admittance) of the system from the point of the fault.

Here's one of the first links that came up on google: http://www.ece.umd.edu/class/enee474.F2003/PDF Files/chp10_3.pdf

If you derive the formulas a couple of times on your own you'll get it right for good (or well for a few years until you need to review). Do you have a power system analysis book of some sort around for use?