# Impedance depending upon sources in 3 phase

Discussion in 'General Electronics Chat' started by elecidiot, Apr 20, 2013.

1. ### elecidiot Thread Starter New Member

Jun 29, 2010
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Hi all,
Does impedance of a linear circuit depend upon the source? Generally we say it DOES NOT, then why does the zero sequence impedance differ from +ve and -ve seq impedance. See in all the three cases the geometry of the circuit remains the same but only the types of the sources change. Any clarifications would be appreciated.

2. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Your question makes me dizzy. What is a zero sequence impedance?

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
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It's part of the method of symmetrical components applied to analysis of AC systems with unbalanced operating conditions - particularly under fault conditions.

4. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Well, that clears things up... NOT!

Sorry I can't answer the question because I still have no idea what we're talking about. I'm going to leave this thread alone.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Last edited: Apr 20, 2013
6. ### elecidiot Thread Starter New Member

Jun 29, 2010
23
0
Hi all,
The doubt I posted is in a logical sense and it does not warrant a detailed study in sequence components of a generator. Referring to t_n_k, in the first hand I googled my apprehensions but was not satisfied with the search results. So only thought of approaching stalwarts like t_n_k here since this forum is like the last resort for people whom the google has given up .

Now I will try to present the problem in a much logical manner.

Consider a star connected load having similar impedance on all legs. Let an impedance be connected from the neutral point of the Y load to the ground.
Now suppose we connect a sinusoidal voltage with same magnitude and phase (called technically zero seq voltage but that is not relevant to the logic here) between every terminal and ground, the voltage source in each phase sees an impedance to the ground called Z0. Suppose now we connect 3 sinusoidal sources with same magnitude and differing by phase angle of 120 deg one each at a terminal, the impedance seen by the source at a phase is called Z+ve or Z-ve which is different from Z0 said above. My question is in-spite of the same geometric distribution of the impedance why this discrimination logically.

Again am pinning my hopes on the stalwarts here.
Thanks again people

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
There must be a difference in the star point potential to ground depending upon whether one is considering the zero sequence current flow or the positive and negative sequence current flows.

Clearly the positive and negative sequence balanced sets you propose will not produce any potential rise of the star point to ground. The latter are balanced sets and therefore have no neutral current component to consider. In contrast, the zero sequence current set must flow entirely via the star point to ground and thereby cause a potential rise of the star point relative to ground.

The zero sequence current set produces a commensurate voltage rise in the load star point whereas neither the positive or negative sequence current sets produce any change in the star point potential relative to ground. The logical conclusion is that the inferred sequence impedances are inherently different since they produce different potential rises.

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