Symmetrical Components: Intuitive Understanding

[111R]

I have quite a bit of literature on symmetrical components including the Blackburn book on symmetrical components in power systems, but I'm having some trouble fully understanding the topic beyond the equations to find the sequence quantities.

With a single line to ground fault, all networks are connected in series. I'm having trouble visualizing this. I understand that positive sequence current flows from the generator to the fault point. At the fault point, I would assume that zero sequence current flows from the fault point back to the generator through the ground return path. How is negative sequence current involved? What is the source of this?

If you have any good papers on symmetrical components, a link will be appreciated.

 

[BJC]

Edith Clarke's books are on google books.

 

[dpc]

I don't think it is useful to "visualize" symmetrical components. They are simply a mathematical construct to allow easier analysis of unbalance faults. The sequence currents are not "real". They just allow an unbalanced system to be modeled as three balanced systems. When the three sequence values are re-combined to an actual phase quantity, then that is when they become re-connected to the real world.

 

[arrehman1]

@111R,

I think it's important to have an intuitive feel for symmetrical components. I wrote this article that attempts to provide an overview of symmetrical component:

http://gpac.link/175IP7B

I'm planning to write symmetrical component analysis for different types of faults - but who knows how long that will take me.

As for visualizing symmetrical components, check out this simulation:

http://personal.strath.ac.uk/steven.m.blair/seq/

- it's very helpful

http://GeneralPAC.com

- Awesome video tutorials for power systems PAC!
The Wye Wye Transformer Connection video:

http://gpac.link/1GdGRSQ

 

[davidbeach]

I tend to agree with dpc, while symmetrical components are an extremely use construct, they have no existence in "reality". The phase domain exists in real circuit, the symmetrical components are only a mathematical construct. The conversions back and forth are easy enough that we generally solve an unbalance fault problem by making the conversion, solving, and converting back to the phase domain. But you'll never find I1, I2, or I0 in a wire.

 

[xnuke]

You can find 3I0, though - in the neutral, of course.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[davidbeach]

Or, do you find I0 + I1 + I2? How do you tell the difference?

Don't get me wrong, I'd have a hell of time doing what I do if it weren't for dear Mr. Fortesque, but it's all just a mathematical construct. Useful, work simplifying, but not measurable in real life. A modern relay can calculate the sequence components and perform highly useful things with them, but you can't make the intuitive connection between phase domain currents and sequence domain currents that the OP desires.

 

[BJC]

You tube has lectures on everthing.

https://www.youtube.com/watch?v=oYj7j5QBlY0

Nothing to do with this but Gilbert Strangs math lectures are great.

https://www.youtube.com/watch?v=oYj7j5QBlY0

 

[dpc]

It is interesting to note that the symmetrical component method is not limited to three-phase systems. It can work for for any multiphase system - it just requires the appropriate corresponding sequence networks. Well, interesting to me, anyway.

 

[jghrist]

I saw oscillography of the primary current in a grd wye - delta transformer (no source on the delta secondary) during a single phase line fault. This showed pure zero-sequence current with each phase equal in both magnitude and phase angle. It was interesting to see the actual measurements of a theoretical current that is not intuitive.

 

[rcw retired EE]

A tool I found useful when instructing new engineers is Alex McEachern's "Power Quality Teaching Toy"

His interactive symmetrical component demo of the rotating positive, negative and zero sequence vectors combining to equal the unbalanced phase & neutarl currents helps visulaizing the concept. It allows you to adjust the per unit value of each symmetrical component and see the effect.
(I don't have time to find and post the link).