Thread: Positive, negative & zero sequence faults & currents

1. Positive, negative & zero sequence faults & currents

Could anybody please explain the above to me in simple terms, if possible.

2. Originally Posted by MGM
.....explain the above to me in simple terms...
Probably not, it's not a particularly simple subject. The basic theory is fairly simple but it rapidly descends into fairly complex vector and phasor mathamatics.

How deep down the rabbit hole do you want to go? I'll start you off so say when you've had enough. I'll also hunt around for some simplified unbalanced network explainations on the net for you, we're going to run into character set issues on a forum board if you want formulas, calcs and phasor diagrams.

Simply... With a star generated system there's a grounded neutral in the centre point and three phases of equal voltage but 120degrees out of sequence. This isa balanced system. What you'reasking about is unbalanced 3-phase systems as caused by faults between a phase and earth or between two phases or inductive / capacitive loads which present an impedance at an angle that lags or leads the angle of the supply voltage. The result is a current that flows out of phase with the supply voltage angle.

3. To assess an unbalanced system you can break it down and redefine it as it's equivalent 3 balanced systems which are the positive sequence component, negative sequence component and zero sequence component, these can be plotted separately as phasor diagrams.

Here's the best laid out explainations I could find. It's one of the few that you don't need to be a direct descendent of Einstein to understand it. http://cdpemosspublic.wsu.edu/Lists/...Components.pdf

I know one of the members here had fairly extensive experience in power factor correction and similar systems, maybe he might be able to chip in with a better explaination.

4. Thank given for this post:

Dave A (20-Sep-12), MGM (19-Sep-12)

5. Hi Andy, thanks for your input so far. Much appreciated. Unfortunately I am fairly busy at the moment, but hopefully by the weekend I can ask some question around this, and with some guidance maybe get a grip on this. Thanks again.

6. Whenever I look at any issue, I take note of the description thereof i.e. positive sequence. This I then compare to other descriptions in the same context i.e. negative and zero sequence. The following are assumptions and I am aware that assumption is the mother of all f^&*-u#\$.

a) On a linear scale from say, -100 to +100, postive will then be greater than 0 to +100, negative smaller than 0 to -100 and zero will be at 0 only.
b) Alternating systems do alternate from zero to positive, back through zero to negative then back to zero continuously repetitive i.e. sine wave. In SA we have a three phase symmetrical generated electrical power frequency of 50Hz.
c) When a three phase system is balanced by the load impedance per phase, this then will be symmetrical if all the phases follows the same pattern continuously and repetitively. The same applies to an unbalanced impedance per phase and then becomes assymmetrical w.r.t. comparison to the three "individual" sine waves.
d) When I refer to a postive sequence in a balanced three phase system, could I then assume it is according to rotation (120° apart) ABC. Negative sequence then to be ACB, if A is the starting phase reference. Zero sequence will then be all three phases in "phase" i.e. no phase displacement.

I will read through the document attached previously, but I am not sure how to approach my "thinking" around this. If my assumption is incorrect w.r.t. c) and d) above, what then constitutes the terms postive, negative and zero sequence, especially in a unbalanced system? Maybe I am completely of the mark here, but I believe it is better to ask a "stupid" question once than to stay uninformed. I am conversant with complex numbers and the application thereof.

7. Still trying to sort this out.

8. Sorry MGM, I've not been ignoring you, I've been out of the country for a while. I'll post back here in the next few days when I have time.

9. Thanks given for this post:

MGM (19-Oct-12)