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GISdude
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Symmetrical Components - Pos., Neg., Zero Sequence

09/27/2015 10:59 AM

Hi all,

I'm still a little fuzzy on symmetrical components. IN A BALANCED 3-PH system, currents and voltages are 120 degrees apart, yes? So, when you're using the equations for finding faults, what are the Pos., neg. zero sequence parts? I'm confused. Is this just the phase sequence of the FAULTED line curents/voltages?

Many thanks for the help.

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lyn
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#1

Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/27/2015 11:33 AM

Tutorial on Symmetrical Components Part 2 - Schweitzer ...
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Legolaz
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#2

Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/27/2015 11:35 AM

What are you using to tell the difference?

Wave got an amplitude and phase.

Yes, balance phase is 1200 between L1,L2,L3 in current and voltages. But, voltage and current lag sometimes in variance to loading.

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Rixter
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#3

Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/27/2015 11:37 AM

Here is a picture that might help you:

Think of a wagon wheel with three equally spaced points on the rim (120 degrees apart), which represent the three phases. The axle is the neutral (which is grounded). As the wheel turns, the voltage from each phase to neutral is the vertical distance from the phase point to the axle (neutral). The voltage between phases is the vertical distance between the phase points. At different times as the wheel turns, these voltages will alternate positive and negative.

If the load is pure resistance in each phase, the current will be in phase with the voltage. If the load is inductive the current will lag behind the voltage in time. If the load is capacitive, the current will lead the voltage in time.
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RAMConsult
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#4

Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/27/2015 12:34 PM

In a balanced three phase system there is no need for symmetrical component analysis, you can just use one phase since the result in the other two phases has the same magnitude but shifted +/- 120º.

Once you introduce any kind of imbalance, whether it's unbalanced loads, an open phase, and/or certain types of faults, then symmetrical components will help you understand what is happening to the voltage and current, which will be different (unbalanced), in each phase and leg of the load.

The positive, negative, and zero sequence components are the result of the mathematical decomposition of the set of phasors that result from the unbalance, whatever its cause. The components don't actually exist but their effects can be calculated, measured, and observed. It is analogous to the decomposition of a nonsinusoidal periodic waveform via Fourier analysis, the original waveform is broken down to a series of sinewaves whose effects can be calculated, measured, and observed, but don't actually exist in the original waveform.

Read the link that Lyn provided, there's plenty more available at Google U.

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lyn
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#5
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Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/27/2015 12:39 PM

Rixter gave an explanation that even I could grasp.No idea why anybody would vote it OT.
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RAMConsult
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#7
In reply to #5

Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/27/2015 10:28 PM

Trouble is it only refers to a balanced set of phasors where symmetrical components aren't necessary. To extend his analogy consider a wagon wheel where the spokes aren't symmetrically spaced and are of different lengths; the distance from the center of rotation (the hub) now varies as the wheel rotates and the distance between the hub and the ground is constantly changing on a cyclic basis.

The instantaneous height of the axle to the ground represents the magnitude of the voltage and the tension/compression (load) on each spoke represents the current. It's a much more involved calculation to figure out the instantaneous value of the time varying quantities under those circumstances, that's where symmetrical components are required. (and no, I didn't vote his answer off-topic, I reserve that for totally off the wall postings)

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lyn
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#8
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Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/28/2015 12:27 AM

I'll leave it to the OP to sort the wheat from the chaff. If they are able.

I didn't think the OT was you.
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SolarEagle
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#6

Re: Symmetrical Components -Pos., Neg., Zero Sequence

09/27/2015 1:35 PM

http://forum.allaboutcircuits.com/threads/unbalanced-3phase-system.44180/

https://en.wikipedia.org/wiki/Symmetrical_components

http://www.electrical4u.com/electrical-fault-calculation-positive-negative-zero-sequence-impedance/

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Crabtree
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#9

Re: Symmetrical Components - Pos., Neg., Zero Sequence

09/28/2015 5:53 AM

The voltages and currents can be 120 degrees apart in an unbalanced system too, Mildred. It's the presence of inductance and capacitance that varies the phase angle between voltage and current and the shape of each waveform.

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RAMConsult
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#10
In reply to #9

Re: Symmetrical Components - Pos., Neg., Zero Sequence

09/28/2015 10:27 PM

No, it's the degree of imbalance caused by the loads, not the nature of the loads themselves. The only way you can have unbalanced voltages and still maintain a symmetric 120º phase spacing is if the machine is unloaded or the load is small relative to the rating of the machine; i.e., a 100 watt bulb across a 100MW generator.

Students will try and argue that a single phase load across a 3Φ generator will have no effect on the output of the remaining unloaded phases, but forcing them to do the phasor/vector math soon convinces them otherwise.

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GISdude
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#11

Re: Symmetrical Components - Pos., Neg., Zero Sequence

10/07/2015 3:31 PM

Thanks again for all the repsones. @RAMconsult, your comments were very useful.

I'll do some of the examples in the workbook to solidify my understanding.

Thanks again.

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