Positive/Negative/Zero Sequence voltages/currents

[jegues]

What does it mean when one refers to,

• Positive
• Negative
• Zero

sequence voltages/currents in relation to 3 phase power system?

My blunt understanding is that in say positive sequence voltages, as we rotate around the phasor diagram in a clockwise manner we see the voltages in the following sequence:

A, B, C, A, B, C... And so on.

In negative sequence voltages the sequence is reversed like so,

C, B, A, C, B, A... And so on.

Am I understanding this correctly?

Why do we chose to make such a distinction anyways?

Also, what does one mean when they refer to zero sequence quantities?

Thanks again!

 

[dlgoff]

I believe these sequences are used when analyzing unbalanced systems with the Symmetrical components method.

In a three-phase system, one set of phasors has the same phase sequence as the system under study (positive sequence; say ABC), the second set has the reverse phase sequence (negative sequence; CBA), and in the third set the phasors A, B and C are in phase with each other (zero sequence). Essentially, this method converts three unbalanced phases into three independent sources, which makes asymmetric fault analysis more tractable.

 

[jegues]

I believe these sequences are used when analyzing unbalanced systems with the Symmetrical components method.

I read the wiki page you linked and it added some clarity, but its still not obvious to me why they are called symmetrical components.

Why the word symmetrical?

What does one mean when they refer to, "symmetrical sets of balanced phasors"?

Thanks again!

 

[SirAskalot]

The reason for using symmetrical components is that a 3 phase unsymetrical system can be analyzed by three symmetrical system/circuits. That is +, - and 0 sequence.

It is (purely?) a mathematical decomposition, and has no physical meaning. (To my knowledge)

 

[dlgoff]

From the wiki Symmetrical components page:

Essentially, this method converts three unbalanced phases into three independent sources, which makes asymmetric fault analysis more tractable.

When looking at electric power system faults,

In a polyphase system, a fault may affect all phases equally which is a "symmetrical fault". If only some phases are affected, the resulting "asymmetrical fault" becomes more complicated to analyse...

bold by me,