Positive/Negative/Zero Sequence voltages/currents

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In summary: The word "symmetrical" is used because all three sets of phasors are in the same sequence, like a choreographed dance. This makes fault analysis easier, because it eliminates the need to consider how the phases are connected (i.e. in a polyphase system, which phases are connected?). Asymmetrical faults, on the other hand, occur when two or more phases are not in the same sequence. This can make fault analysis more complicated, because you need to consider how the phases are connected (and which ones are affected).
  • #1
jegues
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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!
 
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  • #2
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.
 
  • #3
dlgoff said:
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!
 
  • #4
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)
 
  • #5
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,
 

FAQ: Positive/Negative/Zero Sequence voltages/currents

What are positive, negative, and zero sequence voltages/currents?

Positive, negative, and zero sequence voltages/currents refer to the different types of voltage and current components that make up a three-phase power system. Positive sequence refers to the normal or direct rotation sequence of voltages/currents, while negative sequence refers to the reverse or inverse rotation sequence. Zero sequence refers to the common component of all three phases, which is typically used for grounding purposes.

Why is it important to understand positive, negative, and zero sequence voltages/currents?

Understanding these different sequences is crucial for maintaining a balanced and efficient three-phase power system. If there is an imbalance in the positive or negative sequence, it can lead to issues such as increased heating and potential damage to equipment. Additionally, proper understanding of these sequences is necessary for troubleshooting and identifying faults in the system.

What are the differences between positive, negative, and zero sequence voltages/currents?

The main difference between these sequences is their rotation direction and magnitude. Positive sequence voltages/currents have the same magnitude and direction, while negative sequence has the same magnitude but opposite direction, and zero sequence has the same magnitude but no direction as it is a summation of all three phases.

How are positive, negative, and zero sequence voltages/currents calculated?

Positive, negative, and zero sequence voltages/currents can be calculated using mathematical equations, which take into account the phase angles and magnitudes of the three-phase system. These equations involve complex numbers and trigonometric functions, and are used to determine the amplitude and phase angle of each sequence.

Are positive, negative, and zero sequence voltages/currents present in all three-phase systems?

Yes, positive, negative, and zero sequence voltages/currents are present in all three-phase systems. However, the magnitudes and directions of each sequence may vary depending on the type of system and any external factors, such as unbalanced loads or faulty equipment.

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