Fault analysis and symmetrical components in Power Grids

In summary, the conversation discusses fault analysis in three phase systems and addresses some seemingly contradictory points in a video on the topic. It is clarified that in a phase to ground fault, some phase voltages remain unchanged while others are affected and that this is not a contradiction due to the use of symmetrical components. Similarly, in a phase to phase fault, the unfaulted phase voltage may be affected, but this can be explained by the use of symmetrical components. A phasor diagram is provided to further illustrate these concepts.
  • #1
PainterGuy
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69
Hi,

I was trying to learn fault analysis in three phase system from conceptual point of view. I need your help to clarify few seemingly contradictory points in the following video:



1:
At 32: 42, phase to ground fault in introduced. It is said that line voltage Vbc is not affected as is also the case with phase voltages Vbn and Vcn. Then, it is said that Vab and Vca are affected because Van is affected. I agree with this.

After that in a segment on symmetrical components, around 46:51, the analysis of phase to ground fault (between phase A and ground) is shown. At 47:21 it is said that phase voltages on lines B and C remain unchanged and I agree with this. But then surprising it is said that at the fault point positive sequence voltage fall to two-thirds of their pre-fault value. Isn't this a contradiction?

2:
Similarly around 28:40 it was said that in a phase to phase fault, unfaulted phase voltage is not affected. In a segment on symmetrical components around 43:46 phase to phase fault between lines a and b is revisited. Around 44:39 it is said that the voltage at the fault, the unfaulted phase is approximately equal to its pre-fault value. But later it is said, in a seemingly contradictory way, that unfaulted phase voltage Vcn has been affected.

Thank you for your help and time.
 
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  • #2
PainterGuy said:
After that in a segment on symmetrical components, around 46:51, the analysis of phase to ground fault (between phase A and ground) is shown. At 47:21 it is said that phase voltages on lines B and C remain unchanged and I agree with this. But then surprising it is said that at the fault point positive sequence voltage fall to two-thirds of their pre-fault value. Isn't this a contradiction?

Take a look at the phasor diagram below. The phasors in red, Va, Vb and Vc are the phasors of the unbalanced system. Please note that its phase C that has a ground fault now (Vc = 0), instead of phase A as in your video.

The phasors Va and Vb remains unchanged.

From the picture, the unbalanced system can be represented as a vectorial sum of the positive (black), negative (purple) and zero (blue) sequence phasors.

You can see how the positive sequence component can be 2/3, and added together vectorially with the negative and zero sequence component gives the original phasor.
upload_2017-10-22_21-21-55.png
 

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  • #3
Thank you.

Just curious, did you take the picture from some book?
 
  • #4
PainterGuy said:
Just curious, did you take the picture from some book?
It's from some lecture notes I got from a friend some time ago, I don't think they're available online.

This link gives an introduction to symmetrical components, with equations and has a similar picture: http://www.cadickcorp.com/download/symmetricalcomponents.pdf
 
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Related to Fault analysis and symmetrical components in Power Grids

What is fault analysis in power grids?

Fault analysis in power grids is the process of identifying and analyzing faults or abnormalities in the electrical system, such as short circuits, open circuits, and ground faults. This is important for maintaining the reliability and safety of the power grid.

What is a symmetrical component in power grids?

A symmetrical component in power grids refers to the mathematical representation of the three-phase power system as three symmetrical components - positive, negative, and zero sequence components. This allows for easier analysis and understanding of the system's behavior during faults.

Why is fault analysis important in power grids?

Faults in power grids can cause disruptions in the supply of electricity, damage to equipment, and even pose a safety risk. Fault analysis helps in identifying the cause and location of the fault, allowing for quick and efficient restoration of power and prevention of further damage.

What are the methods used for fault analysis in power grids?

The two main methods used for fault analysis in power grids are the symmetrical components method and the fault impedance method. The symmetrical components method is based on the symmetrical components of the system, while the fault impedance method uses the concept of impedance to determine the fault location.

How do symmetrical components help in fault analysis?

The use of symmetrical components allows for easier analysis of the system during faults, as the positive, negative, and zero sequence components can be analyzed separately. This helps in identifying the type and location of the fault, and in determining the appropriate corrective actions to be taken.

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