• A phase-to-ground fault would primarily produce which type of current?
Your Answer: Negative sequence current.
could someone help out with this one?
• A phase-to-ground fault would primarily produce which type of current?
Your Answer: Negative sequence current.
could someone help out with this one?
This may be helpful, illustrated article from GE gives a good basic introduction to Symmetrical Components -> http://www.gedigitalenergy.com/smart...ymmetrical.pdf
If this question hasn't been answered already, the following may be helpful...
Single line to ground (SLG) faults are interesting in that the positive, negative, and zero sequence current are all equal in magnitude and phase. This is because the sequence impedances are connected in series when analyzing a SLG fault.
Line-Line (LL) faults produce (theoretically) no zero sequence current at all. Only positive and negative sequence currents with equal magnitude; phase angles displaced by 180 deg. Only positive and negative sequence impedances are connected in the LL fault.
Line-Line-Ground (LLG) faults will have mostly positive and negative sequence currents with a zero sequence current of smaller magnitude. This is due to a parallel connection of Zpos, Zneg, and 3*Z0.
And last but not least, a 3-phase fault will produce (theoretically) all positive sequence current. This is due to the fact that a 3-phase fault is basically just a REALLY big, balanced load. Balanced loads consist of only positive sequence impedance.
Where do the terms positive, negative, and zero currents come from? Im having a hard time understanding this.
Positive, negative, and zero sequence are terms from the topic of symmetrical components. As a technician, you wouldn't need to know a tremendous amount about the topic, but if you dig into protective relaying it would be very beneficial to you to have a reasonable understanding of the topic.
In short, symmetrical components is a method (mathematical tool) for analyzing unbalanced polyphase systems. Most folks think of 3-phase systems as the only polyphase system, but in reality, we're not limited to 3-phase at all.
Using basic algebra and trigonometry (vetor math, phasor analysis), we can deal with balanced 3-phase systems, but we can't really analyze unbalanced 3-phase systems. It's sorta like trying to drive a nail into a piece of wood with a nut driver; wrong tool for the job.
To really dig into symmetrical components requires some (relatively) heavy duty math which is beyond the scope of this website. However, you can gain an elementary understanding of the topic by doing some google searching and practicing the formulas you find.
If you really want to make your brain sore, get a copy of any old text book on the analysis of faulted power systems. It will explain everything in painful detail.
Thanks for helping me understand this McDaniel but I am still having trouble with where the terms come from. For example what makes a positive sequence "positive," same for negative and zero.
If I understand correctly, zero sequence currents are balanced (sort of like a zero sequence CT?). If this is correct, are positive and negative components unbalanced? What's the difference between positive and negative sequence?
I'm mainly looking for an easy way to remember the difference in each. Thanks again.
There are three sets of independent components in a three-phase system: positive, negative and zero for both current and voltage.
Positive sequence voltages (Figure 1) are supplied by generators within the system and are always present.
A second set of balanced phasors are also equal in magnitude and displaced 120 degrees apart , but display a counter-clockwise rotation sequence of A-C-B (Figure 2), which represents a negative sequence.
The final set of balanced phasors is equal in magnitude and in phase with each other, however since there is no rotation sequence (Figure 3) this is known as a zero sequence.
Hope this helps you adamwilson. The excerpt is from the article I posted earlier in this thread.
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