Re: zero sequence CT versus per phase CT for ground fault pr
It is not possible to explain the concept of ?zero sequence,? without a bunch of math that I don?t remember well and that I wouldn?t inflict on you anyway. But here is a ?sneak preview? of the concept (a spoonful of sugar, if you will), just to help the phrase itself (i.e., the medicine) go down easier.
We all like to work with balanced systems. We try to balance the load on our power panels. If we look at the signals on a scope, we like to see three waves, spaced apart in time by one third of a cycle, and each having the same magnitude. When we have this situation, any analysis of the signals becomes easy. You can use some of the basic trigonometric relationships (about adding and subtracting sines and cosines and such stuff), and get to a solution without arduous labor.
But what if the phases are not balanced? What if the phases are not 120 degrees apart? What if the magnitudes are not equal? Then the trigonometry becomes almost impossible to apply. However, some brilliant guy (who by the way, was given a PhD for his efforts, I understand), come up with a simplifying trick. He proved that you can take three imbalanced phases, and express them with exact accuracy (not just a simulation or an approximation) by using three sets of three phases. You might say that he turned the three-phase system into a nine-phase system.
Why is this simpler? Because the first set of three phases is exactly balanced. All three magnitudes are equal, and the three are 120 degrees apart. This is easy to work with. By the way, the first set of three phases rotates in the same sequence (A ? B ? C) as the original, unbalanced system. This first set of phases is called the ?Positive Sequence.?
The second set of three phases is also balanced. But it rotates in the opposite sequence as the original (A ? C ? B). It is called the ?Negative Sequence.?
The final set of three phases is balanced, but in a difference sense. The three phases have the same magnitude, but there is no phase rotation. All three are exactly in phase with each other. On a scope, you would see one exactly on top of the other, and could not tell them apart. If you drew vectors to model them, there would be three parallel vectors of the same length, as opposed to three vectors at 120 degree angles from each other. Since there is a zero phase angle between the three phases, this set is called the ?Zero Sequence.?
One more tidbit. If you look at balanced phases, and add up all three currents at any point in time, you will get zero. But what if you add up the three ?zero sequence? currents at any point in time? Well you will get a magnitude of three times the size of any one of them. They don?t cancel. This is the reason that harmonics give you problems with neutral currents. The third harmonic gives you zero sequence currents that add up in the neutral, and can overload it.
