Re: Effectively Grounded
Originally posted by poweringtech: What is zero sequence? positive sequence? and negative sequence? How are reactances determined?
What are they? They are the first half of a semester-long course on electrical analysis. There is no easy way to explain the concepts in this type of Forum discussion. But here is a very quick, simplistic introduction:
We all know that we are supposed to balance the loads on each service, switchboard, panel, whatever. Some of us are aware that in a balanced three phase system, the true power is constant (i.e., does not vary form moment to moment). But what happens when the load is not balanced? Also, what happens when there is an unbalanced fault (e.g., single-line-to-ground)?
Some brilliant guy (whose name I could look up, but you all can use Google too) submitted a short mathematical paper (that led to his being awarded a PhD) that proved that any unbalanced system can be modeled with a set of three balanced systems. Instead of the original (unbalanced) Phases A, B, and C, you use the three sets (A1, B1, C1), and (A2, B2, C2), and A0, B0, C0). Each of the three new sets is balanced by itself. The class you take to learn about this stuff tells you how to calculate the values of the three sets, and how to use them to analyze the original (unbalanced) system.
The first set, called the ?Positive Sequence,? rotates in the same pattern as the original set. That is, the sequence of the original is A, then B, then C. The sequence of the new set is A1, then B1, then C1.
The second set, called the ?Negative Sequence,? rotates in the opposite pattern as the original set. That is, the sequence of the original is A, then B, then C. The sequence of the new set is A2, then C2, then B2.
The third set, called the ?Zero Sequence,? does not rotate at all. That is, while the original Phase A voltage is out of phase from the original Phase B voltage by 120 degrees, the three ?Zero Sequence Voltages? VA0, VB0, and VC0 are in phase with each other. It should be noted that the fact that these are in phase is a key reason that the 3rd order harmonic currents add in the neutral, and a key reason for sometimes having to use a 200% neutral conductor.
