NCEES 112

[kris7o2]

Hello everyone, does anyone have any links that demonstrate visualization or drawing representations for this problem? I think I understand the solutions, but I don't completely feel like I truly understand all the variations and intricacies of the question.

When using the method of symmetrical components, which of the following statements is most nearly correct?

(A) The three zero-sequence current phasors are equal in magnitude and displaced by 120°.

(B) The three positive-sequence and the three negative-sequence current phasors are all equal in magnitude, but have opposite phase rotations.

(C) In a wye-connected synchronous generator, the voltage drop through the neutral grounding resistor, R, is equal to ю√3 IoR, where Io is the zero-sequence current.

(D) Positive-sequence and negative-sequence currents cannot pass through the neutral grounding resistor in a wye-connected generator

 

[akyip]

Attached are my notes for the different types of harmonics (positive-sequence, negative-sequence, and zero-sequence harmonics) from Zach Stone's Electrical PE Review, and my notes on problem 112.

The correct answer choice is (D). Positive-sequence currents and negative-sequence currents are each displaced by 120 degrees between each phase. At the neutral connection point where the phase currents combine and add up, the neutral current will be zero because:

1<0 + 1<-120 + 1<+120 = 0

However, zero-sequence currents will pass in the neutral conductor because zero-phase currents have the same phase angles and therefore there magnitudes will add up.

(A) is wrong because zero-sequence current phasors have the same phase angles. This means that they are NOT displaced by 120 degrees.

(B) is wrong because positive-sequence currents do not necessarily have the same magnitudes as negative-sequence currents. However, they will have opposite phase rotations.

(C) is wrong because zero-sequence currents have the same magnitude and the same phase angles. So the neutral current will be triple of one zero-sequence current:

In = Ia0 + Ib0 + Ic0 = 3 * Ia0