# Explanation of NCEES #112



## ElecPwrPEOct11 (Sep 24, 2011)

Hi.

I worked through this problem and am not satisfied with NCEES answer. I understand how to eliminate answers a-c from your choice, but I would like to understand why option D *is *the correct answer. Is it because NO symmetrical components technically pass through the neutral grounding resistor?

I think my problem is that I don't understand how positive, negative, and zero sequence components relate to physical voltages/currents in the 'real' world. Thanks for any help.


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## Flyer_PE (Sep 24, 2011)

The reason D is the correct answer is that the positive sequence currents are balanced and the negative sequence currents are balanced. If your currents are balanced, the neutral current will be zero. By definition, only zero sequence currents will flow through the ground resistor.


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## ElecPwrPEOct11 (Sep 27, 2011)

Flyer_PE said:


> The reason D is the correct answer is that the positive sequence currents are balanced and the negative sequence currents are balanced. If your currents are balanced, the neutral current will be zero. By definition, only zero sequence currents will flow through the ground resistor.


Very succinct and well put. That makes sense, thanks! I had never thought of sequence currents in a context outside of the mathematics. They're still such a weird concept to me.


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## Flyer_PE (Sep 27, 2011)

I think I've posted this link before but I can't remember where. There is an application you can download for free that has some neat graphical representations of what is actually going on with symmetrical components.

Alex McEachern's Power Quality Teaching Toy


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