Following the theme of preparing for qualitative theory and concept questions, which caught many students off guard during the 2016 October Electrical PE Exam, here is another example of how to combine several different formulas to derive a relationship that solves a qualitative question.
Sample Question: True or False: The capacitance of a transmission line decreases as the spacing of phase conductors decreases.
Correct answer: False.
Let’s look at the formula for line capacitance per unit length of a three phase transmission line:
Where GMD, is the geometric mean distance between phases, or:
And D1, D2, and D3 are the distances between three separate conductors:
We know that as D1, D2 and D3 increase, the value of GMD will increase since:
And as the value of GMD increases, the value of capacitance will decrease since GMD is a divisor of the capacitance formula:
Therefore, we know, that increasing conductor spacing will decrease line capacitance and decreasing conductor spacing will increase line capacitance.
The correct answer is False.
Or, The capacitance of a transmission line will not decreases as the spacing of phase conductors decreases.
If you are having a hard time following these steps, or you would like to try a more practical way to solve this question, try solving for capacitance using D1 = D2 = D3 = 1, and comparing it to the capacitance if D1 = D2 = D3 = 100.
You will end up with a smaller value for capacitance with the much larger conductor spacing distance.
Arturo Arias
Thank you!
I have seen also a question related to inductance been increase or decrease as transmission line Increase/decrease lenght
Zach Stone, P.E.
Arturo thanks for commenting.
There are infinite ways qualitative theory questions can be asked.
A great way to practice is to try coming up with your own and seeing if you can solve them using the above method of deriving relationships from the different associated formulas.
Just a quick reminder!
Our policy conforms to the agreement that we have all made with NCEES on not sharing any actual specifics of past questions we have encountered on the PE exam so I am going to assume that you are referring to a problem in practice exam book or text book, of which there are many.
If this is not the case please edit. If not then disregard.
Good luck studying.
Hannah Salafia
This is a really helpful article and hones in on some common areas of confusion. Really thankful that Zach took the time to write this.