Cram for Exam Vol 3 Q 19 - Reducing transmission line inductive reactance

[akyip]

Hi guys,

I just want to make sure I correctly understand something regarding transmission line inductive reactance.

On Cram for Exam Vol 3, question 19 asks which answer choice would not reduce the line's inductive reactance. The correct answer choice is "Lower Transmission voltages."

The explanation in the solution is that raising transmission voltages raises the line's inductive reactance. Is this because the voltage drop across a line will drop, and thus the inductive reactance would drop since Vdrop = I * Xline? I'm just trying to understand this correctly.

Thank you to anyone who helps with this!

 

[Dude99]

Hi guys,

I just want to make sure I correctly understand something regarding transmission line inductive reactance.

On Cram for Exam Vol 3, question 19 asks which answer choice would not reduce the line's inductive reactance. The correct answer choice is "Lower Transmission voltages."

The explanation in the solution is that raising transmission voltages raises the line's inductive reactance. Is this because the voltage drop across a line will drop, and thus the inductive reactance would drop since Vdrop = I * Xline? I'm just trying to understand this correctly.

Thank you to anyone who helps with this!

Lower V does not lower XL but raising V raises XL ?

 

[Orchid PE]

Is this because the voltage drop across a line will drop, and thus the inductive reactance would drop since Vdrop = I * Xline? I'm just trying to understand this correctly.

A) Because of mutual inductance
C) X_L = jwL (lower frequency, lower the reactance)
D) Counteract the inductive reactance

Just by inspection, B has to be false. But I am interested to see an explanation on the reasoning behind why "Increasing voltage reduces the reactance."

 

[Dude99]

2 WAG's

P Max = 3 x Vs x Vr /  XL so if P load is constant raising V increases XL

or does increasing V increase shunt C offsetting or 'lowering' apparent XL???

 

[Orchid PE]

or does increasing V increase shunt C offsetting or 'lowering' apparent XL???

I was kind of thinking something along these lines, but I would've thought Cram's answer would have mentioned it.

 

[akyip]

Guys,

Thank you for your responses. Cram's solutions did not explain this at all (it only said which answer choice was not a true statement), but I should've thought of the P = Vs * Vr / XL equation that Dude99 mentioned. That makes sense now.

 

[Cram For The PE]

Guys,

Thank you for your responses. Cram's solutions did not explain this at all (it only said which answer choice was not a true statement), but I should've thought of the P = Vs * Vr / XL equation that Dude99 mentioned. That makes sense now.

The correct answer is B.

I meant to say that a high voltage line as compared to a low voltage line will have a higher inductance due to the spacing requirements for a high voltage line compared to a lower voltage line. I do not mean to say that raising the voltage raises the inductive reactance. I will get this clarified and re publish. Thank you for brining this to my attention.

 

[Orchid PE]

My initial thought was that voltage has no correlation to the inductive reactance. Also, just because answer B was false, doesn't make the opposite true. But then I read the answer key and was confused. Good to hear the reasoning behind it. 

 

[Cram For The PE]

My initial thought was that voltage has no correlation to the inductive reactance. Also, just because answer B was false, doesn't make the opposite true. But then I read the answer key and was confused. Good to hear the reasoning behind it. 

Yes you are correct about no correlation. That is what I will say in the answer key rather than try to confuse about my previous statement.