Alternator connected to a long line

[jaus tail]

Homework Statement

[/B]

Homework Equations

I think right answer should be A. Long line on no load is like capacitor so due to ferranti effect the terminal voltage is high. Now when alterator is disconnected the terminal voltage will drop to E_f

The Attempt at a Solution

Book says answer is D. It says due to Ferranti effect, terminal voltage does not change and field current decreases. that's the only explanation given.
Am I wrong or is book wrong?

 

[anorlunda]

I agree. The capacitive current of the transmission line has no place to flow except back to the generator. No matter what the voltage regulator does, the current through the sending end breaker can not be zero prior to opening. So if we interrupt that capacitive current, the terminal voltage should drop instantaneously.

The only question should be, "Would the steady state terminal voltage be substantially different than the time 0+ voltage, while holding field voltage constant?" I would have to think about that. Ping @jim hardy.

Anyhow, option D sounds very unlikely.

 

[anorlunda]

I should add that the premise of the problem is unrealistic. It is the receiving end of the long line that will experience the highest voltages; potentially damaging over-voltages. Following radial load rejection, frequency also starts increasing rapidly driving receiving end voltage still higher. We would not allow that to persist until steady state before tripping the sending end breaker.

You just made me remember from 50 years ago, my first patent was for a device that diagnosed that situation and initiated an immediate trip.

 

[jim hardy]

Well it's a pretty safe bet the field gets DC so with constant field voltage one expects constant field current.

Capacitive load current on an alternator produces armature reaction MMF that reinforces the field MMF
so removing that capacitive load will lower flux, in turn lowering terminal voltage.

Is this the same textbook as in your other thread ?
My Mother-In-Law ( a great lady) was a textbook editor for a major publisher.
It is not uncommon for authors to have students write the study example problems .
My Mother-In-Law hired local graduate students to check them for her and she'd resolve mistakes with the author. (There was never a shortage of Grad Students needing part time work in New York City.)
I suggest you approach teacher, if you find the book is fraught with mistakes it reeks of sloppy authoring and editing. Your department head should lodge a complaint with the publisher and the department should go back to a better book.

That's my opinion..

If you find I'm wrong in my analysis please advise.

LATE EDIT see also https://www.physicsforums.com/threads/armature-reaction-drop.826513/#post-5191404