Eng Pro Guides Final Exam Q 74 - Power System Stability

[akyip]

Hey guys,

Re-working Eng Pro Guides April 2020 Final Exam, and on problem 74:

"Which of the following methods is NOT always used to increase a power system's stability?"

The answer is "Increase angle between mechanical angle between generators' rotor and stator fields". I pretty much understand why this is: generator power delivered P = (Ea * Vt / Xs) sin(theta) or P = (Vs * Vr / X) sin(theta). If you increase theta over 90 degrees, you lose max power and machines start to lose synchronism.

But there is another answer choice I see: "Increase generator excitation current." If you continue to increase this current, wouldn't there be a possible issue of overexciting the generator?

Any inputs on this?

 

[Cram For The PE]

There is the field current limit. But the question is asking about stability. If you ever look at the generator capibily curve (GCC) you will see the Steady-State Stability Limit (SSSL). This lies in the under-excitation plane. So as you increase field current you get further away from this limit. See below:


So with all other things being equal an increase in field current gets you further away from SSSL.

As a practical matter, if you think about a generator connected to an infinite system than P=(Vt*Vinf/X)*sin(power angle). The excitation current controls Vt. During a transient condition like a fault a significant reduction in the machine terminal voltage Vt and the ability of the transfer of power occurs.

So as Vt is reduced P is also reduced. To prevent this collapse in P requires your excitation system to increase field voltage very very fast  to hold Vt to a good value. Also when the fault is removed another violent change in the excitation current is required. So in reality the relationship can be complex. But the question is asking "NOT always used". So your Ie moves up and down to keep stability.