Question -
A generator is being run at synchronism with the voltage slightly below system voltage to me (probably should be ‘the’) operational needs of the system. To maintain stability what should be done?

a. Over excite the generator

b. Increase the torque of the prime mover

c. Apply the governor to increase the rotational speed

 

Gary C,

Question 1: What is unstable? The load or the voltage?

Question 2: Is the generator being operated in parallel with other generators or is it operating independently of other generators in Isochronous control mode?

You haven't supplied us with enough information to be of help.

 

Okay; this looks vaguely like a Rainbow Book-type question. It's certainly a poorly-worded question. Probably written by someone charged with writing a test who knew little about the subject matter of the exam and was reading a poorly-written textbook or reference.

By process of elimination:

1) In the condition described by the question statement, the excitation can be described as "under-excited" with the generator operating with a leading VAr indication and a leading power factor. By "over" exciting the writer might mean just increasing the excitation to the level that the generator terminal voltage is equal to the system "requirements", which would zero the VAr indication and put the power factor at unity (1.0), which might be what the writer was getting at. Technically, over-exciting the generator would increase the excitation such that the generator terminal voltage is slightly greater than the system "requirements" and the generator would be operating with a lagging VAr indication and a lagging power factor.

2) Increasing the torque of the prime mover when the generator is in "synchronism" will increase the power (watts) output of the generator over its present level (presuming it is being operated in parallel with other generators and their prime movers).

3) Apply the governor to increase the rotational speed. Now this is really lame; without a governor the prime mover power output would be unstable to begin with (think uncontrolled fuel flow to an engine or steam flow to a turbine). A governor of some kind is required to be able to control the speed to be able to synchronize the generator to the grid or to hold the speed at "synchronism".

Of the three choices (none of which is really very good), my guess would be #1, but that's just a SWAG. And a little experience from having used the Rainbow Book and taking government certification exams written by people with very little knowledge or experience with subject matter (mis)reading texts and reference books (which may not have been well-written to begin with).

Please be sure to let us know what the correct answer was!