Student taking classes from Bismarck State College in Power Plant Technology. Question in my class is why is it important to control the output voltage from a large power producing generator with the Governor controller? Besides the obvious of wanting to supply the voltage level that all the appliances need out there, are there any other reasons to keep the voltage at a set-point in regards to the transmission lines, inductive reactance (VARS) or Capacitance reactance? Talk about some complex, hard to understand concepts!

Help anyone. Thanks a lot.

 

Hello,

This thread, or at least my response to it, is bound to start some bickering (you have to have been reading similar posts on control.com for years to understand, Rick Fisher), but I'm going to dive in anyway. I waited about 24 hours before posting my response to give others a chance to provide their input, and, well, here goes. Hopefully, there won't be any collateral scorching.

I'm going to take exception to one of your statements, the one about using the "Governor controller" to control generator terminal voltage.

In general, a governor is the control system which is used to control the amount of energy going into a prime mover (turbine, reciprocating engine, etc.) that drives a generator. The governor is responsible for ensuring that the torque being produced by the prime mover is stable and can be increased or decreased smoothly as required or as desired.

In general, most synchronous generators (and I presume we are talking about synchronous generators) employ some kind of exciter regulator or excitation control system, also frequently referred to as an AVR (Automatic Voltage Regulator). The AVR might receive some signals (to start or stop the AVR, or to raise or lower generator terminal voltage) from the Governor controller, but the act of maintaining a stable generator terminal voltage is usually the province of the exciter regulator (AVR).

So, it's not the governor that controls the generator terminal voltage, it's the exciter regulator (AVR). If you, or someone else, wants to consider the exciter regulator or excitation control system or AVR as a "governor", then that's your decision and it conflicts with standard definitions and associations.

Synchronous generator terminal voltage is controlled in order to be able to maintain a stable generator terminal voltage (stable anything is usually better than unstable!), and in order to be able to maintain the desired generator power factor. If the power factor of a generator is something other than unity, 1.0, then reactive current will be flowing in the generator stator windings. Most synchronous generators can tolerate "some" reactive current, usually in the lagging direction, without much problem.

If the generator terminal voltage is exactly equal to the system voltage of the grid with with the synchronous generator is connected then the power factor *of the generator* will be unity, or 1.0. In this condition there is ONLY real power flowing in the generator, no reactive current (VArs) (and VARs can be inductive or capacitive, either).

If the generator terminal voltage is allowed to start to drop below the system voltage of the grid with which the synchronous generator is connected then the power factor *of the generator* will be less than unity, less than 1.0 <b>in the leading direction</b>, which is considered to be "capacitive" reactive current (VArs).

If the generator terminal voltage is allowed to start to increase above the system voltage of the grid with which the synchronous generator is connected then the power factor *of the generator* will be less than unity, less than 1.0 <b>in the lagging direction</b>, which is considered to be "inductive" reactive current (VArs).

Power factor and VArs are related. The more reactive current that is flowing in the generator stator the lower the power factor will be. Power factor can never be greater than 1.0 (unity). If the reactive current is "inductive" in nature then the power factor is considered to be lagging, and if the reactive current is "capacitive" in nature then the power factor is considered to be leading.

Hope this helps!

 

I will piggy back CSA's post to help illustrate a point. Hopefully my ASCII diagram is correct and understandable.

Power Factor (PF) never goes above 1.0, but it changes from lagging to leading. This can be confusing in words so look at the diagram below. The voltage examples are just EXAMPLES to prove the concept. The last line is operator slang which I surely will get admonished for around here LOL.

Lower Voltage-------------------------------------GRID voltage------------------------------------Higher Voltage

0.2PF LEAD--------0.8------0.9 LEAD-------------1.0------------0.9 LAG---------0.8-----------0.2PF LAG

11,000V------------------------------------------13,800V-------------------------------------------15,000V

"Taking VARS"------------------------------------Unity-------------------------------------------"Giving VARS"