I am trying to understand a step in frequency control, which I just cannot figure out. Sorry my background is in chemical engineering, and I am just getting into power generation so my knowledge is very limited.
Here is my problem. So in an islanded power system running with one generator (300 MW with 5% droop, setpoint at 150 MW @ 60 Hz), an added load cause grid frequency to drop. Governor arrest frequency decline by ramping up steam flow to generator according to droop setting and "arrest" the frequency drop at 59.9 Hz.
At this point, according to the text I am reading, the governor need to return system to 60 Hz. To do that load reference setpoint is adjusted from 150 MW @ 60 Hz to 160 MW @ 60 Hz. And this will restore grid frequency?
My understanding is that governor monitors frequency and adjust steam throttle valve (MW) according to droop curve. If already operating at 160 MW @ 59.9 Hz, how can simply shifting load reference setpoint add back energy to the system (i suppose this was lost when load was added, due to lost rotational energy of the generator shaft) to cause frequency to rise. My current thinking says that frequency would simply stay at 59.9 Hz since governor would not apply more steam to the turbine because it's already at the throttle setting to output 160 MW.
I know this is wrong, so please help me understand.
>Here is my problem. So in an islanded power system running
>with one generator (300 MW with 5% droop, setpoint at 150 MW
>@ 60 Hz), an added load cause grid frequency to drop.
>Governor arrest frequency decline by ramping up steam flow
>to generator according to droop setting and "arrest" the
>frequency drop at 59.9 Hz.
>At this point, according to the text I am reading, the
>governor need to return system to 60 Hz. To do that load
>reference setpoint is adjusted from 150 MW @ 60 Hz to 160 MW
>@ 60 Hz. And this will restore grid frequency?
So, here's the problem with most texts: The ivory tower egg-heads who write that drivel have never actually operated a prime mover and generator either in parallel with other prime movers and generators or in an island mode, either in single prime mover and generator or two or more prime movers and generators.
There are two basic types of prime mover governor speed control modes: Droop- and Isochronous Speed Control. When operating in islanded mode one of the prime movers (or the lone prime mover and generator) is operated in Isochronous mode. And you cannot set a load reference when for the prime mover and generator operating in Isochronous mode--because the ONLY reference in Isochronous mode is speed (frequency). Unless you know exactly what the actual load is at all times if you use a load reference and the actual load changes before you can change the load reference the speed (frequency) is going to change before you can correct the load reference.
Your problem definition clearly stated load was added to the system after the load reference and the actual load were equal. What do you expect the control system to do--change the load reference by itself? It doesn't do that when you put it in load control.
If the unit were operating in (true) Isochronous mode and load was added to the system (or subtracted from the system) the governor would immediately sense the speed (frequency) change and automatically adjust the energy flow-rate into the prime mover to maintain speed(frequency) automatically increase the prime mover and generator load to match the new actual load.
If the load on a prime mover and generator operating in Droop mode in an islanded situation with the unit in load control mode would do exactly what you described--it would try to maintain the load referenceat the expense of speed (frequency).
So if the load increased by say 10 MW, some of the energy that is required just to maintain 60.0 Hz would be lost to the additional load and the governor would only want to supply 150 MW of load--but the load is actually 160 MW, the difference being made up using some of the energy that is required just to maintain 60.0 Hz--hence the speed (frequency) suffers.
If you want to increase the frequency back to 60.0 Hz you have to add more energy into the prime mover to make up for the energy lost to support the additional 10 MW--and--surprise, surprise!--that additional energy required to return the frequency back to 60.0 Hz will cause the output of the generator output to increase by exactly 10 MW!!! to 160 MW, which is the new actual load.
Do you see how you can't use load control in Islanded mode to automatically control speed (frequency)? If the load changes, you--the operator--have to know precisely when the load is going to change and precisely by how much in order to maintain rated speed (frequency).
However, since speed (frequency) and load are directly proportional--as you noted in your problem description--if the prime mover and generator are operated in Isochronous mode when islanded as the actual load changes--and the speed (frequency) changes proportionally--the governor will immediately respond to the change adjusting the energy flow-rate into the prime mover to maintain speed (frequency), and the load will change to match the new actual load.
Thats another one of the things the ivory tower egg-heads forget to mention about AC power systems: the amount of generation (plus the amount of power required to maintain rated speed (frequency) ) must exactly match the actual load to maintain rated frequency (speed) or else the frequency will be higher or lower than rated.
Now, finally, there are some bastardized versions of Isochronous mode that are not really true Isochronous mode. And if your system is using some kind of "power management system" to maintain frequency by adjusting a load reference to the prime mover governor then, obviously, it's not working properly.
Please be careful when reading texts and reference books on the subject of speed (frequency) control. They most often forget to properly state all conditions when trying (very poorly) to describe speed (frequency) control. It's really not that hard--but if one has zero experience and relies on maths alone, well the results are less than correct.
Hope this helps! The topic of Droop- and Isochronous Speed Control has been covered MANY times here on what sometimes seems like speedcontrol.com. It's really just a balancing act. You must remember that it requires a certain amount of energy just to get to and maintain rated speed (frequency), and the amount of load being produced--when frequency is at rated--equals the amount of energy required to supply the load PLUS the amount required to maintain rated speed (frequency). Change that balance in any way and you adversely affect speed (frequency).
There are two ways throttle valve opening (and therefore steam flow rate and power output and speed in isolated operation) can be adjusted: when speed changes and when speed/ load reference point is changed (by operator).
In the case referred when load increased speed or frequency fell and it remained at the new reduced value. (your understanding is correct). The fallen speed can be restored by increasing throttle (hence steam flow and power) through adjustment of load reference setting. Note new power generation now will match increased load.
Pl refer to my paper for additional info:
This site (scribd.com) wants personal information and eventually money to view documents. Is this document available somewhere else for free and without giving personal information,such as a web-hosting site (like tinypic.com) or Google Drive or OneDrive?
I will also take exception to the statement that a unit in islanded can be operated in fully automatic mode with a load reference and still maintain frequency reliably as load changes. To be fully automatic in islanded operation the reference needs to be speed (frequency), not load, as changes in load will affect speed (frequency). And unless the control system can predict what the load change will be for a given speed (frequency) change the speed (frequency) will be consistently off-rated without manual intervention or additional automatic methods which will usually cause erratic hunting and further instability unless properly tuned, and there are few people who can do that and even fewer plants/owners willing to invest the time and effort to perform the necessary tuning.