Islanded Generating Station 6

Mbrooke · Dec 27, 2019

How does an Islanded 600-2000MW generating station behave? What does typical critical clearing time look like? What is the voltage and frequency divergence during and after a fault for various fault scenarios? How is active and reactive power dispatched/controlled? How does generation behave during trip and reclose events? And how do you "block load"?

FacEngrPE · Apr 3, 2020

Regarding firing rate controls for large boiler this link has a reasonable explanation of cross limiting air fuel ratio controls.

https://www.yokogawa.com/us/library/resources/application-notes/ys1700-boiler-combustion-control/.

The flame safety supervisor is a separate system.

The clip is just the basics. usually there are additional components like air - fuel ratio trim, etc.

The water level controls system is similarly complex.
Tuning a boiler operating control system is a chore.

If you don't care about speed regulation, setting the turbine throttle valve to wide open will make turbine power output respond directly to boiler firing rate. You still want a working throttle (over speed safety) system in case the load trips off line.

Fred

crshears · Apr 4, 2020

The coal-fired steam plant where I worked for just over six years was an interesting place; it had started as a two-unit plant, but was expanded to four units, then six, then eight...with the predictable result that the newer units used a later generation of control technology.

Semi-anecdote:

Units 1&2 had front-fired Babcox-Wilcox boilers with pneumatic Bailey controls; reportedly, these worked well, but could have considerable time lag. An operator in the 1&2 control room I was chatting with said that once he had spun a control knob several revolutions as part of making a load change, then realized he had spun the wrong one. He was sure the unit must be heading for a trip because of his mistake, but discovered with some amusement that the controlled device had barely begun changing its output, so he rapidly spun the knob back to where it had been and carried on. This behaviour was in stark contrast to 3&4, which had tangentially fired Combustion Engineering boilers with solid state controls; the operators said the control response was so rapid they'd almost swear sometimes changes would happen before they'd been dialed in...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

waross · Apr 4, 2020

Another anecdote.
My controls instructor related a incident when commissioning the controls for a new pulp mill.
This was one of the first to go electronic instrumentation.
They were experience a number of failures of new valves.
They traced the failures to operator error.
With the old pneumatic controls condensation in the air lines was an issue.
If an operator suspected a condensation blockage in a control line he would blow it past the trap by cycling the control from setpoint to zero percent, to 100 percent, and back to the set point.
The operators often did this, "just in case".
A demonstration was set up.
A control valve was mounted on a stand and connected to a controller.
This was a pneumatically operated valve controlled by an electronic valve.
120 PSI plant air controlled by an electronic signal, rather than operated by a 3-15 PSI signal.
An engineer cycled the control as the operators normally did on the old systems.
The response was so fast that the valve jumped up off the floor.
The operators took note and there a=was no more damage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

FacEngrPE · Apr 4, 2020

Electronic controls can be very fast, but often if too fast they have control stability problems.

In the coal fired steam plant crshears discusses units 1 and 2 were perhaps a bit slow, 3 & 4 sound like they are faster than they needed to be and likely would have trouble getting air fuel trim to work well. Air Fuel O2 trim always disables itself when the firing rate changes significantly.

On the other hand many Navy marine boilers on needed to pass a test which ramped the firing rate from 20% load to 80% load in 90 seconds with no smoke at the stack (found this in a WW2 B&W type M Boiler manual). The Navy was OK with the high maintenance costs of these very responsive boilers, as the get up and go in a hurry is sometime a lifesaver.

(from

https://maritime.org/doc/destroyer/steam/sec05.htm)

During the Pearl Harbor attack USS_Aylwin_(DD-355)

https://en.wikipedia.org/wiki/USS_Aylwin_(DD-355)

was able to get underway from nearly cold iron while under fire.

https://en.wikipedia.org/wiki/USS_Aylwin_(DD-355)

states that the event starts with one boiler steaming auxiliaries only, 15 minutes to main steam on line, two boilers 25 minutes (8 minutes after main steam was on line) from start of the event and they were away from the pier and heading out of the harbor at full speed.
Per

https://destroyerhistory.org/assets/pdf/generalinformationbooks/348farragut_GIBook.pdf

design shaft horsepower was 2x42800. More background here

https://destroyerhistory.org/goldplater/farragutclass/