Island Mode Frequency Control


We have a manufacturing facility with our power generation through 15MW STG and multiple 2MW DGs. DGs are the only machines capable of providing black start, and the STG is then synchronized afterwards. All generators are running in droop mode (STG droop: 4%, DGs droop: 5%).

In the above scenario with no generator on frequency control, how is the grid frequency controlled at 50Hz?

There would be two (2) ways to accomplish frequency control under the conditions you have described.

The operators would have to be manually adjusting the loads of one or more generator-sets to maintain the desired frequency.

Or, there is some kind of frequency monitoring and control system that sends signals to the prime mover control systems (the "governors") to control frequency. A common name for such a system is Power Management System (PMS), but there are many other names and abbreviations' as well.
Because likely you are connected to the local electrical grid, and you are locked to its frequency.

Only if the local grid has blacked out is there no frequency reference for your generators.

Is your black start from a complete local grid failure or an electrical blackout just on your site?
Can the local grid support your plant load?

PC Load Letter

I saw in the other post that you have a woodward 505e governor control system. Is that just for the steam turbine or for all the DG's as well ? What you need is something called a Isochronous load sharing scheme to maintain the frequency during load variations.

If you have Woodward 505E's & DSLC's (Digital Synchronizer & Load control ) on all the units then you can potentially set up a load sharing system over the "Lon" network. Setup is relatively simple and it works well.

Just call Woodward application group; the ones I know based out of fort collins are a good bunch they will help you out ( might try to sell you something new as 505E's are kind of a retired unit ; you cant buy them for new projects, but they still support them)

If you have different governor then you need to hunt for a third party load sharing controller which will work with all your different governors .... from experience that's a cash pit.
Interesting question. First, let's speak same language.

1. Droop control is when generator reacts of CHANGE of frequency, not controlling it. It's generally done when you're on the grid, some grid operators advise or even demand you to have some spare capacity (like 5% of load) and to be able react at certain speed and droop %.

2. Isochronous control does not care about frequency change it case about actual frequency setpoint (in your case 50 HZ). For small generators you have you will not run isoch on grid (say grid is 49.99HZ and your
setpoint is 50HZ which means you generator will go to MAX load trying to reach setpoint without no effect on grid (it's huge compare to your generator)).

3. Check your knowledge of present control systems of 2MW and 15MW units control systems. It's require to understand ability to add/modify logic to get add functionality you need.

4. Verify your plant load. If it's too low you need to prepare for scenario of going to island mode with all three units at 100% load (15+2+2=19MW) and sudden drop to you plant load (say 1MW). You control have to be able to STOP or quickly reduce load without going to over speed or over frequency(there are generator protection setpoint for that which may need to be checked). Some plant may even have load shedding system or automatic sequence in case of your tieline/ main grid breaker open event (may be you have DCS or some other upper level control control system).

5. Consider black start scenario with following steps:
a. start 2MW on black start to get power for aux systems to start 15MW units. Start 2MW in isoch.

b. start 15MW in isoch. once it's synced 2MW unit shall go to droop. General practice is that the biggest machine is maintaining frequency (isoch) and smaller and more fast loading machines responding to frequency changes (droop).

c. Once grid is back sync main breaker to adjusting speed and excitation voltage of units to sync back.

d. after plant is back on the grid switch all machines back to droop.

5. Read about load sharing and check if it make sense to do that. You can make isoch load sharing when all units run in isoch and not fighting each other, but it require some compex control and may be additional IO between controllers.

6. Check if you can implement control in upper level standalone system (DCS) or add new PLC or whatever control you find acceptable/reliable.

There are so many things to be discovered which require modification and probably testing/tuning to achieve flawless operation. Main things to consider are above, the rest you can figure out on the way.

Have fun! Plan first and implement after.