Synchronization of Multiple Generators onto the Grid


I will do commissioning tests for a plant synchronization system to control a group of turbine generators (2 ~ 3 generators together) to be synchronized onto the grid. FYI, GE CTG rating: 40 MW & Siemens STG rating: 82 MW. For our system, at least 1xCTG, and 1xSTG are to be controlled together for generation bus synchronization with the grid. Of course, the generation bus with multiple generators runs in island mode.

To me, this situation would be quite challenging since CTG & STG controller's responses are not the same. So, I am really concerned if this synchronization would really work although it would take a quite long time.

Do you have any experiences with the synchronization of multiple generators onto the grid? Do you have any thoughts and/or advice? Anything will be very helpful for me.

Thanks in advance.

Hundreds of turbine-generators of all sizes and types are synchronized together on islanded grids in many locations around the world every day. How? Because they are adhering to basic AC (Alternating Current) fundamentals for synchronous generators.

You would need to tell us quite a lot more about the island and how the units are being controlled for us to be able to begin to help with your doubts and concerns. If the island uses some kind of over-all “power management system” to try to control frequency in response to load variations and changes then there will likely need to be some tuning of control parameters involved. But since most “power management systems” are unique and very few are alike and many use unique terms and configurations it would be nearly impossible for anyone to say how the system at this island will work.

A good and proper understanding of Droop Speed Control is the best thing for a situation like this because that is key to understanding interactions between turbine-generator control systems. From your post it would appear there is some other source of steam for the steam turbine (since the exhaust heat from a single 40 MW CTG (Combustion Turbine-Generator) can’t produce 80 MW of steam, so there’s a lot of information you haven’t told.

Again, a good, solid understanding of Droop Speed Control basic AC power generation fundamentals will be key to knowing what is supposed to happen and if it is happening or not. And if there is some kind of “power management system” for frequency and load control at the facility you will likely have to study it closely to determine if it is working correctly or requires adjustment (“tuning”).

Best of luck in your endeavour!

First of all, sorry for the incomplete information. Here is some more information.

This is the combined cycle cogeneration plant which consists of 3 x CTGs / 2 x STGs. Plant load is approx. 180~190MW and will export approx. 80MW to the grid. CTGs were supplied by GE, and Siemens supplied STGs. Of course, we have the power management system (PMS) which controls all TG units in the grid interconnected mode and island mode of the plant., i.e., MW export/PF control in the grid-connected mode and frequency/voltage control in the island mode.

Each TG unit is directly connected to a GSU transformer, then all GSU transformers are connected to EHV SWYD where is defined as a point of interconnection with the grid. EHV SWYD has 2 incoming breakers for grid interconnection and 1 bus tie-breaker. The plant synchronization controller has been designed to close these EHV breakers (of course, one breaker at a time) by controlling multiple generators on the island bus. I also attached an example island bus configuration that we are attempting to close bus tie-breaker but GSU transformers are not shown on the sketch.
Example Island Bus Configuration.JPG
Just for my past experiences, I had experienced with PMS as well as multiple generator control for 2 x island bus synchronization as an engineer in EPC contractor. But all generators were the same rating in that project. So, it was much easier for me to design and operate the island power system but it took much longer in island bus synchronization (i.e., bus tie breaker closure of main generation/distribution bus) when we had controlled multiple generators. I guess that the timing of pulse outputs from the synchronizer to each TG controller might be slightly out of synch, and this slightly misaligned timing might cause a bit longer synchronizing time with multiple generators.

This is why I am concerned about synchronization control and a synch time if we have different sizes of TG units in the island bus, as shown on the sketch, we would not achieve island bus synchronization onto the grid when we will have 1xCTG and 1xSTG or 2xCTGs and 1xSTG on the same island bus.

But in the meantime, I am also thinking it would be OK since all CTGs & STGs will run in speed control and voltage control mode and have the same droop setting but would take much longer than expected. Anyway, I might be getting too worrisome for nothing but better to check before trouble and just want to have more self-confidence on this topic. To me, PMS control is easier than synchronizing control of multiple generators.

I wish all the above information would be sufficient for you, CSA. If you need more information, let me know.
In my experience, the length of the pulses used to change speed reference (and therefore load) are one of the most important things to be “tuned.” All grids have machines synchronized to them and while many have the same percentage droop regulation setpoint and different load capabilities (maximum load) they will all respond a little differently—but they should all respond in a similar manner as regards accepting or reducing load.

In my personal experience, PMS’s are the real problem—especially when they are configured and programmed by people who don’t know droop speed control and don’t understand island ”grids.” And then during commissioning there are all numbers of “experts” on site in the Control Room who don’t really know much at all, except, “That’s not how we did it on the last job!”And they can’t explain how it was done on the last job. And they just have these opinions about how it should work without ever having thought about what they are saying and if it’s even feasible—but they’re certain it’s not operating properly and whatever changes the commissioning person wants to make are incorrect.

I hope you have the plant design and operation plan as well as understand the PMS configuration and programming enough to dig into it and understand the operation and be able to compare it to the intended plant operation so you can identify any changes in programming or se points necessary to make the plant operate smoothly and as intended.

Personally, I am not a fan of most PMS’s. And most of the issues I have had with island operation were the result of poor PMS configuration and programming. But of course the people who provided the PMS are adamant the problem IS NOT with their equipment.

Best of luck. Be safe—and stay healthy!!!

I also want to mention that many STG's used in combined cycle applications operate differently than most other STG's--because they are "captive" to the CTG's and their loads and exhaust heat. Many STG's used in combined cycle applications just open their control valves wide open after an initial warm-up and loading period. They just take whatever steam is available at whatever temperature and produce as much power with it as they can. This can GREATLY complicate matters for islanded operation of combined cycle plants because it's difficult to send a command to load or unload a STG when the control valves are already wide open.

So, you should also be checking with the STG control system programmers to see how they will handle load commands in islanded operation. Will the control valve(s) always be wide open during islanded operation? What happens when GT exhaust flow and temperature suddenly change?

I think this will also complicate re-synchronization with the grid.... And could be the cause of the long period required for re-synch you seem to be talking about. When re-synching with the grid it would probably be best to switch (one of) the CTG('s) to Isochronous Speed Control mode and use it to synch the island with the grid. It would seem that trying to do it with the PMS would be very difficult, trying to hold the CTG AND STG speeds constant long enough to match speed/frequency with the grid and re-close the grid tie breaker. And, once the generator breaker closes the unit running in Isoch mode must quickly switch back to Droop Speed Control or there's going to be problems....!

That's all I can add to this thread. I don't know when you will be commissioning this plant, but it would be great to hear back from you about your experience and what it took to get the plant running smoothly and re-synch'ing smoothly.

I can tell you, that when I'm in a control room when this part of commissioning is going on--I always have a fully-charged torch (flashlight) in my pocket. (I almost ALWAYS carry a torch when working in a power plant..... especially during commissioning, but especially when trying to run in island operation, and re-synch'ing with the grid.)

Best of luck!!!