when our gas turbine generator (GE Fame 5 with mark VI) is supplying the plant and isochronous mode was selected, I wonder whether we can synchronize the machine with the grid or not? in the other word can we change the frequency of the machine in the isochronous mode? if we can how?

 

Dear Shakarami,

I am not sure I am fully clear on your question, but as CSA is keen to say, "I'm sure I'm going to regret this", but here goes anyway.
First, the answer to your question will depend on the application code running in your controller.

Without being able to see the code, I can't really answer for sure how your machine will operate. From your question I am not sure if this is just a personal question or some type of procedure you are trying to develop.

The quick answer to your questions.

1)"Can we change the frequency of the machine in isochronous mode?" Yes you should be able to do this.

2) "If we can, how?" Use the "Raise speed/Load" and "Lower speed/Load" buttons that should be on the HMI to raise or lower the frequency setpoint.

The somewhat longer answer.

If you are running your machine in isochronous mode then the MKVI is monitoring speed (TNH) and the setpoint is the speed reference (TNR). Using the buttons on the HMI for "Raise" or "Lower" you are changing the speed reference to the turbine controller. Since you are in Isoch mode and not tied to the grid, if you change the speed reference by either selecting "Raise" or "Lower" the speed of the machine and your "Island" will change, which will increase or decrease the frequency on your island. There will be some obvious limitations depending on relay settings and the maximum output of the machine versus the total load in your island system.

When your system in in "Droop" speed control mode the MKVI is still monitoring speed, and the setpoint is still the speed reference. But as you raise and lower the speed reference, the speed of the turbine doesn't change since it is tied to the grid. In this case the output of the machine will change as the reference is increased or decreased, as the turbine controller will either increase or decrease fuel input, which will cause an increase or decrease in machine output.

So if you are trying to "resynchronize" your island to the main grid how to best do it? Obviously you will need some type of sync check relay on your line breaker (possibly 52L) since your generator breaker (52G) is already closed. The MKVI can't be used to sync. back to the main grid since it is already sync'ed to your small island. But if all you want to do is raise or lower the frequency of your small island, so that you can use some other controls to close a line breaker and tie back to the grid, then that should be possible.

I do have some concerns about trying to sync back to the grid while in isoch mode. Being in isoch mode when going back to the grid, your unit will react very quickly if there is a difference in frequency(I mean system frequency, not angle). If you rejoin the grid and frequency is low your unit will load very quickly to try and raise the frequency. If you rejoin the grid and your frequency is higher than the grid, then your unit will reduce load very quickly which could cause you to go offline. My suggestion would be to resync to the grid in "droop" mode.

But again it is hard to say exactly how your machine will operate without being able to look at your control code. I hope this helped some, again I suspect CSA or another contributer may have a longer and more defined explanation, but I had to try!

 

When you are connected to a grid in parallel with other generators and their prime movers, the Mark VI should be in Droop Speed Control Mode. So, you would need to switch VERY QUICKLY from Isochronous to Droop when you closed the tie-line breaker to prevent severe load swings after synchronization, possibly severe enough to trip on either exhaust over temperature or reverse power.

The first question I would ask is: Is there a synchronizing circuit and synchronizing check relay across the tie-line breaker that will used to synchronize the plant and turbine-generator to the grid? You will need to be able to match speed and have some idea of voltage differential during synchronization and you will need a synchronizing circuit to get this information, and there should be synch check relay to prevent closing the breaker when it should not be closed.

The second question I would as is: Is there a discrete (contact) input to the Mark VI to automatically switch back to Isochronous control when the tie-line breaker is open? If the tie-line breaker opens and you want the turbine-generator to be able to keep the island running you will need to be able to quickly switch from Droop to Isochronous mode to prevent tripping on under- or over-frequency, and sometimes human operators aren't paying enough attention to be able to do it fast enough to prevent losing the island in the event of a tie-line breaker opening without an automatic switch signal.

Most plants I've been at that synch to a grid through a tie-line breaker wait until the island load is fairly stable, then manually switch the unit to Droop mode, and then synchronize to the grid through the tie-line breaker. Trying to manually switch to Droop after synchronizing can mean that all manner of turbine-generator load instability can be happening after synchronization, because machines operating in parallel with other machines on a grid should almost never be in Isochronous mode because they will "fight" with the other machines to try to control frequency. And that's not a fight you want to see.

You can change turbine speed/generator frequency while in Isochronous mode with the Speed/Load Raise and -Lower buttons. But, again, it's usually best--unless there is an automatic switch from Isoch to Droop on tie-line breaker closure--to first switch to Droop, then synchronize.

 

MIKEVI,

Actually, my post was shorter but we both covered the same concerns.

Just to let you know, Speedtronic turbine control panels can synchronize more than one breaker. All that's required is a second PT for the "other" side of the second breaker (and one that is phase shifted if necessary to match the generator PT), a means of connecting them to the "BUS" PT input of the Speedtronic (through a relay, usually), and some code/configuration in the Speedtronic panel. The Speedtronic can even have different breaker closing times if the two breakers have different characteristics.

Hope this helps!

 

Thanks CSA,

more information always helps. Maybe at some point I will see some code and a system like this.

 

MIKEVI,

I've developed and installed synchronizers that can synchronize a group of generators on one bus to groups of generators on multiple different buses across multiple breakers. CSA is right, all we need are analog or digital PT signals from the buses we are synchronizing, and in this case, the controller was a third party device that sends analog signals to the MarkVI.