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Back to Isochronous and Droop
I understand droop and isochronous modes, but have a question about speedtronic logic.
1 out of 2 members thought this post was helpful...

Older machine GE frame 5 with speedtronic Mark 1. This machine is to operate on the grid but also as a black start (isochronous). On the grid is clear cut - 4% droop, load limited by exhaust temperature.

Isochronous however, I am unclear. We have performed black start testing which goes well, but only have placed minimum load on the machine. Will the unit behave like "normal" when carrying high loads? The manual states " normally operate in parallel with 4% droop. However, when operated alone zero droop may be selected." To my knowledge the operators do not physically select droop or isochronous mode. I think we are always in droop mode. The question is; does speedtronic somehow know when it should be isochronous or parallel? and does it automatically transfer between modes? Or does it just stay in droop mode which inherently works when isochronous?


3 out of 3 members thought this post was helpful...


There was a time when GE-design heavy duty gas turbine control used Isochronous speed control mode during synchronization (FSNL operation), and then was switched to Droop speed control mode when the generator breaker was closed (this was also common for many older GE steam turbine control systems). Exactly when it became common for GE-design heavy duty gas turbines to operate in Droop speed control mode during synchronization (FSNL operation) I'm not sure. I know that in Mark IV digital Speedtronic turbine control systems the gas turbine was operating in Droop speed control mode during synchronization (FSNL), and had to be switched to Isochronous when appropriate (either manually or automatically, again--if Isochronous speed control mode was possible in the control system; Isochronous speed control mode was considered an option on Mark IV Speedtronic turbine control systems).

Without being able to see the Mark I Speedtronic elementary for the site, it's very difficult to say for certain how the control system for the turbine in question knows if it is to be in Droop or Isochronous Speed Control. Typically, there is an input (discrete; contact input) from the utility tie breaker that is used to tell the Speedtronic whether or not it will be, or is, connected to a grid. If the tie breaker is open, the contact input would tell the control system to be in Isochronous; if the tie breaker was closed, it would tell the control system to be in Droop.

HOWEVER, a contact input wasn't always used to automatically switch between Droop and Isochronous. And, some control systems didn't have even a manual switch for the operators to switch between Droop and Isochronous speed control. Many units were only designed to be operated in Droop speed control mode--even black start machines, though most black start machines could be operated in Isochronous mode. But, the selection wasn't always automatic as has been said.

Most GE-design heavy duty gas turbines operating in Droop speed control mode will handle small loads when isolated from a grid (in "island" mode, when Isochronous mode should be selected) pretty well without too much drop in frequency. But, if Isochronous speed control mode is enabled the control system should be able to control frequency all the way from zero load up to the machine's power output rating very well.

But, it would really be necessary to look at the Speedtronic elementary for the machine in question to be able to say whether or not is has the ability to automatically switch between Droop and Isochronous speed control modes, or whether it's incumbent on the operator to know if Droop or Isochronous speed control mode should be selected and to select it when appropriate. Or, whether the unit has the ability for the operator to select between Droop and Isochronous speed control modes. Some units were programmed to use the utility tie breaker status contact input to prevent an operator from selecting Isochronous speed control mode unless the utility tie breaker was open, but wouldn't automatically select Isochronous speed control mode if the utility tie breaker was open or opened.

So, it's really necessary to look at the site-specific Speedtronic elementary, and possibly the Generator Control Panel Elementary as well, to see how the selection of Droop and Isochronous is made--indeed, if it's even possible. (Most GE-design heavy duty gas turbine control systems had the capability for both Droop and Isochronous speed control modes, but Isochronous speed control mode wasn't always enabled as it was sometimes considered a purchased option.)

Hope this helps!

Excellent help.
Thanks CSA!


You're very welcome!

It would be great to know what you find in your research about manual and automatic Droop/Isochronous selection/transfer on your machine!

1 out of 1 members thought this post was helpful...

First I would like to thanks CSA. I have been reading his post on iso and droop mode started from post in 2009 to today. Very useful and practical information. CSA is right - not many practical information available and misleading.

I have a question for CSA. I have 3 hydro generators and they are normally operate in droop mode when connected to grid (grid CB close). Now I would like them to operate in island mode when grid CB is open. Actually they can survive in droop however when load is applied the momentary frequency drop is too great and causes under frequency (UF) protection operation. I would like to increase the frequency to 50.5Hz before loading to avoid UF operation. From your post, you recommend to set one generator to frequency mode and two generators remain in droop mode, by changing frequency setpoint on generator in frequency mode will change frequency for all generators. Is it correct?

Thanks in advance for your feedback.

2 out of 2 members thought this post was helpful...


You're welcome.

Generally, hydro generators are not very fast to respond to load changes, even in frequency control mode. I don't know about the configuration of the inlets to the turbines, but when they need to slow down because of load decrease the water flow has to stop or slow down, or run through some kind of "bypass" which is a waste of water.

And something similar on speeding up; the water flow-rate has to increase quickly if there is a large increase in load.

You would need to know if any of the governors has the capability to be operated in Isochronous Mode (frequency control). And, without some kind of external load control system to maintain frequency by adjusting the flows to one, two or all three control systems the best way to accomplish this would be to put on governor in Isochronous Mode and run the second and third unit with the governor is Droop Mode. You would need to have the operators watch load as the load on the Isoch unit approaches rated they would have to add load to one or both of the Droop machines, which would decrease the load on the Isoch unit, to prevent it from maxing out at rated and be unable to respond to any more load increases.

And, as the load on the Isoch machine approaches zero the operators would have to unload one or both of the Droop machines to raise the load on the Isoch machine to keep it from tripping on reverse power.

This is the part that most people don't understand about Isoch control: It's not automatic, especially when there is more than one unit operating in Droop mode. The operators have to continually monitor the load on the Isoch unit, and change the load on the Droop unit(s) (because operators can't directly change the load on the Isoch unit--only the frequency setpoint). To change the load on the Isoch unit to prevent it from reaching maximum or to prevent it from tripping on reverse power they have to change the load on the Droop unit(s). Which just confuses them immensely.

Or, you can get some kind of PMS (Power Management System) which can be programmed to do that "automatically"--but most such systems don't really work all that well without some manual intervention, or without a lot of diligent tuning and adjustment, most of which no one wants to do because of the cost and effort. They are generally only marginally good, especially when the loads get into the tens or hundreds of MW range.

There is also something which a lot of machines use called "Isochronous Load Sharing" which is just really a detuned Isochronous Mode, or a faster Droop Mode, if you will. But, those usually cause a fair amount of frequency deviation if the load changes by a lot (MW).

Yes; if the operator changes the frequency of the unit in frequency control (Isoch), the frequency for all the machines will change because the frequency control unit (the Isoch) unit controls the frequency for all the machines. Remember--they are synchronized together; no one unit can run at a frequency different than all the other units. The definition of synchronism is that all units run at the same frequency--which is directly related to speed. Magnetic forces inside the synchronous generators keep the generators all running at the same frequency, and speed is related to frequency (F=PN/120, remember; or N=120F/P, where N is speed (in RPM); F is frequency (in Hz); and P is the number of poles (an even number--a multiple of two, that is). If you change the frequency setpoint of the frequency control unit (the Isoch) unit, the Droop machines will follow. They're not really interested in speed (frequency) control; they let other units do that. They are just producing power at a constant rate as long as the grid frequency remains constant--and the frequency is constant on a well-regulated grid of any size, large or small.

It sounds like you are adding a LOT of load (relative to the power output rating of the generator) pretty quickly when you are trying to run in island mode (separated from the grid; grid CB open). And if the governors aren't very fast or the water supply flow-rate can't increase quickly enough then the frequency is going to decrease until the governor/water supply flow-rate can catch up to the load change.

Hope this helps!