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Droop to Isoch Generator Mode Causing Trip

Droop to Isoch Generator Mode Causing Trip

Droop to Isoch Generator Mode Causing Trip

(OP)
An industrial plant with district (internal) load of 60 MW and GE STG 2X11 MW (operated at 8 MW) and GE CGTGs  3X42 MW (operated on preselected load of 28 and droop mode of 5%)and is connected to the power grid. The typical generation is 100 MW total and the power export is 40 MW. When disconnected from the grid, the frequency increases to 61.5 Hz. One of the CGTGs mode was manually changed 65 seconds after disconnection to Isoch to control the frequency. Within 7 seconds The Isoch CGTG tripped on low forward power that is set to trip on 2% load and 3 seconds delay (Mark V alarmed with reverse power). The frequency then changed to 60.8 and the other two CGTGs still on droop started immediately increasing speed and reached 62.2 Hz. What is the reason for the unit on Isoch to trip? Why the droop units started increasing speed? Is there a solution? Is it ok to disable the low forward power trip point and rely on reverse power of the protection relay and the turbine.?

RE: Droop to Isoch Generator Mode Causing Trip

The droop machines under light load will probably idle at 62Hz or so, dropping as they accept load. When you saw the frequency drop the isoch machine was probably acting as a brake with its governor closed because it was being driven above its target speed by the droop machines, and when the isoch machine was disconnected by the protection the droop machines sped up because their load had reduced.

Low forward power is typically a steam turbine trip, reverse power is typically a gas turbine trip so yes, you could probably do as you suggest but you will trip on reverse power instead of low forward power. A gas turbine compressor usually equates to a load roughly 1/3 of the output of the machine, although it depends on things like IGV positions, so you will be motoring a significant load.

One solution is for your machines to run in droop but with a supervisory trim controller to adjust the frequency of the overall group back to the 60Hz nominal. At least, that's how I'd do it in the big power plant world.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Droop to Isoch Generator Mode Causing Trip

(OP)
ScottyUK, Thank you for the reply.

When the Isoch unit tripped the frequency dropped to 60.8 as the droop units picked the load of the Isoch machine (16 MW) equally distributed to the droop units. The question is why the droop units kept increasing frequency for almost  1 minute from 60.8 to 62.2. When loads were changing the machine speed changes in a second or so.

 

"Never Quit, Keep Searching for Solutions, Out of the Box, Sell the Box and Innovate"

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