Our GE Frame-9E gas turbine running with DLN-1 system. Now we want to implement Free governing mode of operation. As it is a DLN machine, we are not able to run approximately 65-85 MW range (TTRF range: 1070 Deg C. to 1120 Deg. C.). Also we have constant speed mode settings to maintain pre-selected load. So what should we do to implement free governor mode of operation?
To start with u got to know if your IBH is tracking properly. If yes let me know the design maximum load for your unit and at what load its transit.
IBH is not commissioned here. Maximum load is 110 MW and around 70-80 MW (approximately) load transition (LLP to PSS) take place.
To operate a GE-design heavy duty gas turbine with a Speedtronic/Mark* turbine control system in Free Governing Mode all one needs to do is use the RAISE and LOWER SPEED/LOAD targets in the HMI to change load. Contrary to wildly popular and false belief--the unit load will not drift or change if Pre-Selected Load Control is not active at all times (unless grid frequency changes--which is common in some parts of the world and which FGMO is intended to help reduce the severity of (grid frequency disturbances)). And, load is supposed to "drift" during grid frequency changes to reduce the severity of disturbances, also contrary to popular and false belief.
When an operator uses the SPED/LOAD targets to change load and stops stops clicking on RAISE or LOWER, the load will remain at the present value. Pre-Selected Load Control is a lazy man's way of operating units--and it contributes to grid disturbance severity and instability. It's great for changing load but once the desired load is attained the operator should click on RAISE- or LOWER SPEED/LOAD to cancel Pre-Selected Load Control--and that places the governor (the Mark*) into Droop Speed Control which is essentially Free Governing Mode.
Most DLN combustor-equipped units have Constant Settable Droop Speed Control--which may not fully meet the strict definition of Free Governing Mode, but to change from Constant Settable Droop Speed Control to "straight" or "pure" Droop Speed Control one would need to work with the unit supplier/packager. In my personal opinion, Constant Settable Droop Speed Control meets the intent of Free Governing Mode, but that's only my personal opinion.
To your other question (because you've really asked two questions), it's simply NOT possible or even realistic to try to or to operate a DLN combustor-equipped machine at or near a combustion mode transfer point. Combustion mode transfers are a function of the combustion reference temperature (TTRF or TTRF1) and NOT load. The typical Lean-Lean to Premix combustion mode transfer temperature setpoint is approximately 1900 deg F during loading, and can be as much as 25-50 deg lower during unloading. So the combustion reference temperature of the unit must remain above the Premix to Lean-Lean transfer temperature--or the unit is going to transfer out of Premix.
If, as most sites are prone to try to do, Pre-Selected Load Control is used to try to maintain a load with a combustion reference temperature that is very close to a combustion mode transfer temperature setpoint then what usually happens is that the unit can "hunt" or oscillate going in and out of the two combustion modes because the operator isn't monitoring combustion reference temperature AND load to keep the unit in the desired combustion mode.
And the same thing can happen if an operator is using Free Governing Mode and not paying attention to the combustion reference temperature. It's the way DLN control works and operates, their supervisors, plant management, plant technicians--AND grid operators and their supervisors--simply must understand that if the desired load for a turbine results in a combustion mode instability then either the desired load has to be increased or decreased to have stable turbine operation.
For example, if the plant is asked to maintain 70 MW, and this causes the unit to transfer out of Premix or, worse, to go back and forth between Premix and Lean-Lean, and it's desired that the unit remain in Premix then the load setpoint is going to have to be raised to, say, 72 or74 MW to keep the unit in Premix Steady State
Hope this helps! Operating DLN combustor-equipped machines requires understanding of how the combustion control works and the "limitations" of load ranges which are dictated by the combustion mode temperature calculation. AND, grid operators and their supervisors must also understand that there must be some leeway or range for certain desired loads when dispatching or setting load setpoints in the lower range of Premix Steady State operation for DLN combustor-equipped units in order for them to remain in low emissions mode (Premix Steady State) and have stable power output.
It's a knowledge and experience thing, operating DLN combustor-equipped machines.
And the Free Governing Mode thing is a "letting go" thing--meaning operators and their supervisors have to let go of the wildly but patently false belief that Pre-Selected Load Contr MUST be used to operate the unit, because it doesn't have to be used to operate at stable load setpoints. And, in fact, Pre-Selected Load Control makes any grid disturbance worse and Free Governing Mode is the grid regulators' way of saying, "You can't operate using Pre-Selected Load Control."