Our company operate Frame 9E engine with DLN1. Fuel type gas only. Last time we have an issue with unstable work on mid load. Engine jump from Premixed to Lean-lean on about 60Mw (55% of full load). you please explain premix mode TO LEAN-LEAN MODE?
Can you explain what you mean by "...unstable work at mid load"?
Does the unit, or do the units, operate while synchronized to a grid with many other prime movers and their generators, or supply a small independent load (sometimes called an "islander system")?
DLN combustion mode switching is (normally) a function of the Combustion Reference Temperature--TRF or TTRF1, depending on the vintage of the control system. As the load is increased and decreased, TTRF/1 increases or decreases (between zero and Base Load--at Base Load TTRF/1 should be relatively stable if speed and load are stable).
However, if the grid frequency is unstable, or the loads on a small, islanded system, are unstable then air flow and/or fuel flow can be unstable and cause the unit to "drop out of" Premix Steady State mode. Or if fuel supply pressure is unstable, or if the IGVs are unstable, or if the IBH (Inlet Bleed Heat) control valve is unstable (not due to grid frequency or islanded load fluctuations) the unit can drop out of Premix--especially if TTRF/1 is close to the transfer temperature, which it likely would be if the load was around 55-60 MW.
Operators need to understand when combustion mode transfers normally occur during start-up and shutdown--in other words, at what TTRF/1 temperatures the combustion mode changes. There are TTRF/2 temperature values for going from Primary to Lean-Lean, and for going from Lean-Lean to Premix (during loading). And there are "deadbands" for unloading (usually 50 deg F) for unloading. So, for example, if the unit transfers from Primary to Lean-Lean at TTRF/1 1600 deg F during loading, it will probably transfer from Lean-Lean to Primary at 1550 deg F TTRF/1 (these are examples, but typical for many GE-design heavy gas turbines with DLN-1 combustion systems).
Again, if the unit is being operated at some load that results in a TTRF/1 that is close to a transfer point AND there is speed or load or fuel valve or IBH or IGV or fuel supply instability, the unit can drop out of Premix (or Lean-Lean) unexpectedly.
So, to properly answer your question with more clarity, we need to understand what was happening when the combustion mode transfer occurred.
Hope this helps on the road to understanding. Actually, this topic (DLN combustion mode transfers) has been covered many times in control.com, and they can all be accessed and reviewed using the 'Search' feature at the far right of the Menu bar at the top of every control.com webpage.
If you have access to the Operation and Service Manuals provided with the turbine and generator, it's important to read and understand how DLN combustion systems work (there are two combustion zones in the combustor, and the fuel is varied between the two zones as the unit transfers (switches) between combustion zones. The should be some brief but informative descriptions with pictures of the combustion liners and fuel nozzles and fuel control valves that should be helpful. If you have (more) questions, we can try to answer them here!