GT Preselected command after temp matching


Thread Starter


I started GT GE Mark VI in simple cycle since load dispatch required additional generation in the grid. Started HRSG, after all pending maintenance work. I gave command to GT for steam temperature matching and IGV control the GT exhaust temperature to 400oC. GT control goes from PRESELECTED to PART LOAD. After HRSG diverter damper fully opened, I tried to select the GT to preselected load but failed for sometime. What are the things that restrict in selecting PRESELECTED LOAD from PART LOAD?
Until temperature matching sequence is complete you will not be able to go to preselected load. Temperature matching usually has 2 parts. First part warms up the HRSG, if it had been depressurized. This typically holds GT exhaust temperature to 371 degC. Second part matches the exhaust temperature to the steam turbine metal temperature plus 100 degC. (I.e., if ST metal temp was 290 degC, GT temp would be set to 390 degC.) Note, these are typical settings for GE steam turbines and when GE supplies the HRSG.

Also note, once the GT exhaust temp is matched to the steam turbine temp, loading will still be inhibited until the steam turbine inlet valve is opened and the ST goes on inlet pressure control (IPC). And, the loading rate can be limited by steam turbine stress control. So, depending on the initial conditions of the HRSG and steam turbine, it could take well over an hour to get back to preselected load (if the ST and HRSG were both cold).
Have been reviewing the logic to find where "loading will still be inhibited until the steam turbine inlet valve is opened and the ST goes on inlet pressure control (IPC)." I have traced out how L90LR -> L90RX5 -> L83JD4 -> TNR.

Would someone indicate which of signal along the way prohibits the raise load until ST goes on inlet pressure control?

can you explain what are these controls?

Generator Control:
Preselected Load - Base Load - External Load.
also i did not see Peak Load!!

Turbine control:
OFF - Cranck - Fire - Manual - Auto - Remote.

When i can use Manual and Auto? What is the different Btw these mode?
Preselected Load - A mode in which the operator can set a specific load setpoint (MW) and the Speedtronic will adjust the fuel to maintain the setpoint. Sometimes there are upper- and or lower limits on the value of the setpoint, and sometimes there is no operator-adjustable setpoint, just a pre-programmed value.

Base Load - When selected, the Speedtronic will monitor compressor discharge pressure and exhaust temperature and will put as much fuel into the machine as possible while trying to maintain a constant <b>firing temperature</b>. ['Firing temperature' is defined as the temperature of the hot combustion gases flowing through the first stage turbine nozzles, not the "flame temperature" in the combustor.] This will result in the optimal power output for the unit for the current operating conditions (ambient temperature, ambient pressure, compressor cleanliness, hot gas path part conditions, etc.), and is approximately rated load at rated conditions.

External Load - This indicates there is some remote (external) ability to adjust the load (power output) of the unit, either through an analog input (typically 4-20 mA) or a combination of RAISE and LOWER discrete inputs. [The definition of "REMOTE" can be varied, but mainly refers to some operator interface other than the turbine control panel operator interface, located in the same room as the turbine control panel or on the turbine control panel itself.]

>also i did not see Peak Load!!

Well, that's because Peak Load is not provided (i.e., was not purchased) on every unit!

OFF - A unit START cannot be initiated.

CRANK - A mode of unit operation where the starting means is used to spin the turbine-generator shaft (of a single-shaft turbine-generator) and fuel is not admitted to the unit. The speed of the turbine-generator shaft during cranking is dependent on the type of starting means, and is not operator-adjustable.

FIRE - A mode of unit operation where the fuel is admitted to the turbine and flame is established, and then fuel is maintained at a constant level and cannot be increased or decrease by the operator. This is basically a "test" mode, and is also sometimes used to "dry" a turbine after an Off-line Water Wash has been completed. The speed of the unit is not operator-adjustable during Firing.

MANUAL - This mode has largely been abandoned with "newer" units (since about 1980, or so), but was a method of allowing the operator some control of turbine speed during starting and acceleration.

AUTO - When this mode is selected and a START is initiated, the unit will go through a purge sequence, fuel will be admitted and flame established, and the unit will be automatically accelerated to FSNL (Full Speed-No Load), or 100% of rated speed in preparation for loading. This is the typical, "normal" mode selected for operation.

REMOTE - This is a mode that allows a "remote" location to start, stop, and raise/lower load. The definition of "remote" can be many things; it can mean a Central Control Room, or a location hundreds of meters, or even further, away. In some cases, selecting REMOTE will disable "local" control ("local" typically meaning from the turbine control panel or the operator interface located in the same room as the turbine control panel).

For typical operations, use AUTO mode. MANUAL mode would only be used for testing or troubleshooting purposes.

I would think the turbine control system you are finding these modes on would have a document supplied with the control system called 'Control Specification - Operating Instructions and Sequences'. If so, you might find more information on MANUAL mode in that document, which can usually be found in the Instruction Manual provided with the unit.
There is a logic signal in the ST Mark VI that is true when the unit is on inlet pressure control. I believe it is called L83IPC. For GE supplied combined cycle controls, this is one of the signals that is used to turn off the temperature matching in the gas turbine control. The logic is usually in the HRSG control. I believe the command signal from the HRSG control to the GT control is L43TMOFF_CPB (or something close to that).
Within the GT Mark VI (or Mark VIe) there is logic that effectively keeps the GT out of preselected or base load control mode while it is in the temperature matching mode.

Base Load is maximum load (temperature control limit). In this mode the GT loads up to the exhaust temperature control limit and stays there. The actual MW output will vary as the ambient inlet air temperature varies.
Preselected Load is control to a specific MW setpoint.
External Load is control to an externally supplied load setpoint. It is essentially preselected mode with the preselected setpoint controlled by an external load signal.
Peak Load is like Base load, but the temperature control limit is higher to allow greater output. This will reduce the life of the hit gas path components in the gas turbine, and there are limits on how long this may be used. This control mode is NOT available on combined cycle applications.

Turbine Control:

OFF: If the GT is shut down, unit will not respond to a start command.

CRANK: Unit will go to purge speed following as start command and will stay there. It will not fire.

FIRE: A unit that is on CRANK, will fire when the switched to FIRE mode. The unit will not accelerate, but will hold there until MANUAL, AUTO or REMOTE is selected. MANUAL will bring the unit to FSNL (full speed no load = synchronizing speed), but will not automatically synchronize. Operator can manually synchronize and load. AUTO will automatically synchronize and load. REMOTE is like AUTO, but the unit will respond to remote command inputs (if this capability is provided).

In general, the control would be place in either AUTO or REMOTE and left there. The other modes are normally used for commissioning, maintenance and troubleshooting.
I find this information in GE manual

1)Manual Mode
In the MANUAL Mode a start signal will result in an automatic start-up and loading to the minimum operating speed, then the shaft speed is controlled by raise and lower commands on HMI.

2) Auto Mode
In the AUTO Mode a start signal will result in an automatic start-up and loading to the selected value, an external signal from DCS is taken as shaft speed set point.

3) Remote Mode
In REMOTE Mode position, the set point of load may be varied from outside through the commands from DCS. The increase and decrease variation speed of the turbine shaft depends on the value imposed by the automatic ramp and can be modified through the Operator Interface.