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Manual 35018V1
505XT Digital Control System for Steam Turbines
Woodward
93
Figure 3-21. Extraction/Admission Control Panel
Manual P Control
The output of the Ext/Adm controller that feeds into the Ratio Limiter logic is referred to as the P demand.
Manual P control is a feature that allows the operator to take manual control of this value. The transition
from Ext/Adm PID control into Manual P is always bumpless. However, it is the operator’s responsibility to
create a bumpless return to closed loop control, but insuring that the Ext/Adm process variable and
setpoint match and that the output of the PID is very near the Manual P demand value.
This feature is always available and is very usefully in troubleshooting issue related to the stability of the
Ext/Adm controller or one of the steam valves. The demanded position for this control can be
programmed to come from an analog input, so that it can be adjusted remotely.
If the Ext/Adm Process Variable (PV) milliamp input signal is out of range (below 2 mA or above 22 mA)
an alarm will occur, Ext/Adm control will be inhibited and the unit will fall back into Manual P control. The
operator can then decide to maintain the LP valve position relating to this value, raise or lower it, or
disable this mode which will ramp the LP valve demand to 100%
Extraction/Admission Control Dynamics
The Ext/Adm PID control uses a single set of dynamic settings. These values are programmable and may
be tuned at any time. If the unit is expected to run normally in both fully decoupled and coupled modes
(uncommon), it is important to understand that the dynamic settings will be only optimum for one of these
conditions.
In tuning this controller during commissioning, it is acceptable to initially start in fully decoupled mode and
tune this loop after the correct speed loop controller dynamics have been set. These will provide a good
starting point for tuning the Ext/Adm controller in coupled mode (on ratio limiter control). Once this is done
the unit can be further optimized by using the OPTI_RATIO feature that will automatically tune both the
speed loop and ext/adm loop together.
Refer to the PID Dynamic Adjustments section in this manual for more specifics.
Extraction/Admission Droop
When sharing control of a parameter with another external controller, the Ext/Adm control amplifier can
also receive a programmable DROOP feedback signal for control loop stability. This feedback signal is a
percentage of the LSS bus (control valve position). By including this second parameter into the control
loop, the Ext/Adm PID becomes satisfied, and does not fight with the other external controller over the
shared parameter. The Droop % fed back to the Ext/Adm PID is equal to the following defaulted settings: