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Manual 35018V1
505XT Digital Control System for Steam Turbines
Woodward
107
Figure 3-32. Ext/Adm Mode Control Priorities
Fully Decoupled Mode –
It is possible to run in a fully decoupled mode in which there is no interaction between the two PID’s and
the two valve demand outputs. In this mode, the HP valve demand is directly driven by the Speed PID
output and the LP valve demand is directly driven by the Extraction PID output. In this mode the Ratio
Limiting is not used, so no operational map limiters are enforced. Since the PID’s are independent of
each other there is a high probability of the control demands conflicting with each other (fighting) and one
controller tending to slowly oscillate around its desired setpoint (hunting). Typically the dynamic response
of only 1 parameter can be optimized in this mode.
Under the Extraction/Admission Configuration menu, there is a selection to “Allow the use of Full
Decoupled Mode”. If that box is checked then the screen below will be available in the Service menu
under the Extraction/Admission Control menu. The permissives listed apply to either enabling or disabling
full-decoupled mode. If the ACTIVE LED is on, it indicates that when Ext/Adm control is enabled that the
control will transition into Full Decoupled mode, rather than transitioning into Ratio Limiter base mode
(Mode 0).