Manual 35018V1 505XT Digital Control System for Steam Turbines
Woodward 53
A maximum override time limit is provided as an extra level of protection, in the event the contact input is
left closed. A ten-minute maximum time limit is applied to a Manual override command (as defaulted in
the Service Mode). This time starts when the RUN command is initiated and re-arms the loss-of-speed
detection when the time expires. The 505XT will execute a system shutdown if turbine speed is not above
the ‘Failed Speed Level’ setting when the time expires.
Automatic Speed Override
If the Manual Speed Override option is not programmed, the Automatic Speed Override logic is used by
the 505XT to override the loss-of-speed signal shutdown logic during a turbine start-up. With the
Automatic Override logic, the loss-of-speed signal failure is disarmed when the turbine trips and remains
disarmed until the sensed turbine speed exceeds the programmed (‘Failed Speed Level’ setting + 50
rpm). Once turbine speed exceeds this level, the loss of speed detection circuit is re-armed and the
control will execute a system shutdown if sensed speed drops below the ‘Failed Speed Level’ setting.
For added protection, a timed limit is available on the automatic speed override function. The timed speed
override timer deactivates the loss-of-speed override logic after the programmed time expires. If
programmed, this timer starts counting down once a ‘START’ command has been issued. This timer
when programmed provides an extra level of protection, in the event both speed input probes are failed
when the unit is started. This timer may be programmed in the 505’s Service mode.
Acceleration Limiter
The acceleration limiter is available for startup to significantly reduce speed overshoot to the minimum
controllable speed setpoint at turbine start. The Acceleration Limiter is disabled after reaching the
minimum controllable speed and the 505XT will continue the start sequence via the Speed PID.
If the Acceleration Limiter is set to be used via the Service menu, then it will control speed at the ‘Rate to
Min’ rate (RPM/s), as set in the Turbine Start configuration, until the Speed PID is in control at the
Minimum Controllable Speed. When the Start Sequence is set to “No Start Sequence”, the Minimum
Controllable Speed is Minimum Governor. When the Start Sequence is set to “Idle/Rated Sequence” or
“Auto Start Sequence”, the Minimum Controllable Speed is the Lowest Idle setting.
Acceleration Control
Using the Acceleration Limiter feature carries some risk without
proper tuning of the Acceleration PID. Please monitor actuator
behavior at turbine start. Verify there is no instability in the
Acceleration Controller that may result in system damage during
oscillation of the actuator/valve if it is unstable. When the response of
the Acceleration Controller is over-damped, it may follow the speed
setpoint very slowly at startup or even move the valve closed but,
after this, acceleration should be controlled fairly well until the
Acceleration Limiter logic is disabled.
Turbine Start Mode Procedures
Manual Start Mode
The following start-up procedure is employed when the Manual start mode is configured:
1. Issue a RESET command (to reset all alarms and shutdowns). The LP demand and LP Limiter
behavior is determined by the turbine type.
a. Extraction Only: The LP Limiter will ramp to 100%
b. Admission Only or Extraction/Admission: The LP Limiter will ramp to 100%. The LP valve
position will vary to maintain zero admission flow, defined by the steam map.
i. For Full-Decoupled configurations with any admission control, the LP Valve Limiter
will remain at the LP Minimum demand.
To Next Page
Loading...
Need help?
General
