Manual 35018V1 505XT Digital Control System for Steam Turbines
Woodward 50
Start Permissive
An external contact may be used as a turbine start-up permissive. When programmed for this
functionality, the contact input must be closed in order for a ‘RUN’ command to be executed. Should the
contact be open when a ‘RUN’ command is given, an alarm will be issued and the 505 display will
indicate that the start permissive was not met (Start Perm Not Met). The alarm does not need to be
cleared but the contact must be closed before the 505 will accept the ‘RUN’ command. After ‘RUN’ has
been accepted, the start permissive contact will have no effect on operation.
Example: This input could be connected to a Trip & Throttle valve’s closed limit switch to verify that it is in
the closed position before a turbine start-up is performed.
Open Wire Detection on MPU Speed Signals
The 505XT automatically verifies continuity of the speed MPU circuits each time it prepares to issue a
“Ready to Start” status message. If an open wire is detected it will issue an alarm for this input, if all
MPU’s appear to have open wires a Trip is initiated. This open wire test can be done manually any time
the turbine is stopped and the speed is zero. It is also possible to disable the automatic test at start the
Speed Sensor screen.
The screen is located under Analog Inputs/Speed Signal X/Open Wire Test.
Figure 3-1. Open Wire Detection Test
Zero Speed Signal Override
The 505XT issues a shutdown if no speed signal is detected (magnetic pickup voltage less than 1 Vrms
or speed is less than the ‘Failed Speed Level’). To allow the control to start with speed not being sensed,
this shutdown logic must be overridden. The control can be configured to provide a manual or an
automatic speed override. For added protection, a timed limit on the override is available. The status of
the MPU override logic may be viewed in the Service mode or through Modbus communications. The
override logic applies to both passive and active speed probes.
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