Manual 35018V1
505XT Digital Control System for Steam Turbines
Woodward
83
Remote Cascade Set Point
If desired, the Cascade set point can be positioned through an analog signal. Optionally, one of the 505’s
six analog inputs can be programmed to position the Cascade PID set point. This allows the Cascade set
point to be positioned remotely by a process control or distributed plant control system.
The Remote Cascade Set Point (RCS) range is determined by the programmed Analog input’s 4 mA and
20 mA settings. The Remote Cascade Set Point range is tunable in the Service mode, but cannot be set
outside of the min and max Cascade Set Point settings.
The Remote Cascade Set Point input may be enabled from the 505 keypad, contact input, or Modbus
communications. The last command given from any of these three sources dictates enable/disable.
If the milliamp signal to the Remote Cascade set point input is out of range (below 2 mA or above 22 mA)
an alarm will occur and the Remote Cascade set point will be inhibited until the input signal is corrected
and the alarm is cleared. Depending on configuration and system conditions, the Remote Cascade Set
Point may be in one of the following states (505 front panel screen messages):
Disabled - The Remote Set Point function is not enabled and will have no effect on the Cascade set
point.
Enabled - The Remote Set Point has been enabled but cascade control is not active. The breakers
are not closed, speed < min gov, or cascade has not taken control.
Active - The Remote Set Point has been enabled but Cascade is not in control. Cascade has been
enabled and the Remote Cascade set point is in control of the set point but the Speed PID is not in
control of the LSS bus.
In Control - Cascade is in control of the LSS bus (via the Speed PID) and the Remote Cascade Set
Point is positioning the Cascade set point.
Inhibited - Remote Set Point cannot be enabled; input signal is failed, Cascade input signal is failed,
a controlled stop is selected, the unit is shut down, or the remote cascade control is not
programmed.
When enabled, the Remote Cascade set point may not match the Cascade set point. In this case, the
Cascade set point will ramp to the Remote Cascade set point at the programmed ‘Casc Setpt Rate’
setting (as defaulted in the Service mode). Once in control, the fastest the Remote Cascade set point will
adjust the Cascade set point is at the programmed ‘Rmt Cascade Max Rate’ setting. If the ‘Rmt Cascade
Max Rate’ was set at 10 and the Remote Cascade set point analog input instantly moved from 0 units to
1000 units, the Remote Cascade set point will move to 1000 units at 10 units/s).
Remote Cascade Enable Logic
There are three different options for enabling Remote Cascade Set Point and Cascade control as follows:
One Remote Enable contact input or function key command
Both enable commands programmed; Remote Casc Enable and Cascade Enable
No enable commands programmed
When only one Remote Enable command is programmed (either F-key or contact input), selecting
‘Enable’ will enable both Cascade control and Remote Cascade control. This configuration allows both
functions to be enabled with one command if this is the normal operation mode. If ‘Disable’ is selected,
both control modes are disabled.
A contact input can be programmed to enable and disable the Remote Cascade Set Point (RCS)
input/function. When this contact is open the RCS is disabled, and when it is closed the RCS is enabled.
The contact can be either open or closed when a trip condition is cleared. If the contact is open it must be
closed to enable the RCS input. If the contact is closed it must be opened and re-closed to enable the
RCS input.