Manual 35018V1
505XT Digital Control System for Steam Turbines
Woodward
78
The 505XT is capable of being configured to use an analog input (feed-forward signal) from an anti-surge
controller. This input allows the 505XT controller to decouple the response of its Speed and Cascade PID
controls from that of the anti-surge controller, allowing for increased system stability in all conditions.
Refer to this manual’s Speed Controller block diagram to better understand how this input is applied
within the 505’s controller logic. This signal should represent the anti-surge controller’s anti-surge valve
demand where 0% = 4 mA = Closed and 100% = 20 mA = Open. Delays in this signal should be kept to a
minimum.
This Feed-Forward function is only active when operating within the normal operating range (between Min
Gov and Max Gov) and when it has been enabled. This function can be enabled/disabled through a
contact input, programmable function key, or Modbus communications.
When enabled, if the feed forward analog input increases or decreases, the speed set point will be
increased or decreased respectively. This is an offset added to or subtracted from the speed set point.
After a feed-forward event, this speed offset will slowly ramp back to zero based on the delay time
configured (typically 120 seconds). For example, if the max offset is 100 rpm with the max forward rate at
50%/s and the min offset is –75 rpm with the min forward rate at
–25%/s and “Action Delay” is set to 120 seconds, then the following events describe the feed-forward
action:
1. The speed set point is at Rated speed, X rpm.
2. The Feed-Forward Analog Input increases by 50% in one second.
3. The speed set point is instantly increased by 100 rpm.
4. The set point slowly decreases back to X rpm (at least 120 seconds).
5. The Feed-Forward Analog Input decreases by 25% in one second.
6. The speed set point is lowered by 75 rpm.
7. The set point slowly increases back to X rpm taking at least 120 seconds.
The feed-forward loop can be configured for a temporary response, as described here, or as a direct
action (continuous deviation based on the incoming signal).
Emergency Loop
In case of a compressor surge event, a large speed upset may occur, and recovery may be very difficult.
If this event occurs, Emergency Feed-Forward action can be programmed to instantly bias the control’s
speed reference using a larger offset than the regular feed-forward loop.
When configured, the Emergency Feed-Forward bias action will increase the anti-surge controller’s effect
for a short period, configured as the ‘Emergency Action Delay’, to assist the anti-surge controller with
protecting the compressor. The Emergency Action takes effect when the Feed-Forward Analog Input
increases or decreases faster than the configured ‘FW Rate to Activate’ (this should be larger than the
‘Min/Max Forward Rate’ used for the regular Feed Forward loop). The ‘Emergency Max Speed Offset’ will
be added to the speed reference. This offset immediately begins ramping down and will reach zero after
the configured ‘Emergency Action Delay’. At this point, only the normal feed forward action offset will be
in effect since the emergency will typically have a much shorter duration.
Direct Action
When configured for direct action, the feed-forward loop will offset the speed set point proportional to its
4–20 mA calibration. This offset will not ramp back to zero; it remains active and offsets the speed
reference based on the actual value of the Feed-Forward Analog Input. For example, if configured for a
max offset of 150 rpm and a min offset of –50 rpm, then Direct Feed-Forward action offsets the set point
by 0 rpm if the analog input is at 8 mA. The offset is 100 rpm if the analog input is at 16 mA, and the
offset will not ramp down to 0; it remains at 100 rpm as long as the analog input is at 16 mA.
The direct action cannot be used to decrease speed below Min governor and above Max Governor.