Manual 35018V1
505XT Digital Control System for Steam Turbines
Woodward
69
Figure 3-13. Speed Relationships
Once turbine speed is equal to or greater than the ‘Min Governor Speed Set Point’ setting, the speed set
point may be adjusted through discrete raise and lower commands. When a raise or lower speed
command is issued, the set point moves at the programmed ‘Speed Set Point Slow Rate’. If a speed
raise/lower command is selected for longer than three seconds, the speed set point will then move at the
fast rate which is three times the speed set point slow rate. The speed set point slow rate, fast rate delay,
and fast rate can all be adjusted in the Service mode.
The shortest length of time a set point will move for an accepted raise or lower command is 40
milliseconds (120 milliseconds for a Modbus command). If the speed set point slow rate is programmed
for 10 rpm/s, the smallest increment it will move is 0.4 rpm (1.2 rpm for Modbus).
The speed set point may be set to a specific level, by directly entering a set point value through the 505
keypad or Modbus communications.
To “enter” a specific set point from the 505 keypad, follow these steps:
1. Go to the Speed Control page – select Commands/Entered Setpoint
2. Dialog Box will open with target value – click Enter (to highlight value)
3. Use numeric keypad to enter desired setpoint – click Enter (to accept)
4. Click on the GO button to ramp from current setpoint to the new setpoint
If a valid number was entered, the setting will be accepted as the new target setpoint. If an invalid number
is “entered” the setting will not be accepted and the 505’s screen will momentarily display a value out-of-
range message. This ‘Entered’ rate is tunable through the Service mode.
Requirements for valid entries are the following:
Speed must be below the maximum governor setting
Speed must be above the idle setting and not within any critical speed avoidance band
Once the speed set point is above the minimum governor setting, the set point cannot be lowered
below minimum governor
If the unit is driving a generator and the unit is on-line, the speed set point cannot be set below
the minimum load setting (set in Service)
The speed set point can also be directly entered from Modbus/OPC, however, the allowed range is
between the minimum governor and the maximum governor speed settings. The allowed set point range
is limited to between the minimum load and the maximum governor settings if the unit is driving a
generator and the unit is on-line.
When the 505 is configured for a generator application, a special speed set point rate (Sync Window
Rate) is used to increase set point resolution around synchronous speed. This allows for tighter set point
control to accommodate synchronizing manually or by an automatic synchronizer which interfaces to the
505 using discrete contacts. This Sync Window Rate is defaulted to two rpm/s and is only used when the
generator breaker is open and the speed set point is within 10 rpm of rated speed. Both the synchronizing
rate and the synchronizing window are tunable in the Service mode.