Manual 35018V1
505XT Digital Control System for Steam Turbines
Woodward
49
Chapter 3.
505XT Control Description
Introduction
Previous models of the 505 family were separated into dedicated model part numbers for either single
valve (the 505 or 505 Enhanced) or multi-valve turbines (the 505E).
The 505XT is designed so that it can be applied to both single and multi-valve turbines.
The configurations support the following turbine types:
Type 0 Single Valve or Split-Range Valves turbines
Type 1 Single Extraction (only) turbines
Type 2 Single Admission (only) turbines
Type 3 Single Extraction/Admission turbines
The selection of turbine type is found under the Steam Map header under the Configuration menu. The
factory default is Type 0 for this selection.
The start modes and the start sequence operations for all of these types of turbines is exactly the same.
The only difference with extraction and/or admission type turbines is the handling of the LP valve (also
referred to as the V2 valve). Once warm-up is complete and the unit is ready to start, the LP will ramp to
100% (full open to the exhaust end of the turbine) or ramp to an operator defined demand position, in the
case of an extraction/admission type turbine.
Once the turbine has reached minimum governor or rated speed it can then be placed in operation –
referred to as going ‘On-Line’. For a generator application this is initiated by closing the breaker and on a
mechanical drive it occurs once unit reaches minimum governor speed. If the unit is a turbine type 1, 2,
or 3, this is the point at which the 505XT will can be transitioned from speed control to ratio-limiter control.
In ratio limiter control both the HP and LP valves are controlled to maintain 2 different parameters,
selectable by the user.
Turbine Start Modes
The 505XT has three turbine start modes (manual, semi-automatic or automatic) from which to choose
from. One of these start modes must be chosen and programmed to perform a system start-up. Once a
‘RUN’ command is issued, the speed set point and valve limiter are manipulated automatically by the
505XT or manually by the operator, depending on which start mode is selected. After a turbine start
sequenced has been completed, turbine speed will be controlled at a minimum controlling speed. The
minimum controlling speed may be idle if idle/rated is being used, low idle if the auto start sequence is
used, or minimum governor if neither idle/rated or auto start sequence is used.
A ‘RUN’ command may be issued from the 505 keypad, an external contact, or through Modbus
communications. If an ‘External Run’ contact is programmed, a ‘RUN’ command is issued when the
contact is closed. If the contact is closed prior to start-up it must be opened and re-closed to issue a
‘RUN’ command.
If turbine speed is sensed when a ‘RUN’ command is issued, the control will instantly match the speed set
point to the sensed speed and continue towards the minimum controlling speed. In the event the sensed
turbine speed is greater than the minimum controlling speed setting, the speed set point will match this
sensed speed, the Speed PID will control at this point, and the control will wait for further action to be
taken by the operator. If turbine speed is first sensed within a critical speed avoidance band when a ‘Run’
command is received, the speed set point will match the actual speed, decrease to the lower end of the
critical avoidance band, and wait for action to be taken by the operator.