505E Digital Governor
Manual 85018V1
8
Woodward
Control Overview
The 505E Digital Governor is designed to control extraction, extraction/
admission, or admission steam turbines. The difference between these turbines
is the capability of the turbine to allow low pressure steam, which is at a lower
pressure than the inlet, to enter and/or exit the turbine. An extraction turbine
allows the lower pressure (extraction) steam to exit the turbine only and will have
a non-return valve in the extraction header/line to prevent steam from entering
the turbine. An admission turbine (also called induction) will allow excess header
steam to enter the turbine through the low pressure inlet. An extraction/admission
turbine will allow low pressure header steam to enter or exit the turbine
depending on system pressures. A turbine with admission capability will have a
stop valve or trip-and-throttle valve in the low pressure line to prevent steam from
entering the turbine when the unit is tripped. The type of turbine used will depend
on the system requirements and must be designed by the turbine manufacturer
to perform the functions required.
The 505E has two independent control channels available, the speed/load and
auxiliary controllers. The outputs of these two controllers are low-signal-selected
(LSS) to provide to speed/load demand signal to the ratio/limiter. In addition to
these channels, the speed/ load controller can be manipulated by another
controller, the cascade controller. The cascade controller is ‘cascaded’ into the
speed controller, whereby the speed controller setpoint is changed directly by the
cascade controller output. The auxiliary controller can act as either a control
channel or as a limiting channel. All three of these PID controllers have the
option of utilizing an analog input signal to remotely position their setpoints.
Additional features of the 505E include frequency control, isochronous
loadsharing, critical speed avoidance, idle/rated control, and an automatic start
sequence. There are two serial communications ports which can be used to
monitor and control the turbine using Modbus protocol.
Extraction Turbines
The 505E control can be configured to operate single automatic extraction
turbines by controlling the interaction of the governor (HP or high pressure) valve
and the extraction (LP or low pressure) valve. (The 505E can also operate the
governor valve and the first extraction valve of multiple extraction turbines).
Single automatic extraction turbines have a high pressure stage and a low
pressure stage, each controlled by a valve. Steam enters the turbine through the
HP valve (see Figure 1-2). At the downstream end of the HP turbine stage and
before the LP valve, steam can be extracted. The LP valve controls the entry of
steam into the LP turbine stage, and the diverting of steam through the extraction
line. As the LP valve is opened, more steam enters the LP stage and less is
extracted.
In most cases, the operator of an extraction turbine needs to maintain both
turbine speed/ load and extraction pressure/flow at constant levels. Changing the
position of either the HP valve or the LP valve affects both turbine speed/load
and extraction. If either the load on the turbine or the extraction demand
changes, both the HP valve position and the LP valve position must be changed
to maintain speed/load and extraction. The movement of both valves is
automatically calculated by the 505E’s ratioing logic based on the turbine
performance parameters to minimize valve/process interaction.
