DSLC Digital Synchronizer and Load Control
Manual 02007
78
Woodward
Chapter 7.
Process Control Description
Introduction
The process control is operational in full-function DSLC models only.
The process control function of the DSLC control will control any process where
the controlled parameter is determined by the generator load, and the controlled
parameter can be monitored as a 4–20 mA or 1–5 Vdc input signal.
The control compares the input signal to the process set point, or the external
load reference signal if it is used, and adjusts the generator load to maintain the
desired set point.
Description
Figure 7-1 shows a block diagram of the process control function. The process
control mode is selected when the process input, base load, CB Aux, and
load/unload switch contacts are closed. The process input signal is compared
with the process reference, which may be either the internal fixed reference or
the 4–20 mA remote process reference input. In process control mode, the Raise
Load and Lower Load contact inputs operate on the process control reference
rather than on the load reference. When the internal reference is used, the Raise
Load and Lower Load contact inputs raise and lower the process reference
rather than generator load. The 4–20 mA remote reference input becomes the
process reference and is selected when both the Raise Load and Lower Load
contact inputs are closed.
When the process function is initially selected, the reference is set equal to the
internal or remote process reference. If the process input and process reference are
not equal, the control ramps the load reference in the appropriate direction to reduce
the difference error. When the process error reaches zero or the load reference
reaches the maximum or minimum values, the process control is enabled.
When the process control is enabled, the error signal between the process
reference and process signal is input to a PID (Proportional, Integral, Derivative)
controller operating in cascade with the load control. The output of the controller
is a load reference which is limited by the high and low load limit set points to
prevent overload or reverse power on the generator. The load setting signal is
output from the load control to the speed control to set control at the required
load to maintain the desired process level.
Additional features of the process control are an adjustable process input signal
filter and adjustable deadband on the integrator. The adjustable filter allows
reducing bandwidth when controlling a noisy process such as experienced in
digester gas fuel applications. The deadband is useful in both noisy applications
as well as for very slow processes.
The process control function is configurable for direct and inverse action. Direct
process control is where the sensed input signal increases as the load increases
(such as where the sensed input is exhaust pressure or export power). An
inverse action control is where the sensed input signal decreases as the load
increases (such as when controlling import power where the import power will
decrease as the generating system picks up more of the local load).