.. _section-6.1:

Introduction
------------

The |SAS| control system model was developed for the design and
analysis of control systems in LMR plants. The model is described in
Ref. 6-1 while application of the model is described in Ref. 6-2. In
this chapter, the model is described and guidelines for using the model
are given. Essentially, the user should be able to set up an input deck
and run the model using the material in this section and in :numref:`section-A6.1`.

The model is very flexible, allowing the user to select any number of
plant variables for input to the control system as measured quantities.
These signals can then be processed by a user defined network of
mathematical blocks that implement the control equations. The output
from these blocks can then be used to drive various actuators already
existing in |SAS| or they can be used to directly control plant
variables in |SAS|. The model has a steady- state solution
finder that can be used to determine initial values for demand signals
and state variables that place the control system in a steady state that
is consistent with the plant steady state as calculated by
|SAS|. The control system model can also be used to calculate
auxiliary variables and print their values.

The model is an integral component of |SAS| and is accessed
through the input deck in a manner similar to the other reactor
component models. Before using the model however, one must write the
mathematical equations that describe the desired plant control system
and identify the plant variables that are to be measured and controlled.
The user then transforms the equations and variables into a block
diagram where the individual component blocks are basic mathematical
elements such as integrators and summers. The input deck is prepared
directly from this block diagram with each block definition occupying an
input card and each plant variable that links with the control system
also occupying an input card. Several other cards must also be entered
to specify how the control system initial conditions are to be
calculated and to assign values to parameters that control the accuracy
and stability of the transient solution. A set of parameters also exists
for controlling the printing of debug data. This output is useful for
diagnosing input errors.

This section describes the basic model; it also gives some general
guidelines for using the model. The section assumes the reader has a
knowledge of power plant control systems and is able to write the
equations that describe their system. The organization of the material is
as follows. In :numref:`section-6.2` the general equation form that can be
represented is given. It is very probable that the user's model fits
this form but this should be verified. The solution techniques used to
solve the block diagram equations are described in :numref:`section-6.3`. :numref:`section-6.4` presents some general guidelines for selecting values of solution
control parameters and describes some of the model features and how they
are used. The input description is given in :numref:`section-A6.1`.