scope:

Scope and Field of Application

In their task and effect, hmiting controls lie between the actual operating controls and the protective de¬ vices. Operating controls have the task of Controlling the power Station process such that the generated Out¬ put always corresponds to the predetermined set point, and the individual sub-processes are thus carried out in accordance with economic and environ¬ mental criteria. Protective devices have the task of protecting the environment, along with the plant and plant components, against damage, principally by means of shutdown. Limiting controls are used to Sup¬ port the operating controls, and hence enable continued operation - possibly under restricted conditions - in the event of transient-processes, e.g. during runup and run-down, and in the event of anomalous operating states.

Limiting controls frequently make use of the actuators of the operating controls. They can be switched off in the same way as the operating controls. When they are switched off, however, the possibility no longer exists of early and automatic hmiting of process values before the protective de vice is triggered. The limiting controls thus differ from the protective devices, which cannot be switched off and which mostly function with their own actuators. Operating controls and protective devices are not dealt with in this Guideline.

The tasks for control of sub-processes may vary as a function of the operating State of the whole plant. Structure shifts are carried out to adapt the control to the various tasks. These are devices used to select structures

- between operating and limiting controls

- within limiting controls

- within operating controls.

Structure shifts are thus not independent control solutions to process tasks. They are dealt with in the Guideline in connection with the limiting controls (see examples in Appendix).