In an industrial power station for the chemical industry DDC has been used for the first time in Germany in power plants.
The processing of the control circuits is based on the processing of analog and digital measured values organized in lists. These lists are completed by means of the keyboard of a data display. In the same way, the control circuits are structured by means of the data display and control parameters given to the control algorithms.
A three-term control algorithm has been selected as it ensures optimum control results under consideration of the storage size and the computer time required.
High flexibility is reached due to the fact that analog and digital values which are obtained by means of freely programmable functions in the computer can be processed like analog and digital values of the hardware. Thus, the sequential control can act on the reference value control and the control circuit structure. In addition, the controller parameters may depend on selectable process variables by means of software analog functions.
26 out of the 38 control circuits of the steam generator are working unter DDC. 45 control functions including cascade control and feed-forward control with time behaviour are operating under DDC. In addition, 21 control circuits have been equipped with simple analog back-up controllers.
The conditions for changing-over from “DDC” to “back-up” and “hand” have been determined in co-operation with the user under consideration
of safety and economical aspects.
The DDC-position-algorithm consists of the three-term control algorithm and the balancing algorithm. The latter is used for the control circuits operating under “back up” or “hand” to facilitate the smooth transition to the control algorithm at any time.
Finally, the experience gained during the commissioning of direct digital control circuits and the operational results obtained up to now are described.