Usage of Single Computer for control of all the process variables of a plant would result in the loss of control activities in the plant if a failure occurs in the master single computer. Also, the time required for accessing large data, processing the received data and controlling of the entire plant by single computer will take more time and slows down the processing system.
Distributed Control System (DCS)
In a distributed control system, different process systems in the plant will have their control system loop. Thus in DCS number of individual control loops will be present. Several Microprocessors were employed at the different locations of the power plant and dedicated to provide services for individual process system of the plant and monitor and control each process of the plant (This is unlike SCADA master-slave architecture, in DCS all microprocessors will be masters). Every processor at different locations of the plant acquire the data from its process system, process the received data and provides control functions and also stores the required data. These microprocessors are connected to each other and also connected to the station master computer through which operator can see the information and take control actions if necessary.
Control system in power plant will have distribution in both topologically and functionally. The functional distribution such as an operation of boiler side values, fans, turbine values, boiler oil firing, boiler coal firing for carrying out such different functions Microprocessors is distributed.
Topological distribution is achieved by employing closed loops for the operation of different equipment such as boiler steam A equipment, boiler steam B equipment, boiler oil firing AB/CD/EF elevations will have dedicated control loop.
This type of distribution results in an increase in speed and reliability of the control system.