Microprocessor Based DCS:

Microprocessor Based DCS – Modern plants nowadays are very complex, large scale systems with a very high degree of automation. The designs and the operation of these plants are determined by the criteria of economy availability and safety. For example, in thermal power plants, greater unit capacity with higher temperatures and higher pressures are being adopted.

Thermal power plants are required to be extremely fuel efficient because of the increasing fuel costs. Designs of instrumentation and control systems are being oriented to achieve these goals. Today Microprocessor Based DCS provide reliable and satisfactory control. Distributed Control Systems (DCS) are characterized by

• Microprocessor-based self contained functional hardware modules operating in parallel.
• Data buses for fast data transmission.
• Shared video display units for operator communication and monitoring.

The DCS provides physical means of achieving optional economy, maxi­mum reliability and safety and minimum environmental impact. Software, that is, concepts and algorithms are available to achieve advanced power plant con­trol schemes. The benefits of improved control are:

(a) Improved efficiency

• by better combustion
• by reduction of losses during start-up or cycling and
• improved thermodynamic efficiency by higher mean final steam and

(b) Longer lifetime of components (especially critical parts) due to reduced transients stresses.

(c) Reduced maintenance on actuation due to smoother action.

In addition to the economic benefits of improved control, following advantages are gained.

• Lower cost per function (software function modules).
• Less electronic hardware (less number of cabinets required).
• Lower cabling and installation cost (multiplexing, data buses).
• Lower costs for documentation (self documentation).
• Reduced maintenance costs (self diagnosis, less spare parts).
• Shorter design and commissioning phase (flexibility, CAD).
• Easy changes in structure, e.g. after plant changes (flexible firmware).
• Higher availability, less disturbances and faults, shorter outages designed redundancy, self checking, error locating fast repair, etc.

Control system using totally distributed digital control system (DCS) is designed to achieve the following goals.

1.Reliable Plant Operation: To diagnose system conditions and to continue plant operation in case of faults. Provision of redundancy is done, as appropriate at various levels. This can be obtained by the use of decentralized functions.

2.Man Power Saving: This can be achieved by having a fully automated centralized operation and quick recovery from faults by centralised control and supervision.

3.Rapid Load Response: This can be achieved by reducing steam temperature error which is due to slow response of fuel loop.

4.Dynamic Balancing Control: This is obtained by predictive control.