Basic design of SCDA – Supervisory control and data acquisition

SCDA means – supervisory control and data acquisition. It is a computer-based system, which performs measurement, data acquisition, data transmission, operating and control function. In modern power generation, transmission and factory with automation system has a computer-based interface where operator can watch the data in real time updated, interact with it, operate-control system by clicking or putting settings data.

SCDA design of a simple steam generation unit

Let’s say a simple steam generation unit is to be watched and control. As we know that, a steam boiler unit has two basic safety- the steam pressure, level of water inside boiler (high, low). Now we can design a basic SCDA to control this small steam unit as follows.

    Fig: SCDA animation- courtesy Wikipedia

The first concept is to monitor the steam pressure, for this, we can set a vertical display unit where to display the steam pressure value and the graph in real time updated. The graph will be divided in to three section. Green section is the safe zone, purple section is the alert zone and red is the danger zone. So a boiler rated for 7 bar will be settled as follows. 0-7 bar green zone, 7-7.5 bar alert zone, <7.5 danger zone.

Thus, we get the real time update of boiler pressure. Now the next thing is to control or cope up situation. To control the boiler pressure we can design scda to few settings like start another system that consume the steam so that the steam pressure is reduced from danger to green, stop the boiler burner to stop producing further steam. Therefore, boiler pressure is covered.

Now to monitor & control the high and low level of water inside boiler, firstly we can set a graph in SCDA to monitor the water level. There will be green zone, purple zone, red zone in water level. Say as per the manufacturer- a certain boiler is designed to perform safely with 50% to 80% of water inside. Thus we can set the SCDA as follows – 50%-80% green zone, (>50%, 80%>) alert zone, (>40%, 85%>) red zone.

To control the boiler water level , if the alert signal is raised or displayed in SCDA – the next thing the operator is to do start the boiler feeder pump to fill water in (if it is set to operate via SCDA) or send an operator to the boiler immediately. In case of uncontrolled situation, the last thing is to do- shut down the boiler burner.

                                         

                                                                             Fig: SCDA System

                                                         

SCDA – Supervisory control and data acquisition example

  

                                                                          Fig: SCDA example