Introduction to Industrial Control Systems
How SCADA Works
SCADA and Polling
SCADA (Supervisory Control and Data Acquisition) is a 70-year-old technology and is, fundamentally, the most common terminology used for industrial control systems. It is the ability to monitor the physical world, acquire data in an automated way, and to provide control type signals back into the system to advise an action based on the data. The action may be automated, or in the form of dispatch.
SCADA in Utilities is the engine behind monitoring voltage, power, currant, phase. This functionality can be applied to other Industries as well, where the monitoring points may be the flow rate in water or the pressure in an oil pipeline.
SCADA acquires data through a process called polling. A message is sent out to a remote telemetry unit (RTU) and waits for a reply. That unit has wires that connect to the physical world to measure voltage, current, and phase. The response back is the specific values the poll is requesting.
The poll can contain a general request such as a voltage reading, or it or it can simply be focused on very specific metrics. An example could be a request for the temperature of a transformer. Has it overheated? Has it seen events that would cause you to believe that its life cycle is being degraded? Do actions need to be taken to protect and extend it’s life cycle? Polling can provide this information.
That process of polling can happen every couple of seconds, every couple of minutes, every couple of hours, days, months or years. It is dependent on the asset, the role the asset plays in a system, and the risk that asset presents.
The reverse side of this is when, for example, there is a polling process that is every 30 minutes and at minute 15 a fault occurs. The asset, which will measure an over-voltage condition or over-pressure condition in a water pipe, will tell the RTU, which will then issue a request for a poll, called an unsolicited poll response. This response will then be sent through the SCADA system.
The SCADA system will recognize that this is an unsolicited request and respond with what is a fault condition. That process is an extremely common process. In fact, it is the most common use for situational awareness or understanding the state of the system you’re measuring.