HOW SCADA SYSTEMS WORK

SCADA stands for Supervisory Control And Data Acquisition.

SCADA systems are typically used to perform data collection and control at the supervisory level.

A SCADA System performs four functions.

1.Data acquisition. (Example: Water level of a reservoir)

2.Networked data communication.

3.Data presentation

4.Control

The above functions are performed by four kinds of components:

1.Sensors

Sensors are digital or analog control relays that directly interface with the system. For example there are sensors to detect the water level of a reservoir and determine the distance between the sensor and the water level. The information will be transmitted to the Computer System through the network.

2.Remote telemetry units

These are small computerized units deployed in the field at specific locations. They serve as local collection points for gathering information. Then they deliver such information to the main control system.

The RTU connects to physical equipment, and reads status data such as the open/closed status from a switch or a valve, reads measurements such as pressure, flow, voltage or current. By sending signals to equipment the RTU can control equipment, such as opening or closing a switch or a valve, or setting the speed of a pump.

The RTU can read digital status data or analog measurement data, and send out digital commands or analog set points.

An important part of most SCADA implementations are alarms. An alarm is a digital status point that has either the value NORMAL or ALARM.

Alarms can be created in such a way that when their requirements are met, they are activated. An example of an alarm is the "fuel tank empty" light in a car. The SCADA operator's attention is drawn to the part of the system requiring attention by the alarm. Emails and text messages are often sent along with an alarm activation alerting managers along with the SCADA operator.

Important to know that quality SCADA RTUs must have 3 important characteristics: Data Networking, Data Reliability and Data Security.

3.SCADA Master Station

These are larger computer systems that serve as the central processor for the SCADA system. They provide human interface to the system. Facility is provided to automatically control the system depending on signals provided by the sensors.

The term "Master Station" refers to the servers and software responsible for communicating with the field equipment (RTUs, PLCs, etc), and then to the HMI software running on workstations in the control room, or elsewhere. In smaller SCADA systems, the master station may be composed of a single PC. In larger SCADA systems, the master station may include multiple servers, distributed software applications, and disaster recovery sites. To increase the integrity of the system the multiple servers will often be configured in a dual-redundant or hot-standby formation providing continuous control and monitoring in the event of a server failure.

The SCADA system usually presents the information to the operating personnel graphically, in the form of a mimic diagram. This means that the operator can see a schematic representation of the plant being controlled. For example, a picture of a pump connected to a pipe can show the operator that the pump is running and how much fluid it is pumping through the pipe at the moment. The operator can then switch the pump off. The HMI software will show the flow rate of the fluid in the pipe decrease in real time. Mimic diagrams may consist of line graphics and schematic symbols to represent process elements, or may consist of digital photographs of the process equipment overlain with animated symbols.

The HMI package for the SCADA system typically includes a drawing program that the operators or system maintenance personnel use to change the way these points are represented in the interface. These representations can be as simple as an on-screen traffic light, which represents the state of an actual traffic light in the field, or as complex as a multi-projector display representing the position of all of the elevators in a skyscraper or all of the trains on a railway. Initially, more "open" platforms such as Linux were not as widely used due to the highly dynamic development environment and because a SCADA customer that was able to afford the field hardware and devices to be controlled could usually also purchase UNIX or OpenVMS licenses. Today, all major operating systems are used for both master station servers and HMI workstations.

4.Communication networks

The communication network provides connectivity between SCADA System and Remote Telemetry Units.

SCADA systems have traditionally used combinations of radio and direct serial or modem connections to meet communication requirements, although Ethernet and IP over SONET is also frequently used at large sites such as railways and power stations.

The remote management or monitoring function of a SCADA system is often referred to as telemetry. SCADA protocols are designed to be very compact and many are designed to send information to the master station only when the master station polls the RTU.