What Is Supervisory Control And Data Acquisition

Published Date: 02 Nov 2017

SCADA The definition of SCADA is ‘Supervisory Control and Data Acquisition’.

High level definitions

Supervision: Computer processes and personnel supervise, or monitor, the conditions and status of the power system and other processes using this acquired data. Operators and engineers monitor the information remotely on computer displays and graphical wall displays or locally, at the device, on front-panel displays and laptop computers.

Control: Control refers to sending command messages to a device to operate the I&C (Instrumentation and Control) and power system devices. Traditional supervisory control and data acquisition (SCADA) systems rely on operators to supervise the system and initiate commands from an operator console on the master computer. Field personnel can also control devices using front-panel push buttons or a laptop computer.

Data Acquisition

Data acquisition refers to acquiring, or collecting, data. This data is collected in the form of measured analog current or voltage values or the open or closed status of contact points. Acquired data can be used locally within the device collecting it, sent to another device in a substation, or sent from the substation to one or several databases for use by operators, engineers, planners, and administration.

2. Advantages of SCADA

a) Increase Efficiency

-Minimize Fault Response Time

-Reduce Planned Downtimes

-Isolate and Precisely Locate Faults

b) Maximize Profitability

- Reduce Failures / Unplanned Downtimes

- Reduce Operations Overhead

- Reduce Manpower Requirement

- Maximize (Achieve Expected) Equipment Life Time

c) Maximize Safety

- Public Safety

- Site Safety

3. Applications

SCADA systems are used to automate complex industrial processes where human control is impractical - systems where there are more control factors, and more fast-moving control factors, than human beings can comfortably manage.

Electric power generation, transmission and distribution: Electric utilities use SCADA systems to detect current flow and line voltage, to monitor the operation of circuit breakers, and to take sections of the power grid online or offline.

Water and sewage: State and municipal water utilities use SCADA to monitor and regulate water flow, reservoir levels, pipe pressure and other factors.

Buildings, facilities and environments: Facility managers use SCADA to control HVAC, refrigeration units, lighting and entry systems.

Manufacturing: SCADA systems manage parts inventories for just-in-time manufacturing, regulate industrial automation and robots, and monitor process and quality control.

Mass transit: Transit authorities use SCADA to regulate electricity to subways, trams and trolley buses; to automate traffic signals for rail systems; to track and locate trains and buses; and to control railroad crossing gates.

Traffic signals: SCADA regulates traffic lights, controls traffic flow and detects out-of-order signals.

4. Specific functions of SCADA

Key Functions of SCADA

SCADA and Substation Automation systems can provide specific key functions, such as:

a) Measurements- include real time measurements with required accuracy and resolution.

b) Status monitoring- Change of state as well as external contact status.

c) Control- Control commands issued to the actuators by system or operators.

d) Ancillary services-includes information about ancillary devices.

e) Time synchronism-Central time keeping and stamping for disturbance/fault reporting.

f) Programmed logic functions- Controls interlocks, sectionalizing and reclosing.

5. Four Levels of SCADA

Overall structure of a SCADA system

There are four distinct levels within SCADA, these being;

a) Field instrumentation

b) PLCs / RTUs and IED’s

c) Communications networks

d) SCADA host software.

Field Instrumentation

Most field devices such as valves are fitted with actuators, enabling a PLC or RTU to control the device rather than relying on manual manipulation. This capability means the control system can react more quickly to optimize production or shutdown under abnormal events.

PLC/RTU/IED’s

A Programmable Logic Controller or PLC is a computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines. A Remote terminal unit (RTU) is a microprocessor controlled electronic device that interfaces objects in the physical world to a distributed control system or SCADA (supervisory control and data acquisition) system by transmitting telemetry data to a master system. Programmable Logic Controllers (PLCs) and Remote Telemetry Units (RTUs) used to be distinctly different devices but over time they are now almost the same. I&C devices built using microprocessors are commonly referred to as intelligent electronic devices (IEDs). Microprocessors are single chip computers that allow the devices into which they are built to process data, accept commands, and communicate information like a computer. Automatic processes can be run in the IEDs, and communications are handled through a serial port like the communications ports on a computer.

Communication Networks

The remote communication network is necessary to relay data from remote RTU/PLCs/IED’s, which are out in the field, to the SCADA host located at the field office or central control center. With assets distributed over a large geographical area, communication is the glue or the linking part of a

SCADA system and is essential to its operation. How well a SCADA system can manage communication to remote assets is fundamental to how successful the SCADA system is.

Protocols are electronic languages that PLCs and RTUs use to exchange data, either with other PLCs and RTUs or SCADA Host platforms. In recent years, protocols have appeared that are truly non-proprietary, such as DNP (Distributed Network Protocol). These protocols have been created independently of any single manufacturer and are more of an industry standard

SCADA Host software

It has been the mechanism to view graphical displays, alarms and trends. Control from the SCADA Host Software became available with the development of control elements for remote instruments. Initially, these systems were isolated from the outside world and were the domain of operators, technicians and engineers with the responsibility to monitor, maintain and engineer processes and SCADA elements. With advancements in Information Technology (IT) this is no longer the case and many different stake holders now require real time access to the data that the SCADA Host software generates. Accounting, maintenance management and material purchasing requirements are preformed or partly preformed from data derived from the SCADA system.

6. Architecture

Centralized Architecture:

In less complex applications such as, electric distribution network monitoring and control, and applications where cost may be an important consideration, a centralized architecture (refer figure 1) is an effective approach. In a centralized configuration, all the applications are running on a single or redundant hardware platform, including the software that supports the operator displays. Having the entire software applications resident on the same computer and running on one operating system makes the development of such a system easier and therefore less costly. The interface between the various

software applications also becomes simpler since data exchange between applications is handled by the

inter-process communications utility provided by the operating system. Hardware maintenance of a fewer number of computers is also less costly.

Figure 1. Centralized Architecture

Distributed Architecture:

When complex applications require more extensive data processing and availability requirements, a

distributed architecture (refer Figure 2) is more appropriate. In this configuration, Supervisory Control and Data Acquisition (SCADA) master stations have both software and hardware in a distributed architecture. The processing power is distributed among various computers and servers that communicate with each other through a real-time dedicated LAN in the control center.

The most common form of distributed systems shares the load between the data acquisition functions and the user interface functions. In such configurations, the data acquisition and processing applications are resident on a server platform(s) that handle(s) all the communications functions with the substations and other systems as well as most of the data processing. The user interface functions are handled by clients that are connected to the servers via a network.

There are many reasons for using a distributed architecture:

-Using different operating systems for different applications: a real-time operating system (UNIX®, VMS) is used for data acquisition and processing while Microsoft Windows is used for a graphical user interface.

-There is this need to distribute the applications to achieve performance and availability objectives.

Distributed systems have many advantages over centralized systems:

-Since the data processing is shared by multiple servers on the network, the various servers require less processing power than a centralized system. In this way, the cost of the individual hardware platforms can be reduced.

-It is also easier to upgrade or to add servers if additional processing power is required.

-In distributed systems, the failure of one server does not make the whole system inoperative.

Figure 2. Distributed Architecture

Credits:

www.ieee.org (IEEE Std C37.1-2007)

www.schneider-electric.com

www.cyber.st.dhs.gov

www.selinc.com

www.time-i.com