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      SCADA system 

A SCADA (supervisory control and data acquisition) is an automation control system that is used in industries such as energy, oil and gas, water, power, and many more. The system has a centralized system that monitors and controls entire sites, ranging from an industrial plant to a complex of plants across the country. A SCADA system works by operating with signals that communicate via channels to provide the user with remote controls of any equipment in a given system. It also implements a distributed database, or tag database, that contains tags or points throughout the plant. These points represent a single input or output value that is monitored or controlled by the SCADA system in the centralized control room. The points are stored in the distributed database as value-timestamp pairs. It's very common to set up the SCADA systems to also acquire metadata, such as programmable logic controller (PLC) register paths and alarm statistics.While these systems simplify a given infrastructure, their components are quite complex. 

There are five essential composing parts of a SCADA system:

 Human Machine Interface (HMI)

 supervisory system

 Remote Terminal Units (RTUs)

 Programmable Logic Controllers (PLCs)

 communication infrastructures

The HMI processes data from each tag and sends it to a human operator, where he or 

she then can monitor or control the system. The supervisory system gathers the data sent from each tag and sends commands or operations to the process. The RTUs connect sensors and convert their signals to digital data and send it to the supervisory system, 

where it can be stored in a distributed database. PLCs are used as field devices because they are much more versatile and economical than process-specific RTUs. Finally, the communication infrastructure delivers connectivity to the supervisory system and then to the RTUs and PLCs for the user to command. The communication infrastructure is necessary to relay data from remote RTU/PLCs, which run along electric grids, water supplies, and pipelines. Communication is the absolute most essential link for a SCADA system to operate properly; however, how well the system manages communication from HMI to RTUs and PLCs fundamentally determines how successful a SCADA system can be. Below is a figure of what a basic SCADA system might look like for a given infrastructure.

Supervisory control and data acquisition (SCADA) is a system of software and hardware 

elements that allows industrial organizations to:

 Control industrial processes locally or at remote locations

 Monitor, gather, and process real-time data

 Directly interact with devices such as sensors, valves, pumps, motors, and more 

through human-machine interface (HMI) software

 Record events into a log file

SCADA systems are crucial for industrial organizations since they help to maintain 

efficiency, process data for smarter decisions, and communicate system issues to help mitigate downtime.The basic SCADA architecture begins with programmable logic controllers (PLCs) or 

remote terminal units (RTUs). PLCs and RTUs are microcomputers that communicate with an array of objects such as factory machines, HMIs, sensors, and end devices, and 

then route the information from those objects to computers with SCADA software. The SCADA software processes, distributes, and displays the data, helping operators and 

other employees analyze the data and make important decisions.

For example, the SCADA system quickly notifies an operator that a batch of product is showing a high incidence of errors. The operator pauses the operation and views the SCADA system data via an HMI to determine the cause of the issue. The operator reviews the data and discovers that Machine 4 was malfunctioning. The SCADA system’s ability 

to notify the operator of an issue helps him to resolve it and prevent further loss of product.

Who Uses SCADA?

SCADA systems are used by industrial organizations and companies in the public and private sectors to control and maintain efficiency, distribute data for smarter decisions, and communicate system issues to help mitigate downtime. SCADA systems work well in many different types of enterprises because they can range from simple configurations   to large, complex installations. SCADA systems are the backbone of many modern 

industries, including:

 Energy

 Food and beverage

 Manufacturing

 Oil and gas

 Power

 Recycling

 Transportation

 Water and waste 

water

 And many more

Virtually anywhere you look in today's world, there is some type of SCADA system running behind the scenes: maintaining the refrigeration systems at the local supermarket, ensuring production and safety at a refinery, achieving quality standards at a waste water treatment plant, or even tracking your energy use at home, to give a few examples.

Effective SCADA systems can result in significant savings of time and money. Numerous case studies have been published highlighting the benefits and savings of using a modern SCADA software solution such as Ignition.

      SCADA components

A SCADA system usually consists of the following main elements:

Supervisory computers

This is the core of the SCADA system, gathering data on the process and sending control commands to the field connected devices. It refers to the computer and software responsible for communicating with the field connection controllers, which are RTUs and PLCs, and includes the HMI software running on operator workstations. In smaller SCADA systems, the supervisory computer may be composed of a single PC, in which case the HMI is a part of this computer. In larger SCADA systems, the master station may include several HMIs hosted on client computers, multiple servers for data acquisition, 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 malfunction or breakdown.

Remote terminal units

Remote terminal units, also known as (RTUs),connect to sensors and actuators in the process, and are networked to the supervisory computer system. RTUs have embedded control capabilities and often conform to the IEC 61131-3 standard for programming and support automation via ladder logic, a function block diagram or a variety of other languages. Remote locations often have little or no local infrastructure so it is not uncommon to find RTUs running off a small solar power system, using radio, GSM or satellite for communications, and being ruggedized to survive from -20C to +70C or even 

-40C to +85C without external heating or cooling equipment.

Programmable logic controllers

Also known as PLCs, these are connected to sensors and actuators in the process, and are networked to the supervisory system. In factory automation, PLCs typically have a high speed connection to the SCADA system. In remote applications, such as a large water treatment plant, PLCs may connect directly to SCADA over a wireless link, or more commonly, utilize an RTU for the communications management. PLCs are specifically designed for control and were the founding platform for the IEC 61131-3 programming languages. For economical reasons, PLCs are often used for remote sites where there is a large I/O count, rather than utilizing an RTU alone.

Communication infrastructure

This connects the supervisory computer system to the RTUs and PLCs, and may use 

industry standard or manufacturer proprietary protocols. Both RTU's and PLC's operate autonomously on the near-real time control of the process, using the last command given from the supervisory system. Failure of the communications network does not necessarily stop the plant process controls, and on resumption of communications, the operator can continue with monitoring and control. Some critical systems will have dual redundant data highways, often cabled via diverse routes.

Human-machine interface

The human-machine interface (HMI) is the operator window of the supervisory system. It presents plant information to the operating personnel graphically in the form of mimic diagrams, which are a schematicrepresentation of the plant being controlled, and alarm and event logging pages. The HMI is linked to the SCADA supervisory computer to provide live data to drive the mimic diagrams, alarm displays and trending graphs. In many installations the HMI is the graphical user interface for the operator, collects all data from external devices, creates reports, performs alarming, sends notifications, etc.Mimic diagrams consist of line graphics and schematic symbols to represent process elements, or may consist of digitalphotographs of the process equipment overlain with animated symbols.Supervisory operation of the plant is by means of the HMI, with operators issuing commands using mouse pointers, keyboards and touch screens. For example, a symbol of a pump can show the operator that the pump is running, and a flow meter symbol can show how much fluid it is pumping through the pipe. The operator can switch the pump off from the mimic by a mouse click or screen touch. The HMI will show the flow rate of the fluid in the pipe decrease in real time.

The HMI package for a 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.

       SCADA hardware 

SCADA hardware :

A SCADA system consists of a number of remote terminal units (RTUs) collecting field data and sending that data back to a master station, via a communication system. The master station displays the acquired data and allows the operator to perform remote control tasks. 

The accurate and timely data allows for optimization of the plant operation and process. Other benefits include more efficient, reliable and most importantly, safer operations. This results in a lower cost of operation compared to earlier non-automated systems. 

On a more complex SCADA system there are essentially five levels or hierarchies: 

• Field level instrumentation and control devices 

• Marshalling terminals and RTUs 

• Communications system 

• The master station(s) 

• The commercial data processing department computer system

The RTU provides an interface to the field analog and digital sensors situated at each 

remote site. The communications system provides the pathway for communication between the master station and the remote sites. This communication system can be wire, fiber optic, radio, telephone line, microwave and possibly even satellite. Specific protocols and error detection philosophies are used for efficient and optimum transfer of data. 

The master station (or sub-masters) gather data from the various RTUs and generally 

provide an operator interface for display of information and control of the remote sites. In large telemetry systems, sub-master sites gather information from remote sites and act as a relay back to the control master station.

    

Landlines for SCADA 

Even with the reduced amount of wire when using a PC to IED system, there is usually a lot of wire in the typical SCADA system. This wire brings its own problems, with the main problem being electrical noise and interference. Interference and noise are important factors to consider when designing and installing a data communication system, with particular considerations required to avoid electrical interference. Noise can be defined as the random generated undesired signal that corrupts (or interferes with) the original (or desired) signal. This noise can get into the cable or wire in many ways. It is up to the designer to develop a system that will have a minimum of noise from the beginning. Because SCADA systems typically use small voltage they are inherently susceptible to noise. 

The use of twisted pair shielded cat5 wire is a requirement on most systems. Using 

good wire coupled with correct installation techniques ensures the system will be as noise free as possible. Fiber optic cable is gaining popularity because of its noise immunity. At the moment most installations use glass fibers, but in some industrial areas plastic fibers are 

increasingly used.

Fig:Glass fibre optic cables

Future data communications will be divided up between radio, fiber optic and some infrared systems. Wire will be relegated to supplying power and as power requirements of electronics become minimal, even the need for power will be reduced.

SCADA and local area networks 

Local area networks (LAN) are all about sharing information and resources. To enable all the nodes on the SCADA network to share information, they must be connected by some transmission medium. The method of connection is known as the network topology. Nodes need to share this transmission medium in such a way as to allow all nodes access to the medium without disrupting an established sender. A LAN is a communication path between computers, file-servers, terminals, workstations, and various other intelligent peripheral equipments, which are generally referred to as devices or hosts. A LAN allows access for devices to be shared by several users, with full connectivity between all stations on the network. A LAN is usually owned and administered by a private owner and is located within a localized group of buildings. 

Ethernet is the most widely use LAN today because it is cheap and easy to use. Connection of the SCADA network to the LAN allows anyone within the company with the right software and permission, to access the system. Since the data is held in a database, the user can be limited to reading the information. Security issues are obviously a concern, but can be addressed.

Modem use in SCADA systems 

Often in SCADA systems the RTU (remote terminal unit (PLC, DCS or IED)) is located at a remote location. This distance can vary from tens of meters to thousands of kilometers. One of the most cost-effective ways of communicating with the RTU over Long distances can be by dialup telephone connection. With this system the devices needed are a PC, two dialup modems and the RTU (assuming that the RTU has a built in COM port). The modems are put in the auto-answer mode and the RTU can dial into the PC or the PC can dial the RTU. The software to do this is readily available from RTU  manufacturers. The modems can be bought off the shelf at the local computer store.