Scada Components

7/31/2019 Scada Components

1/28

SCADACOMPONE

NTS

Prepared By:-

Animesh Ghosh

Roll no- 4

M.Tech(EE)


2/28

INTRODUCTI

ONSupervisory Control and Data Acquisition (SCADA) is a system that allows an operator at a

master facility to monitor and control processes that are distributed among various remote sites.

A properly designed SCADA system saves time and money by eliminating the need for service

personnel to visit each site for inspection, data collection/logging or make adjustments. Real-timemonitoring, system modifications, troubleshooting, increased equipment life, automatic report

generating . . . these are just a few of the benefits that come with todays SCADA system.

Other benefits SCADA Systems provide:

Reduces operational costs

Provides immediate knowledge of system performance

Improves system efficiency and performance

Increases equipment life

Reduces costly repairs

Reduces number of man-hours (labor costs) required for troubleshooting or service

Frees up personnel for other important tasks Facilitates compliance with regulatory agencies through automated report generating

SCADA (supervisory control and data acquisition) has been around as long as there

have been control systems. The first SCADA systems utilized data acquisition by means


3/28

of panels of meters, lights and strip chart recorders. The operator manually operating

various control knobs exercised supervisory control. These devices were and still are used

to do supervisory control and data acquisition on plants, factories and power generating

facilities. The following figure shows a sensor to panel system.

The disadvantages of a direct panel to sensor system are:

The amount of wire becomes unmanageable after the

installation of hundreds

of sensors

The quantity and type of data are minimal and rudimentary

Installation of additional sensors becomes progressively harder

as the system

grows


4/28

Re-configuration of the system becomes extremely difficult

Simulation using real data is not possible

Storage of data is minimal and difficult to manage

No off site monitoring of data or alarms

Someone has to watch the dials and meters 24 hours a day

The main components of a SCADA system are:-

Remote Terminal Unit (RTU)

Master Terminal Unit (MTU)

Communication Equipment & Software

REMOTE

TERMINAL UNIT

(RTU)The Remote Terminal Unit is usually defined as a communication satellite

within the SCADA system and is located at the remote site. The RTU gathers

data from field devices (pumps, valves, alarms, etc.) in memory until the MTU

initiates a send command. Some RTUs are designed with microcomputers and

programmable logic controllers (PLCs) that can perform functions at the

remote site without any direction from the MTU. In addition, PLCs can be

modular and expandable for the purpose of monitoring and controlling

additional field devices. Within the RTU is the central processing unit (CPU) that

receives a data stream from the protocol that the communication equipment

uses. The protocol can be open like Modbus, Transmission Control Protocol and

Internet Protocol (TCP/IP) or a proprietary closed protocol. When the RTU sees

its node address embedded in the protocol, data is interpreted and the CPU


5/28

directs the specified action to take. 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 the system

and/or altering the state of connected objects based on control messages

received from the system. Another term that may be used for RTU is remotetelemetry unit, the common usage term varies with the application area

generally.

A short discussion follows on the individual hardware

components.

Typical RTU hardware modules include:

Control processor and associated memory

Analog inputs

Analog outputs

Counter inputs

Digital inputs

Digital outputs

Communication interface

Power supply

RTU rack and enclosure

Control processor (or CPU):

This is generally microprocessor based (16 or 32 bit) e.g. 68302

or 80386. Total memory capacity of 256 kByte (expandable to 4

Mbytes) broken into three types:

1 EPROM (or battery backed RAM) 256 kByte
http://en.wikipedia.org/wiki/Microprocessorhttp://en.wikipedia.org/wiki/Distributed_control_systemhttp://en.wikipedia.org/wiki/SCADAhttp://en.wikipedia.org/wiki/Telemetryhttp://en.wikipedia.org/wiki/Microprocessorhttp://en.wikipedia.org/wiki/Distributed_control_systemhttp://en.wikipedia.org/wiki/SCADAhttp://en.wikipedia.org/wiki/Telemetry


6/28

2 RAM 640 kByte

3 Electrically erasable memory (flash or EEPROM) 128 kByte

A mathematical processor is a useful addition for any complex

mathematical

calculations. This is sometimes referred to as a coprocessor.

Communication ports typically two or three ports either RS-

232/RS-422/RS-485 for:

Interface to diagnostics terminal

Interface to operator station

Communications link to central site (e.g. by modem)

Diagnostic LEDs provided on the control unit ease

troubleshooting and diagnosis of problems (such as CPU

failure/failure of I/O module etc).

Another component, which is provided with varying levels of

accuracy, is a real-time clock with full calendar (including leap

year support). The clock should be updated even during power off

periods. The real-time clock is useful for accurate time stamping

of events. A watchdog timer is also required to provide a check

that the RTU program is regularly executing. The RTU program

regularly resets the watchdog time. If this is not done within a

certain time-out period the watchdog timer flags an error

condition (and can reset the CPU).

Analog input modules:


7/28

There are five main components making up an analog input

module. They are:

The input multiplexer

The input signal amplifier

The sample and hold circuit

The A/D converter

The bus interface and board timing system

A block diagram of a typical analog input module is shown in the

following figure .


8/28

Analog input modules

These have various numbers of inputs. Typically there are:

8 or 16 analog inputs

Resolution of 8 or 12 bits

Range of 420 mA (other possibilities are 020 mA/10 volts/0

10 volts)

Input resistance typically 240 k to 1 M

Conversion rates typically 10 microseconds to 30 milliseconds

Inputs are generally single ended (but also differential modesprovided)

For reasons of cost and minimization of data transferred over a

radio link, a common configuration is eight single ended 8-bit

points reading 010 volts with a conversion rate of 30

milliseconds per analog point.

An important but often neglected issue with analog input boards

is the need for sampling of a signal at the correct frequency. The

Nyquist criterion states that a signal must be sampled at a

minimum of two times its highest component frequency. Hence

the analog to digital system must be capable of sampling at a

sufficiently high rate to be well outside the maximum frequency

of the input signal. Otherwise filtering must be employed to

reduce the input frequency components to an acceptable level.

This issue is often neglected due to the increased cost of

installing filtering with erroneous results in the measured values.

It should be realized the software filtering is not a substitute for

an inadequate hardware filtering or sampling rate. This may

smooth the signal but it does not reproduce the analog signal

faithfully in a digital format.


9/28

Analog outputs

Typically the analogue output module has the following features:

8 analogue outputs

Resolution of 8 or 12 bits

Conversion rate from 10 seconds to 30 milliseconds

Outputs ranging from 420 mA/ 10 volts/0 to 10 volts

Care has to be taken here on ensuring the load resistance is not

lower than specified (typically 50 k ) or the voltage drop will be

excessive. Analog output module designs generally prefer to

provide voltage outputs rather than current output (unless poweris provided externally), as this places lower power requirements

on the backplane.

Digital inputs

It is important with alarm logic that the RTU should be able to

distinguish the first

alarm from the subsequent spurious alarms that will occur.

Most digital input boards provide groups of 8, 16 or 32 inputs per

board. Multiple

boards may need to be installed to cope with numerous digital

points (where the count of a given board is exceeded).

The standard, normally open or normally closed converter may be

used for alarm. In general, normally closed alarm digital inputs

are used where the circuit is to indicate an alarm condition.

The input power supply must be appropriately rated for the

particular convention used, normally open or normally closed. For


10/28

the normally open convention, it is possible to de-rate the digital

input power supply.

Digital output module

A digital output module drives an output voltage at each of the

appropriate output

channels with three approaches possible:

Triac switching

Reed relay switching

TTL voltage outputs


11/28

The TRIAC is commonly used for AC switching. A varistor is often

connected across

the output of the TRIAC to reduce the damaging effect of

electrical transients.Three practical issues should also be observed:

A TRIAC output switching device does not completely switch on

and off but

has low and high resistance values. Hence although the TRIAC is

switched off

it still has some leakage current at the output. Surge currents should be of short duration (half a cycle). Any

longer will

damage the module.

The manufacturers continuous current rating should be

adhered to. This often

refers to individual channels and to the number of channels.

There are situations

where all the output channels of the module can be used at full

rated current

capacity. This can exceed the maximum allowable power

dissipation for the

whole module.


12/28

Communication interfaces

The modern RTU should be flexible enough to handle multiple

communication media such as:

RS-232/RS-442/RS-485

Dialup telephone lines/dedicated landlines

Microwave/MUX

Satellite

X.25 packet protocols

Radio via trunked/VHF/UHF/900 MHz

Interestingly enough, the more challenging design for RTUs is the

radio communication interface. The landline interface is

considered to be an easier design problem. These standards will

be discussed in a later section.


13/28

Power supply module for RTU

The RTU should be able to operate from 110/240 V AC 10% 50

Hz or 12/24/48 V DC 10% typically. Batteries that should be

provided are lead acid or nickel cadmium. Typical requirementshere are for 20-hour standby operation and a recharging time of

12 hours for a fully discharged battery at 25C. The power supply,

battery and associated charger are normally contained in the RTU

housing.

Other important monitoring parameters, which should be

transmitted back to the central site/master station, are:

Analog battery reading

Alarm for battery voltage outside normal range

Cabinets for batteries are normally rated to IP 52 for internal

mounting and IP 56 for external mounting.

PLCs used as RTUs

A PLC or programmable logic controller is a computer based solidstate device that controls industrial equipment and processes. It

was initially designed to perform the logic functions executed by

relays, drum switches and mechanical timer/counters. Analog

control is now a standard part of the PLC operation as well.

The advantage of a PLC over the RTU offerings from various

manufacturers is that it can be used in a general-purpose role and

can easily be set up for a variety of different functions.


14/28

MASTERTERMINAL

UNIT (MTU)The Master Terminal Unit is usually defined as the master or heart

of a SCADA system and is located at the operators central control

facility. The MTU initiates virtually all communication with remote

sites and interfaces with an operator. Data from remote field

devices (pumps, valves, alarms, etc.) is sent to the MTU to beprocessed, stored and/or sent to other systems. For example, the

MTU may send the data to the operators display console, store

the information, and then send an operators initiate command to

a field pumps RTU.

The central site/master station can be pictured as having one or

more operator station(tied together with a local area network)

connected to a communication system consistinof modem and

radio receiver/transmitter. It is possible for a landline system tobe used iplace of the radio system, in this case the modem will

interface directly to the landline.

Normally there are no input/output modules connected directly to

the master stationalthough there may be an RTU located in close


15/28

proximity to the master control room. The features that should be

available are:

Operator interface to display status of the RTUs and enable

operator control Logging of the data from the RTUs

Alarming of data from the RTU

As discussed earlier, a master station has two main functions:

Obtain field data periodically from RTUs and submaster stations

Control remote devices through the operator station

It may also be necessary to set up a submaster station. This is to

control sites within a specific region. The submaster station has

the following functions:

Acquire data from RTUs within the region

Log and display this data on a local operator station

Pass data back to the master station


16/28

Pass on control requests from the master station to the RTUs in

its region

The master station has the following typical functions:

Establishment of communications

Configure each RTU

Initialize each RTU with input/output parameters

Download control and data acquisition programs to the RTU

Operation of the communications link

If a master slave arrangement, poll each RTU for data and write

to RTU

Log alarms and events to hard disk (and operator display if

necessary)

Link inputs and outputs at different RTUs automaticallyDiagnostics

Provide accurate diagnostic information on failure of RTU and

possible

problems

Predict potential problems such as data overloads

General interfaces:

Operator interfaces, or human machine interface (HMI) for SCADA

systems provide the functions of status indication, alarm

reporting, operator intervention in control action, and data

storage and programming. Several levels or layers of operator


17/28

interfaces are required to provide a reliable and maintainable

system: equipment level, controller level, and supervisory level.

At the controller and supervisory level, HMI may also provide

capability to modify the controller program.

Master station software:

There are three components to the master station software:

The operating system software

The system SCADA software (suitably configured)

The SCADA application software

There is also the necessary firmware (such as BIOS) which actsas an interface between the operating system and the computer

system hardware. The operating system software will not be

discussed further here. Good examples of this are DOS, Windows,

Windows NT and the various UNIX systems.

System SCADA software:

This refers to the software put together by the particular SCADA

system vendor and then configured by a particular user.

Generally, it consists of four main modules:

Data acquisition

Control

Archiving or database storage

The man machine interface (MMI)

This software is discussed in more detail in the next chapter. As

discussed earlier, a successful SCADA system design implies

considerable emphasis on the central site structure. Hence, this


18/28

will be assessed under the next section. However, one of the

features of a central site is the use of LANs.

COMMUNICATION COMPONENTS

Communication networks may be used in SCADA systems to

pass data between field devices and PLCs, between different

PLCs, or between PLCs and personal computers used for

operator interface, data proc-essing and storage, or

management information. Although a communications

circuit can involve only two pieces of equipment with a

circuit between them, the term network typically refers to

connecting many devices together to permit sharing of data

between devices over a single (or redundant) circuits. Data

is transmitted over a network using serial communication, in

which words of data called bytes consisting of individual

logical zeros and ones (bits) are transmitted sequentially

from one device to another. The col-lection of data in a

single transmission is often called a packet. The rate at

which data can be transmitted over a network is defined in

bits-per-second or bps, but typically expressed in thousands

(Kbps) or millions (Mbps).


19/28

In large SCADA systems, there is usually a

communications network of some type connecting the

individual PLCs to the operator interface equipment at

the central control room. There may also be networks

used at lower levels in the control system architecture,for communications between different PLCs in the same

subsystem or facility, as well as for communications

between field devices and individual PLCs. The

following Figure shows the various levels of network

communications in a typical large SCADA system.

b. Although not widely applied to SCADA systems, two

terms that are commonly used with respect to

management information systems communication are

local area network (LAN) and wide area network (WAN).

A LAN consists of all of the devices, typically PCs and


20/28

servers within a particular facility or site. A WAN is

created by providing a connection between LANs,

typically over a long geographic distance using

telecommunications facilities. Large SCADA systems

may be required to interface to LANs or WANs toprovide data transfer to management information

systems or to permit internet access to SCADA system

data.

The two main components of SCADA communication

are:

Fieldbus

Ethernet

FIELDBUS:

Fieldbus is a standard for digital field instrumentation enabling

field instruments to not only communicate with each other

digitally, but also to execute all continuous control algorithms

(such as PID , ratio control, cascade control, feedfor -ward control,

etc.) traditionally implemented in dedicated control devices . Inessence, Fieldbus extends the general concept of a distributed

control system(D CS) all the way to the field devices themselves .

In this way, Fieldbus sets itself apart as more than just another

digital communication bus for industry it truly represents a

new way to implement measurement and control systems .

Its main advantages are:-

The star configuration of individual lines is replaced by a

digital bus known as the fieldbus. It is cabled to the

various field devices and transmits digitized data via

digital interfaces.

Such a bus connects some tens of field devices among

themsclves and to a plant computer. If the devices are


21/28

close together, the cost of cabling is substantially less

than for individual cabling.

The growing intelligence of field devices calls for the

transmission not only of process measurements and controlinformation but also of digital status and control data

between field devices and control computers. The additional

cabling that used to bc required for his purpose can be

omitted when the fieldbus is employed.

The conversion of analog signals to digital locally, that

is, at the connection to the fieldbus, prevents interference

to the signal on route to the control computer. The

redundancy of digital codes also makes it possible toreconstruct erroneous data.

The standardization of field bus protocols permits

standard information interfaces between field devices and

a wide variety of control computers, so that open

communication results at the field level. This is not possible

at present with the wide range of information

transmission media at the field level

Diverse media can be used for digital data transmission

(e.g., copper twisted pair, coaxial cable, optical fiber, radio

waves), so that economic and technical optimization is

possible (transmission rate, noise immunity, energy

expenditure/intrinsic safety. electrical isolation, etc.).

Topologies:-

Bus

An alternative topology is the bus layout, where short spur

cables connect instruments to a longer trunk cable.

Terminal blocks or even quick-disconnect couplings

within each junction box provide a convenient means of


22/28

disconnecting individual devices from the segment without

interrupting data communication with the other devices.

The ideal arrangement for a bus network is to minimize

the length of each spur cable, so as to minimize the delay of

reflected signals off the unterminated ends of the drops .

Remember that only two termination resistors are allowed in

any electrically continuous network segment, and so thisrule for bids the addition of terminator s to the end of each

spur cable.

Tree

Yet another alternative topology for H1 networks is the so

called tree or chicken-foot arrangement, where a long trunkcable terminates at a multi-point junction along with s ever

al field devices and their spur cables.


23/28

Most Fieldbus systems resemble a combination of bus and chicken-foot topologies , where multiple junction

devices serve as connection points for two or more field

instruments per junction.

Daisy Chain

The simplest way to connect Fieldbus H1 devices together is

the so-called dais y-chain method, where each instrumentconnects to two cable lengths , forming an uninterrupted

chain network from one end of the segment to the other.


24/28

As simple as this topology is , it suffers from a major

disadvantage: it is impossible to disconnect any device in

the segment without interrupting the networks continuity.

Disconnecting (and reconnecting for that matter ) any device

necessarily results in all downstream devices losing

signal,if only for a brief time. This is an unacceptable state of

affairs for most applications .


25/28

ETHERNET:

During the mid-seventies, Xerox Corporation (Palo Alto)

developed the Ethernet network concept, based on work done byresearchers at the University of Hawaii. The Universitys ALOHA

network was set up using radio broadcasts to connect sites on the

islands. This was colloquially known as their Ethernet since it

used the ether as the transmission medium and created a

network between the sites. The philosophy was straightforward.

Any station wanting to broadcast would do so immediately. The

receiving station then had a responsibility to acknowledge the

message, advising the original transmitting station of a successfulreception of the original message. This primitive system did not

rely on any detection of collisions (two radio stations transmitting

at the same time) but depended on an acknowledgment within a

predefined time. The initial Xerox system was so successful that it

was soon applied to other sites, typically connecting office

equipment to shared resources such as printers and large

computers acting as repositories of large databases.

In 1980, the Ethernet Consortium consisting of Xerox, Digital

Equipment Corporation and Intel (a.k.a. the DIX consortium)

issued a joint specification, based on the Ethernet concepts,

known as the Ethernet Blue Book 1 specification. This was later

superseded by the Ethernet Blue Book 2 (Ethernet V2)

specification, which was offered to the IEEE for ratification as a

standard. In 1983, the IEEE issued the IEEE 802.3 standard for

carrier sense multiple access/collision detect (CSMA/CD) LANs

based on the DIX Ethernet standard.

An engineer named Bob Metcalfe conceived the idea of Ethernet

in 1973, while working for the Xerox research center in Palo Alto,

California. His fundamental invention was the CSMA/CD method of

channel arbitration, allowing multiple devices to share a common


26/28

channel of communication while recovering gracefully from

inevitable collisions . I n Metcalfes vision, all of the network

intelligence would b e built directly into controller devices

situated between the DTE devices (computers , terminals ,

printers , etc.) and a completely passive coaxial cable network.Unlike some other networks in operation at the time, Metcalfes

did not rely on additional devices to help coordinate

communications between DTE devices . The coaxial cable linking

DTE devices together would b e completely passive and dumb,

performing no task but the conduction of broadcast signals

between all devices . In that sense, it served the same purpose as

the luminiferous ether once believed to fill empty space:

conducting electromagnetic waves between separated points .

Metcalfes original network design operated at a data rate of 2.94

Mbps , impressive for its time. By 1980, the three American

computer companies DEC (Digital Equipment Corporation), Intel,

and Xerox had collaborated to revise the Ethernet design to a

speed of 10 Mbps , and released a standard called the DIX

Ethernet standard (the acronym DIX representing the first letter

of each companys name). Later , the IEEE Local and Metropolitan

Networks Standards Committee codified the DIX Ethernet

standard under the numeric lab el 802.3. At the present time

there exist many supplemental standards underneath the basic

802.3 definition, a few of them listed here:

802.3a-1985 1 0 BASE 2 t hin Ethernet

802.3d-1987 FOIRL fiber -optic link

802.3i-199010 BASE T twisted -pair cable Ethernet

802.3u-1995100 BASE -T Fa st Ethernet and Auto

-Negotiation

802.3x-1997 Ful l -Duplex standard


27/28

802.3ab-19991000 BASE -T Gigabit Ethernet over twisted

-pair cable

The IEEE 802.3 standard is limited to layers 1 and 2 of the OSI

Reference Model: the Physical and Data link layers . I n thephysical layer (1), the various supplements describ e all the

different ways in which bits are electrically or optically

represented, as well as permissible cable and connector types . In

the data link layer (2), the IEEE standard describes how devices

are addressed (each one with a unique identifier known as a MAC

address, consisting of a 48-bit binary number usually divided into

six bytes , each byte written as a two-character hexadecimal

number), and also how data frames are organized for Ethernet

transmissions .

CONCLUSI

ONUsing the above components, we can build up an efficient

and reliable SCADA system for supervisory control of field

processes.


28/28

BIBLIOGRAPHY

Practical SCADA for industry- Bailey/Wright (Newnes)

Lessons in Industrial Instrumentation- Tony R. Kuphaldt

Advanced Industrial Control Technology- Peng Zhang

(Elsevier)

Scada Components

Documents

Transcript of Scada Components