PLC Quick Working Explain

8/6/2019 PLC Quick Working Explain

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UGC scholar ship based research program:

PLC Based Substation Control And Its Linking

WithThe New Automation Ssheme

REPORT 3

By: Malik Sameeullah (08EES31) B.Tech electrical

Date :27:02:2011

PLC: working in substation with help of illustration.

Much attention given to the use of PLC (Programmable Logic Controller) insubstation and distribution automation application in recent years. Day to day

new application based on plc and SCADA is found. PLC is very cost competitive in

compression of traditional RTU and have many benefits in substation automation.

So need to understand PLC is much essential to develop new system which leads

PLC.

PLC parts:

y Input and output port: input device such as switches, pushbutton andother sensors like RTD (Resistance Temperature Detector) are connected to

the terminal strip under the bottom cover of PLC. Input is required so that

PLC work on input and generate logic. Generally input is either 1 or 0, which

is a case when we have to work on only two conditions (logic). But


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sometimes analog input is present which is need to sense as it is so we

need converter to convert it into equivalent digital signal. For this type of

input we need specific PLC module to serve this condition. Similarly output

port is provided to implement the result in field circuits.

y Main part (Processor):The processor is an self contained unit is generallyspecified according to memory required for the program to be

implemented. The processor is consist of microprocessor, system memory,

serial communication port for printer, PLC LAN link, supply port and

external port for I/O. Mounting rack is usually a metal framework with a

printed circuit board backplane which provides meanly for mounting the

PLC I/O modules and processor. There is a provision of adding a extra I/O by

adding a rack.

y Program loader/monitor section:Remote control unit known asworkstation from where a technical expert monitor substation and do

appropriate change in logic to control in better way. Program is written in

specific software packages and loaded in PLC. So, there is a gateway

between PC and PLC.

Ladder Logic: Programming of PLC:A ladder logic program is exactly similar to an electrical ladder diagram. It is

readily understood and maintained by skill workers familiar with relay logic. It

simplifies training ladder language, however, it generally lack good flow control.

Modern PLC is user friendly a graphical type programming with a help menu to

easily use any instruction.

The program symbols for a PLC input will look like a normally open or normally

closed contact used in typically electrical diagrams. Two types of switches areused:

(1)Normally Open (NO) signal state of contact is 1 then power can flowotherwise power cannot flow.


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(2)Normally Closed (NC) signal state of contact is 1 then power cannot flowotherwise power is flow.

The output instruction is OE ( )- i.e. Output energies. This is used to turn on bit

when circuit condition is satisfied. The real world switches are connected to theinput terminal and the outputs relays are connected to the output terminal of the

PLC. The I/O terminals are grouped together and placed in modules. There

arehundred of modules, it is important to have some kind of numbering system to

identify each module and all its input and output terminals. The generalized

numbering of I/O is following pattern:

Input/output Module number Rack number Slot number Word number

Terminal number

i.e. - I:0.0/1

AND gate using ladder logic: For two input C1(I:0/1),C2(I:0/1) and output

Y(O:0/1)

In this case output is high only when both inputsare high. Therefore, the AND gate

is equivalent to a series combination of normally open switches in a ladder as

shown in fig. Similarly different logic is made using different combination of

switch. For the digital circuit, if we want to develop equivalent ladder logic three

steps is follow: (1) Design description by truth table (2) Logic minimization (3)

Logic realization. There is also a provision of timers and counter which help to

provide delay on output. Timers have a great industrial application for

automation.

The contacts on the left side of the timer function block are the timer enable

contacts. When they are closed, power passes to the left terminal of the timer, it

clock is enabled and it starts timing. When they open power stop flowing through


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the terminal, and timer stop functioning. A timer function blocks has three output

contacts. When the timer is timed out, DN is set. The enable bit follow the input

enable status. If the enable contact is true then output EN is true. TT is set when

the timer is operating.

Communication:

For the communication between various PLC and PC station in substation. These

is a need of communication system. Different vendor use different protocol.

IEEE -488 bus: To connect and control programmable instruction and to provide a

standard interface for communication b/w instrument from difference sources.It

consists of 16 signal lines and 8 ground line. The 16 signal lines are divided into 3

group, namely 8 data lines, 3 handshake lines, 5 interface, management lines.

Serial interface RS-232C:


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RS for recommended standard and C for third version. RS 232C is a standard

interface approved by the electronic industries. All once for connecting serial

device. The Rs 232 interface standard specification of connecting two device, the

data terminal equipment (DTE) & Data Communication Equipment (DCE).

Ethernet:

This has become least expensive most popular way of communication. Speed LAN

alternative use standard Ethernet, twisted pair Ethernet. Ethernet refer to the

cable.Four type of cabling are commonly used. These are:10Base5, 10Base2,

10Base-T, 10Base-F. The electromagnetic type ray which contain message may

propagate through some medium like coaxial cable cause the development of

Ethernet. One of the problems in earlier day is that computer listening before

transmitting. So other wait until the current transmission complete then all start

at once. The solution is that each computer listen during its own transmission and

if it detects interface, jam the ether to alert all senders. And after random time

resend data. If data again collide than waiting time go double and so on and give

competing transmission and give one of them a chance to go first.

Profibus: Root of PLC Communication

General features of Profibus

y Straight per topologyy Token passing between up to three mastery Length from 960m/9.6kbps with 7 repeaters. With fibre optic cable the

length can over 80 km.

The Profibus familiar consist of three compatible version: DP, FMS, PA

PA:

Use layer 1,2 and user interface of the OSI model and layer 3 to 7 are not define.

Direct data link mapper allows access between the user interface and layer 2. The

user interface specified both application function and device behavior. RS-485

and fibre optic are available physical media for Profibus DP.


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FMS:

Layer 1,2,and 7 are defined. The application layer consist of fieldbus message

specification (FMS) and lower layer interface (LLI). FMS contain the application

protocol and provides the user with a wide selection of communication services.Application layer provides with a user communication services, mate it possible to

access variable, transmit program and control programmable execution.

PA:

Designed for process control and automation, using the international fieldbus

standard physical layer, IEC 1158-2 for bus powered sensor and actuator to be

operated intrinsically safe area. This is a hybrid communication system use a

function of master slave system.

PLC working in substation with the help of example

At a generation station, generator is the main part as without it no power is

available. So need to regulate properly, so that reliable flow of power is maintain.

PLC is easily utilized for this purpose. Generator connected to the turbine which is

rotate by steam so alternator also rotates. For regulating the speed of alternator

and stopping in case of renovation or in any emergency, we need to have a

control over a entry of steam. For control the over current, we need to trip some

line. In case of fault on main line, we need to activate the main circuit breaker.

Temp control of generator is also necessary. Temperature regulation of generator

by sensing the temp of coil using RTD and convert it to specify signal to produce

input at PLC. If the signal crosses the safe limit then we immediate stop the entry

of steam and so generator is stop. Relays are connected from PLC so that it

reliability increase because know we able to control relay in wide range.


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For the control we have input and output connected with a PLC. They are as

follow:

Input

y I0:1, start bottom for start generator.y I0:2, stop bottom for shutdown generator.y I0:3, temp sensor device to sense temperaturey I0:4, CT for sense overload and trip line 1y

I0:5, CT for sense overload and also trip line 2

Output

y O0:1, motor supply who stem entryy O0:2, input for a relay to trip line 1y O0:3, input for a relay to trip line 2y O0:4, for closing main ckt after generator starty O0:5, for activating relay in cause of ground fault due to which

overcurrent is flow


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END

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PLC Quick Working Explain