Automatic Overload Protection System

MINI PROJECT 2010 AUTOMATIC OVERLOAD PROTECTION SYSTEM

DEPT. OF E&C MZCE, KADAMMANITTA

1

AUTOMATIC OVERLOAD PROTECTION SYSTEM

Mini Project Report submitted in the partial fulfillment of the award of the degree

Of

Bachelor of Technology

In

ELECTRONICS & COMMUNICATION ENGINEERING

Of

Mahathma Gandhi University

By

RaHuL R

[email protected]

March 2010

Department of Electronics &Communication Engineering

Mount Zion College of Engineering, Kadammanitta,

Pathanamthitta-689649, Phone: 0468 2216325, 2274363 Fax: 2217425

Email: mountzion01@ sify.com Website: www.mountzioncollege.org



2

MOUNT ZION COLLEGE OF ENGINEERING

(Affiliated to M.G University&Approved by A.I.C.T.E)

Kadammanitta, Pathanamthitta

Kerala-689649 Email: mountzion01@ sify.com Website: www.mountzioncollege.org

DEPARTMENT OF ELECTRONICS& COMMUNICATION

CERTIFICATE

Certified that this is a bonafide record of the mini project

work done by RAHUL R of sixth semester, Electronics &Communication

Engineering, under Mahatma Gandhi University during the year 2010.

Project Guide HOD

Internal Examiner External Examiner



3

ACKNOWLEDGEMENT

To bring something into existence is truly the work of

ALMIGHTY. We thank GOD ALMIGHTY for making this venture a success.

We express our wholehearted thanks to the Management of

the college, Mr. Abraham Kalammannil, Chairman, for providing us an opportunity to do

our studies in this esteemed institution. We thank our Principal, Prof.P.C.Mohandas for

providing the facilities for our studies and constant encouragement in all achievements.

At the outset we wish to place on record our sincere thanks to

quite a few people without whose help, this venture would not have been a success.

We would like to express profound gratitude to our Head of

the department, Mr. Rangit Varghese, for his encouragement and for providing all

facilities for carrying out this project. We express our highest regard and sincere thanks to

our project Co-ordinators, Mr. Sreeji Krishnan, who provided the necessary guidance and

serious advice to carry out this project. We also express my gratitude to our Project Guide,

Mr. Sudheesh S.R, for her apt suggestions and support. Our sincere thanks to all the staff



4

members of the department of Electronics and Communication who guided as throughout

the entire course

PREFACE

Because of the differences in our college level studies

and industry level requirements, we are allotted a project to get knowledge about the

on goings at industries. I did the mini project that covered up a practical knowledge

of what I have studied so far in books. I did experienced an exposure to various

electronics devices and equipments which I would not have able to get easily

anywhere else. In electric power distribution, an automatic overload protection system

is a circuit breaker equipped with a mechanism that can automatically close the

breaker after it has been opened due to a fault. Automatic overload protection systems

are used in coordinated protection schemes for overhead line power distribution

circuits.

. All the topics which were dealt with in the project duration are mentioned in an

easy manner here in the report which I am submitting to our college for reference

purpose. I am highly thankful to the college faculty and the management for the

insertion of such a training period in our curriculum.



5

CONTENTS

1. INTRODUCTION 1

2. BLOCK DIAGRAM 2

3. BLOCK DIAGRAM EXPLANATION 3

3.1. CURRENT TRANSFORMER

3.2. OVER LOAD DETECTOR

3.3. TIMER

3.4. COUNTER

3.5. DISPLAY

3.6. RELAY

4. CIRCUIT DIAGRAM 4

5. CIRCUIT DIAGRAM EXPLANATION

5.1. CURRENT SENSOR 5

5.2. TRIP TIMER 5

5.3. TRIP COUNTER 6

5.4. RESET & AUTORESET SWITCH 6

5.5. OUTPUT CONTROL SWITCH 6

5.6. OUTPUT SECTION 7

5.7. POWER SUPPLY 7

6. WORKING 8

7. PCB LAYOUT

7.1. SOLDER SIDE 10

7.2. COMPONENT SIDE 11

8. PCB FABRICATION 12

9. COMPONENTS STUDY



6

9.1. RELAYS 19

9.2. REGULATED POWER SUPPLY 20

9.3. CURRENT TRANSFORMER 22

10. ADVANTAGES 26

11. DISADVANTAGES 26

12. APPLICATIONS 26

13. COMPONENTS REQUIRED 27

14. CONCLUSION 28

15. BIBLIOGRAPHY 29

16. APPENDIX: DATASHEETS



7

LIST OF FIGURES

1. POWER SUPPLY……………………………….7

2. PCB LAYOUT: SOLDER SIDE…...…………...10

3. PCB LAYOUT: COMPONENT SIDE…...…..…11

4. PCB FABRICATION BLOCK DIAGRAM……12

5. RELAYS…………………………………….…..19

6. LM 7805 VOLTAGE REGULATOR IC……….21

7. CURRENT TRANSFORMER…………………22

LIST OF TABLES

1. COMPONENTS REQUIRED ……………..27



8

1. INTRODUCTION

In electric power distribution, an automatic overload protection system is a

circuit breaker equipped with a mechanism that can automatically close the breaker

after it has been opened due to a fault. Automatic overload protection systems are

used in coordinated protection schemes for overhead line power distribution circuits.

These circuits are prone to transitory faults such as shorting or overload. With a

conventional circuit breaker or fuse, a transient fault would open the breaker or blow

the fuse, disabling the line until a technician could manually close the circuit breaker

or replace the blown fuse. But an automatic overload protection system will make

several pre-programmed attempts to re-energize the line. If the transient fault has

cleared, the automatic overload protection system circuit breaker will remain closed

and normal operation of the power line will resume. If the fault is some sort of a

permanent fault (downed wires, tree branches lying on the wires, etc.) the automatic

overload protection system will exhaust its pre-programmed attempts to re-energize

the line and remain tripped off until manually commanded to try again. 90% of faults

on overhead power lines are transient and can be cured by automatic overload

protection system. The result is increased availability of supply.



9

2. BLOCK DIAGRAM



10

3. BLOCK DIAGRAM EXPLANATION


The current transformer in the system is connected in series with the power line

and sense the current flow through the power line. The output of CT is connected to

the overload sensor circuit.

3.2. OVER LOAD DETECTOR

The over load detector circuit compares the signal from the CT with the preset

value. If it is greater than the preset value then the comparator generates a trigger to

start the timer circuit.

3.3. TIMER

The timer circuit will change its output level and keeps it for a preset time when

it get triggered. The output determines the load relay function and clocks the counter.

3.4. COUNTER

The counter counts the line tripping. When it reaches the predetermined value it

turns the fault indicator on and disconnects the load permanently. The technician can

reset it after removing the fault. If the trip count is lesser than the predetermined value

then the reset timer resets the counter.

3.5. DISPLAY

The display shows the status of each line.

3.6. RELAY

The relays are used to make/break the line-load connections. A relay driver

circuit is there to drive the relays.



11

4. CIRCUIT DIAGRAM



12

5. CIRCUIT DIAGRAM EXPLANATION

An automatic overload protection system has the following circuits section.

1. Current sensor

2. Trip times

3. Trip counter

4. Reset & auto reset

5. Output control switch

6. Output

5.1. CURRENT SENSOR

This section has current transformer, rectifier, filter and a comparator. The line is

connected to the load through a current transformer. Whenever the load increases

current also increases. The output voltage of current transforms is also varies in

proportional to the input current. The output voltage is given to the comparator after

rectification and filtering. The comparator compares this voltage with the reference

voltage. If it is greater than the rectified voltage the comparator output goes low

and triggers the trip timer. LM324 is used as the comparator. Rectified transformer

output is connected to the negative input and reference is given to the positive input.

5.2. TRIP TIMER

A 555 times is configured in astable multivibrator mode performs the trip

timer function. The comparator in the current sensor section will trigger this timer.

The output goes to high (quasi stable). This gives a clock to the counter and to the

o/p control switch through NOT gate to terminate the load from the line. After the

time determined by the value of resistor & capacitor connected on pins 6 & 7 of the

555 the output goes low. This connects the load to the line.



13

5.3. TRIP COUNTER

The trip counter is constructed with CD4017 counter IC. The trip timer gives clock to

pin 14 when it detect over load signal. Now the counter increment to the next value

from its reset state i.e. the HIGH output goes to Q1 from Q0, on the next trips it. Goes

from Q1 to Q2 and the final trip the Q3 will become high. This is fed to the output

control switch through a NOT gate Q1 & Q2 are ORed with two diodes and gives to the

auto reset circuit. So, if the fault has been cleared during these count stage, than the

auto reset circuit reset the counter after a certain time. If 3 trips occur counter will be on

Q3 and this will keep the circuit open. It requires a manual reset after solving the

problem on the faulty line. All the first 4 outputs of the counter are connected to LED to

show the status. Q0shows normal, Q1 & Q2shows tripped status and Q3 shows open

(faulty) status. The reset pin is connected to the auto reset section and to the reset

switch.

5.4. RESET & AUTORESET SWITCH

The reset section has a RC network to provide an initial delay during the start up time.

Switch in series with a 22E resistor can be used to reset the count.

An auto rest switch constructed using an astable multivibrator (with 555) and an

integrated digital switch CD4066. The astable multivibrator generates a reset pulse in a

certain interval. The output is connected to the digital switch (CD4066). The switch is

controlled by the output of counter. When the counter output is on Q1 or Q2 then the

switch is ON and the reset pulse from 555 reaches at reset pin 4017(counter).

5.5. OUT PUT CONTROL SWITCH

CD4066 a digitally controllable switch performs the output controlling function by

enabling & disabling the control signal from trip timer section to output section

according to the signal from counter state. If the counter is on Q3 then it open the

switch other vise the switch is being closed.



14

5.6. OUTPUT SECTION

The output section has a ULN2004 driver to driver the 12V relay. It

drives the relay according to the signal from the output control switch. On the relay

the load is connected on N/C and connected the load from the line.

5.7. POWER SUPPLY

A 12V step down transformer is used to step down the 230V to 12V. The bridge

rectifier is constructed with 1N4007 diodes. A 1000MFD capacitor is used to filter

the DC.

Fig. 1



15

6. WORKING

The automatic overload protection system circuit has a current sensing unit to detect the

over current through the power line. The basic two reasons for the over current in a

power line are overload and short circuit. So, when these faults occur the current sensor

gives a signal to the line controller circuit. Now the line controller disconnects the load

and starts a timer. After a certain time, determined by the timer, the control unit again

tries to connect the load. If the fault retains there again it disconnects the load. It repeats

this until the number of times determined by a counter in the control unit. After all it

terminates the load permanently. A manual reset after the rectification of the fault will

reconnects the load.

This system is designed for the automatic trip management of the faulty electric power

line. It is also able to handle 3 phase line. The system has an overload detection section

to sense the over load (or) short circuit on the line. The current transforms in this

section deliver voltage on secondary according to the current through the primary. A

diode and a capacitor is used rectify and filters the AC volt. Than a comparator

(LM324) compare it with the reference volt. If it exceeds the reference level, than the

comparator output goes low and it trigger the trip timer. The trip timer circuit is a

monostable multivibrator using IC 555. The trigger pin (2) of this IC will receive the

trigger signal and change its output to high. Time period of this quasi stable state is

determined by the RC value. After that it resets to low. The output of this section is

connected to two sections. One is to the trip counter and other is to the output control

switch on the positive going edge the trip counter increments to the next the output

control switch routes the signal to the output section. If the control signal from Q3

(through a NOT gate) is high, the counter CD4017 counts each trip. LEDs are used

show the counter status on its output Q0 to Q3. These output are used for managing

status display LEDS, output control and auto reset control section. Output Q0 controls

the green LED to show normal status. Q1 & Q3 control the yellow LED to show the



16

trip status and the same time it enables the auto reset control.Q3 control the RED LED

to show the permanent fault status and to disable the output control switch. The reset

pin receives reset commands from manual (or) auto reset section. In short, during the

reset condition the status of trip counter is on Q0. When a fault occurs trip timer trigger

the counter and the counter moves to Q1. Meanwhile the load is disconnected from the

line. After a short time system connects the load to the line. If the fault existing the trip

times again triggers and counter goes to Q3. The Q1 &Q2 output have same effect on

all process. This time the auto reset circuit is enabled. So if the faulty is temporary the

circuit will be automatically reset on permanent fault the counter moves to Q3. Than it

requires a manual reset.



17

7. PCB LAYOUT

7.1. SOLDER SIDE

Fig. 2



18

7.2. COMPONENT SIDE

Fig. 3



19

8. PCB FABRICATION

The PCB design process typically involves placing and connecting parts;

specifying how they're to be packaged; uniquely identifying them; adding information

for simulation, synthesis, board layout, purchasing, or other external functions; and

incorporating information from external functions.

Fig. 4



20

Once you finish a first pass at placing and connecting parts, use the commands

on the Tools menu in the project manager to complete the process. Click on the

command names in the figure for information about the tool commands.

As shown in the figure, you use Annotate, Design Rules Check, and Cross

Reference to package the parts in your design and make sure there are no unconnected

parts, unwanted connections, or other invalid design conditions. In practice, you

might run these tools several times before moving on to the next phase.

Generally, you should run Design Rules Check to verify your design before you

generate a net list. This allows for more efficient net list creation, and you can

concentrate on net list-specific problems if they should occur during the Create Net

list process. Design Rules Check warns you if certain conditions exist in your design.

The severity of the specific problem may prevent completion of the design. Other

conditions are subject to your judgment, and may be of no consequence. Once you are

satisfied with the results of design tests like Design Rules Check, and then proceed

with the creation of a net list.

You can add properties or change their values at any point, and there are several

ways to do this. If you want to change the value of one or two properties, just edit

them on the schematic page. To edit properties on many parts at the same time, use

Update Properties or Capture's built-in spreadsheet editor (from the Edit menu,

choose Browse and then Parts). If you're more comfortable editing in a full-featured

spreadsheet or database program, use Export Properties to write design data out and

Import Properties to read the changes back in.



21

Once you're satisfied with your design use Create Net list to create a net list in

any of the formats supported by Capture. This is often the point at which you use Bill

of Materials to create a list of parts used in the design.

Use Back Annotate to incorporate any packaging changes necessary because of

routing or manufacturing constraints. You may need to add or modify properties again

or make other changes in the design, as shown in the figure.

Preparing a Capture design for Layout is a two-part process. First, you must

create a valid design and then create a net list in an .MNL format for Layout. After

you have prepared your Capture design, you can create a new Layout design using the

.MNL net list.

You can bring Capture net list information into Layout in two ways. You can

choose one of the AutoECO options to merge the net list with the board file, or you

can select the Run ECO to Layout option in Capture (in the Create Net list dialog

box) to automatically communicate modifications to Layout. If the board file is open

when you update the net list file, Layout automatically displays a dialog box asking if

you want to load the new net list file. If the board file is not open when the net list

changes, Layout prompts you to load the modified net list when you re-open the

board file. Then after auto placing or manual placing auto-routing will complete all

routing works. If not completed then we can route manually

Laser printers and photocopiers use plastic toner, not ink, to draw images. Toner

is the black powder that ends up on your clothes and desk when replacing the printer

cartridge. Being plastics, toner is resistant to etching solutions used for making PCBs

- if only we could get it on copper! Modifying a printer for working with copper is out

of question, but we can work around it with the toner-transfer principle. Like most

plastics, toner melts with heat, turning in a sticky, glue-like paste. So why not print on

paper as usual, place the sheet face-down on PCB copper, and melt toner on copper

applying heat and pressure.



22

The perfect paper should be: glossy, thin, and cheap. Cut the paper to a size

suitable for your printer. Try to get straight, clean cuts, as jagged borders and paper

dust are more prone to clog printer mechanism. An office cutter is ideal, but also a

blade-cutter and a steady hand work well.

Laser printers are not designed for handling thin, cheap paper, so we must help

them feeding the sheets manually instead of using the paper tray. Selecting a straight

paper path minimizes the chances of clogging. This is usually achieved setting the

printer as if it were printing on envelopes.

We want to put as much toner on paper as possible, so disable “toner economy

modes” and set printer properties to the maximum contrast and blackness possible.

We want to print your PCB to exact size, so disable any form of scaling/resizing (e.g.

“fit to page”). If your printer driver allows, set it to “center to page” as it helps to get

the right position using a non-standard size sheet.

Print the PCB layout as usual, except we must setup the printer as described

above and you must print a mirrored layout.

PCB material is fiberglass like, and a trick to cut it effortlessly is to score a

groove with a blade cutter or a glass cutter. The groove weakens the board to the point

that bending it manually breaks it along the groove line. This method is applicable

only when cutting the whole board along a line that goes from side to side, that is we

can’t cut a U or L shaped board with it.

For small boards, lock the PCB material in a vice, aligning vice edge and cut line. Use

an all-aluminum vice which is soft and doesn’t scratch copper, if we use a steel vice

protect copper surface with soft material.



23

Using the vice as a guide, score BOTH board sides with a blade cutter (be careful) or

another sharp, hardened tool (e.g. a small screwdriver tip). Ensure to scratch edge-to

edge. Repeat this step 5-6 times on each side.

Bend the board. If groove is deep enough, the board will break before reaching 30

degrees bend. It will break quite abruptly so be prepared and protect our hands with

gloves.

To make paper alignment easier, cut a piece of PCB material that is larger (at least

10mm/0, 39 inch for each side) than the final PCB.

It is essential that the copper surface is spotlessly clean and free from grease that could

adverse etching. To remove oxide from copper surface, use the abrasive spongy scrubs

sold for kitchen cleaning. It’s cheaper than ultra-fine sandpaper and reusable many

times. Metallic wool sold for kitchen cleaning purposes also works. Thoroughly scrub

copper surface until really shiny. Rinse and dry with a clean cloth or kitchen paper.

To make paper alignment easy, cut excess paper around one corner (leave a small

margin though). Leave plenty of paper on the other sides to fix the paper to the desk. As

the board is larger than the final PCB, there is large margin for easy placement of paper

on copper.

Turn the iron to its maximum heat (COTTON position) and turn off steam, if present.

While the iron warms up, position the materials on the table. Don’t work on an ironing

board as its soft surface makes it difficult to apply pressure and keep the PCB in place.

Protect table surface with flat, heat-resistant material (e.g. old magazines) and place the

board on top, copper face up. Lock the board in place with double-adhesive tape.

Position the PCB printout over the copper surface, toner down, and align paper and



24

board corners. Lock the paper with scotch tape along one side only. This way, we can

flip the paper in and out instantly.

Flip out the paper, and preheat copper surface placing the iron on top of it for 30

seconds. Remove the iron; flip back paper into its previous position over the copper.

It is essential that paper does not slip from its position. We can also cover with a second

sheet of blank paper to distribute pressure more evenly. Keep moving the iron, while

pressing down as evenly as we can, for about one minute.

Remove the iron and let the board to cool down.

This is the fun part. When the board is cool enough to touch, trim excess paper and

immerge in water. Let it soak for 1 minute, or until paper softens.

Cheap paper softens almost immediately, turning into a pulp that is easy to remove

rubbing with your thumb. Keep rubbing until all paper dissolves (usually less than 1

minute). Don’t be afraid to scratch toner, if it has transferred correctly it forms a very

strong bond with copper.

The board with all paper removed. It is OK if some microscopic paper fibers remain on

the toner (but remove any fiber from copper), giving it a silky feeling. It is normal that

these fibers turn a little white when dry.

There are many alternatives for etching liquids, and we can use the one that suits your

taste. Using ferric chloride (the brown stuff): it’s cheap, can be reused many times, and

doesn’t require heating. Actually, moderate heating can speed up etching, but find it

reasonably fast also at room temperature (10…15 minutes).



25

The down side of this stuff is that it’s incredibly messy. It permanently stains

everything it gets in contact with: not only clothes or skin (never wear your best clothes

when working with it!), but also furniture, floor tiles, tools, everything. It is

concentrated enough to corrode any metal – including your chrome-plated sink

accessories. Even vapors are highly corrosive: don’t forget the container open or it will

turn any tool or metallic shelf nearby into rust.

For etching, place the container on the floor (some scrap cardboard or newspaper to

protect the floor from drops). Fit the board on the hanger, and submerge the PCB. Stir

occasionally by waving the hanger.

First impression may be that nothing happens, but in less than 10 minutes some copper

is removed, making first tracks to appear. From now on, stir continuously and check

often, as the process completes rather quickly. We don’t want to overdo it, otherwise

thinner tracks start being eroded sideways. As a rule of thumb, stop 30 seconds after we

don’t see any copper leftovers over large areas. Rinse the board with plenty, plenty,

plenty of water.



26

9. COMPONENTS STUDY

9.1. RELAYS

Fig. 5

Basically, a relay is an electrically operated switch, and actually the predecessor of the

transistor. Solenoids are relays also but the very large types which carry huge amounts of

current. Relays are the smaller types. Relays come in three types: electro mechanical, solid-

state, and so-called hybrids which are a combination of the first two. There are also some

specialized types that fall into neither category but I will deal with them later in this

tutorial. Lets take electro-mechanical types first, they are available in three main models;

armature, plunger, and reed. The Armature Relays are the elegant. Plenty turns of very

fine magnet-wire are wound around an iron core to form an electro-magnet. The movable

metal armature has an electrical contact that is positioned over a fixed contact attached to

the relay frame. A spring holds the armature up so that the movable and fixed contacts are

normally separated (open). When the coil is energized, it attracts the pivoting armature and

pulls it down, closing (make) the SPST contacts and completes the power circuit. Vice-

versa, this relay can be made to open the contacts instead of closing them, or can do both

IEA REFERENCE



27

either way. The armature relay is pretty old and no longer used in new applications; they do

still exist however and are being used still at the time of writing this document.

Relays are components which allow a low-power circuit to switch a relatively high

current on and off, or to control signals that must be electrically isolated from the

controlling circuit itself. Newcomers to electronics sometimes want to use a relay for this

type of application, but are unsure about the details of doing so. Here’s a quick rundown to

make a relay operate, you have to pass a suitable .pull-in. and holding current (DC) through

its energizing coil. And generally relay coils are designed to operate from a particular

supply voltage often 12V or 5V, in the case of many of the small relays used for electronics

work. In each case the coil has a resistance which will draw the right pull-in and holding

currents when its connected to that supply voltage. So the basic idea is to choose a relay

with a coil designed to operate from the supply voltage you are using for your control

circuit and then provide a suitable .relay driver circuit so that your low-power circuitry can

control the current through the relays coil. Typically this will be somewhere between 25mA

and 70mA Often your relay driver can be very simple, using little more than an NPN or

PNP transistor to control the coil current. All your low-power circuitry has to do is provide

enough base current to turn the transistor on and off, as you can see from diagrams A and

B.

9.2. REGULATED POWER SUPPLY

Most digital logic circuits and processors need a 5 volt power supply. To use these parts

we need to build a regulated 5 volt source. To make a 5 volt power supply, we use a

LM7805 voltage regulator IC (Integrated Circuit). The IC is shown below.



28

Fig. 6

The LM7805 is simple to use. You simply connect the positive lead of your unregulated

DC power supply (anything from 9VDC to 24VDC) to the Input pin, connect the negative

lead to the Common pin and then when you turn on the power, you get a 5 volt supply from

the Output pin. Sometimes the input supply line may be noisy. To help smooth out this

noise and get a better 5 volt output, a capacitor is usually added to the circuit, going

between the 5 volt output and ground (GND). We use a 220 uF capacitor. 12V supply is

also made in a same manner



29


Fig. 7

A CT for operation on a 110 kV grid

In electrical engineering, a current transformer (CT) is used for measurement of electric

currents. Current transformers, together with voltage transformers (VT) (potential

transformers (PT)), are known as instrument transformers. When current in a circuit is

too high to directly apply to measuring instruments, a current transformer produces a

reduced current accurately proportional to the current in the circuit, which can be

conveniently connected to measuring and recording instruments. A current transformer also

isolates the measuring instruments from what may be very high voltage in the monitored

circuit. Current transformers are commonly used in metering and protective relays in the

electrical power industry

http://en.wikipedia.org/wiki/Electrical_engineering

http://en.wikipedia.org/wiki/Power_system_protection

http://en.wikipedia.org/wiki/Relay

http://en.wikipedia.org/wiki/File:Leistungsschalter-110KV.jpg



30

Like any other transformer, a current transformer has a primary winding, a magnetic core,

and a secondary winding. The alternating current flowing in the primary produces a

magnetic field in the core, which then induces current flow in the secondary winding

circuit. A primary objective of current transformer design is to ensure that the primary and

secondary circuits are efficiently coupled, so that the secondary current bears an accurate

relationship to the primary current.

The most common design of CT consists of a length of wire wrapped many times around a

silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore

consists of a single 'turn' of conductor, with a secondary of many hundreds of turns. The

primary winding may be a permanent part of the current transformer, with a heavy copper

bar to carry current through the magnetic core. Window-type current transformers are also

common, which can have circuit cables run through the middle of an opening in the core to

provide a single-turn primary winding. When conductors passing through a CT are not

centered in the circular (or oval) opening, slight inaccuracies may occur.

Current transformers used in metering equipment for three-phase 400 ampere electricity

supply

Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical

examples of low voltage single ratio metering current transformers are either ring type or

plastic moulded case. High-voltage current transformers are mounted on porcelain bushings

to insulate them from ground. Some CT configurations slip around the bushing of a high-

http://en.wikipedia.org/wiki/Transformer

http://en.wikipedia.org/wiki/Magnetic_core

http://en.wikipedia.org/wiki/Alternating_current

http://en.wikipedia.org/wiki/Electricity_meter

http://en.wikipedia.org/wiki/Three-phase

http://en.wikipedia.org/wiki/File:Stromwandler_Zeichnung.svg

http://en.wikipedia.org/wiki/File:CurrentTransformers.jpg



31

voltage transformer or circuit breaker, which automatically centers the conductor inside the

CT window.

The primary circuit is largely unaffected by the insertion of the CT. The rated secondary

current is commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would

provide an output current of 5 amperes when the primary was passing 4000 amperes. The

secondary winding can be single ratio or multi ratio, with five taps being common for multi

ratio CTs. The load, or burden, of the CT should be of low resistance. If the voltage time

integral area is higher than the core's design rating, the core goes into saturation towards the

end of each cycle, distorting the waveform and affecting accuracy.

Current transformers are used extensively for measuring current and monitoring the

operation of the power grid. Along with voltage leads, revenue-grade CTs drive the

electrical utility's watt-hour meter on virtually every building with three-phase service, and

every residence with greater than 200 amp service.

The CT is typically described by its current ratio from primary to secondary. Often,

multiple CTs are installed as a "stack" for various uses. For example, protection devices

and revenue metering may use separate CTs; stacking them provides severability while

consolidating the high voltage interface. Similarly, potential transformers such as the CVT

are used for measuring voltage and monitoring the operation of the power grid.

http://en.wikipedia.org/wiki/Saturation_%28magnetic%29

http://en.wikipedia.org/wiki/Power_grid

http://en.wikipedia.org/wiki/Capacitor_voltage_transformer

http://en.wikipedia.org/wiki/Power_grid



32

Safety precautions

Care must be taken that the secondary of a current transformer is not disconnected from its

load while current is flowing in the primary, as the transformer secondary will attempt to

continue driving current across the effectively infinite impedance. This will produce a high

voltage across the open secondary (into the range of several kilovolts in some cases), which

may cause arcing. The high voltage produced will compromise operator and equipment

safety and permanently affect the accuracy of the transformer.

http://en.wikipedia.org/wiki/Electrical_impedance

http://en.wikipedia.org/wiki/Electric_arc



33

10. ADVANTAGES

1. It prevents complete outages followed by transient fault

2. Lowers the manual efforts

3. Saves time.

4. It avoids accidents due to electric shocks

11. DISADVANTAGES

1. This circuit is made up of discrete components

12. APPLICATION

1. It is used for overhead line power distribution circuits.



34

COMPONENTS REQUIRED

Table. 1

Sl. No Name of the component Quantity

1 Current Transformer 3

2 Relay 3

3 NE 555 4

4 CD 4069UB 3

5 CD 4066 BC 2

6 ULN 2004 1

7 LM 324 3

8 CD 4017 3

9 Diode 1N4007 3

10 Diode 1N4148 6

11 Resistance (100Ω) 3

12 Resistance (10Ω) 3

13 Resistance (1.5kΩ) 3

14 Resistance (5.6kΩ) 3

15 Resistance (1kΩ) 9

16 Resistance (22kΩ) 3

17 Capacitor (0.01µF) 4

18 Capacitor (22µF) 3

19 LED 9

20 Potentiometer (5k) 4



35

13. CONCLUSION

Controllers for the automatic overload protection system range from the original

electromechanical systems to digital electronics. The digital circuit based automatic

overload protection systems are designed to overcome down time from transient

faults. These are very compact reliable device consisting programmable counter logic,

timer and status monitoring.

Here the current sensing concept is adapted from the original electromechanical

systems and added a logical system to it.



36

14. BIBLIOGRAPHY

1. Electronic Devices and Circuits. By B.L. Theraja.

2. Principles of Electronics. By V.K. Mehta

3. Digital Fundamentals. By Floyd

4. Digital Principles and Applications. By Malvino and Leach.

5. Analog Electronics. By Eastern Books

6. Basic Electronics and Linear Circuits. By Bhargava

DATASHEET LINKS

1. www.bellwindonline.com

2. www.national.com

3. www.fairchildsemi.com

4. www.ti.com

5. www.nxp.com

Documents

Automatic Overload Protection System