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EE04 704: POWER SYSTEMS III Resource Materials

By

Sasidharan Sreedharan

(www.sasidharan.webs.com)

MODULE - I

Circuit Breakers

Vidya Academy of Science and Technology

Trissur, Kerala, India (August- 2011)

8/3/2011

1

EE 09 506 ELECTRICAL MATERIAL SCIENCE

CLASS 1: MODULE 1EE04 704: POWER SYSTEM III

Class 1

SUBJECT INTRODUCTIONModule I

Circuit Breakers : Principles of operation, different

types and their operations, ABCB, oil CB, SF6,vacuum

CB, circuit breaker ratings, cause of over voltages,

surges and traveling waves, voltage waves on

transmission line, reflection and attenuation,

protection against lightning, earth wires, lightning

diverters, surge absorbers, arcing ground, neutral

earthing , basic concepts of insulation levels and their

selection, BIL, coordination of insulation

Module II

Protective Relays: Protective zones, requirement of

protective relaying, different types of relays and their

applications, generalized theory of relays, protection

scheme for generator, transformers, lines and bus bars,

static relays, amplitude and phase comparators, lock

diagrams of static relays, microprocessor based

protective relaying- overcurrent & impedance relays

Module III

• Electric Traction: Systems of traction, speed

time curve, mechanics of traction, braking,

power supply, systems of current collection.

• Electric Heating : Advantage of electric

heating, resistance and induction arc furnaces,

construction and field of application, high

frequency power supply and the principle and

application of dielectric heating - .

Module IV

• Introduction to SCADA systems - block

diagram -communication between various

control centers –three level control systems -

functions and features. . Introduction to HVDC

transmission – kinds of DC links – comparison

with HVAC systems – PQ problems -

introduction to FACTS – FACTS controllers –

SVC - STATCOM - UPFC

Books

1. Sunil S Rao :Switch gear protections ; Khanna

Publishers(11th edition)

2. 2. Soni, Gupta & Bhatnagar :A course in Electrical Power ;

Dhanpat Rai & Sons.

3. A.R.Van.C.Warrington :Protective Relays Vol 1 & 2 ;

Chappman & Hall

4. Ravindranath M. Chander:Power System Protection and

Switchgear.

5. G. T. Haydt :Electric Power Quality.

6. Badriram : D.N Viswakarma : Power system protection &

switchgear .Tata McGraw Hill

7. Narain .G. Hingorani: Understanding FACTS. IEEE PRESS.

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Exam Scheme and PatternSessional work assessment

2Assignments 30%

2 tests 60%

Regularity &Participation in class 10%

Total marks = 50

Exam Pattern

Q I - 8 short type questions of 5 marks, 2 from each module

Q II - 2 questions A and B of 15 marks from module I with choice to answer

any one

Q III - 2 questions A and B of 15 marks from module II with choice to answer

any one

Q IV - 2 questions A and B of 15 marks from module III with choice to answer

any one

Q V - 2 questions A and B of 15 marks from module IV with choice to answer

any one

Resource Materials

• All resource materials including class power points,selected notes, reference books, assignments, doubtclearings etc will be regularly posted in the website.

• Students are requested to go through the same.

www.sasisreedhar.webs.com

[email protected]

Contents

• 1. Circuit Breakers

• 2. Protective Relays

• 3. Electric Traction

• 4. Introduction to SCADA Systems

Power System Protection

Disturbances: Light or Severe

• The power system must maintain acceptable

operation 24 hours a day

– Voltage and frequency must stay within certain limits

• Small disturbances

– The control system can handle these

– Example: variation in transformer or generator load

• Severe disturbances require a protection system

– They can put in danger the entire power system

– They cannot be overcome by a control system

Power System Protection

Operation during severe disturbances:

– System element protection

– System protection

– Automatic reclosing

– Automatic transfer to alternate power supplies

– Automatic synchronization

Electric Power System Exposure to External

Agents

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Damage to Main Equipment Protection System

A series of devices whose mainpurpose is to protect persons andprimary electric power equipment fromthe effects of faults

Blackouts

• Loss of service in a large

area or population

region

• Hazard to human life

• May result in enormous

economic losses

• Overreaction of the

protection system

• Bad design of the

protection system

Characteristics Main Causes

Short Circuits Produce High Currents

FaultSubstation

abc

I

IWire

Three-Phase Line

Thousands of Amps

Electrical Equipment Thermal Damage

I

t

In Imd

Damage Curve

Short-Circuit Current

Damage Time

Rated Value

Mechanical Damage During

Short Circuits• Very destructive in busbars, isolators, supports,

transformers, and machines

• Damage is instantaneous

i1

i2

f1 f2

Rigid Conductors f1(t) = k i1(t) i2(t)

Mechanical Forces

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The Fuse

Fuse

Transformer

Protection System Elements

• Protective relays

• Circuit breakers

• Current and voltage transducers

• Communications channels

• DC supply system

• Control cables

Three-Phase Diagram of the Protection Team DC Tripping Circuit

+

Circuit Breakers Current Transformers

Very High Voltage CTMedium-Voltage CT

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Voltage Transformers

Medium Voltage

High Voltage

Note: Voltage transformers are also known as potential transformers

Protective Relays

Examples of Relay Panels

Old Electromechanical

Microprocessor-Based Relay

How Do Relays Detect Faults?

• When a fault takes place, the current, voltage,

frequency, and other electrical variables behave in a

peculiar way. For example:

– Current suddenly increases

– Voltage suddenly decreases

• Relays can measure the currents and the voltages

and detect that there is an overcurrent, or an under

voltage, or a combination of both

• Many other detection principles determine the

design of protective relays

The Future

• Improvements in computer-based protection

• Highly reliable and viable communication

systems (satellite, optical fiber, etc.)

• Integration of control, command, protection,

and communication

• Improvements to human-machine interface

Diesel Traction - 1912

The diesel engine was invented in the year 1893, by ayoung German Engineer, called Rudolf Diesel. But it wasonly nineteen years later, that the first Diesel locomotivecame into existence.

Since then, diesel traction has grown from strength to strength. Over 89,000 Diesel locomotives have been built in the world so far, the General Motors, USA alone contributing to as many as 56,000 Locomotives.

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Electric Traction - 1881

After many decades of satisfactory performance, the steamengines were to give way to more modern locomotives.The year 1881 saw the birth of the first electric Railway runby a German Engineer Werner Van Siemens using boththe rails to carry the current. Finding this a little toodangerous, Siemens soon adopted the overhead electricwires. Electric locomotives today raun on Rail roads inmany countries.

SCADA

• What is SCADA

• Where and Why are SCADA systems used

• What do SCADA systems Provide?

• Evolution

• Benefits

Traditional Control Traditional Control

• Dedicated Consoles

• Point to point communication

• No network

– No remote access

– No remote diagnostic

Distributed Control

Network Protocol

Field Bus

Field Bus-Integrator

PLC’s

Distributed Control

• Advantages:– Distributed databases/ programs created from a

single development environment (also in frontend processor)

– Distributed access

– Distributed diagnostic

– Display everything everywhere

• Difficult:– Integration of various field bus components

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Data Server

SCADA ?SCADA ?SCADA ?

Supervisory

ControlAnd

Data

Acquisition

Graphics and Batch processingArchiving, Logging,

Access Control, Alarms

Distributed database

PLC’s

Field Bus

Data Server

Control Programs

What, Where and Why

• What is “SCADA” and where is it used

– Supervisory Controls And Data Acquisition

– Application area :

• Industrial processes: chemical, power

generation and distribution, metallurgy etc.

• Nuclear processes: reactors, nuclear waste, ...

What do SCADA Provide?

• Flexible and open architecture

• Basic SCADA functionality

• MMI

• Alarm Handling and Trending

• Access Control

• Automation

• Logging, Archiving, Report

Generation

• Interfaces to H/W and S/W

• Development Tools

SCADA functions MMI

Configuration

of SCADA Systems

Data Server PLC’sData Server

Is SCADA the only Future ? ...New Technology: JetWeb

Each node is an individual Web Server

All nodes and all I/O hooked up to the Ethernet

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Kerala Load Dispatch SCADA functions

SCADA functions

Type Parameter Controlled FACTS Devices Functions

Series Controllers Series P TCSC, SSSC, TCPARTo alleviate OLL

Increase TC

Shunt Controllers Shunt Q SVC, STATCOMCompensate V

by injecting Q

Combined Series-Shunt

ControllersSeries P & Shut Q UPFC

Combination of series

& shunt controller

FACTS Devices

FACTS devices are solid-stateconverter that have the capability ofcontrol of various electricalparameters in transmission circuit [13,15,16]:

Thyristor Controlled SeriesCompensator (TCSC)

Static VAR Compensator (SVC)

Unified Power Flow Controller(UPFC)

Static Compensator (STATCOM)

Static Syncronous SeriesCompensator (SSSC), etc

47

etcetc

tcr

Types of FACTS Devices

Regards

sasidharan.webs.com

8/3/2011

1



Fundamentals of Fault Clearing

Module I










House Wiring

Distribution Board

Miniature Circuit Breakers

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Why we need a circuit breaker1. During the operation of the power systems, it is often desirable and

necessary to switch on or off the various circuits (e.g., transmission lines,

distributors, generating plants etc.) under both normal and faulty

conditions.

2. Previously this function was performed by a switch and a fuse in series

with the circuit.

3. However, such method has two disadvantages.

Firstly, when fuse blows out, it takes quite sometime to replace it and restore the supply

to the customers.

Secondly, a fuse cannot successfully interrupt the heavy fault currents that occur on the

modern high voltage power systems and large capacity circuits.

4. Therefore, with the advancement in power systems, there was a need to

develop a more reliable means of control.

5. The circuit breaker was developed to switch on and off the various circuits

of a power system

Air Circuit Breaker

SF6 Circuit Breaker

Devices used for circuit breaking

1.Fuse and ironclad switches

2.Isolators

3.Circuit breakers

Fuses

1. Fuses and Iron Clad Switches

• Fuse is an over current switch which operates when

the current exceeds a preassigned value

• When the limit exceeds, it melts and causes the

current interruption.

• The supply is restored only when a healthy fuse

replaces the damaged (melted) one in the line.

• To permit this without any danger of shock to the

operator, fuses are connected on the load side of an

ironclad switch.

Fuses

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Distribution Board Fuse Cut Outs

Iron Clad Cut Outs Metal Clad Switch Fuses Switch Disconnector

2. Isolators

• An isolator is a switch connected after a circuit

breaker.

• When a circuit or a busbar is taken out of service by

tripping the circuit breaker, the isolator is then open

circuited and the isolated line is earthed through

earth switch so that the trapped line charges are

safely conducted to ground.

Isolators

110 KV Isolator

3. Circuit Breakers• A circuit breaker is a complex circuit-breaking device with the

following duties:

I. Makes or breaks both normal and abnormal currents

II. Appropriately manages the high-energy arc associated with

current interruption.

III. The problem has become more acute due to interconnection

of power stations resulting in very high fault levels.

IV. Effects current interruption only when it is called upon to do

so by the relay circuits. In fact they are required to trip for a

minimum of the internal fault current and remain

inoperative for a maximum of through fault current

V. Rapid and successive automatic breaking and making to aid

stable system operation

VI. Three pole (3-pole) and single pole (1-pole) auto-reclosing

arrangement

Fuses/ Circuit Breakers

• Difference between fuse/circuit breaker protection

and overload protection:

– Fuses and circuit breakers protect circuit from

grounds and short circuits only.

– Protect Motor Circuit and Power system from a

short in the motor Circuit.

– C.B.’s are more expensive but can be reset, Fuses

are less expensive but can be real pain to

replace.

1.Contact.

3.Operating

Mechanism.

4.Arc quenches medium.

2.Insulation.

Basic Elements of Circuit Breaker

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Circuit Breaker - Switching

• In addition to the making and breaking

capabilities, a circuit breaker is required to do

following tasks under the following typical

conditions:

– Short-circuit interruption

– Interruption of small inductive currents

– Capacitor switching

– Interruption of short-line fault

– Asynchronous switching

Circuit Breaker - Duties

A circuit breaker is a switching i.e. current interrupting ormaking device in switchgear.

It is defined as a piece of equipment which can do anyone of the following tasks:

• Makes or breaks a circuit either manually or byremote control under normal conditions.

• Breaks a circuit automatically under fault conditions

• Makes a circuit either manually or by remote control

under fault conditions

• Thus a circuit breaker is used for incorporating manual as well

as automatic control for the switching function.

• Automatic control of the circuit breaker is incorporated with

the help of relays

• The main advantage associated with the use of circuit breaker

is that, unlike a fuse, which operates once and then has to be

replaced, a circuit breaker can be reset (either manually or

automatically) to resume normal operation.

• Circuit breakers are made in varying sizes, from small devices

that protect an individual household appliance up to large

switchgear designed to protect high voltage circuits feeding

an entire city.

What are Relays?

• Relays are electrical

switches that open or

close another circuit

under certain

conditions.

Relay Purpose

Isolate controlling circuit from controlledcircuit.

Control high voltage system with lowvoltage.

Control high current system with lowcurrent.

Logic Functions

Relay Types• Electromagnetic Relays (EMRs)

– EMRs consist of an input coil that's wound to accept aparticular voltage signal, plus a set of one or morecontacts that rely on an armature (or lever) activatedby the energized coil to open or close an electricalcircuit.

• Solid-state Relays (SSRs)

– SSRs use semiconductor output instead of mechanicalcontacts to switch the circuit. The output device isoptically-coupled to an LED light source inside therelay. The relay is turned on by energizing this LED,usually with low-voltage DC power.

• Microprocessor Based Relays

– Use microprocessor for switching mechanism.Commonly used in power system monitoring andprotection.

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How a Relay Works

The fault clearing process

Arc Voltage:

The Voltage drop across the arc is called Arc Voltage.

Arc Extinction

The Fault Clearing Process• The primary function of the circuit breakers

mechanism is to provide the means for opening andclosing the contacts.

• When a fault occurs on any part of the system, thetrip coils of the circuit breaker get energized and themoving contacts are pulled apart by somemechanism, thus opening the circuit.

• When the contacts of a circuit break are separatedunder fault conditions, an arc is struck betweenthem.

• The production of the arc generates enormous heat

• Therefore, the main problem in the circuit breaker isto extinguish the arc within the shortest possibletime

Arcing Phenomena• When the current-carrying contacts are being separated, arc starts.

• This phenomena of arcing is common to both dc and ac circuit breakers.

• Arc gets extinguished every time the current wave passes through zero

• Arc can restrike only if the transient recovery voltage across the

electrodes already separated and continuing to separate, reaches a

sufficiently high value causing breakdown.

• The function of an ac circuit breaker is to prevent restriking of the arc,

which depends upon the following important factors:

1. The nature and pressure of the medium of arc

2. The external ionizing and de-ionizing agents present

3. The voltage across the electrodes and its variation with time

4. The material and configuration of the electrodes

5. The nature and configuration of the arcing chamber

Types of Arc

• Arcs in the circuit breakers are categorized as:

– a) High-pressure arcs: with ambient pressures of 1

atm and above

– b) Vacuum arcs

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• During the arcing period, the current flowingbetween the contacts depends upon the arcresistance.

• The greater the arc resistance, the smaller isthe current that flows between the contacts.

• The arc resistance depends upon thefollowing factors:

– Degree of Ionization

– Length of the Arc

– Cross-section of the Arc

a. Degree of Ionization

The arc resistance increases with the decrease in

the number of ionized particles between the

contacts.

b. Length of the Arc

The arc resistance increases with the length of

the arc i.e., separation of contacts.

c. Cross-section of the Arc

The arc resistance increases with the decrease in

the area of cross-section of the arc.

Principle of Arc Extinction

• Prior to discussing the methods of arc

extinction, it is essential to scrutinize the

factors accountable for the maintenance of

arc between the contacts. These are:

• Potential difference between the contacts

• Ionized particles between the contacts

Potential Difference between the

Contacts

• When the contacts have small separation, thepotential difference between them is sufficient tomaintain the arc.

• One way to extinguish the arc is to separate thecontacts to such a distance that potentialdifference becomes inadequate to maintain thearc.

• However this method is impracticable in highvoltage systems where a separation of manymeters may be required.

Ionized Particles between the

Contacts

• The ionized particles between the contacts

tend to maintain the arc.

• If the arc path is de-ionized, the arc extinction

will be facilitated.

• This may be achieved by cooling the arc

removing the ionized particles from the space

between the contacts.

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Methods of Arc Interruption

1. High Resistance Interruption:

Arc resistance is increased with time so that the current is reduced to a value insufficient to maintain it.

Used in DC Circuit breakers(1) Arc lengthening.(2) Arc cooling.(3) Arc splitting.(4) Arc constraining.

Methods of Arc Interruption

2. Low Resistance Interruption/current zerointerruption:

Arc resistance is kept at low value until thecurrent zero.

At current zero Arc gets extinguishes by itselfnaturally

Used in AC circuit breakers(1) Cooling. (2) Gap lengthening.(3) Blast effect.

Recovery Voltage and restriking

voltage

Recovery Voltage

The normal frequency rms voltage that appears

across the breaker poles after final arc extinction

has occurred is termed as recovery voltage.

Restriking Voltage

The transient voltage that appears across the

contacts at the instant of arc extinction is called

the restriking voltage.

Recovery Voltage and restriking

voltage

Regards

sasidharan.webs.com

8/3/2011

1



Class 3: June 28,2011

Types of Circuit Breakers

SUBJECT INTRODUCTION

Module I










Based on Voltage

Based on Location

Indoor Circuit Breaker:

Medium and low voltage breakers arecategorized as Indoor circuit breakers,

Outdoor circuit breaker

Circuit breakers which have air as externalinsulating medium are classified as outdoorcircuit breakers.

Based on External DesignDead tank circuit breakers

• In the dead tank circuit breakers, the switching device is located,

with suitable insulator supports , inside a metallic vessel at ground

potential and filled with insulating medium.

• In dead tank circuit breakers, the incoming and outgoing conductors

are taken out through suitable insulator bushings, and low voltage

type current transformers are located at lower end of both insulator

bushings, i.e. at the line side and the load side.

Live tank circuit breaker

• In live tank circuit breakers, the interrupter is located I an insulator

bushing, at a potential above ground potential.

• The live tank circuit breakers are cheaper (with no current

transformer), and require less mounting space.

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Based on Interrupting Media• air and oil was predominant interrupting media till late 70s.

• But today, vacuum and SF6 are the only dominant interrupting

technologies, for medium and high voltage segments of circuit

breaker

The medium used for the arc extinction can be:

Oil

Air

Vacuum

Sulphur Hexafluoride (SF6)

Accordingly, the circuit breakers may be classified into following

categories (which will be treated in detail in the present report):

Oil Circuit Breakers

Air-blast Circuit Breakers

Sulphur Hexafluoride (SF6) Circuit Breakers

Vacuum Circuit Breakers

High resistance interruption.(1) Arc lengthening.(2) Arc cooling.(3) Arc splitting.(4) Arc constraining.

Low resistance interruption.(1) Cooling. (2) Gap lengthening.(3) Blast effect.

Method of arc extinction in circuit breaker.

Regards

sasidharan.webs.com

8/3/2011

1



SF6 and Vacuum Circuit Breaker


Module I










Introduction

• Only SF6 and vacuum circuit breakers are

currently being installed, but some air-blast

and oil circuit breakers are still in place in

distribution substations

• Vacuum CBs are used for distribution voltages

• SF6 CBs with the ‘puffer’ mechanism are used

for transmission voltages.

Fixed

contact

Moving

contact

ARC

Fixed

contact

Moving

contact

ARCARC IS QUENCHED BY

MEDIUM

IN A CIRCUIT BREAKER

OPERATING

PRINCIPLE OF

BREAKER

OVERVIEW OF ARCS IN BREAKERS-:

During the separationof contacts, due tolarge fault current andhigh current densityat the contact regionthe surroundingmedium ionizes andthus a conductingmedium is formed.This is called the ARC.

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CLASSIFICTION OF CIRCUIT BREAKERS

OILOILOILOIL AIRAIRAIRAIR VACCUMVACCUMVACCUMVACCUM SF6SF6SF6SF6

OIL CIRCUITBREAKER

AIR BLASTCIRCUITBREAKER

VACCUMCIRCUITBREAKER

SF6CIRCUITBREAKER

SF6 CIRCUIT BREAKER

OVERVIEW OF SF6 GAS:-

High dielectric strengthHigh dielectric strengthHigh dielectric strengthHigh dielectric strength1. High dielectric strengthHigh dielectric strengthHigh dielectric strengthHigh dielectric strength

Excellent arc quenching abilityExcellent arc quenching abilityExcellent arc quenching abilityExcellent arc quenching ability2. Excellent arc quenching abilityExcellent arc quenching abilityExcellent arc quenching abilityExcellent arc quenching ability

Excellent thermal stabilityExcellent thermal stabilityExcellent thermal stabilityExcellent thermal stability3. Excellent thermal stabilityExcellent thermal stabilityExcellent thermal stabilityExcellent thermal stability

Good thermal conductivityGood thermal conductivityGood thermal conductivityGood thermal conductivity4. Good thermal conductivityGood thermal conductivityGood thermal conductivityGood thermal conductivity

Electrical properties

Chemically inertChemically inertChemically inertChemically inert1. Chemically inertChemically inertChemically inertChemically inert

NonNonNonNon----toxictoxictoxictoxic2. NonNonNonNon----toxictoxictoxictoxic

NonNonNonNon----corrosivecorrosivecorrosivecorrosive3. NonNonNonNon----corrosivecorrosivecorrosivecorrosive

NonNonNonNon----flammableflammableflammableflammable4. NonNonNonNon----flammableflammableflammableflammable

Physical & Chemical properties

5. High density

6.High electro-negativity

Dielectric properties of SF6 gas:-

DS of sf6 is 2.5 times of air

DS of sf6 is 30% less than oil

DS of sf6 is equal to oil at 63700 N/m2

And 15% higher at 122500 N/m2

(D S - Dielectric Strength)

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Advantages of SF6 gas:-

Increased safetyIncreased safetyIncreased safetyIncreased safety1. Increased safetyIncreased safetyIncreased safetyIncreased safety

Reduced sizeReduced sizeReduced sizeReduced size2. Reduced sizeReduced sizeReduced sizeReduced size

Easy installationEasy installationEasy installationEasy installation3. Easy installationEasy installationEasy installationEasy installation

Low maintenanceLow maintenanceLow maintenanceLow maintenance4. Low maintenanceLow maintenanceLow maintenanceLow maintenance

SF6 CIRCUIT BREAKEROPERATION

SF6 CIRCUIT BREAKEROPERATION

SF6 DECOMPOSITION PRODUCTS

SF6

SF4 + 2F

SOF2 + 2HF MFn

SO2 + HFSiF4 + 2 H2O

SPARK

H2O

H2O SiO2

M

magnitude & duration of discharge

materials of construction of equipmentmaterials of construction of equipmentmaterials of construction of equipmentmaterials of construction of equipment

contamination levels of moisture and air inside thecontamination levels of moisture and air inside thecontamination levels of moisture and air inside thecontamination levels of moisture and air inside theequipmentequipmentequipmentequipment

Amount of each decomposition product

depends on :-

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REMOVAL OF SF6 BYPRODUCTS:-

Byproducts are corrosive & likely to affect the organic materials.

Absorbent materials used in circuit breakers

activated alumina( effective for SOF2,H2S,SF2)

molecular sieves( sodalime-CaO.NaOH)-removes stable gases such as SF4,SiF4 & S2F2

So suggested mix is 50/50 of sodalime & alumina.

The suggested weight of absorbent is 10% of theweight of the gas

ADVANTAGES OF SF6 CIRCUIT BREAKERS

Very short arcing period

Can interrupt much larger currents as compared to other breakers

No risk of fire

Low maintenance, light foundation, minimum auxiliary equipments

No over voltage problem

The green/blue block does not move

• The blue outline piston moves right

• The arc is blown away by the blast of SF6

SF6 PUFFER TYPE CIRCUIT BREAKER

VACCUM CIRCUIT BREAKER

• Vacuum is used as the arc quenching medium.

• Basic principle is that arc wont sustain in vacuum

• Employs the principle of contact separation

• There is no ionization due to medium.

• The initial arc caused by field and thermionic emissions during

the contacts separation, will die away soon, since there is no

further ionization because of vacuum.

• Since vacuum offers the highest insulating strength and far

superior arc quenching properties than any other medium.

• When the contacts in the vacuum circuit breakers are opened

in vacuum, an arc is produced between the contacts by the

ionization of metal vapors of contacts.

• However, the arc is quickly extinguished because the metallic

vapors, electrons and ions produced during arc rapidly

condense on the surface of the circuit breaker contacts.

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Construction

• The vacuum circuit breaker consists of fixed contact,moving contact and arc shield mounted inside thevacuum chamber.

• The movable member is connected to the controlmechanism by stainless steel bellows.

• This enables the permanent sealing of the vacuumchamber so as to eliminate the possibility of leak.

• A glass vessel or ceramic vessel is used as the outerinsulating body.

• The arc shield prevents the deterioration of theinternal dielectric strength by preventing metallicvapors falling on the inside surface of the outerinsulating cover.

Advantages

• Compact and Durable.

• Low Operating energy since mechanism need to drive only small

masses at moderate speed for very short distances.

• Because of the very low voltage across the metal vapor arc, energy

is very low.

• (Arc voltage is between 50 and 100V.)

• Metal vapor re-condenses on the contact and hence contact

erosion is extremely small.

• No generation of gases during and after the circuit breaker

operation.

• It can break any heavy fault current

• They can successfully withstand lightning surges.

Regards

Sasisreedhar.webs.com

8/3/2011

1



ABCB and Oil Circuit Breaker

SUBJECT INTRODUCTIONIntroduction

• High pressure air-blast is used as arc quenching medium.

• The contacts are opened in a flow of air-blast establishedby the opening of the blast valve.

• The air-blast cools the arc and sweeps away the arcingproducts.

• Consequently, the arc is extinguished and flow of current isinterrupted.

• Whenever current at high voltages needs to be interrupted,more breaking units are used, in series.

• Dry and clean air supply is one of the most essentialrequirements for the operation of the air-blast circuitbreakers.

Sequence of Operation of ABCB

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AXIAL BLAST ABCB

Advantages• The risk of fire is eliminated in these circuit breakers.

• The arcing products are completely removed by the

blast whereas the oil deteriorates with successive

operations.

• The size of these breakers is reduced, as the

dielectric strength grows so rapidly

• Due to the rapid growth of the dielectric strength,

the arcing time is also very small.

• The arc extinction is independent of the fault current

to be interrupted.

Disadvantages

• These circuit breakers are very sensitive to the

variations in the rate of rise of restriking

voltage.

• The air-blast is supplied by the compressor

plant that needs considerable maintenance

Conclusion

• Other gases such as Nitrogen, Carbon dioxide, andHydrogen can also be used. But air is preferred becauseof the fact that the Carbon dioxide tends to freeze, andthe hydrogen gas is very expensive.

• This type of circuit breaker has been used earlier foropen terminal HV applications, for voltages of 245 kV,and 400 kV up to 765 kV, especially where fasterbreaker operation was required.

• The interrupting capability of air circuit breaker isusually increased by increasing the normal pressurerange. Normally, the pressure level is around 30 to 35bars.

• In order to maintain the insulation level and reliabilityof operation, it is required that the air to be very dry.

• Currently, ABCBs are replaced by SF6 circuit breakers

Oil Circuit Breaker

Plain Oil Circuit Breaker

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Minimum

Oil

Circuit Breaker

• Insulating oil (i.e., transformer oil) is used as an arc quenching

medium.

• The contacts are opened under oil and an arc is struck between

them.

• The surrounding oil dissociates into hydrogen.

• The large volume of the hydrogen gas pushes the oil away from the

arc.

• In an oil circuit breaker, the arc quenching process is entirely

dependent on arc energy generated.

• The arc drawn across the contacts is contained inside the

interrupting pot, and thus the hydrogen gas formed by the vaporized

oil (gas) is also contained inside the chamber.

• As the contacts move, and the moving contact separates, nozzle exit

allows escape of the hydrogen gas trapped inside the interrupting

chamber.

• The escaping high pressure hydrogen gas, having a high thermal

conductivity, takes away the heat, thus making the contact gap cool

and free from ionization, immediately after current zero.

The arc extinction is facilitated by two processes:

1. Firstly, the hydrogen gas cools the arc, aidingthe de-ionization of the medium between thecontacts.

2. Secondly, the gas sets up turbulence in the oiland forces it into the space between contacts, thereby eliminating the arcing products from the arcpath.

This results in extinguishing the arc and as a resultthe circuit current is interrupted.

Advantages

1. Oil absorbs the arc energy to produces

hydrogen gas during arcing.

2. The hydrogen has excellent cooling properties

and helps to extinguish the arc. (In addition to hydrogen gas, a small

proportion of methane, ethylene, and acetylene are also generated in oil decomposition.)

3. The oil provides insulation for the live exposed

contacts from the earthed portions of the

container.

4. Oil provides insulation between the contacts

after the arc has been extinguished.

5. The oil close to the arc region provides cooling

Disadvantages

1. Oil is inflammable and may cause fire hazards.

2. When a defective circuit breaker fails underpressure, it may cause explosion.

3. The hydrogen gas generated during arcing,when combined with air, may form an explosivemixture.

4. During arcing, oil decomposes and becomespolluted by carbon particles, which reduces itsdielectric strength.

5. Requires periodic maintenance andreplacement.

Types of Oil Circuit Breakers

Oil Circuit Breakers can be classified based on

the quantity of oil used. The two popular types

are:

i. Bulk Oil Circuit Breakers

ii. Minimum Oil Circuit Breakers

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Bulk Oil Circuit Breaker

These circuit breakers use a large quantity of oil. The oilhas to serve two purposes:

1. It extinguishes the arc during opening of contacts.

2. It insulates the current conducting parts from oneanother and from the earthed tank.

In the bulk oil circuit breakers, the interrupting unit isplaced in a tank of oil at earth potential and the incomingand outgoing conductors are connected through insulatorbushings.

Minimum Oil Circuit Breaker (MOCB)

• These circuit breakers (MOCB) uses only asmall quantity of oil.

• In such circuit breakers, oil is used only for arcextinction; the current conducting partsinsulated by air or porcelain or organicinsulating material.

• In these circuit breakers, the oil requirement

can be minimized by placing the interruptingunits, in insulating chambers at live potential,

on an insulator column.

Regards

sasidharan.webs.com

8/3/2011

1



Circuit Breaker Ratings


Introduction

• A circuit breaker must operate under all

conditions, but its operation becomes critical

when there is a fault in the system where

breaker is used.

• During fault conditions, a circuit breaker must

open the faulty circuit and break the fault

current.

• The ratings of the circuit breakers are also for

breaking and making capabilities.

Circuit Breaker Ratings

• There are three ratings for breakers as:

1. Breaking Capacity

2. Making Capacity

3. Short-time Rating

The circuit breaker ratings carefully selectedbased on a particular application.

Breaking Capacity• Breaking capacity is defined as the r.m.s. current that a circuit breaker is

capable of breaking at given recovery voltage and under specified

conditions (i.e. power factor, rate of rise of restriking voltage).

• The breaking capacity is always stated at the r.m.s. value of fault current

at the instant of contact separation.

• When the fault occurs, there is a considerable asymmetry in the fault

current due to the presence of a d.c. component.

(In the Britain, it is a usual practice to take breaking current equal to the symmetrical

breaking current. However, in America, the practice is to take breaking current equal

to asymmetrical breaking current. Therefore, the American rating given to a circuit

breaker is higher than the British rating.)

• Breaking capacity in MVA in terms of the rated breaking current (I) and

rated service voltage (V) in three-phase circuit is:

Breaking Capacity = sqrt 3 x V x I x 10-6 MVA

• However, the agreed international standard of specifying breaking

capacity is defined as the rated symmetrical breaking current at a rated

voltage.

Making Capacity• It is the peak value of current (including d.c. component) during

the first cycle of current wave after the closure of circuit breaker.

• There is always a possibility of closing or making the circuit

breaker under the short circuit conditions.

• The capacity of a circuit breaker to make current depends upon

its ability to withstand and close successfully against the effects of

electromagnetic forces.

• Making capacity is stated in terms of a peak value of current

instead of r.m.s. value.

Making Capacity = 2.55 x symmetrical breaking capacity

Short Time Rating• The period for which the circuit breaker is able to carry fault

current while remaining closed is known as short-time rating.

• This rating is needed because sometimes a fault on the system is

of temporary nature and persists only for a second or two after

which the fault is automatically cleared (transient fault). For the

sake of continuity of the supply, the breaker should not trip in

such situations.

• This means that the circuit breakers should be able to carry high

current safely for some specified period while remaining closed.

• Circuit Breakers should have a specified short-time rating.

• If the fault persists for a duration longer than the specified time

limit, the circuit breaker will trip, disconnecting the faulty section.

• The short-time rating of a circuit breaker depends upon its ability

to withstand:

– The electromagnetic force effects

– The temperature rise

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Arc Voltage

Arc voltage is defined as the voltage that appearsacross the contacts of the circuit breaker during thearcing period (the period in which the arc persists).

[As soon as the contacts of the circuit breaker areseparated, an arc is formed between them. The voltagethat appears across the contacts during this period,until the arc is extinct, is called the arc voltage. Thevalue of this voltage is highest at the zero current point.This peak value of the arc voltage helps maintain thecurrent flow in the form of arc].

Recovery Voltage• Recovery voltage is defined as the normal (50 Hz) voltage that

appears across the contacts of the circuit breaker immediately after

the final arc extinction. It is approximately equal to the system

voltage.

[When the contacts of the circuit breaker are opened, current drops to

zero after every half cycle. At some current zero, the contacts are

separated adequately apart and dielectric strength of the medium

between the contacts attains high value due to the elimination of

ionized particles. At such an instant, the medium between the contacts

is strong enough to prevent the breakdown by restriking voltage.

Consequently, the final arc extinction takes place and circuit current is

interrupted. Immediately after the final current interruption, the

voltage that appears across the contacts has a transient part. However,

these transient oscillations cave in rapidly due to the damping effect of

the system resistance and normal circuit voltage appears across the

contacts. The voltage across the contacts is of normal frequency and is

identified as recovery voltage].

Restriking Voltage

• Restriking voltage is the transient voltage that appearsacross the contacts at or near current zero during thearcing period.

[The current interruption in the circuit depends upon thehigh frequency transient voltage, the restriking voltage. Ifthe restriking voltage rises more rapidly than thedielectric strength of the medium between the contacts,the arc will persist for the next half-cycle. On the otherhand, if the dielectric strength of the medium builds upmore rapidly than the restriking voltage, the arc fails torestrike and the current will be interrupted].

Rate of Rise of Restriking Voltage

• It is the rate of increase of restriking voltage and is

abbreviated by R.R.R.V. its unit is kV/m sec

• It is R.R.R.V, which decides whether the arc will re-

strike.

• If R.R.R.V is greater than the rate of rise of dielectric

strength between the contacts, the arc will re-

strike.

• The value of R.R.R.V depends on:

1. Recovery voltage

2. Natural frequency of oscillations

Regards


8/3/2011

1



Over Voltages in Power Systems


Module I


types and their operations, ABCB, oil CB,

SF6,vacuum CB, circuit breaker ratings, cause of

over voltages, surges and traveling waves, voltage

waves on transmission line, reflection and

attenuation, protection against lightning, earth

wires, lightning diverters, surge absorbers, arcing

ground, neutral earthing , basic concepts of

insulation levels and their selection, BIL,

coordination of insulation

Causes of Over Voltages

Lightning Over Voltage

Lightning is an attempt by nature to maintain a

dynamic balance between the positively charged

ionosphere and the negatively charged earth.

Over fair-weather areas there is a downward transfer

of positive charges through the global air-earth

current.

This is then counteracted by thunderstorms, during

which positive charges are transferred upward in the

form of lightning.

During thunderstorms, positive and negative charges

are separated by the movements of air currents

forming ice crystals in the upper layer of a cloud and

rain in the lower part.

The cloud becomes negatively charged andhas a larger layer of positive charge at its top.

As the separation of charge proceeds in thecloud, the potential difference between thecenters of charges increases and the verticalelectric field along the cloud also increases.

The total potential difference between the

two main charge centers may vary from l00to 1000 MV.

Only a part of the total charge-severalhundred coulombs-is released to earth bylightning.

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Lightning Protection System

STEPPED LEADER

STREAMERS

Lightning Discharge• The channel to earth is first established by a stepped

discharge called a leader stroke.

• The leader is initiated by a breakdown between polarized

water droplets at the cloud base caused by the high electric

field, or a discharge between the negative charge mass in the

lower cloud and the positive charge pocket below it.

• As the downward leader approaches the earth, an upward

leader begins to proceed from earth before the former

reaches earth.

• The upward leader joins the downward one at a point

referred to as the striking point.

• This is the start of the return stroke, which progresses upward

like a travelling wave on a transmission line.

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Lightning Phenomenon• At the earthing point a heavy impulse current

reaching the order of tens of kilo amperesoccurs, which is responsible for the knowndamage of lightning.

• The velocity of progression of the return stoke isvery high and may reach half the speed of light.

• The corresponding current heats its path totemperatures up to 20,000°C, causing theexplosive air expansion that is heard as thunder.

• The current pulse rises to its crest in a few microseconds and decays over a period of tens orhundreds of microseconds.

Lightning FACTS

• A strike can average 100 million volts of

electricity

• Current of up to 100,000 amperes

• Can generate 54,000 Degree F

• Lightning strikes somewhere on the Earth

every second

• Kills hundreds of people every year.

Protection Against Lightning

1) Air terminal

2) Conductors

3) Ground termination

4) Surge protection

Switching Over Voltage

With the increase in transmission voltages, switching

surges have become the governing factor in the design

of insulation for EHV and UHV systems.

Overvoltage produced on transmission lines by

lightning strokes are only slightly dependent on the

power system voltages.

According to the International Electro-technical

Commission (IEC) recommendations, all equipment

designed for operating voltages above 300 kV should be

tested under switching impulses (i.e., laboratory-

simulated switching surges).

Origin of Switching Over Voltage There is a great variety of events that would initiate a switching surge

in a power network.

The switching operations of greatest relevance to insulation design canbe classified as follows

1. Energization of transmission lines and cables.

a. Energization of a line that is open circuited at the far end

b. Energization of a line that is terminated by an unloaded transformer

c. Energization of a line through the low-voltage side of a transformer

2. Reenergization of a line.

This means the energization of transmission line carrying charges trappedby previous line interruptions when high-speed reclosures are used.

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Origin of Switching Over Voltage3. Load rejection.

• This is affected by a circuit breaker opening at the far end of theline.

• This may also be followed by opening the line at the sending end inwhat is called a line dropping operation.

4. Switching on and off of equipment.

All switching operations involving an element of the transmissionnetwork will produce a switching surge.

• a. Switching of high-voltage reactors

• b. Switching of transformers that are loaded by a reactor on their tertiary winding

• c. Switching of a transformer at no load

5. Fault initiation and clearing

Temporary Over voltage

• Temporary overvoltage's (sustained overvoltage)differ from transient switching overvoltage in thatthey last for longer durations, typically from a fewcycles to a few seconds.

• They take the form of undamped or slightlydamped oscillations at a frequency equal or closeto the power frequency.

• The classification of temporary overvoltage asdistinct from transient switching overvoltage ismainly due to the fact that the responses of powernetwork insulation and surge arresters to theirwave shapes are different.

Events leading to the generation of temporary

overvoltage

1. Load Rejection:

When a transmission line or a large

inductive load that is fed from a power station is

suddenly switched off, the generator will speed

up and the bus bar voltage will rise.

Load Rejection

Ferranti Effect

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Ground Fault

• A single line-to-ground fault will cause the voltages toground of the healthy phases to rise.

• In the case of a line-to-ground fault, systems with neutralsisolated or grounded through high impedance may developover voltages on healthy phases higher than normal line-to line voltages.

• Solidly grounded systems will only permit phase-to-ground overvoltage well below the line-to-line value.

• An earth fault factor is defined as the ratio of the higher ofthe two sound phase voltages to the line-to-neutral voltageat the same point in the system with the fault removed.

Harmonic Overvoltage Due to

Magnetic Saturation

• Harmonic oscillations in power systems areinitiated by system nonlinearities whose primarysource is that of the saturated magnetizingcharacteristics of transformers and shuntreactors.

• The magnetizing current of these componentsincreases rapidly and contains a high percentageof harmonics for voltages above the ratedvoltage.

• Saturated transformers inject large harmoniccurrents into the system.

Regards


8/3/2011

1



Protection Against External Over Voltages

Module I










What is a Voltage Surge?

• High amplitude, short duration overvoltage

– Any voltage level that is short in duration and isalso 10% greater than the systems normaloperating AC, RMS or DC voltage level. A voltagesurge is also known as a voltage transient.

Types of Voltage Disturbances• The most common voltage

disturbance is a surge or spike involtage

• Less common types of disturbancesare:

– Swell – An increase in thepower frequency AC voltagewith durations from one halfcycle to a few seconds

– Sag – A rms reduction in thepower frequency AC voltagewith durations from one halfcycle to a few seconds (alsoknown as dip)

Outages1%

Swell / Sag11%

Surge / Spike88%

Protective Ratio and Protective

Margin

Protective Ratio:

Impulse withstand level of equipment

------------------------------------------------------

Protective level of surge arrestor

Protective ratios are usually above 1.2

Protective Margin = Equipment withstand level – protective level of surge arrestor

Protective levels are different for lightning impulse and switching impulse

Lightning Impulse withstand

------------------------------------------------------ = 1.2

Switching Impulse with stand level

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9

The Enemy

Types of Lightning Stokes

• A Type Stroke: Discharge between a charged cloud and earth

• B Type Stroke: If cloud 1 discharges to cloud 2, there is a sudden

change in the charge on cloud 3. The discharge between cloud 3 and

earth is called B type stroke

• B Type Stroke does not hit earth conductor or earth wire and hence no

protection can be provided to the OH line against such strokes

Protection Against Lightning

Power Stations and substations from direct

strokes

Overhead transmission lines from direct

strokes

Electrical Apparatus from Travelling Waves

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Traveling Wave/ Overvoltage Protection

• Lightning hits mid-span

• Current divides and then

propagates

• V = I x R

Protection devices against lightning

Device Where Applied Remarks

Rod Gaps Across Insulator string ,

bushing insulators

• Difficult to Coordinate

• Create dead short circuit

• Cheap

Over Head Ground Wire Above Over head lines

Above the sub station area

Provide effective protection

against direct stroke on the

line conductors tower

substation equipment's

Vertical Masts in Substation In Substations Instead of providing over

head shielding wires

Lightning Masts/Rods

(Earthed)

Above Tall buildings Protect buildings against

direct strokes

Angle of Protection = 30

Protection devices against lightning

Device Where Applied Remarks

Lightning Arrestors

(Surge Arrestors)

On incoming line in each

substation

Near Terminals of

transformers and generators

Diverts over voltage to earth

with out causing short

circuit.

Used at every voltage level

in every substation for each

line

Surge Absorbers Near motor and generator

terminals

Near rotating machines or

switch gear

Resistance Capacitance

combination absorbs the

over voltage surge and

reduces steepness of the

wave

Lightning Shielding of Substation

• Ground wires above Substation areFrequently equipped by

1. Lightning rods above structural steel work.(Earthing Rods/MASTS, lightning conductors)

2. O/H ground cage solidly bounded to Groundmat to provide a low resistance groundeconomically.


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Lightning

Rod

Earth rods are installed in tall building and are also connected to

earth

Positive charges accumulate on the sharp points of the lightning

rods there by lightning strokes are attracted to them

Transmission Tower Earthing

At every tower earth wire is grounded

Positive charges accumulate on the wire

In the absence of earth wire lightning stroke would strike the line conductors

causing

Lightning Stroke on Overhead LinesDirect stoke on line conductor

Harmful; voltage of the order of several million volts

Insulation flash over

Travelling waves spreads in both directions shattering

line insulators

Direct stroke in Tower Top

‘L’ is the inductance of the tower; ‘I’ is the current in tower; ‘R’ is the effective

resistance of the tower and ‘e’ is the voltage between tower top and earth

Direct stroke on Ground Wire

Can cause flash over between line conductor and earth

wire or line conductor and tower

Indirect stroke or B Stroke on OH Line Conductor

Same effect as direct stroke on conductor

Over head shielding wires does not offer 100% protection

d ie L R i

d t= +

Over Head Shielding Screen

Lightning Masts and Over Head

Shielding Screens• Lightning Masts are installed in strategic location in the

switchyard

• The tower top is earthed.

• Lightning Masts are preferred for outdoor switchyards up to 33kV

• For 66kV and above, lightning masts become too tall and

uneconomical.

• Over head Shielding screens protect the outdoor substation and

overhead lines approaching substation

• The entire switchyard is provided with earthed overhead

shielding screens with conductor of 7/9 SWG.

• Over head shielding screens are preferred because they provide

adequate protection and the height of the structures in the

substation provided with overhead shielding wires is

comparatively less than that of lightning masts.

• The shielding angle is maintained as 30 – 45 degrees


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Rod Gaps or Spark Gaps It is a very simple type of diverter and consists of two 1.5 cm rods,

which are bent at right angles.

One rod is connected to the line circuit and the other rod is connected

to earth.

The distance between gap and insulator must not be less than one

third of the gap length so that the arc may not reach the insulator and

damage it.

Generally, the gap length is so adjusted that breakdown should occur

at 80% of spark-voltage in order to avoid cascading of very steep wave

fronts across the insulators.

The string of insulators for an overhead line on the bushing of

transformer has frequently a rod gap across it.

Under normal operating conditions, the gap remains non-conducting.

On the occurrence of a high voltage surge on the line, the gap sparks

over and the surge current is conducted to earth.

In this way excess charge on the line due to the surge is harmlessly

conducted to earth

Horn Gap

The gap between horns is less at the bottom and large at the top

An arc is produced at the bottom during high voltage surge

The arc commutes along the horn due to electromagnetic field action and

length increases

Impulse Ratio:

Impulse ratio of a protective device is the ratio of breakdown voltage

on specified impulse wave to break down voltage at power frequency

Visit http://www.arresterworks.com/resources/photo.php for more photos

132 KV SF 6 CIRCUIT BREAKER

LA

132/25 KV TRANSFORMER

HV BUSHING

RADIATORS

MARSHALLING BOX

TAP CHANGER

BUCHHLOZ RELAY

8/3/2011

6

25 KV SINGLE POLE ISOLATOR

MOVING ROAD FIX JAW

PEDASTAL INSULATOR

ARCING HORN

TIE-ROD INSULATOR

Surge Suppressors & Lightning Arresters

(a) Surge diverter,

(b) Surge suppressor,

(c) Lightning arrester

These are placed in parallel & permanently connected by Spark over of a series gap .

Usually connected between phase and ground

Usually near the terminals of the large medium voltage rotating machines and inHV/EHV.HVDC substations to protect the apparatus insulation from lightning andswitching surges

Discharges current impulse surge to earth and dissipates energy in the form of heat

Provides protection against impulse voltage wave

Definition

“A protective device for limiting surge voltages by discharging or bypassing surge

current, and it also prevents the flow of follow current while remaining capable of

repeating these functions”.

SUB SECTIONONG AND PARRALING POST

DC SECTION

CT

PT

CB

LA

BUS BAR

Traditional Lightning Arresters

• Traditional lightning arresters uses nonlinear

resistance elements as before

• However have a gap or gaps series with them

• So resistor is isolated from circuit under normal

conditions & is introduced when a surge appears by

spark over of gap

• It is possible to design resistor element from energy

dissipation & voltage-limiting under surge

conditions

Types/Classifications

Originally, there were three types of surge arresters. They are:

1. Expulsion type

2. Nonlinear resistor type with gaps (currently silicone-carbide gap type)

3. Gapless metal-oxide type.

There are four (3) classifications of surge arresters. They are:

1. Station Type : Highest capacity for energy dissipation

2. Line Type (Intermediate type) : Generally used for protectinglarge transformers, intermediate substations (>5000A Rating)

3. Distribution class (heavy, normal, and light duty) Secondarytype: Intended for pole mounting in distribution circuits for theprotection of distribution transformers

• Expulsion types are no longer being used .

• Nonlinear resistor type with gaps was utilizedthrough the middle of the 1970s and iscurrently being phased out

• The conventional gap type with silicone-carbide blocks/discs are still being used andthe gapless metal-oxide type are the mostwidely used today.

• Gapless metal-oxide surge arrester (MOSA),since it provides the best performance andreliability

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Magnetically Blown Surge arrestor

• The gap assembly of magnetically blown out gaps comprises ofZircon Porcelain plates forming a chamber which encloses correctlyspaced electrodes.

• Blow out coils are connected such that during spark over the coilscome into the circuit.

• The magnetic field due to the blow out coil current extends thearc in the gap to cool and elongate it and the arc gets quenchedwithin half cycle.

• In mutli stage surge arrestors there is a need to equalize the powerfrequency voltage distribution between units in series

• For this purpose, main gaps are shunted by ceramic , non lineargrading resistors.

• These resistors is high enough at normal service and just beforespark over , the voltage across the grading resistors become veryhigh and assists in the sparking process.

• The surge arrestor is filled with nitrogen and is hermetically sealedin order to avoid ingress of moisture and dirt.

• A pressure relief diaphragm is provided at each end in order toavoid pressure build up during discharge so as to protect theporcelain housing.

Surge arrestor Popular Types

1. Gapped Silicon Carbide (SiC) arrestor

Commonly called as valve type or conventional

Gapped arrestors

Consists of silicon carbide discs in series with the gap

units.

2. Zinc Oxide Gapless Arrestor

Called Zno arrestors or Metal oxide arrestors

These are gapless arrestors and consists of Zinc oxide

discs in series

They have superior voltage current characteristics and

are preferred in EHV and HVDC installations

Surge Arresters (Lightning Arrestors)

• Surge causes traveling voltage wave

• Voltage would be enough to flash-over the insulation

• Surge arrester high resistance at L-G voltage

• Surge arrester low resistance at surge voltage

• Surge is diverted to ground

• Surge arrester high resistance again after surge

• Conduction time is too short for breakers to react.

• Surge arrestors are usually connected between phase

and ground in the distribution system near the terminals

of large medium voltage rotating machines and in

HV,EHV,HVDC sub station to protect the apparatus

insulation from lightning and switching surges.

SiC Surge Arrestors(Gapped Arrestors)

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SiC Surge Arresters

(Gap Arrestors)• Developed in the 1930’s.

• The silicon carbide (SiC) surge arrester consists of a series

combination of silicon carbide non-linear resistive blocks with a

set of spark gaps .

• On application of a surge the gaps spark over allowing surge

current to flow through the SiC blocks that limit the voltage

produced across the arrester.

• Apart from dissipating some surge energy the primary purpose

of the spark gaps is to ensure the resistive blocks are not

damaged by continuous power frequency current.

• Arresters vary depending on their voltage class & duty

• They are stacked in series & hermetically sealed in a porcelain

housing . ( 5)xI KV x= =

Metal Oxide Arresters(Gapless arrestor)

Developed in 70s'

Made of zinc oxide varistor

Consists of zinc oxide discs in series

ZNO arrestors have superior VI Characteristics and

higher energy absorption level

Preferred for EHV and HVDC installation

The material used is Zinc oxide, Bismuth oxide and

cobalt oxide. ( 40)xI KV x= =

High Value of x gives superior characteristics for the surge

arrestor

Metal Oxide Surge Arrestor

Parts of Metal Oxide Arresters

1. ZnO, (Zinc Oxide) varistors

2 Silicone housing

3 Flame retardant structure

4 Corrosion resistant aluminum

fittings

8/3/2011

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Comparison of Surge Arrestors

Metal oxide arrestors have superior V-I

Characteristics

Preferred Type of Arrester

• Preferred material for application is Zinc Oxide(ZnO)

• However traditionally SiC used

• Traditional type still in a vast number are in service

• A different approach relates to a type of surgesuppressor, in which when suppressor operates andan arc is established in gap this arc must bequenched when surge passed or resistor will bedestroyed by current that flow

Surge Absorber/Surge Absorber

• Power surges, both voltage and current, areoccurring continually in today’s power systems

• Whether they occur naturally, such as fromlightning and static electricity; or are man made,such as inductive surges from motor,transformers, solenoids, etc. power surges are afact of life.

• These power surges have a very high voltageand current level as compared to electricalnoise

Surge Absorber

• Surge suppressors reducesthe steepness of the wavefront.

• A capacitance connectedbetween line and earth oran inductor connected inseries with the line resultsin reduction of steepnessof the wave front.

• An oscillatory condition canbe eliminated byconnecting a resistoracross the inductor.

• Lightning surges have precipitous dv/dt values and huge

electrical charge.

• Surge absorbers must assimilate this surge.

• This limiting voltage capability varies depending upon the type

of absorber.

Ferranti Surge Absorber

• Ferranti surge absorber consists of an inductor which is coupled

magnetically to metal tank enclosure

• The coil of the inductor has a metal shield inside it in which

current is induced

• The terminal bushings are made or porcelain

• The tank is filled with transformer oil

• The steep wave can be considered to be combination of high

frequency waves

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• The surge absorber reduced the steepness of

the wave front and the energy is transferred to

the tank by mutual induction.

• Pure condenser used as surge absorber cannot

dissipate the energy in the wave The energy is

dissipated in the resistance connected in series

with the condenser

Current Surge Absorber

Lightning Diverter

8/3/2011

1



Arcing Grounds and Neutral Earthing

Module I










Introduction

• Consider a high voltage line connected to supply with outload

• Even if no currents are drawn by the load, the conductorsof the system continue to charge the system capacitancealternatively to positive and negative polarity.

• The distributed capacitance between the phases and earthdraw charging currents from the source

• For high voltage systems the charging currents aresignificant and the reactive KVA may be of the order ofhundred of KVA and the reactive KVA influences the totalKVA of the system.

• The reactive KVA causes substantial flow of capacitancecurrent with a ground as return path

• Neutral grounding is a simple method of controlling suchcurrents

Arcing grounds

• A temporary fault caused by falling on a

branch, lightning surge etc. creates an arc

between an over head line and ground.

• The arc extinguishes and restrikes in a

repeated regular manner .The phenomena is

called arcing ground.

• Arcing grounds are common in ungrounded

systems.

Charging currents IR

and IY are neutralized

by IL

The current flowing

through the neutral

connection is

0R Y LI I I+ + =

There by the arc is

extinguished

• Each line has an inherent distributed capacitance with respect

to earth.

• Consider an earth fault on line B. the distributed capacitance

discharges through the fault when the gap between F and

ground breaks down.

• The capacitance again gets charged and again discharged.

• Such repeated charging and discharging of the line to ground

capacitance resulting in repeated arcs between line and

ground is called arcing grounds.

• Arcing ground produces several voltage oscillations reaching

to three to four times normal voltage.

• A temporary fault grows into a permanent fault due to arcing

grounds

• The problem of arcing grounds can be solved by earthing the

neutral through a coil called Peterson coil or Arc suppression

coil connected between neutral and earth.

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Neutral Earthing

• All modern power systems operate withgrounded neutrals.

• Neutral point of the generator, transformersystem ,circuit, rotating machines etc isconnected to earth either directly or indirectlythrough reactance.

Why

• Limit the potential of current carrying conductorwith respect to the general mass of earth

• Provide a current return path for earth faults inorder to allow protective devices to operate.

Importance of neutral earthing• Earth Fault protection is based on neutral earthing

• System voltage during earth fault depends onneutral earthing

• Neutral earthing has associated switchgear

• Neutral earthing is provided basically for thepurpose of protection against arcing grounds,unbalanced voltages with respect to earth,protection from lightning and for the improvementof the system.

• The term earthing and grounding have samemeaning (Earthing in UK and grounding in USA)

Equipment Earthing• Equipment earthing is connecting to earth the non current

carrying metallic parts in the neighborhood of electricalcircuits.

• The non current carrying parts include

– Motor body, switchgear metal enclosure, transformer tank,conduits of wiring etc.

– Support structures, tower, poles etc.

– Sheath of cables

– Body of portable equipment such as iron, oven etc

• The potential of earthed body does not reach to dangerouslyhigh value above earth since it is connected to the earth.

• Secondly the earth fault current flows through the earthingand may readily cause operation of fuse or an earth faultprotection

• Equipment earthing is a safety measure.

Terms and Definitions1. Earthing/Ground: Connecting to earth or ground

2. Neutral earthing/system neutral earthing (grounding). Connectingto earth the neutral point or the star point of generator,transformer, rotating machine, neutral point of a groundingtransformer.

3. Reactance earthing: Connecting the neutral point to earth througha reactance

4. Resistance earthing: Connecting the neutral point through aresistance.

5. Non effective earthing: When an intentional resistance orreactance is connected between neutral point and earth

6. Solid earth or effective earthing: Connecting the neutral point toearth without intentional resistance or reactance

7. Resonant earthing: Earthing through a reactance of such a valuethat power frequency current in the neutral to groundconnection is almost equal and opposite to power frequencycapacitance current between unfaulted line and the earth.

8. Petersen coil , arc suppression coil, ground fault

neutralizer

• Adjustable reactor connected between neutral and earth

• The reactance is such that power frequency current

between line and earth due to capacitance of healthy lines

and earth is equal and opposite to the current in the earth

connection

• The reactor used in the resonant earthing is called

Peterson coil or arc suppression coil or earth fault

neutralizer.

9. Un grounded system:

System whose neutral points are not earthed.

8/3/2011

3

10. Earth Fault Factor

• It is calculated at the selected point of the

system.

V1 is the highest rms phase to phase power frequency

voltage of sound phases during earth fault on another

phase

V2 is the rms phase to earth power frequency voltage at the

same location with fault on the faulty phase removed.

1

2

VEFF

V=

Disadvantages of Ungrounded Systems

1. Arcing ground.

2. In ungrounded systems, the voltage of thehealthy line above earth is increased by

times when an earth fault occurs on a line.

• This causes stress on the insulation of all themachines and equipment connected to thesystem.

• The voltage rise of the line above earth issustained and there by insulation failure islikely to occur through fault current.

3

3. In ungrounded systems earth fault cannot be

sensed and the earth fault relaying becomes

complicated.

• In grounded system earth fault is enough to

operate the earth fault relay.

• The current in neutral circuit can be used to

operate the earth fault relay.

4. Over Voltages due to induced static charges

are not conducted to earth in ungrounded

systems.

• The voltages due to lightning surges do not

find path to earth.

8/3/2011

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Advantages of Neutral Grounding

1. Arcing grounds are reduced or eliminated

2. The voltage of healthy lines with respect to earth remains at the harmlessvalue.

3. The life of the insulation is long due to prevention of voltage surges orsustained over voltages. There by maintenance, repairs, breakdowns andhence improved continuity.

4. Stable neutral point

5. The earth fault relaying is relatively simple. Useful amount of earth faultcurrent is available to operate the earth fault relay.

6. The over voltages due to lightning are discharged to earth.

7. By employing resistance or reactance in earth connection , the earth faultcurrent is available to operate the earth fault relay.

8. Improved service reliability due to limitation of arcing grounds andprevention of unnecessary tripping of circuit breakers

9. Greater safety to personnel and equipment due to operation of fuses orrelays on earth fault and limitation of voltages

10. Life of equipment's, machines, installation is improved due to limitation ofvoltage. Hence overall economy.

Types of Grounding1. Ungrounded System: The neutral is not connected to

earth. Also called insulated neutral system.

2. Solid Grounding or Effective Grounding: The neutral isdirectly connected to ground with out intentionalimpedance between neutral and ground.

3. Reactance grounding: Reactance is connectedbetween neutral and ground.

4. Resonant Grounding: Adjustable reactor of correctlyselected value to compensate the capacitive earthcurrents is connected between neutral and earth.The coil is called Petersen coil or Arc suppression coilor Earth Fault Neutralizer.

Reactance in Neutral Connection

Arc Suppression Coil(Ground Fault Neutralizer)

Earthing Transformer

• The neutral point (star point) is usually

available at every voltage level from the

generator or transformer neutral.

• If neutral point is not available the alternative

is to go for a zigzag transformer.

• Such transformer have no secondary.

• If grounding transformer is not available a star

delta transformer can be used.

8/3/2011

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Earthing Transformer

Additional Concepts

• Earth Mat: Mesh of steel pipes or rods laid atdepth of 0.5m in the entire substation area(Excluding foundations)

• Touch Potential: Touch potential is defined as thepotential between the figures of a raised hand(2m from the ground) touching a sub-stationstructure and the feet.

• Step Potential: Step potential is defined as thepotential difference between two steps of aperson standing on the ground with feel apartduring the flow of earth fault current.

Regards


8/3/2011

1



Lightning Diverter & Surge Absorber


Module I

Circuit Breakers : Principles of operation,

different types and their operations, ABCB, oil

CB, SF6,vacuum CB, circuit breaker ratings,

cause of over voltages, surges and traveling

waves, voltage waves on transmission line,

reflection and attenuation, protection against

lightning, earth wires, lightning diverters, surge

absorbers, arcing ground, neutral earthing ,

basic concepts of insulation levels and their


What is insulation coordination

• Insulation Coordination is the process ofdetermining the proper insulation levels of variouscomponents in a power system as well as theirarrangements.

• It is the selection of an insulation structure thatwill withstand voltage stresses to which thesystem, or equipment will be subjected to,together with the proper surge arrester.

• The process is determined from the knowncharacteristics of voltage surges and thecharacteristics of surge arresters.

Insulation coordination deals with the selection

of the following.

• Selection of voltage levels

• Selection of impulse withstand levels

• Selection of protective levels for each voltage levels

• Coordination of protective levels between

consecutive voltage levels

• Choice of protective level characteristics of surge

arrester with respect to basic impulse withstand level

of the apparatus at each voltage level

Basic Insulation Level (BIL)

• This is the referenceinsulation level expressed asan impulse crest (or peak)voltage with a standardwave not longer than a 1.2 x50 microsecond wave.

• A 1.2 x 50 microsecondwave means that theimpulse takes 1.2microseconds to reach thepeak and then decays to 50%of the peak in 50microseconds.

Terms and Definitions1. Withstand Voltage This is the BIL level that can repeatedly applied to an equipment without flashover, disruptive charge or other electrical failure under test conditions.

2. Chopped Wave Insulation Level

• This is determined by using impulse waves that are of the same shape as that of the BILwaveform, with the exception that the wave is chopped after 3 microseconds.

• Generally, it is assumed that the Chopped Wave Level is 1.15 times the BIL level for oilfilled equipment such as transformers.

• However, for dry type equipment, it is assumed that the Chopped Wave Level is equal to the BIL level.

3. Critical Flashover Voltage

This is the peak voltage for a 50% probability of flashover or disruptive charge.

4. Impulses Ratio

• This is normally used for Flashover or puncture of insulation.

• It is the ratio of the impulse peak voltage to the value of the 60 Hz voltage that causesflashover or puncture.

• Or, it is the ratio of breakdown voltage at surge frequency to breakdown voltage atnormal system frequency (60 Hz).

8/3/2011

2

Insulation Coordination

• The protective level of the surge arresters are

selected such that these are below the impulse

withstand level of the protected apparatus.

• The insulation level of the equipment or machine

is expressed in terms of curve value of the

specified impulse withstand level and rms value

of the one minute power frequency voltage

which is the apparatus can withstand and during

the tests made under specified conditions.

• The rms value of this voltage is called power

frequency voltage withstand level

• Correlation of insulation characteristics P with characteristics Q of the protective

device.

• The lightning arrester will spark over at a voltage less than the insulation withstand

voltage of the equipment if curve Q lies below curve P.

• Protective device must have a lower protective level characteristics of the

protective equipment

Explanation

• Basic insulation level of 550 kV is chosen

• The line insulation can withstand standard impulsewave of 860kV Crest.

• The breakdown voltage of line lightning arrester is500kV

• Transformer impulse voltage withstand level is 650kV

• High voltage surge coming from transmission line willbe discharged to earth by lightning arrester

• The residual voltage being less than breakdown voltage, the transformer insulation is protected.

• The surge arrestor should have the lowest spark overvoltage.

Selection of surge arrestor1. Determine the continuous arrester voltage. This is usually the system rated

voltage.

2. Select a rated voltage for the arrester.

3. Determine the normal lightning discharge current. Below 36kV, 5kA rated

arresters are chosen. Otherwise, a 10kA rated arrester is used.

4. Determine the required long duration discharge capability.

For rated voltage < 36kV, light duty surge arrester may be specified.

For rated voltage between 36kV and 245kV, heavy duty arresters may be

specified.

For rated voltage >245kV, long duration discharge capabilities may be

specified.

5. Determine the maximum prospective fault current and protection tripping

times at the location of the surge arrester

and match with the surge arrester duty.

6. Select the surge arrester having porcelain creepage distance in accordance

with the environmental conditions.

7. Determine the surge arrester protection level and match with standard IEC

99 recommendations.

8/3/2011

3

Regards


8/3/2011

1



Surges and Travelling Waves

Module I










3

Transmission Lines

Transmission Line Equations for a Lossless Line

The transmission line consists of two parallel and uniform conductors, not

necessarily identical.

Types of Transmission Lines

I(z,t) +

V(z,t) - z

I(z,t) +

V(z,t) -Coaxial Line

Two-Wire Line (Twisted Pair)

Electric and magnetic fields around single-phase

transmission line

Properties of Transmission Lines

• Two wires having a uniform cross-section in one

(z) dimension

• Electrical quantities consist of voltage V(z,t) and

current I(z,t) that are functions of distance z along

the line and time t

• Lines are characterized by distributed capacitance

C and inductance L between the wires

– C and L depend on the shape and size of the

conductors and the material between them

8/3/2011

2

Capacitance of a Small Length of Line

dt

tdVCltI

C

l

)()(

Then er.Farads/met

beality proportion ofconstant Let the wires. theof length the

toalproportion is current, thehence and charge, total theSince

current. theis derivative time whose wires,on the charge a induces

wires the toapplied Voltage capacitor. a asact wires twoThe

=

I(t) +

V(t) -

l

Open circuit

E

Inductance of a Small Length of Line

The wire acts as a one- turn coil. Current applied to the wires induces

a magnetic field throught the loop, whose time derivative generates the

voltage. The amount of magnetic flux (magnetic field × area), and hence

the voltage, is proportional to the length l of the wires. Let the constant

of proportality be L Henrys/meter. Then

V(t) = LldI(t)

dt

I(t) +

V(t) -

l

Short circuitB

Two-Port Equivalent Circuit

I(z,t) +

V(z,t)

-

z z+∆z z

L∆z C ∆z

I(z,t) +

V(z,t)

-

+ I(z +∆z,t)

V(z+∆z,t)

-

Kirchhoff circuit equations

V(z,t) = L∆z∂I(z,t)

∂t+ V (z + ∆z,t) I(z,t) = C∆z

∂V (z + ∆z,t)

∂t+ I(z + ∆z,t)

or

V(z + ∆z,t) − V (z,t)∆z

= −L∂I(z,t)

∂t

I(z + ∆z,t) − I(z,t)∆z

= −C∂V (z + ∆z,t)

∂t

Transmission Line Equations

Taking the limit as ∆z → 0 gives the Transmission Line Equations

∂V (z,t)

∂z= −L

∂I(z,t)

∂t

∂I(z,t)

∂z= −C

∂V (z,t)

∂tThese are coupled, first order, partial differential equations whose solutions

are in terms of functions F( t - z/v) and G(t + z /v) that are determined by

the sources. The solutions for voltage and current are of the form

V(z,t) = F( t - z/v) +G(t + z /v) I(z,t) =1

ZF (t - z/v) - G(t + z /v)[ ]

Direct substitution into the TL Equations, and using the chain rule gives

− 1v

F '(t - z/v) - G'(t + z /v)[ ] = −L1Z

F '(t - z/v) - G'(t + z /v)[ ]

−1

vZF '(t - z/v) +G'(t + z /v)[ ]= −C F '(t - z/v) +G'(t + z /v)[ ]

where the prime (' ) indicates differentiation with respect to the total variable

inside the parentheses of F or G.

Conditions for Existence of TL Solution

For the two equations to be satisfied

1v

= LZ

and 1

vZ= C

Multiplying both sides of the two equations gives 1

v2Z=

LC

Z or

v =1

LC m/s

Dividing both sides of the two equations gives vZ

v=

L

ZC or

Z =L

C Ω

v and Z are interpreted as the wave velocity and wave impedance.

Junctions Between Two Regions

0 z

I(0-,t) I(0+,t)

TL 1 V(0-,t) + V(0+,t) TL 2

Ex(0-,t) Ex(0

+,t)

Hy(0-,t) Hy(0

+,t)

Medium 1 Medium 2

x

z

Terminal condtions for the

Junction of two TL' s

V(0− ,t) = V (0+,t)

I(0− ,t) = I (0+,t)

Boundary conditions at the

interface of two media

Ex(0−,t) = Ex (0+ ,t)

Hy (0−,t) = Hy (0+,t)

Plane wave propagation and

boundary conditions are analogus

to junctioning of two TL' s

8/3/2011

3

Reflection and Transmission

Incident wave

ExIn(z,t)=F1(t-z/v1)

HyIn(z,t) Transmitted wave

Reflected wave

v1 and η1 v2 and η2

x

z

A source creates an incident wave whose electric field is given by the known

function F1(t - z/v1). Using the boundary conditions we solve for the unknown

functions G1(t +z/v1) and F2 (t - z/v2) for the electric fields of the reflected

and transmitted waves : E x (0−,t) = F1(t) +G1(t) = F2 (t)= Ex (0+ ,t)

Hy(0− ,t) =

1

η1

F1(t) - G1(t)[ ]=1

η2

F2(t) = H y(0+,t)

Reflection and Transmission Coefficients

Solution of the boundary condition equations for G1(t) and F2(t) in terms of F1(t)

G1(t) = ΓF1(t) F2(t) = ΤF1(t)

The reflection coefficient Γ and transmission coefficient Τ are given by:

Γ = η2 −η1

η2 + η1

Τ =1+ Γ = 2η2

η2 + η1

Examples:

I. Suppose medium 1 is air so that η1 =η ≡ µo εo = 377 and medium 2 has

relative dielectric constant εr = 4 so that η2 = µo εrεo = 0.5η. Then going

from air- to - dielectric Γad = 0.5η −η0.5η +η

= − 13

and Τad =1− 13

= 23

Reflection and Transmission, cont.

II. Now suppose the wave is incident from the dielectric onto air so that medium 1

is the dielectric η1 = 0.5η ( ) and medium 2 is air η2 = η( ). Then going from

dielectic- to - air, Γda =η −0.5ηη + 0.5η

= +1

3 and Τad =1+

1

3=

4

3

Note that :

1. Γda = −Γad

2. Since T is the ratio of fields, not power, it can be greater than 1.

Termination of a Transmission Line

I(0-,t)

TL V(0-,t) + RL

0 z

Terminal condtions

V(0,t) = RLI(0,t)

F(t) + G(t) =RL

ZF(t) −G( t)

Solving for G(t) in terms of F (t),

G( t) = ΓF( t) where the reflection

coefficient is Γ = RL − ZRL + Z

Special cases :

1. Matched termination, RL = Z and Γ = 0. Simulates a semi - infinite TL

2. Open circuit, RL → ∞ and Γ =1. Total reflection with V (0,t) = 2F (t).

3. Short circuit, RL = 0 and Γ = −1. Total reflection with V (0,t) = 0.

Regards

www.sasisreedhar.webs.com

VIDYA ACADEMY OF SCIENCE AN TECHNOLOGY DEPARTMENT OF ELECTRICAL AND ELECTRONICS

EE04 704: POWER SYSTEM - III ASSIGNMENT- 1

Date of Submission: 10/08/2011 Short Answer Type:

1. Explain voltage – time curves in power system studies

2. Discuss the principle of arc interruption in circuit breakers

3. Give the classification of circuit breakers

4. Explain in detail DC current breaking

5. Explain the phenomenon of arc formation in circuit breakers in case of abnormal conditions

6. Explain the following

a) Restriking Voltage

b) Recovery Voltage

c) RRRV

7. Explain the following terms

a) Symmetrical breaking current

b) Making Current

8. Compare the merits of SF6 circuit breaker over air blast circuit breaker

9. What are the causes of over voltages in power system

10. Describe the working principle of SF6 circuit breaker

11. Compare the relative performance of the following

a) Rod Gap

b) Expulsion Gap

c) Value Type LA

12. What is BIL? Explain its significance in power system studies

13. What is a ground wire? Discuss the location with respect to power conductors

14. Differentiate between surge arrestor and surge diverter.

15. Explain the process of arc extinction in high vaccum. What is current chopping?

16. State the difference between equipment earthing and neutral earthing

17. What are the merits and demerits of reactance earthing compared to the solid earthing?

18. Write short notes on substation earthing

19. List out the characteristics of ideal surge diverter.

Essay Type:

20. Discuss the principle of arc interruption in a) Oil Circuit breaker b) SF6 circuit breaker and

compare between them.

21. Describe with near sketches the principle of medium voltage air blast circuit breaker by

incorporating resistance switching.

22. Describe the construction principle of operation of a) Rod Gap b) Explusion gap c) Value type

lightning arrestor.

23. What is neutral earthing? With the help of suitable diagrams explain the various neutral

earthing schemes.

24. Explain the phenomena of arcing ground on overhead transmission lines. How neutral earthing

does opposes arcing ground currents.

25. What are the basic requirements of lightning arrestor? Differentiate between

a) Lightning arrestor and lightning Conductor

b) Surge Arrestor and surge diverter

Student Question Distribution

Student Class Number Short Answer Question No:

Essay Type question No:

1-10 1-6 20,21

11-20 7-13 22,23

21-30 14-19 24,25

31-40 1-6 20,21

41-50 7-13 22,23

51-60 14-19 24,25

61-70 1-6 20,21

71-80 7-13 22,23

Note:

1. Each student has to answer 6 short answer type and 2 essay type question approximately

2. All the students are to discuss and study the questions from other groups so that no student

misses any question.

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