EE 09 506 ELECTRICAL MATERIAL SCIENCE

CLASS 1: MODULE 1EE04 704: POWER SYSTEM III

SF6 and Vacuum Circuit Breaker

SUBJECT INTRODUCTION

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

selection, BIL, coordination of insulation

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.

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)

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 :-

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.

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

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