SUBJECT INTRODUCTION EE 09506 ELECTRICAL MATERIAL SCIENCE EE04 CLASS 704: 1: POWER MODULE SYSTEM 1 III SF6 and Vacuum Circuit Breaker
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 ARC IS QUENCHED BY ARC MEDIUM Moving contact OPERATING PRINCIPLE OF BREAKER IN A CIRCUIT BREAKER
OVERVIEW OF ARCS IN BREAKERS-: During the separation of contacts, due to large fault current and high current density at the contact region the surrounding medium ionizes and thus a conducting medium is formed. This is called the ARC.
CLASSIFICTION OF CIRCUIT BREAKERS OIL AIR VACCUM SF6 OIL CIRCUIT BREAKER AIR BLAST CIRCUIT BREAKER VACCUM CIRCUIT BREAKER SF6 CIRCUIT BREAKER
SF6 CIRCUIT BREAKER
OVERVIEW OF SF6 GAS:- Electrical properties 1. High dielectric strength 2. Excellent arc quenching ability 3. Excellent thermal stability 4. Good thermal conductivity
Physical & Chemical properties 1. Chemically inert 2. Non-toxic 3. Non-corrosive 4. Non-flammable 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:- 1. Increased safety 2. Reduced size 3. Easy installation 4. Low maintenance
SF6 CIRCUIT BREAKER OPERATION
SF6 CIRCUIT BREAKER OPERATION
SF6 DECOMPOSITION PRODUCTS SF6 SPARK SF4 + 2F H2O M SOF2 + 2HF MFn H2O SO2 + HF SiO2 SiF4 + 2 H2O
Amount of each decomposition product depends on :- magnitude & duration of discharge materials of construction of equipment contamination levels of moisture and air inside the equipment
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 the weight 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
SF6 PUFFER TYPE CIRCUIT BREAKER The green/blue block does not move The blue outline piston moves right The arc is blown away by the blast of SF6
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 the vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapors falling on the inside surface of the outer insulating 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 isverylow. (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.
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