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Monday, August 5, 2013

PPT On Power Distribution Equipments


Power Distribution Equipments Presentation Transcript: 
1.Power Distribution & Utilization

2.Contents To Be Covered
Circuit Breakers & their Types
Lightening protection
Protective Relay Introduction
Disconnect Switches
Metering Equipment

Circuit breakers are high voltage, high current devices that automatically disconnect faulted equipment to protect people, prevent damage to equipment and minimize the effects of damage.
Range in size from 125V,15A to over 800kV,thousands of Amperes.

4.Arc in Circuit Breakers:
Opening of the contacts of circuit breaker causes ionization of gas present between contacts, as a result, producing arc.
It is due to the presence of high electric field between the current carrying contacts.
Severe in case of inductive loads(Mostly Industrial & faulted lines).
Arc must be extinguished to interrupt the current in case of Fault.

5.Methods to Extinguish Arc:
Firstly, by lengthening the arc which causes  rise in resistance, drop in current & temperature rise, thus reducing the arc.
Using an arc quenching medium e.g Air, Oil or an Insulating gas.
DC arcs are harder to break than Ac, but can be extinguiushed using the same principles.

6.Methods used to break the arc in air are:
Arc Trip
Arc Horn
Interrupting Fins

It causes an arc, which is hot, to rise if the contacts are properly oriented.
Increased surface area is exposed to cooler air due to stretching in rising arc, causing its temperature to drop.
Longer an arc can be drawn, easier it is to extinguish.

8.Arc Tip:
Arc tips, also called arcing contacts, break after the main contacts break.
Prevents pitting of the main contacts.
Works on same principle as Convection.
Only difference is that, it spreads the arc further, thus making it extinguish earlier.

9.Interrupting Fins:
These are placed in the path of rising arc.
Stretch the arc further
Cool it more
Extinguish it faster.

10.Types of Circuit Breakers Air Circuit Breakers:
Use air as arc interrupting medium.
Used mostly as low voltage breakers.
These are driven open by strong springs compressed by rachet mechanism.

11.Air Blast Circuit Breakers:
Special purpose medium voltage breakers.
Used at voltages between 14.4kV and 34.5kV.
Blast of compressed air(800 psi) is blown across breaker contacts as they are open.
Opening mechanism is provided by powerful springs.
Contacts are closed either by a motor or compressed air.
Must have a resistor across the arc to dampen high voltage oscillations which may re-ignite the arc.

12.Vacuum Circuit Breakers:
Contacts are enclosed in a container with a high vacuum.
No significant arcing can occur because no air is present to be ionized.
Operated by spring force.
Available for voltages from 480V to 34.5V.

13.Oil Circuit Breakers:
Use oil as arc interrupting medium.
Oil has a dielectric strength far in excess of air.
Opening of contacts in oil causes the oil to disassociate which absorbs energy.
Hydrogen is used as a product of disassociation.
Can be used up to voltage levels of 230kV.
Use of arcing chamber increases capacity by a factor of 500, to 10,000 MVA.

14.Sulphur Hexaflouride(SF6) Circuit Breakers:
SF6 is used as an arc quenching medium.
Used for extremely high voltage(EHV, above 345kV) applications.
These are usually live tank designs supported on insulators.

15.Circuit Breaker Ratings:
User must consider a number of ratings to select a circuit breaker.
Continuous Voltage Rating(Decrease at altitudes above 3000 ft.).
Rated Impulse Voltage.
Continuous Current Rating.
Maximum Fault Current.
Interrupting Time.
Mechanical considerations such as size, foundation and space required.

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