Engr. Syed Muhammad Munavvar Engr. Syed Muhammad Munavvar HussainHussain

Circuit BreakersCircuit Breakers

DefinitionDefinitionA circuit breaker is defined as a piece of equipment which can do any one of the following tasks:

Makes or breaks a circuit either manually or by remote control under normal conditions

Breaks a circuit automatically under fault conditions

Makes a circuit either manually or by remote control under fault conditions

Circuit Breaker with Internal Circuit Breaker with Internal StructureStructure

Switching FunctionSwitching FunctionThus a circuit breaker is used for

incorporating manual as well as automatic control for the switching function.

The automatic control of the circuit breaker is incorporated with the help of relays.

The automatic control is only done in case of fault conditions

Advantage over FuseAdvantage over FuseA fuse operates once and then has

to be replaced.The main advantage associated

with the use of circuit breaker is that a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Circuit Breaker ContactsCircuit Breaker Contacts

There are two types of contacts:Primary contact Arcing contact

Circuit Breaker ContactsCircuit Breaker ContactsThe primary contact is always

made of a high conductive material such as copper (Cu).

The arcing contact is made of arc resistance material such as tungsten or molybdenum, which has a much lower conductivity than those used for primary contacts.

Normal ConditionNormal Condition

Under the normal operating conditions, these contacts remain closed and are not open automatically until and unless the system becomes faulty.

Faulty ConditionFaulty Condition

When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.

Arc PhenomenonArc Phenomenon

When the contacts of a circuit break are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases.

Categories of ArcsCategories of ArcsArcs in the circuit breakers are categorized as:

High-pressure arcs:(with ambient pressures of 1 atm and

above)Vacuum arcs:(with ambient pressures below 10-4

torr)

Arc is Useful?Arc is Useful?The arc is useful in a way as it provides a

low resistance path for the current after contact separation. It prevents current chopping and associated abnormal switching over-voltages in the system.

The arc provides a gradual, but quick, transition from the current-carrying to the current-breaking states of the contacts.

Arc ExtinctionArc Extinction

The arc depends upon the following factors:

Degree of IonizationLength of the ArcCross-section of the Arc

Classification of Circuit Classification of Circuit BreakersBreakers

Circuit BreakersCircuit Breakers

1. Based 1. Based on on VoltageVoltage•Low•Medium•High/Extra High•Ultra High

3. Based 3. Based on on External External DesignDesign•Dead Tank•Live Tank

4. Based 4. Based on on InterruptiInterrupting Mediang Media•Air Blast•Oil•SF6

•Vacuum

2. Based 2. Based on on LocationLocation•Indoor•Outdoor

1. Based on VoltageOn the basis of the voltage levels for which they are used, the circuit breakers are classified as:

Category Range of Voltage

Low voltage Less than 1 kV

Medium voltage 1 kV to 52 kV

High/Extra High voltage

66 kV to 765 kV

Ultra High voltage Above 765 kV

2.2. Based on LocationBased on Location

Circuit breakers are, based upon where they are located, classified as:

Indoor typeOutdoor type

3.3. Based on External DesignBased on External Design

From the point of view of their physical structural design, outdoor circuit breakers can be identified as either:

dead tank typelive tank type

4.4. Based on Interrupting Based on Interrupting MediaMedia

The circuit breakers may be classified into following categories:

Oil Circuit BreakersAir Circuit BreakersAir-blast Circuit BreakersSulphur Hexafluoride (SF6) Circuit

BreakersVacuum Circuit Breakers

1.1. Oil Circuit BreakersOil Circuit Breakers

The circuit breakers in which some insulating oil (i.e., transformer oil) is used as an arc quenching medium.

Arc Extinction in OCBsArc Extinction in OCBs

The arc extinction is facilitated mainly by two processes:

Cont…Cont… Arc Extinction in Arc Extinction in OCBsOCBs

Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionization of the medium between the contacts.

Cont…Cont… Arc Extinction in Arc Extinction in OCBsOCBs

Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path. This results in extinguishing the arc and as a result the circuit current is interrupted.

AdvantagesAdvantagesThe oil provides insulation for the

live exposed contacts .The hydrogen produced during

arcing has excellent cooling properties and helps extinguish the arc.

The oil close to the arc region provides cooling surface.

DisadvantagesDisadvantagesOil is inflammable and may cause fire

hazards. The hydrogen, when combined with air,

may form an explosive mixture.During arcing, oil becomes polluted by

carbon particles, which reduces its dielectric strength. Hence, it requires periodic maintenance and replacement.

Types of Oil Circuit BreakersTypes of Oil Circuit Breakers

They can be classified with the reference to the quantity of oil used.

Bulk Oil Circuit BreakersMinimum Oil Circuit Breakers

Bulk Oil Circuit BreakersBulk Oil Circuit Breakers

These circuit breakers use a large quantity of oil.

Cont…Cont… Bulk Oil Circuit BreakersBulk Oil Circuit Breakers

The oil has to serve two purposes:It extinguishes the arc during opening of contacts.

It insulates the current conducting parts from one another and from the earthed tank.

Minimum Oil Circuit BreakersMinimum Oil Circuit Breakers

These circuit breakers use a small quantity of oil.

In such circuit breakers, oil is used only for arc extinction; the current conducting parts insulated by air or porcelain or organic insulating material.

2.2. Air-Blast Circuit BreakersAir-Blast Circuit Breakers

These circuit breakers employ a high pressure air-blast as an

The contacts are opened in a flow of air-blast established by the opening of the blast valve. arc quenching medium.

Arc ExtictionArc Extiction

The air-blast cools the arc and sweeps away the arcing products of the atmosphere. Consequently, the arc is extinguished and flow of current is interrupted.

Air-Blast Circuit BreakerAir-Blast Circuit Breaker

Uses of ABCBsUses of ABCBs

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

AdvantagesAdvantagesThe risk of fire is eliminated.The arcing products are completely removed by the blast,

so the expenditure of oil replacement is avoided.The size of these breakers is reduced.Due to the rapid growth of the dielectric strength, the

arcing time is also very small. It causes less burning of oil.The arc energy is also very small fraction of that in oil

circuit breakers.The arc extinction is facilitate by the high pressure air,

and is independent of the fault current to be interrupted.

DisadvantagesDisadvantages

These circuit breakers are very sensitive to the variation s in the rate of rise of restriking voltage.

The air-blast is supplied by the compressor plant that needs considerable maintenance.

3.3. Sulphur Hexafluoride (SFSulphur Hexafluoride (SF66) )

Circuit BreakersCircuit Breakers

In these circuit breakers, Sulpher hexafluoride gas (SF6) is used as the arc quenching medium.

The SF6 is an electronegative gas and has a strong tendency to absorb free electrons.

This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.

Uses of SFUses of SF66 CBs CBs

These circuit breakers are available for complete range of medium voltage and high voltage application up to 800 kV and above.

This medium is most suitable for metal-clad and hybrid HV sub-stations.

AdvantagesAdvantagesDue to the low energy the contact erosion is small.The gaseous medium SF6 possesses excellent dielectric and

arc quenching properties. Due to the superior arc quenching property of the SF6 gas,

such circuit breakers have very short arcing time. Furthermore, they can interrupt much larger current.

These breakers give noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike the air-blast circuit breaker.

The SF6 gas is not inflammable, so there is no risk of fire and explosion in SF6 breakers.

DisadvantagesDisadvantagesThese circuit breakers are expensive due to the

high cost of SF6 gas.Since SF6 gas has to be reconditioned after

every operation of the breaker, additional equipment is required for this purpose.

The SF6 gas has been identified as a greenhouse gas, so can be harmful for population in its area of operation.

4.4. Vacuum Circuit BreakersVacuum Circuit Breakers

In these circuit breakers, the vacuum is used as the arc quenching medium.

The degree of vacuum in these circuit breakers is in the range from 10-7 to 10-5 torr.

Vacuum Circuit BreakerVacuum Circuit Breaker

AdvantagesAdvantagesThe vacuum circuit breakers are compact in size and

have longer lives.Because of the very low voltage across the metal

vapor arc, energy is very low.There is no generation of gases during and after the

circuit breaker operation.The outstanding feature of these breakers is that it

can break any heavy fault current perfectly just before the contacts reach a definite open position.

They can successfully withstand lightning surges.

5.5. Air Circuit BreakersAir Circuit Breakers

Air circuit breaker is defined as a circuit breaker, in which the contacts open and close in air at atmospheric pressure.

Arc InterruptionArc InterruptionThe principles of arc interruption used in an

air circuit breaker are rather different from those in any other type of circuit breaker.

This can be achieved in three ways:1. Intense cooling of the arc plasma, so that

the voltage gradient is very high2. Lengthening the arc path to increase the

arc voltage3. Splitting up the arc into a number of

series arcs

Circuit Breakers RatingsCircuit Breakers Ratings

There are three ratings for breakers as:Breaking CapacityMaking CapacityShort-time Rating

Breaking CapacityBreaking CapacityBreaking 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).

Cont…Cont… Breaking CapacityBreaking Capacity

It is a common practice to express the breaking capacity in MVA by taking into account the rated the rated breaking current and rated service voltage.

Thus if I is the rated breaking current in Amperes and the rated service voltage is V in volts, the breaking capacity for three-phase circuit is:

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

Making CapacityMaking CapacityIt is the peak value of current (including d.c. component) during the first cycle of current wave after the closure of circuit breaker.

Mathematically stated, making capacity is given as:

Making Capacity = 2.55 x symmetrical breaking capacity

Short-time RatingShort-time RatingThe period for which the circuit

breaker is able to carry fault current while remaining closed is known as short-time rating.

The short-time rating of a circuit breaker depends upon its ability to withstand:1. The electromagnetic force effects2. The temperature rise

Thank You … !