Circuit Breakers

02/04/15Dr.Latif Shaikh

Introduction Circuit Breaker The Arc Isolators Air Circuit Breakers Air Blast Circuit Breakers Vacuum Circuit Breakers Oil Circuit Breakers Sulfur Hexafluoride Circuit Breakers Circuit Breaker Ratings Circuit Breaker Controls High Voltage Circuit Breakers Comparison Reclosures Sectionalizers Fuses Fuse application


A circuit breaker is a piece of equipment which can Make or break a circuit either manually or by remote control under normal conditions.

Break a circuit automatically under fault condition Make a circuit either manually or by remote under

fault condition


CB consists of fixed and moving contacts called electrodes

Under normal operating condition these contacts remain closed and will not open automatically unless the system becomes faulty .These contacts can be opened manually or by remote control.

When a fault occurs in a circuit the trip coils of the circuit breaker get energised and the moving contacts are pulled apart by some mechanism ,thus opening the circuit.




When contacts of circuit breaker starts separating the contact resistance starts increasing. This increases the (I square r) loss which is heat produced .

This heat increases the energy of electrons in the contact areas and the ionized particles tries to maintain the current when contacts are separated.This flow of charged particles form one contact to other is called an arc .

The medium surrounding the arc also contains ions .Due to this charged particles the arc continues even if the

breakers contacts are separated.The voltage (potential gradient) across the arc is less and

so it continues even for low voltages.


DC arcs are to be interrupted by increasing the resistance interruption method in which resistance of the arc is increased so that the arc voltage can no longer maintain the current and the arc is extinguished.

Size of DC circuit breaker increases as the voltage level increases.

AC arcs current reduces to zero in each cycle (2 times) If the circuit breaker contacts are opened at time when

the current passed through zero and dielectric strength of the medium is build up rapidly so that arc cannot strike again then arc can be extinguished successfully.

Size of AC circuit breaker can be small compared to same voltage DC circuit breaker.


Separate the contacts of circuit breaker such that the arc resistance increases to a very high value. The pd between the contacts is unable to maintain the arc current. For high voltage circuit breakers this method is impracticable since a separation of many meters will be required.(High Resistance Method)

The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized ,the arc extinction is facilitated .This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.(Low Resistance Method)


Arc Voltage – It is the voltage that appears across the contacts of the circuit breakers during the arcing period as the contacts are opened.

Recovery voltage – It is the normal frequency voltage that appears across the contacts of circuit breaker after final arc extinction.

Rate of rise of restriking voltage – (RRRV) It is the rate of increase of restriking voltage .RRRV depends upon 1) recovery voltage and 2) Natural frequency of oscillation


It is the transient voltage that appears across the contacts at or near current zero during arcing period.

If dielectric strength rise is greater than the rise of restriking voltage then the arc will not restrike.


It is the phenomena of current interruption before natural current zero is reached. It occurs in air blast circuit breaker because they retain same extinguishing power irrespective of the magnitude of current to be interrupted.

When interrupting low inductive current e.g.magnetising current of transformer, a rapid deionizing effect causes current , to fall below its zero value before natural current zero is called current chopping.





The switching Resistor (R) is connected in parallel with the CB contacts. Current chopping produces high voltage oscillations which can be prevented by this method.

During arc interruption CB contacts separate first and after arc gets extinguished ‘S’ opens depending upon the time delay provided to it.

When the fault occurs the CB contacts open and arc is struck between them. Since R is in parallel with Cb contacts ,a part of arc current flows through this resistance so arc current decreases and deionization rate increases. The arc resistance also increases so current through R increases. This continue till the arc current is insufficient to maintain the arc.


Breaking capacity – It is the current (r.m.s.) that a Cb is capable of breaking at given recovery voltage and under specified conditions.

Making Capacity – The peak value of current (including DC component) during the first cycle of current wave after closure of circuit breaker is known as making capacity. Making capacity = 2.55 × symmetrical breaking capacity

Short time rating – It is the period for which the CB is able to carry fault current while remaining closed.

Normal current rating – It is the r.m.s. value of current which the CB is capable of carrying continuously at its rated frequency under rated specified conditions.



1.Air Circuit Breakers

2.Air Blast Circuit Breakers

3. Vacuum Circuit Breakers

4.Oil Circuit Breakers

5.Sulfur hexafluoride circuit breakers


1.In this Circuit Breaker the arc is elongated using arc runners and arc splitters so as to increase the resistance of the arc.

3.This increases the voltage required to maintain the arc and if the available voltage cannot sustain the arc ,the arc gets extinguished.

2.At current zero ,the recovery voltage across the contacts becomes less than the arc voltage and the arc gets extinguished.

4. The energy in the system inductance at current zero is zero .Hence arc interruption is easier.



1 Main contacts 2 Arcing Contacts 3.Arc Rising in the direction of Arrow 4.Arc getting Split 5.Arc Splitter Plates 6.Current Carrying Terminals 7.Arc Runners


Air Circuit Breakers

1.Used For low voltage levels and current levels

2.As voltage level increases, the size of breaker becomes large so not convenient for higher voltage and current levels.

3.Air is used as medium to extinguish the arc which have inferior extinguishing properties compared to SF6 or Vacuum circuit breakers

4.Operating control is manual as well as automatic.5.It is used up to 6.6kV with a breaking capacity of

15MVA.6.Suitable for repeated operation because medium of

arc extinction is air.


In this breaker, a high pressure air blast is used as an arc quenching medium.

The contacts are opened and a flow of air blast is maintained by opening the blast valve.

The air blast cools the arc and takes away the arcing products to atmosphere .

This rapidly increases the dielectric strength of the medium between the contacts and the arc is extinguished and the flow of current is interrupted.



High speed of operation Short arcing time High speed reclosing Less weigh as compared to oil circuit breakers Very less maintenance No possibility of explosion

Cost is more For complete compress air installation is

required These breakers are more sensitive to RRRV. For operation and maintenance ,highly

skilled persons are required



Air is admitted in the arc extinction chamber it pushes the moving contact. This air blast takes away the ionized gases along with it. Afterwards the arc gets extinguished. High pressure air has higher dielectric strength.

The design is such that the air expands into the low pressure (atmospheric pressure zone).The air at high speed removes heat from the arc, thus arc is quenched. Diameter of arc is reduced.

Uses1.Arc Furnaces2.Traction Syetems


When two contacts of this circuit breaker are separated in vacuum an arc is struck and hot spots are formed on the surface of the contacts .These hot spots produce metal vapor and plasma. The amount of vapor in plasma depends on how rapidly the vapor is emitted from contact surface which depends on the arc current. The current is of alternating nature, it pass through zero several times, so the rate of vapor emission also becomes zero, and the vapor already emitted gets condensed . During this process the dielectric strength builds up rapidly and the restriking of arc is prevented.



It consists of fixed contact, moving contact and arc shield mounted inside a 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 leakage,.

A glass vessel or ceramic vessel is used as outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing the deterioration of the internal dielectric strength.

Applications – Outdoor application where maintenance required is

minimum.In the high voltage system from 22 KV to 66kV power Circuits.


When two contacts of circuit breaker are separated in vacuum arc is struck and hot spots are formed on the surface of the contacts. These hot spots produce metal vapour and plasma.

At current zero the rate of vapour emission becomes zero.

The vapour already emitted gets condensed .During this process the dielectric strength builds up and

the restriking of arc is prevented.


Compact in size Reliable and long life Heavy fault can be interrupted effectively No gas is generated after arc extinction

operation Operation is not noisy Arc energy is low No risk of fire

Vacuum has to be maintained at desired level always



This breaker makes use of oil for quenching the arc.The circuit breaker which uses more oil or which is

bulky is called bulk oil circuit breaker.The construction is simple and it consists of fixed and

moving contacts enclosed in a strong weather –tight earthed tank containing oil up to a certain level and an air cushion above the oil level.

Application These breakers are used up to 11 KV with an

interrupting capacity of 250MVA.


Under normal working conditions, the fixed and moving contacts are closed. On the occurrence of fault, the moving contacts come down and an arc is struck between the contacts. The oil between the contacts gets decomposed and hydrogen gas bubble is formed around the contacts. The hydrogen gas cools the arc and the turbulence effect cause the lengthening of arc. The deionization of medium between contacts takes place and at some critical length of gap between the contacts ,the arc is extinguished.The hydrogen gas bubble produces a very high pressure in the oil. The tank is therefore made strong to withstand a large pressure. The oil moves upwards when hydrogen bubble is formed. The air is present between the oil level and tank top and acts as cushion and absorbs mechanical shock produced due to upward oil movement.


oil has high dielectric strength Oil absorbs arc energy while

decomposing Good cooling property of the gas formed

due to decomposition It acts as an insulator between the live

parts and earth

Long arcing time Do not permit high speed of interruption Arc interruption control can be obtained

only by increasing the length of arc Chance of fire and explosion.


Operation When the contacts are

separated in oil arc is formed. The heat of arc decomposes oil and gases are formed. These gases expand due to heating of the arc. The gas flowing near the contact zone cause cooling and splitting of the arc and the arc gets extinguished.


In this breaker, the supporting ,current interruption and top chamber are made of porcelain .Hence, clearance between live parts is small and requires less quantity of oil, hence the breaker is called Minimum Oil Circuit Breaker. The chambers are completely filled with oil. The oil from upper chamber does not come to lower.

The fixed contact is enclosed in the quenching chamber. Moving contact makes sliding contact with the lower fixed contact. The operating rod is operated by operating mechanism, the three poles operate simultaneously.The voltage ratings are from 3.6kV to 420 KV.


Requires less quantity of oil Requires smaller space Maintenance is less Cost per breaking capacity in MVA is less Suitable for both manual and automatic

operation

Possibility of fire and explosion. Difficult to remove gases from the space

between contacts. Oil deteriorates rapidly due to

carbonization Smaller quantity of oil, so carbonization

increases.


Electronegative –It has the ability of an atom to attract and hold electrons. Such gas have high dielectric stremngth.SF6 is electronegative .It forms negative ions ,Negative ions are heavy and immobile so they do not flow easily .Hence SF6 gas has high dielectric strength.

Rate of rise of dielectric strength is very high. Can be liquefied and stored in steel tanks Dielectric strength increase linearly with pressure. Gas is inert. Therefore contacts will not get eroded. Gas is non –inflammable , Colorless ,odorless, Non-toxic Thermally stable up to 55 degrees



Under normal operating conditions the contacts are closed . On occurance of fault contacts are opened. The movable

contact moves away from the fixed contact. The arc is struck between the fixed and moving contacts. High pressure SF6 gas now flows over the arc and it absorbs

the free electrons from the arc. This builds up the dielectric strength between the gap very

fast and the arc is extinguished


Advantages Silent operation, compact

size Vary short arcing time No risk of fire No reduction in dielectric

strength due to operation No moisture problem No current chopping

problem Can interrupt larger

currents Suitable for explosive

environment due to totally enclosed body

Disadvantages

Costly Requires conditioning of SF6

gas from time to time SF6 gas is suffocating ,so its

leakage can cause suffovcation of the persons in surrounding areas.



Isolator(disconnecting switch) operates under no load condition. It does not have any current breaking capacity or current making capacity. Isolator is not even used for breaking load currents.

Isolators are used in addition to circuit breakers ,and are provided on each side of every circuit breaker to provide isolation and enable maintenance.

Sequence of operationWhile opening –Open circuit breaker first and then

isolatorsWhile closing –Close isolators first and then close

circuit breakers


Different types of controls are required for successful operation of circuit breakers.

1.Relays – These are required to give a trip signal to circuit breaker in case of fault condition. Different types of relays are available like over current, over voltage ,under voltage, loss of excitation,reverse power etc.

2.Sensor equipments are required to check the condition of circuit breakers arc extinguishing medium .

The controls are pressure sensors to sense the pressure of air in case of air blast circuit breakers .

In case of Sulfur hexafluoride circuit breakers also the pressure sensors are required.

In case of vacuum circuit breakers also sensors are required to check the vacuum level in the breaker.



The faults in power system may be temporary or permanent.

The temporary faults like touching of overhead conductors due to wind are cleared automatically after some time .

In case of temporary faults auto reclosing helps to maintain the system supply .

The breaker opens on a fault. After some time delay ,the breaker is closed . If the fault is temporary and it is removed ,then the

breaker will close successfully If the fault still persists then the breaker opens.



Sectionalizers are used in the bus bars so that fault on any section of bus bar will not cause complete shut down.

Advantages of sectionalizers1.If fault occurs on any section of bus bars then that section

is isolated from other sections without affecting the system.

2.Fault current is much lower than in case of un sectionalized system (as the fault is fed from only one section)

3.Repairing and maintenance on one section can be carried out by de energizing that section only eliminating the possibility of complete shut down.

The breaker in the bus bars is acting as sectionalizing breaker in the shown figure.


Fuse is a device used in circuit for protecting electrical equipments against overloads and /or short circuits.

Fuse element or fuse wire is that part of the fuse device which melts when an excessive current flows in the circuit and thus isolates the faulty device from the supply circuit.

Desirable qualities of fuse elements1.Low melting point2.Low ohmic losses3.High conductivity4.Free from deterioration due to oxidation5.Low cost




It consists of a heat resisting ceramic body having metal end caps. A fusible wire is welded to it. The space within the body, surrounding the element is completely packed with a powder, usually quartz, which acts as an arc extinguishing medium.

Under fault condition, the current flowing through the element produces very large amount heat and the fuse element melts before the fault current reaches first peak value. The heat produced in the process vaporizes the melted fuse element. The chemical reaction between the silver vapor and filling power results in the formation of high resistance which helps quenching the arc.



Engineering

Circuit Breakers