Circuit Breakers: Principles, Operation, and Arc Interruption Methods - Prof. Patel, Slides of Electrical and Electronics Engineering

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Circuit Breakers

Switchgear is a device that allows operating devices like

Generators, Transmission lines, and Electrical equipment to

function.

Switchgear is classified into different types based on voltage and

application, and has different insulating mediums and continuous

current ratings:

Low Voltage Switchgear (LV):

This type of Switchgear is preferred for residential ,medium to

large Industrial systems.

It uses low-voltage power circuit breakers to enhance reliability and

coordination.

Medium Voltage Switchgear (MV):

This type of Switchgear protects equipment and circuits that operate

at voltages between 1 kV and 36 kV.

It's used in commercial, and industrial electrical installations to

control power distribution.

Switchgear:

2

A Circuit Breaker is an electrical switch designed to protect an electrical

circuit from damage caused by overcurrent/overload or short circuit.

Its basic function is to interrupt current flow after protective relays

detect a fault.

It protects the circuit by interrupting the current flowing in the

line, transformer etc…

A breaker interrupts the current by Mechanically moving electrical

contacts apart inside an interrupter, causing an arc to occur that is

immediately suppressed by the high-dielectric medium inside the

interrupter.

Circuit Breaker:

2

A Circuit Breaker is a mechanical

switching device, capable of making,

carrying and breaking currents

under normal circuit conditions.

It is also capable of making and

carrying currents for a specified

time and breaking currents.

Circuit Breaker:

Fixed

contac

t

ARC

Moving

Fixed

contac

t

ARC IS

QUENCHED BY

MEDIUM

Moving

contact

2

Circuit Breaker:

Circuit breaker consists of two

coils :

Closing Coil- Used to Close the

circuit

Tripping Coil- Used to Trip the

circuit

These coils activate the stored energy

and directs the Moving

contact (MC) to Open or Close.

DC batteries are used to energize these coils.

Solenoid are used to Close or Trip it. ➢ CBs are usually arranged with pilot devices to sense a fault current

and to operate the Trip opening mechanism.

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Operating principle:

A Circuit Breaker as a switching and current interrupting

consist of fixed and moving contacts, which are touching each

other and carry the current under normal condition.

When the Circuit breaker is closed, the current carrying

contacts, called the electrodes, engage each other under the

pressure of a Spring.

Whenever a fault occurs on any part of the Power System,

the Trip coils of the breaker get energized and the Moving

contact are pulled apart by some mechanism, thus opening

the circuit.

The separation of current carrying production an Arc. The

current is thus able to continue until the discharge ceases.

2

Terms related to CBs:

Recovery Voltage:- It is the normal

frequency (50 Hz) r.m.s. the Voltage

that appears across the contacts of the

circuit breaker after final Arc

extinction.

It is approximately equal to the system

Voltage.

When contacts of the Circuit breaker

are opened, the current drops to

zero after every half cycle.

When Current zero, the contacts are separated sufficiently apart.

At such an instant, the medium between the contacts is strong enough

to prevent the breakdown by the Restriking Voltage.

2

Terms related to CBs:

Rate of Rise of Restriking Voltage

(RRRV):-

It is the ratio of the Peak Transient

Recovery Voltage to the total time

from Zero Voltage to peak Voltage.



As the length of line is increases the value of line Capacitance & line

Inductance changes.



RRRV depends on length of the Transmission line, RRRV decreases as

the length of Line decreases.

2

Arc extinction methods are:

By lengthening the gap.

Cooling the Arc.

Inserting medium of high dielectric strength.

Reducing the cross-section of Arc.

Splitting of Arc

Arc:

Arc Interruption in Circuit Breakers:

2

The two modes of Arc Interruption Methods in CBs are:

High Resistance interruption

Low Resistance or current Zero interruption.

High Resistance Interruption:

In this case the Arc is controlled in such a way that its effective

Resistance increases with time so that the current is reduced to

a value insufficient to maintain it.

The Arc Resistance may be increased with lengthening,

cooling and splitting the Arc.

The main drawback with this type of Arc Interruption is that the

Energy dissipated is High and so it is only used in Low and

medium power A.C. Circuit breakers and in D.C. circuit

breakers.

Low Resistance or Current Zero Interruption:

Slepian’s Theory:

In this theory, at each current Zero, there is a Race between

RRRV and the rate which insulating medium recovers its

dielectric strength.

If the rate at which

the dielectric

strength progresses

is faster than the rate

at which voltage

rises, the Arc will be

quenched;

Otherwise, the Arc

Energy Balance Theory (Cassie’s Theory)

Low Resistance or Current Zero Interruption:

The space between the contacts contains some ionized gas

immediately after current Zero and hence, it has a finite

post – Zero moment, power is Zero because Restriking

voltage is Zero.

When the Arc is finally extinguished, the Power gain

becomes Zero, the gap is fully de-ionized and its Resistance

is infinitely high.

In between these two limits, first the

Power increases, reaches a

maximum value, then decreases

and finitely reaches Zero value.

Construction:

2

Construction: