Adigrat University College of Engineering and Technology Department of Electrical and Computer Engineering Engineering Stream
Adigrat University College of Engineering and Technology Department of Electrical and Computer Engineering Engineering Stream
Adigrat University College of Engineering and Technology Department of Electrical and Computer Engineering Engineering Stream
ENGINEERING
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CIRCUIT BREAKERS
1.1 INTRODUCTION
Electrical circuit breaker is a switching device which can be operated manually as
well as automatically for controlling and protection of electrical power system
respectively. As the modern power system deals with huge currents, the special
attention should be given during designing of circuit breaker to safe interruption of
arc produced during the operation of circuit breaker. This was the basic definition of
circuit breaker
By the arc extinction medium, the circuit breaker is categorized into four types. They are
the air break circuit breaker, air blast circuit breaker, Sulphur hexafluoride circuit breaker
and vacuum circuit breaker. The classification of the circuit breaker is shown in the figure
below
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Fig; classification of circuit breaker
AC circuit breaks
DC circuit breakers
1.2.1 AC Circuit Breaker
The AC circuit breaker is sub-classified into two types, i.e., the low voltage circuit
breaker and the high voltage circuit breaker. The circuit breaker whose value lies
below the 1000V is known as the low voltage circuit breaker, and above 1000V it
is known as a high voltage circuit breaker
1.2.2 Types according to the medium of installation:
Oil circuit breaker.
Air Blast circuit breaker.
SF6 circuit breaker.
Vacuum circuit breaker.
1. Oil circuit breaker.
Oil circuit breaker is such type of circuit breaker which used oil as a dielectric or
insulating medium for arc extinction. In oil circuit breaker the contacts of the
breaker are made to separate within an insulating oil. When the fault occurs in the
system the contacts of the circuit breaker are open under the insulating oil, and an
arc is developed between them and the heat of the arc is evaporated in the
surrounding oil.
Due to this arc, a large amount of heat is liberated, and a very high temperature is reached
which vaporizes the surrounding oil into gas. The gas, thus liberated surrounds the arc and
its explosive growth around it displace the oil violently. The arc is extinguished when the
distance between the fixed and moving contact reaches a certain critical value, depends on
the arc current and recovery voltage
The oil circuit breaker is very reliable in operation, and it is very cheap. The most important
feature of oil circuit breaker is that no special devices are used for controlling the arc caused
by moving contact
Air blast circuit breakers employ a high-pressure air blast as an arc quenching
medium. Under normal condition the contacts are closed. When a fault occurs, contacts are
opened and an arc is struck between the them. The opening of contacts is done by a flow
of air blast established by the opening of blast valve (located between air reservoir and
arcing chamber). The air blast cools the arc and sweeps away the arching products in to the
atmosphere. Thus, the dielectric strength of the medium is increased, prevents from re-
establishing the arc. The arc gets extinguished and flow of current is interrupted. The circuit
breakers are classified on the basis the direction of air blast to the arc. They are classified
into
1. Axial Blast Type - air blast is directed along the arc path.
2. Cross Blast Type - air blast is directed at right angles to the arc path.
3. Radial Blast Type - air blast is directed radially.
In air break circuit breaker the arc is initiated and extinguish in substantially static air in which the
arc moves. Such breakers are used for low voltages, generally up to 15KV and rupturing capacities
of 500MVA
The SF6 gas absorbs the free electrons in the arc path and forms ions which do not act as a charge
carrier. These ions increase the dielectric strength of the gas and hence the arc is extinguished.
This process reduces the pressure of the SF6 gas up to 3kg/cm^2 thus; it is stored in the low-
pressure reservoir. This low-pressure gas is pulled back to the high-pressure reservoir for re-use.
Now a day puffer piston pressure is used for generating arc quenching pressure during an
opening operation by mean of a piston attached to the moving contacts
In vacuum circuit breakers, vacuum is used as the arc quenching medium. Vacuum offers
the highest insulating strength. so, it has far superior arc quenching properties than any
other medium.
For example, when contacts of a breaker are opened in vacuum, the interruption occurs at first
current zero with dielectric strength between the contacts building up at a rate thousands of times
higher than that obtained with other circuit breakers.
The technology is suitable for mainly medium voltage application. For higher voltage vacuum
technology has been developed but not commercially viable.
A breaker which used vacuum as an arc extinction medium is called a vacuum circuit breaker. In
this circuit breaker, the fixed and moving contact is enclosed in a permanently sealed vacuum
interrupter. The arc is extinct as the contacts are separated in high vacuum. It is mainly used for
medium voltage ranging from 11 KV to 33 KV
Working principles of Vacuum Circuit Breaker
When the fault occurs in the system, the contacts of the breaker are moved apart and hence the arc
is developed between them. When the current carrying contacts are pulled apart, the temperature
of their connecting parts is very high due to which ionization occurs. Due to the ionization, the
contact space is filled with vapor of positive ions which is discharged from the contact material.
The density of vapor depends on the current in the arcing. Due to the decreasing mode of current
wave their rate of release of vapor fall and after the current zero, the medium regains its dielectric
strength provided vapor density around the contacts reduced. Hence, the arc does not restrike again
because the metal vapor is quickly removed from the contact zone.
The HVDC circuit breaker is a switching device that interrupts the flow of abnormal direct
current in the circuit. When the fault occurs in the system, the mechanical contacts of the
circuit breaker are pulled apart and thus their circuit is open. In HVDC circuit breaker,
circuit breaking is difficult because the current flow through it is unidirectional and there
is no zero current.
The main application of the HVDC circuit breaker is to interrupt the high voltage direct
current flows in the network. AC circuit breaker easily interrupts the arc at natural current
zero in the AC wave. At zero current, the energy to be interrupted is also zero. The contact
gap has to recover the dielectric strength to withstand natural transient recovery voltage.
With DC circuit breakers, the problem is more complex as the DC waveform does not have
natural current zeros. Forced arc interruption would produce high transient recovery
voltage and restrike without arc interruption and ultimate destruction of the breaker
contacts. In designing of HVDC circuit breakers, there are three main problems to be
overcome. These problems are
Creation of artificial current zero.
Prevention of restrikes arc.
Dissipation of stored energy
The artificial current zero principles are used in HVDC circuit breakers for arc
extinction. By introducing a parallel L-C circuit, the arc current is subjected to
oscillations. These oscillations are severe and have several artificial current zeros.
The breaker extinguishes the arc at one of the artificial current zeros. The crest
current of the oscillation must be greater than the direct current to be interrupted.
A series resonant circuit with L and C is connected across the main contact M of a
conventional DC circuit breaker through an auxiliary contact S1, and resistor R is
connected through contact S2. Under normal operating conditions, main contact M
and charging contact S2 remain closed, and the capacitor C is charged to line voltage
through the high resistance R. Contact S1is open and has line voltage across it
For interrupting main circuit current Id, the operating mechanism open contact S2 and closes
contact S1. This indicates discharge of capacitor C through inductance L, main contact M
and auxiliary contact S1 setting up an oscillatory current shown in the figure below. Thus,
artificial current zeros are created, and the circuit breaker main contact M is opened at a
current zero. After that, contact S1 is opened, and contact S2 is closed.
Another way of interrupting the main direct current is by its diversion to the capacitor so that the
magnitude of current to be interrupted by the circuit breakers becomes smaller. This is shown in
the figure below. The capacitor C is initially uncharged.
When the main contact M opens, the main circuit current is diverted to the capacitor C.
Thus, the current to be interrupted by the main contacts M becomes smaller. The nonlinear
resistor R absorbs energy without greatly adding to the voltage across the main contact M.
The problem of prevention of restrikes is very intense in oscillating current DC circuit breakers
where the time in which the current is chopped is very small. Thus, the steep surge of the restriping
voltage across breaker terminals is produced, and the circuit breaker must be capable of
withstanding this voltage
Detects and isolates faults on transmission and distribution lines by opening and closing
circuit breakers
Isolating the controlling circuit from controlled circuit when the two are at different
potential
It is applicable in logic function
It is applicable in delay function
Controls high voltage circuits with the help of low voltage signals
2. Instrument transformers
an instrument transformer is a device used to reduce the current or the voltage values
in to values can handled by other equipment’s.
Is a device to transform the power system current and voltage to lower magnitudes,
and provide an isolation between the power network and the relay and other
instruments connected to the transformer secondary winding
These are special types of transformers used for the measurement of voltage, current,
power & energy.
current transformer
potential transformer
They generally used to step down current in a very predictable fashion with respect to
current and phase the current (or series)
transformer has a primary coil of one or more turns of thick wire connected in series with
the line whose current is to be measured.
The secondary consists of a large number of turns of fine wire & is connected across the
ammeter terminals
the grounding of CTs is important to both the safety and the correct operating of
protective relays
to assure safe and reliable operation, the neutral of the CT secondary should have a
for an instrument transformer, it is necessary that the transformer ratio must be exactly equal to turns
ratio and phase of the secondary terms (voltage and current). two types of errors affect these
ratio error
Ratio error
In practice it is that current transformation ratio I2/I1 is equal to the ratio N2/N1. but actually, it is not so. the
current ratio is not equal to turns ratio because of magnetizing and core loss components of the exciting
current. it also gets affected due to the secondary currents and its power factor. the load current is not a
constant fraction of the primary current. similarly, in case of potential transformer, the voltage ratio V2/V1 is
also not exactly equal to N2/N1 due to the factors mentioned above. thus, the transformation ratio is not
constant but depends on the load current, power factor of load and exciting current of the transformer. due to
this fact, large error is introduced in the measurements done by the instrument transformers such an error is