power system 1 unit 5 contains for various faults & circuit breakers
1. Department of Electrical Engineering
POWER SYSTEM 1
UNIT 5
Fault Analysis & Protection Systems
From:
Dr. Monika Vardia
Associate Professor
2. Method of Symmetrical Components
When the system is unbalanced the voltages, currents
and the phase impedances are in general unequal.
The method of symmetrical components simplified the
problems of the unbalanced three-phase system.
The balanced set of components can be given as a
positive sequence component, negative sequence
component, and zero phase sequence component.
The phase sequence of the positive component is Va, Vb
and Vc and the phase sequence of negative components
is Va, Vc, and Vb.
4. The positive sequence component of
the unbalanced three phase system
• In positive phase sequence component, the set of three
phasors are equal in magnitude, spaced 120° apart from each
other and having the same phase sequence as the original
unbalanced phasors.
5. Negative Phase Sequence Component
In negative phase sequence component, the set of the
three phasors are equal in magnitude, spaced 120° apart
from each other and having the phase sequence opposite
to that of the original phasors. The negative phase
sequence is shown in the figure below
6. Zero Phase Sequence Components
• In zero phase sequence components, the set of three phasors
is equal in magnitude to zero phase displacement from each
other. The zero phase sequence component is shown in the
figure below.
• The three phase balanced system is a special case of a general
three-phase system in which zero and negative sequence
components are zero.
7. Symmetrical & Unsymmetrical Faults
• Normally, a power system operates under balanced
conditions. When the system becomes unbalanced
due to the failures of insulation at any point or due
to the contact of live wires, a short–circuit or fault,
is said to occur in the line.
• Faults may occur in the power system due to the
number of reasons like natural disturbances
(lightning, high-speed winds, earthquakes),
insulation breakdown, falling of a tree, bird
shorting, etc.
8. Cont....
Faults that occurs in transmission lines are
broadly classified as
Symmetrical faults
Unsymmetrical faults
9. Symmetrical faults
In such types of faults, all the phases are short-circuited to each
other and often to earth.
Such fault is balanced in the sense that the systems remain
symmetrical, or we can say the lines displaced by an equal angle (i.e.
120° in three phase line).
It is the most severe type of fault involving largest current, but it
occurs rarely.
10. Unsymmetrical faults
• Unsymmetrical faults involve only one or two
phases.
• In unsymmetrical faults the three phase lines
become unbalanced.
• Such types of faults occur between line-to-
ground or between lines.
• An unsymmetrical series fault is between
phases or between phase-to-ground.
11. fault in the three phase system can be
classified as;
• Single line-to-ground fault (LG).
• Line-to-line fault (LL).
• Double Line-to-ground fault (LLG).
• Three-phase short circuit fault (LLL).
• Three-phase-to-ground fault (LLLG).
18. Neutral Grounding
• In neutral grounding system, the neutral of the system or
rotating system or transformer is connected to the
ground.
• The neutral grounding is an important aspect of power
system design because the performance of the system
regarding short circuits, stability, protection, etc., is
greatly affected by the condition of the neutral.
A three phase system can be operated in two possible ways
• With ungrounded neutral
• With a grounded neutral
19. Ungrounded Neutral System
• In an ungrounded neutral system, the neutral is not
connected to the ground.
• In other words, the neutral is isolated from the ground.
• Therefore, this system is also known the isolated neutral
system or free neutral system .
20. Grounded System
• In neutral grounding system, the neutral of
the system is connected to the ground.
• Because of the problems associated with
ungrounded neutral systems, the neutrals are
grounded in most of the high-voltage systems.
21. Advantages of neutral grounding
• Voltages of phases are limited to the line-to-ground
voltages.
• Surge voltage due to arcing grounds is eliminated.
• The overvoltages due to lightning discharged to
ground.
• It provides greater safety to personnel and
equipment.
• It provides improved service reliability.
22. Method Of Neutral Grounding
• The methods commonly used for grounding
the system neutral are
• Solid grounding (or effective grounding)
• Resistance Grounding
• Reactance Grounding
• Peterson-coil grounding (or resonant
groundings)
23. computation of fault currents
• Fault calculations are one of the most
common types of calculation carried out
during the design and analysis of electrical
systems.
• These calculations involve determining the
current flowing through circuit elements
during abnormal conditions – short circuits
and earth faults.
24. Types of Fault
• A fault is an abnormal or unintended connection of live elements of a system
to each other or to earth.
• The impedance of such connections are often very low, resulting in large
currents flowing.
• The energy contained in fault currents can quickly heat components, creates
excessive forces and can result in devastating explosions of equipment.
• There are three types of faults:
• Three Phase Faults
• Phase to Phase Faults
• Earth Faults
• Typically highest fault current is given by a three phase fault
26. Cont....
• In a three phase fault, all three phases (L1, L2 and
L3) are shorted together.
• To find the fault current at any point in the
network, a sum is made of the impedances in the
network between the source of supply (including
the source impedance) and the point at which the
fault is occurs.
• To find the fault current Ik, the nominal applied
voltage, U0 is divided by the summed impedance
Z.
27. Phase to Phase Faults
• Phase to Phase Fault In a power system (L1 to
L2 for example), two phases are connected
together.
• The fault current is again, the nominal applied
voltage divided by the summed impedance.
28. Earth Faults
• To find the value of earth fault current at any point in a
network, a sum is made of the earth fault impedances in the
network between the source of supply (including source
impedance) and the return path impedances.
• In an earth fault, one phase is directly connected to earth (L1
to earth for example).
29. What is a Circuit Breaker?
• A circuit breaker is defined as a switching device that can
be operated manually or automatically for controlling
and protecting an electrical power system.
• It consists of two main contacts: a fixed contact and a
moving contact.
• The contacts are normally closed and allow current to
flow through the circuit.
• When a fault occurs, such as a short circuit or an
overload, the contacts are separated by a mechanism
that releases stored potential energy.
• This mechanism can be spring-operated, pneumatic,
hydraulic, or magnetic.
30. How A Circuit Breakers Works
(Working Principle)
• Detection of Fault: The circuit breaker detects a fault
condition, such as an overload or a short circuit in the
electrical system.
• Activation of Mechanism: Once a fault is detected, the
circuit breaker activates a mechanism that triggers the
release of stored potential energy.
• Separation of Contacts: This energy then separates the
two main contacts of the circuit breaker: a fixed
contact and a moving contact.
• Arc Formation: The separation of contacts creates a
highly conductive and ionized plasma known as an arc.
31. Cont...
• Arc Quenching: The circuit breaker uses various
methods to extinguish or quench the arc, such as
cooling, compressing, or replacing the ionized medium
with fresh gas.
• Current Interruption: This process interrupts the flow
of current, stopping any damage from the fault
condition.
• Resetting: Once the fault is cleared, the circuit breaker
can be manually or automatically reset, allowing the
current to flow again.
• Re-establishment of Contacts: Resetting the breaker
brings the fixed and moving contacts back together, re-
establishing the flow of current through the circuit.
32. circuit breakers are classified by their
arc quenching medium
• Oil circuit breaker
• Air circuit breaker
• SF6 circuit breaker
• Vacuum circuit breaker
33. OCB – Oil Circuit Breaker –
Construction and Working Principle
• An Oil circuit breaker OCB is a type of circuit breaker that
uses insulating oil as a dielectric medium to quench the
arc and break the circuit safely.
• The oil used is insulating oil used usually transformer oil
that has better dielectric strength than air.
• The heat produced by the arc vaporizes the oil, producing
a hydrogen gas bubble surrounding the arc.
• The pressure of the oil compresses the gas bubble
increasing its dielectric strength which extinguishes the
arc during the zero-crossing.
34.
35. Construction of Oil Circuit Breaker
• The construction of this kind of circuit breaker is simple.
• It includes current-carrying contacts that are surrounded in a strong
and metal tank.
• Here the tank is loaded with transformer oil.
• The transformer oil works as an insulator & arc extinguishing
medium among the existing element & earth.
• At the peak of the transformer oil, the air can be filled within the
tank that acts as a pad to manage the transferred oil on the
creation of gas in the region of the arc.
• It absorbs the mechanical shock of the rising movement of oil.
36. Working Principle
• In normal operation of this circuit breaker, the contact in
the breaker will be closed as well as carries the current.
• Once the fault happens within the system, then the
contacts will move apart & an arc will be struck among
the contacts.
• Because of this arc, a huge amount of heat will be
released & high temperature can be achieved to vaporize
the nearby oil to gas.
• So this gas will be surrounded by the arc & its unstable
increase around it will move the oil violently.
37. Advantages
The advantages of the oil circuit breaker include the
following.
• It uses less amount of oil
• Dielectric strength of oil is high
• Oil in the breaker will absorb the energy of arc when
decomposing.
• Less space
• Risk of fire can be reduced
• Maintenance also reduced
38. Disadvantages
The disadvantages of the oil circuit breaker include the following.
• It uses less quantity of oil so that carbonization amount will be increased
• Removing of the gases within the contact space is difficult
• The dielectric strength will declines the oil quickly because of the high
quantity of carbonization.
• Arcing time is high
• Do not allow interruption with high-speed
• Controlling of arc interruption can be done based on the length of arc.
• It can form any volatile mixture through the air.
39. Air Circuit Breaker
• An Air Circuit Breaker (also known as an Air Blast Circuit Breaker or
ACB) is an automatically operated electrical switch that uses air to
protect an electrical circuit from damage caused by excess current
from an overload or short circuit.
• Its primary function is to interrupt current flow after a fault is
detected.
• When this happens, an arc will appear between the contacts that
have broken the circuit.
• Air circuit breakers use compressed air to blow out the arc, or
alternatively, the contacts are rapidly swung into a small sealed
chamber, the escaping of the displaced air, thus blowing out the
arc.
40. Working Principle of Air Circuit Breaker
• The working principle of this breaker is rather
different from those in any other types of circuit
breakers.
• The main aim of all kind of circuit breaker is to
prevent the reestablishment of arcing after current
zero by creating a situation where in the contact
gap will withstand the system recovery voltage.
• For interrupting arc it creates an arc voltage in
excess of the supply voltage. Arc voltage is defined
as the minimum voltage required maintaining the
arc.
42. Air Circuit Breaker Construction
• The external parts of ACB mainly include the
ON & OFF button, an indicator for the position
of the main contact, an indicator for the
mechanism of energy storage, LED indicators,
RST button, controller, rated nameplate,
handle for energy storage, displays, shake,
fault trip rest button, rocker repository, etc.
43. Air Circuit Breaker Working
• Air circuit breakers operate with their contacts in free air.
• Their method of arc quenching control is entirely different from that of oil circuit-breakers.
• They are always used for the low-voltage interruption and now tends to replace high-voltage
oil breakers.
• Air Circuit breakers generally have two pairs of contacts. The main pair of contacts (1) carries
the current at normal load and these contacts are made of copper metal.
• The second pair is the arcing contact (2) and is made of carbon. When the circuit breaker is
being opened, the main contacts open first. When the main contacts opened the arcing
contacts are still in touch with each other.
• As the current gets a parallel low resistive path through the arcing contact. During the
opening of the main contacts, there will not be any arcing in the main contact. The arcing is
only initiated when finally the arcing contacts are separated. Each of the arc contacts is fitted
with an arc runner which helps.
44. Applications of Air Circuit Breakers
• Air Circuit Breakers are used for controlling the power station auxiliaries and
industrial plants. They offer protection to industrial plants, electrical machines
like transformers, capacitors, and generators.
• They are mainly used for the protection of plants, where there are possibilities
of fire or explosion hazards.
• The air brake principle of the air breaker circuit arc is used in DC circuits and
AC circuits up to 12KV.
• The air circuit breakers have high resistance power that helps in increasing the
resistance of the arc by splitting, cooling, and lengthening.
• An air circuit breaker is also used in the Electricity sharing system and NGD
about 15kV
45. SF6 Circuit Breaker
• Sulphur Hexafluoride (SF6) Circuit Breaker
• A circuit breaker in which SF6 under pressure
gas is used to extinguish the arc is called SF6
circuit breaker.
• SF6 (sulphur hexafluoride) gas has excellent
dielectric, arc quenching, chemical and other
physical properties which have proved its
superiority over other arc quenching mediums
such as oil or air.
46. Properties of Sulphur hexafluoride
Circuit Breaker
• Sulphur hexafluoride possesses very good insulating and arc quenching
properties. These properties are
• It is colourless, odourless, non-toxic, and non-inflammable gas.
• SF6 gas is extremely stable and inert, and its density is five times that of air.
• It has high thermal conductivity better than that of air and assists in better
cooling current carrying parts.
• SF6 gas is strongly electronegative, which means the free electrons are easily
removed from discharge by the formation of negative ions.
• It has a unique property of fast recombination after the source energising
spark is removed. It is 100 times more effective as compared to arc quenching
medium.
47. Construction of SF6 Circuit Breakers
• SF6 circuit breakers mainly consist of two parts,
namely (a) the interrupter unit and (b) the gas
system.
• Interrupter Unit – This unit consists of moving and
fixed contacts comprising a set of current-carrying
parts and an arcing probe.
• It is connected to the SF6 gas reservoir. This unit
consists slide vents in the moving contacts which
permit the high-pressure gas into the main tank.
49. Gas System
• The closed circuit gas system is employed in SF6 circuit
breakers.
• The SF6 gas is costly, so it is reclaimed after each
operation.
• This unit consists low and high-pressure chambers with a
low-pressure alarm along with warning switches.
• When the pressure of the gas is very low due to which
the dielectric strength of gases decrease and an arc
quenching ability of the breakers is endangered, then this
system gives the warning alarm.
50. Working Principle of SF6 Circuit
Breaker
• In the normal operating conditions, the contacts of the breaker are
closed.
• When the fault occurs in the system, the contacts are pulled apart,
and an arc is struck between them.
• The displacement of the moving contacts is synchronised with the
valve which enters the high-pressure SF6 gas in the arc interrupting
chamber at a pressure of about 16kg/cm^2.
• 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.
51. Advantage of SF6 circuit breaker
• SF6 gas has excellent insulating, arc extinguishing and many other properties
which are the greatest advantages of SF6 circuit breakers.
• The gas is non-inflammable and chemically stable. Their decomposition
products are non-explosive and hence there is no risk of fire or explosion.
• Electric clearance is very much reduced because of the high dielectric strength
of SF6.
• Its performance is not affected due to variations in atmospheric condition.
• It gives noiseless operation, and there is no over voltage problem because the
arc is extinguished at natural current zero.
• There is no reduction in dielectric strength because no carbon particles are
formed during arcing.
• It requires less maintenance and no costly compressed air system is required.
52. Disadvantages of SF6 circuit breakers
• SF6 gas is suffocating to some extent. In the case of
leakage in the breaker tank, the SF6 gas being heavier
than air and hence SF6 are settled in the surroundings
and lead to the suffocation of the operating personnel.
• The entrance of moisture in the SF6 breaker tank is very
harmful to the breaker, and it causes several failures.
• The internal parts need cleaning during periodic
maintenance under clean and dry environment.
• The special facility requires for transportation and
maintenance of quality of gas.
53. Vacuum circuit breaker
• 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.
54. The vacuum circuit breaker has mainly
two phenomenal properties
• High insulating strength: In comparison to various other
insulating media used in circuit breaker vacuum is a
superior dielectric medium. It is better than all other
media except air and SF6, which are employed at high
pressure.
• When an arc is opened by moving apart the contacts in a
vacuum, an interruption occurs at the first current zero.
With the arc interruption, their dielectric strength
increases up to a rate of thousands time as compared to
other breakers.
55. Construction of Vacuum Circuit
Breaker
• It is very simple in construction as compared to any other circuit
breaker.
• Their construction is mainly divided into three parts, i.e., fixed
contacts, moving contact and arc shield which is placed inside the
arc interrupting chamber.
• The outer envelope of vacuum circuit breaker is made up of glass
because the glass envelope help in the examination of the breaker
from outside after the operation.
• The fixed and moving contacts of the breaker are placed inside the
arc shield.
57. Working 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 vapour of positive ions which is discharged
from the contact material.
• The density of vapour depends on the current in
the arcing.
58. Advantages of Vacuum Circuit Breaker
• Vacuum circuit breaker does not require any additional
filling of oil or gas. They do not need periodic refilling.
• Rapid recovery of high dielectric strength on current
interruptions that only a half cycle or less arcing occurs
after proper contact separation.
• Breaker unit is compact and self-contained. It can be
installed in any required orientation.
• Because of the above reasons together with the
economic advantage offered, vacuum circuit breaker has
high acceptance.
59. Disadvantage of Vaccum Circuit
Breaker
• Requirements of high technology for
production of vacuum interrupters.
• It needs additional surge suppressors for the
interruption of low magnetizing currents in a
certain range.
• Loss of vacuum due to transit damage or
failure makes the entire interrupter useless,
and it cannot be repaired on site.
61. Cont....
• The sequence impedance network is defined as a balance
equivalent network for the balance power system under an
imagined working condition so that only single sequence
component of voltage and current is present in the system.
• The symmetrical components are useful for computing the
unsymmetrical fault at different points of a power system
network.
• The positive sequence network determines the load flow studies
in power system.
