Transcript chapter2_circuit_breakers_jan_2014

2. Circuit Breakers and Recloser
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Dr.Latif Shaikh
Course Outline
• 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
• 4/8/2015
Fuse application
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Circuit Breaker
•
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
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Operating Principle
• Circuit Breaker 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
energized and the moving contacts are pulled apart by some mechanism
,thus opening the circuit.
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Simplified Diagram of Circuit Breaker Control
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Electric Arc
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Electric Arc
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.
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ARC in AC and DC circuits
• 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.
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Principles of Arc extinction
• 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)
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Important terms
 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
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Restriking Voltage
• 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.
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Current Chopping
• 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.
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Current Chopping
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Resistance Switching
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Resistance Switching
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.
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Circuit Breaker Rating
 Breaking capacity – It is the current (r.m.s.) that a Circuit Breaker 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.
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IEEE Standards for Selection of Circuit Breakers
Step 1.Calculate highest value of initial RMS current considering symmetrical
fault. The current can be obtained by sub -transient reactance of synchronous
generators and transient reactance of synchronous motors and induction
motors are neglected.
Following Multiplying factors are applied to take into account dc components
and decrement of dc components in current. If short circuit KVA exceeds
5000,000 ,then add 0.1 to the given factors
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8 cycles or slow breaker
1.0
5 cycle breaker
1.1
3 cycle breaker
1.2
2 cycle breaker
1.4
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IEEE Standards for Selection of Circuit Breakers
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1) CB rated 1500A,1000MVA,33kV,3sec,3phase oil CB.Find a)rated
normal current b) breaking capacity c)rated symmetrical current
d)Rated making current e) short time rating f)rated service voltage
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Problem -A generator connected through a 5 cycle circuit breaker to a
transformer is rated 8000KVAand 13.8kV with the reactance of
Xd”=10%,Xd’=16%,and Xd=100% .It is operating at no load and rated voltage
when 3 phase short circuit occurs between breaker and transformer. Find
1.Sustained short circuit current in the breaker
2.The initial symmetrical rms current in the breaker
3.Maximum possible dc component in the breaker
4.Current to be interrupted by the breaker
5.The interrupting KVA
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Requirements of Circuit Breaker
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Classification of Circuit Breakers
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Air 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.
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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 . So
commonly used in Industrial Switchgears . Auxiliary switchgear Generating
Stations
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Air Blast Circuit Breakers
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.
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Air Blast Circuit Breaker(Radial Flow)
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Advantages and Disadvantages
Advantages
 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
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Disadvantages
 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
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Air Blast Circuit Breakers (Axial Flow)
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Axial Blast ABCB
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.
Uses
1.Arc Furnaces
2.Traction Syetems
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Modification of Air Blast Circuit Breakers
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Advantages of Air Blast Circuit Breaker
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Disadvantages of Air Blast Circuit Breakers
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Applications of Air Blast Circuit Breakers
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Vacuum Circuit Breakers
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.
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Vacuum Circuit Breaker
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Vacuum Circuit Breakers
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.
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Working of Vacuum Circuit Breaker
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.
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Vacuum Circuit Breakers
Advantages
 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
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Disadvantages
 Vacuum has to be
maintained at desired level
always
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Oil Circuit Breakers
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Oil Circuit Breakers
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.
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Oil Circuit Breaker (Working)
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 rthe
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.
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Bulk Oil Circuit Breaker
Advantages
• 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
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Disadvantages
• Long arcing time
• Do not permit high speed of
interruption
• Arc interruption control can
be obtained only by
increasing the length of arc
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Low Oil circuit Breakers
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.
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Minimum Oil Circuit Breakers
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.
Applications –Minimum oil circuit breakers are available in for all voltages and
highest breaking capacity hence they are preferred in almost all protection
schemes
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Maintenance of Oil Circuit Breakers
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Minimum Oil Circuit Breaker
Advantages




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
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Disadvantages
 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.
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SF6 circuit Breaker
(Properties of SF6 gas)
 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
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Sulfur Hexafluoride Circuit Breaker
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Operation of SF6 Circuit Breaker
• 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
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Advantages and Disadvantages of SF6 Breaker
Advantages
 Silent operation, compact
size
 Vary short arcing time
 No risk of fire
 No reduction in dielectric
strength due to operation
 No current chopping problem
 Can interrupt larger currents
 Suitable for explosive
environment due to totally
enclosed body
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Disadvantages
 Costly
 Requires conditioning of SF6 gas
from time to time
 SF6 gas is suffocating ,so its
leakage can cause suffocation of
the persons in surrounding
areas.
 Special facilities are required for
transporting gas
 Additional equipments are
required for reconditioning
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Isolators
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Isolators
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 operation
While opening –Open circuit breaker first and then isolators
While closing –Close isolators first and then close circuit breakers
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Circuit Breaker Controls
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.
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Automatic Reclosing
Many faults (80-90%) in the overhead distribution system like flash over of
insulators, crow faults, temporary tree contacts , etc are temporary in nature.
Thus, taking a feeder or line permanent outage may lead to unnecessary long
loss of service to customers. Hence, many utilities use fast automatic reclosers
for an overhead radial feeder without synchronous machines or with
minimum induction motor load. Presence of synchronous machines will
require additional problem of synchro-check to be addressed. The almost
universal practice is to use three and occasionally four attempts to restore
service before lock out .
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 Subsequently, energization is by manual intervention. The initial reclosure can be
high speed (0.2 - 0.5sec) or delayed for 3 - 5 seconds. This allows for de-ionization
time for fault arc. If the temporary fault is cleared, then the service is restored.
Otherwise, the relay again trips the feeder. Then one or two additional time
delayed reclosures are programmed on the reclosing relay. Typical schedule might
be instantaneous, followed by 30sec, or 35sec, followed by 15sec. If the circuit still
continues to trip, the fault is declared as permanent and the recloser is locked out.
Reclosers use three phase and single phase oil or vacuum circuit breakers for
overhead distribution lines.
 With underground network, faults tend to be more often permanent and
reclosers are not recommended. In case of large synchronous motors, distributed
generators or induction motor loads, it is recommended that sufficient time is
allowed for underfrequency relays to trip these sources of back emf out-of-thecircuit.
•
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Reclosures
Application of reclosers in distribution systems requires selection of its ratings
such as minimum trip current, continuous current, symmetrical interrupting
current etc.
For a single phase system, single phase reclosers can be used whereas for a
three phase system, one three phase recloser or three single phase reclosers
can be used. Reclosers have to be selected by considering the following
factors.
Voltage Rating.
Continuous current Rating : This is the maximum load current the recloser
has to carry.
Maximum Symmetrical Interrupting Rating: The maximum symmetrical fault
current should not exceed this rating.
Minimum Tripping current : This is the minimum fault current that a recloser
will clear. It is equal to two times the continuous current rating. Usually
tolerance is ±10%. This decides the sensitivity of the recloser
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Example on Reclosure Selection
• Consider a three phase distribution system with a single phase tap as
shown in fig . Maximum load on this single phase tap is 40A and that on
three phase line is 200A. Fault currents at F1,F2, F3 and F4 are also shown
in the fig. Table shows the available standard rating of single phase and
three phase reclosers. Select the ratings of reclosers at B.
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IEEE Standard Table for Reclosure Selection
Table (Source : IEEE Tutorial Course 80EH0157 - 8 - PWR)
Current Rating (Amperes)
Rated Maximum Voltage kV
rms
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Continous Current Rating
Symmetrical Interrupting Rating
at Maximum Volts
15.0
50
1250
15.5
100
2000
15.5
280
4000
15.5
560
8000
27.0
100
2500
27.0
280
4000
38.0
560
8000
15.0
50
1250
15.5
100
2000
15.5
280
4000
15.5
400
4000
15.5
560
8000
15.5
560
16000
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Answer
Recloser at B
Maximum load current on this single phase line = 40A.
Continuous current rating of this recloser must be 1.25 - 1.5 times the maximum load
current to account for anticipated load growth.
i.e. Continuous current rating of this recloser at B = 40 × 1.5 = 60A.
From the table 1, any recloser with continuous current rating of 100A and above is
acceptable.
Maximum fault current at B = 1750A.
Interrupting current rating must be greater than 1750A. From the table 1, we see that
recloser with 100A continous current rating has 2000A symmetrical rms short circuit
current rating. Hence, we can choose this recloser.
Minimum tripping current = Continuous current rating × 2 ± 10% tolerance
= 100 × 2 ± 10% of 200 = 220A
Since the minimum trip current 220A is less than the minimum fault current 250A at
the line end, it can protect the entire line.
Voltage rating of the line is 11kV. So we can select the maximum voltage rating of
15.5kV (from the table).
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Sectionalizers
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Sectionalizers
Sectionalizers are used in the bus bars so that fault on any section of bus bar
will not cause complete shut down.
Advantages of sectionalizers
1.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.
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Fuses
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 elements
1.Low melting point
2.Low ohmic losses
3.High conductivity
4.Free from deterioration due to
oxidation
5.Low cost
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Thermal Characteristic of Fuse
•
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As the magnitude of the current
increases, melting time reduces. It
should be obvious that larger
magnitude currents will lead to higher
power dissipation (I2R) in the fuse and
hence faster rise in temperature of the
element. This would imply that melting
time of the fuse should be inversely
proportional to magnitude of square of
current. The relationship between the
magnitude of the current that causes
melting and the time needed for it to
melt is given by the fuse's melting time
current characteristics (TCC). To cover a
wide range of currents and operating
time, TCC is plotted on a log-log paper.
Current Voltage Time relationship of
Non Current Limiting Fuse (Expulsion
Fuse)
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Current Voltage Time relationship of
Current Limiting Fuse
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Expulsion Fuse
• The expulsion type fuse is used where expulsion gases cause no problem
such as in overhead circuits and equipment. These fuses can be termed as
current awaiting types; and the function of interrupting medium is similar
to that of an ac circuit breaker. The temperature of arc is of the order of
4000-5000K. At this temperature special materials located in close
proximity to fuse element rapidly create gases. Preferred gas generating
materials are fiber, melamine, boric acid and liquids such as oil or carbon
tetrachloride. These gases help to create a high pressure turbulent
medium surrounding the arc, thus when the current does reach to zero
and the arc channel reduces to a minimum; the ablated gases rapidly mix
with remaining ionized gas and thereby deionize them as well as remove
them from ‘arc area'. In turn, this leads to rapid build up of dielectric
strength that can withstand the transient recovery voltage (TRV) and
steady state power system voltage.
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Vacuum Fuses
• Vacuum fuse is a non expulsive fuse but still a current zero awaiting type.
The design, operation and current-voltage-time relationship of this fuse
closely matches with that of an expulsion fuse. The main difference is that
it is a completely sealed unit and no expulsion action. Interruption occurs
because of rapid dielectric build up that occur in a vacuum after current
zero is reached
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Current Limiting Fuse
•
Basically, the current limiting fuses
attempt to constrict the arc and it is
cooled by sand.
A typical current limiting fuse is shown
in fig . In this case, the fusible element
is very long. The element is completely
surrounded with filler material,
typically silica sand, to contain the arc
as well as maintain a very high
pressure in the long restricted arc area
caused by the practically simultaneous
melting of the full length of element.
This then allows the fuse to produce a
very high resistance in the circuit in a
very short period of time (typically
hundreds of µsec).
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