2. Circuit Breakers and Recloser

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2. Circuit Breakers and Recloser
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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
• 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

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
Xd10,Xd16,and Xd100 .It is operating at
no load and rated voltage when 3 phase short
circuit occurs between breaker and transformer.
Find1.Sustained short circuit current in the
breaker2.The initial symmetrical rms current in
the breaker3.Maximum possible dc component in
the breaker4.Current to be interrupted by the
breaker5.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

• Disadvantages

• 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

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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

• Disadvantages

• 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

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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

• Disadvantages

• 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

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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

• Disadvantages

• 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.

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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

• 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-the-circuit.

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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
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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

• 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.

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• Current Voltage Time relationship of Non Current
  Limiting Fuse (Expulsion Fuse)

• 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).