Specification of CTs:

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• Specification of CTs
• CTs are defined in terms of rated burden,
  accuracy class and accuracy limit.
• Saturated values of rated burden are
• 2.5, 5, 7.5, 10, 15 30 VA
• Saturated accuracy limit factors are
• 5,10,15, 20 30
• Two accuracy classes (5 10 ) are quoted 5P
  10P which give a composite error at rated
  accuracy limit. The method of describing at CT is
  as following.
• 15 VA class 5P20
• It means the burden is 15 and will not have more
  than 5 error at 20 times its rated current.
• Design requirements of CT are specified in
  terms of knee point voltage, magnetizing current
  at knee point and secondary resistance. There are
  known in general as "class x" CTs.

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Application In specifying CT's, the variation
of impedance over the range of setting any relays
should be taken into consideration .

• Example
• The normal burden of an over current relay is VA
  at setting, where the normal setting range of the
  relay is 50 to 200 of nominal current. There
  for a 1 A relay set to 50 would have
• Current setting 501 0.5 A
• Voltage across the coil 6V
• Relay impedance 12 ?
• At 200
• Current setting 2001 2 A
• Voltage across the coil 1.5V
• Relay impedance 0.75 ?
• If the characteristic of the relay should be
  maintained up to 20 times the relay setting, then
  the Vkp not less than 20 6 V120 V for a 50
  setting
• Or 20 1.5V 30 V for a
  200 setting.

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• However, the relay operating at 20 times its
  setting will have saturated magnetically and
  therefore the impedance will be reduced. Hence,
  in the case of the lowest setting core must be
  taken when specifying.
• The impedance at setting, which means that Vkp
  60 would be satisfactory because the relay would
  have saturated before times. For an earth fault
  relay having minimum setting of 20 would have
  voltage at setting of

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Voltage transformers VTsOne requirement should
be used with protection, which is that the
secondary voltage must be accurate representation
of the primary voltage in both magnitude and
phase. To meet this requirement, they are
designed to operate at low flux densities so that
Ie , ration and phase angle errors are small.
This larger than that of power transformer, which
increase the overall size of the unit. The
nominal secondary voltage is sometimes 110V (line
voltage).
Accuracy VTs usually have range of voltage from
80 to 120 and range of burden from 25 to 100
. In protection when voltage suppressed,
accuracy measurement may be important during
fault condition.
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ProtectionThe primary side of VTs are usually
protected by HRC fuses and fuses or miniature
circuit breaker on the secondary side

• Residual Connection
• During earth fault of any one of the three
  phases, it is not possible to derive a voltage in
  the conventional manner. Therefore, the residual
  (broken delta) connection as shown in figure
  (3.5) must be used. Under the three-phase
  balanced conditions the three voltage sum to
  zero. If one voltage is absent during fault
  condition, then the difference in voltages
  between the phases will be delivered to the relay

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• Capacitor VTs
• The cost of the electromagnetic VTs at voltages
  of 132 kV or more is very high. Thus, the
  capacitor VTs is proposed to be the more
  economical equipment. It is virtually a
  capacitance voltage divide with a tuning
  inductance and an auxiliary transformer as shown
  in figure (3.6).

Fig. 3.6 Capacitor voltage Transformer transformer
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Power system protectionText protective Relay
by J.Lewis Blackburn.

• General philosophies
• What is a relay?
• (IEEE) define a relay as an electric device that
  is designed to interpret input condition in a
  prescribed manner and after specified condition
  are met to respond to cause contact operation
  .Relay are utilized in all as pacts of activity,
  the home ,communication , industry..etc.
• A protective relay is defined as a relay whose
  function is to detect defective line or apparatus
  or other power system condition of an abnormal or
  dangerous nature and to initiate appropriate
  control circuit condition. Fuse are also used in
  protection and define as an over current
  protective device with in a circuit opening
  fusible part that is heated and severed by the
  passage of the over current thought it.

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• A primary objective of all power system is to
  maintain a very high level of condition of
  service, and to minimize the outage times when
  intolerable conditions occur. Loss of power, dip
  of voltage and over voltage will occur due to
  consequences of natural events, physical
  accident, equipment failure a disoperation by
  human error.
• Protection is the science, skill, and art of
  applying and setting and / or fuses to provide
  maximum sensitivity to fault and undesirable
  condition.
• ypical power circuit breaker
• Protective relays provide the "brains" to same
  trouble ,but as low energy device they are able
  to open and isolate the problem area of the power
  system . CBs and varions types of circuit
  interrupters are used to provide the "muscle" for
  fault isolation .
• Thus protective relays and interrupting devices
  are "team" . protective relays without CBs have
  no basic value except for alarm. On the other
  hand , CBs without protective relays are only
  energized or de energized manually. Different
  type of voltages of typical CBs are shown in fig
  (1-7 1-8).

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• Typical relay CB connection
• Usually protective relays are connected to
  power system through CT and/or VT. The circuit
  can be represented by a typical "one-line'" ac
  schematic and dc trip circuit schematic as shown
  in fig (1-9) . in normal operation and when
  CB(52) is closed , it is contact closes to
  energize the CB trip coil 52T, which function to
  open breaker main contact and de energize the
  connected circuit. The relay contacts are not
  designed to interrupt the CB trip coil current so
  an auxiliary relay is used to "seal in" or by
  pass the protective relay. Then 52a will open to
  de energize the breaker coil.

Fig-1.9 Typical single line ac connection of a
protective relay with its de trip schematic
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• Basic objectives of system protection
• Protection does not mean prevention, but
  minimizing the duration of the trouble, the five
  basic objectives are
• i) Reliability assurance that the protection
  will perform correctly.
• ii) Selectivity maximum continuity of service
  with minimum system disconnection.
• iii) Speed of operation minimum fault duration
  and consequent equipment damage.
• iv) Simplicity minimum protective equipment and
  associated circuitry to achieve the protection
  objectives.
• v) Economics maximum protection at minimum total
  cost.

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• Classification of Relays
• Classification can be done by different ways,
  such as by function, input, performance
  characteristics an operating be divided into five
  types
• i) Protective Relays
• Protective relays and fuses operate on the
  intolerable power system conditions. They are
  applied to all parts of the power system
  generates, buses, TFs, TLs, distribution lines
  and feeds, motors, loads, capacitors banks and
  reactors. Fuses are usually used for low voltage
  level (480 V).
• ii) Regulating Relays
• Regulating relays are associated with tap
  changer of TFs, on governor of generating
  equipment to control the voltages level with
  varying load (used during normal conditions).

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• iii)Reclosing, synchronism check, synchronizing
  relays
• Relays of this type are used in energizing or
  restoring lines to service after an outage and in
  interconnecting pre-energizing parts of the
  systems.
• iv) Monitoring Relays
• Relays of this type are used in energizing or
  restoring lines to service after an outage and in
  interconnecting pre-energizing parts of the
  systems.
• Ivv) Auxiliary Relay
• There are two categories contact
  multiplication (repeat contactors) and circuit
  isolation.

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Other relay classification Protective relays
classified by input are known as current,
voltage, power, frequency and temperature relays.
Those classified by operating principles are
electromechanical, solid state etc. those
classified by performance are distance,
reactance, over current .etc.
Induction Relays
Torque id produced by applying two alternating
fields to a moveable disk, which are displaced in
space and time.
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The applied torque would accelerate the disc to a
speed limited only by friction and windage
control can be done by two ways 1- By
permanent magnet whose field passes through the
disk and produces a breaking force , which
control the time characteristic of the relay . 2-
By control spring which produces a torque
proportional to disc angular disc placement.
Which is an inverse time characteristic
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Over current protection
The standard relay characteristic t 3(log
M)-1 (3)/(log M) where, M multiple of setting.
At twice setting current, operation time 10
sec. And at 10 the setting current, operation
time 3 sec. As shown in fig. (5.1).
Fig.5.1 Characteristic Curve. Inverse-Minimum
Time Relay
The seven plug bridge positions would be marked
7 6 5 4 3 2 1
200 175 150 125 100 75 50
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Grading Principles (GEC text)
Relay coordination can be achieved either by
time or over current or a combination of both
time and over current in order to chive correct
discrimination .
i Discrimination by time
This method uses time intervals to give the relay
nearest to the fault to operate first. Figure
(9.1) Shows that the radial feeders have CBs at
the in feed end of each section, where each
protection comprises a definite time delay over
current relay in which the operation of current
sensitive element initiates the time delay
element.
Figure (9.1)
Discrimination by current Fault currents
varies with the fault positions due to the
difference in impedances relays are set at a
tapered values such that only the relay nearest
to the fault trips its CB, Fig (9.2).
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iii- Discrimination by Both Time and Current-
Because of the limitation imposed by two previous
methods, a time / current characteristic has
evolved . The following example illustrates this
method of coordination clearly .
Graphs
Fig.5.3 Time/Current Curves. IDMT Relay
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Fig.1.1 Distance protection, (a) Schematic. (b)
Time/Distance graph.
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Fig.1.2 Distance Relay Circuit (Moving Coil
Relay)
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