Latest / Silent Sentinels- Protective Relays for A-C and D-C Systems / 004 - Alternating-Current Systems part 1

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004 - Alternating-Current Systems part 1 · Silent Sentinels- Protective Relays for A-C and D-C Systems

• 0:00
  Speaker 1: Chapter three, Part one of Silent Sentinels by Westinghouse Electric
• 0:06
  Speaker 1: and Manufacturing Company. This LibriVox recording is in the public domain.
• 0:13
  Speaker 1: Alternating current Systems, Section one General theories of line troubles.
• 0:22
  Speaker 1: Alternating current systems suffer from more severe short circuits than
• 0:25
  Speaker 1: do direct current systems, primarily because of their better inherent
• 0:29
  Speaker 1: regulation and often because of the greater power concentration. Modern
• 0:33
  Speaker 1: polyphase systems have become so large and complicated that they
• 0:37
  Speaker 1: present many difficult problems due to the various unbalanced faults
• 0:40
  Speaker 1: which can occur. Alternating current transmission systems may be divided
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  Speaker 1: into two general classes, first those having the neutral insulated
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  Speaker 1: from ground, second those having the neutral connected to ground metallically.
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  Speaker 1: The second classification may be further subdivided into four groups,
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  Speaker 1: known respectively as solidly grounded neutral systems, low resistance grounded
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  Speaker 1: neutral systems, medium resistance grounded neutral systems, and high resistance
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  Speaker 1: grounded neutral systems. The solidly grounded neutral system is one
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  Speaker 1: having its connected metallically to ground without any limiting resistance
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  Speaker 1: in the circuit. The terms low resistance, medium resistance, and
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  Speaker 1: high resistance refer to the relative values of limiting resistance
• 1:27
  Speaker 1: inserted in the neutral connection between the neutral point of
• 1:30
  Speaker 1: the system and ground, and are dependent on the neutral
• 1:33
  Speaker 1: voltage of the system under consideration. As a general rule,
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  Speaker 1: a system may be considered as low resistance grounded neutral
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  Speaker 1: if the grounding resistor is of such a value as
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  Speaker 1: to produce in any circuit for a fault to ground
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  Speaker 1: a current which, under minimum generating capacity conditions is at
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  Speaker 1: least four times the line relay current setting. A medium
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  Speaker 1: resistance grounded neutral system has a grounding resistor of such
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  Speaker 1: a value as to limit the current for a fault
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  Speaker 1: to ground to less than four times the current setting
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  Speaker 1: as to limit the current for a fault to ground
• 2:05
  Speaker 1: to less than four times the current setting for the
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  Speaker 1: line relays of that circuit. This fault current will, still, however,
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  Speaker 1: be greater than the maximum load current over the feeder,
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  Speaker 1: even under abnormal operating conditions. A high resistance grounded neutral
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  Speaker 1: system has its grounding resistor of such a value that
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  Speaker 1: the current for a fault to ground is on the
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  Speaker 1: order of full load current of the grounding transformer. Bank
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  Speaker 1: or less. Protection for faults from wires to ground may
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  Speaker 1: or may not be possible, depending on certain local conditions,
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  Speaker 1: which will be discussed under methods of clearing ground faults
• 2:41
  Speaker 1: Solidly grounded systems. Systems having their neutrals solidly grounded are
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  Speaker 1: subject to short circuits between one wire and ground whenever
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  Speaker 1: that wire becomes accidentally grounded. They can also suffer from
• 2:54
  Speaker 1: short circuits between only two wires of the system, as
• 2:57
  Speaker 1: well as between two wires and ground. In addition to
• 3:01
  Speaker 1: these unbalanced short circuits, such a system can suffer from
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  Speaker 1: a balanced three phase short circuit. In considering a system
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  Speaker 1: which has its neutral grounded, it is customary to assume
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  Speaker 1: that the ground resistance between the neutral and the fault
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  Speaker 1: is comparatively low, so that it does not decrease the
• 3:16
  Speaker 1: value of the short circuit current by any great amount. However,
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  Speaker 1: in some localities the ground resistance may be high, but
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  Speaker 1: such conditions are not of common occurrence. In calculating the
• 3:27
  Speaker 1: quantity of ground current likely to flow, it is important
• 3:30
  Speaker 1: to bear in mind that the impedance of the circuit
• 3:32
  Speaker 1: is high because of the great separation between the line
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  Speaker 1: wire and the path through the ground unngrounded systems. On
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  Speaker 1: systems having an isolated neutral, the three wire short circuit
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  Speaker 1: and two wire short circuit are the same as on
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  Speaker 1: grounded systems, but an accidental ground does not produce a
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  Speaker 1: short circuit as it does in the solid the grounded system.
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  Speaker 1: If the system is small, an accidental ground will make
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  Speaker 1: little disturbance, or at least it will not cause any
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  Speaker 1: abnormal value of current to flow. But if the system
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  Speaker 1: is large in extent so that the electrostatic capacity between
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  Speaker 1: conductors and ground is high, a noticeable charging current will
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  Speaker 1: flow into the fault. On a large system, there is
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  Speaker 1: always a charging current flowing in the conductors, but such
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  Speaker 1: a charging current is normally balanced and is distributed throughout
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  Speaker 1: the network. When an accidental ground occurs on one wire,
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  Speaker 1: the potential between each of the other two conductors and
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  Speaker 1: ground throughout the system is increased beyond its normal value
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  Speaker 1: by the factor one point seventy three, and these wires
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  Speaker 1: therefore require a greater charging current than that which they
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  Speaker 1: normally carry. As shown in Fig. Ten, each of these
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  Speaker 1: wires represents the plate of a condenser, the ground being
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  Speaker 1: the other plate, and the wire leading to the fault
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  Speaker 1: represents the connection between the source of potential and the
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  Speaker 1: ground plate. Therefore, this grounded wire must carry all the
• 4:51
  Speaker 1: charging current for the two condensers. The value of this
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  Speaker 1: current is difficult to calculate accurately, but an approximate value
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  Speaker 1: can be very easily obtained, and it is about twice
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  Speaker 1: the normal charging current of the system. The normal charging
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  Speaker 1: current can be calculated from tables which take into account
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  Speaker 1: the size and spacing of the conductors and the frequency
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  Speaker 1: of the circuit. Reason for grounding through resistance. If an
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  Speaker 1: unngrounded system is small with a consequently small charging current,
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  Speaker 1: it is possible to operate the system when one of
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  Speaker 1: the conductors is accidentally grounded, so that the necessity for
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  Speaker 1: cutting out the disabled section is not urgent. Furthermore, most
• 5:30
  Speaker 1: grounds are usually of a temporary nature, such as a
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  Speaker 1: flashover on an insulator, and they will clear themselves without damage.
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  Speaker 1: If the system is large, these disturbances will become more frequent,
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  Speaker 1: and the charging current will increase due to the greater
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  Speaker 1: electrostatic capacity of the system, to such a value that
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  Speaker 1: an arc over the insulator will not break, but will
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  Speaker 1: continue until serious damage has been done and until the
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  Speaker 1: line probably is burned down. Therefore, on large systems it
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  Speaker 1: is necessary to provide some means of cutting out a
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  Speaker 1: disabled section, and this usually requires some method of grounding
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  Speaker 1: the neutral grounding is also necessary in order to relieve
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  Speaker 1: strains on insulators, because, as has been shown, when one
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  Speaker 1: line is accidentally grounded in an unngrounded system, the voltage
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  Speaker 1: between the other two lines and the ground is increased
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  Speaker 1: one point seventy three times. The solidly grounded neutral has
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  Speaker 1: been considered, and it was shown that an accidental ground
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  Speaker 1: on a conductor results in a short circuit. This causes
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  Speaker 1: considerable disturbance, which is frequently so severe that some of
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  Speaker 1: the load, particularly synchronous motors, will be forced to drop
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  Speaker 1: off the system before the fault can be cleared. This
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  Speaker 1: disadvantage may be overcome by inserting sufficient resistance in the
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  Speaker 1: neutral connection to limit the current to a small value
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  Speaker 1: so that an accidental ground will not seriously disturb the load.
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  Speaker 1: Sometimes this resistance is so low that the ground current
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  Speaker 1: approaches a short circuit in magnitude and can therefore be
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  Speaker 1: handled by ordinary relays intended for short circuit protection. It
• 7:00
  Speaker 1: is becoming a common practice to use a resistance of
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  Speaker 1: such magnitude that the ground current is kept below the
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  Speaker 1: full load value of the current in any of the feeders.
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  Speaker 1: This is particularly useful in cable systems because cable faults
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  Speaker 1: usually start between one conductor and the grounded lead sheath,
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  Speaker 1: and as the ground current is held to a small value,
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  Speaker 1: it is possible by means of suitable relays to disconnect
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  Speaker 1: the faulty cable before the trouble is developed into a
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  Speaker 1: short circuit with a consequent interruption to the nearby load.
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  Speaker 1: Nature of open wire short circuits, the nature of a
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  Speaker 1: short circuit and its effect upon relay operation have been
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  Speaker 1: determined both by experiment and years of experience on overhead lines.
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  Speaker 1: Short circuits are usually arcs, and the resistance across them
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  Speaker 1: is small and of a fairly stable value. Short circuits
• 7:47
  Speaker 1: of this nature sometimes cause extensive trouble wherever there are
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  Speaker 1: several transmission lines on the same structure, due to the
• 7:53
  Speaker 1: blowing of the arc from one circuit to the adjacent ones.
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  Speaker 1: Flashovers caused by lightning are a particular annoying because the
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  Speaker 1: lightning usually strikes on the windward side of the structure.
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  Speaker 1: The lightning coming with the storm, and the wind then
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  Speaker 1: may blow the arc across all the other circuits. For
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  Speaker 1: this reason, it is advantageous if possible, to disconnect the
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  Speaker 1: first line in trouble before the arc has had time
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  Speaker 1: to involve the neighboring lines, but this is always difficult
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  Speaker 1: and usually impossible to accomplish. Several cases are known where
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  Speaker 1: trouble has communicated itself in this manner to several parallel circuits,
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  Speaker 1: each of which was equipped with practically instantaneous relays, and
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  Speaker 1: yet the circuit breakers could not open fast enough to
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  Speaker 1: clear one line before the arc had blown across to
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  Speaker 1: the other lines. In checking some of these faults, the
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  Speaker 1: Weather Bureau records were consulted, and it was found that
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  Speaker 1: the average wind velocity at that station was about four
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  Speaker 1: miles per hour, which corresponds to approximately six feet per second.
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  Speaker 1: When it is considered that an arc has no inertia,
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  Speaker 1: it is easy to see why trouble on one circuit
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  Speaker 1: will involve the neighboring circuit, even on a quiet day.
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  Speaker 1: It sometimes happens that in testing lines, after an extensive interruption,
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  Speaker 1: the switchboard operator closes in the good lines first and
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  Speaker 1: then closes in the bad line, thus starting trouble which
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  Speaker 1: involves all the lines and trips them out again. Therefore,
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  Speaker 1: in testing circuits it is good policy to vary the
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  Speaker 1: sequence of closing the breakers after one sequence of operation
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  Speaker 1: is failed to hold. On systems having a large connected
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  Speaker 1: generating capacity, the length to which an arc can be
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  Speaker 1: drawn out by the wind is almost unbelievable. Arcs more
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  Speaker 1: than twenty feet long within a station have been seen,
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  Speaker 1: and authentic cases are recorded of arcs having a length
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  Speaker 1: of two hundred feet or more on high voltage lines.
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  Speaker 1: These long arcs usually occur on lines of sixty six
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  Speaker 1: thousand volts or above, but where towers are used to
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  Speaker 1: support the structure, it is surprising how seldom they jump
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  Speaker 1: from one circuit to another. This is probably because the
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  Speaker 1: grounded steel tower acts as a shield between the circuits
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  Speaker 1: at the point where the trouble starts, namely at the insulators.
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  Speaker 1: Nature of trouble in cables. Because trouble in cables usually
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  Speaker 1: occurs at the joint in manholes where there is danger
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  Speaker 1: of injuring adjacent cables, it is essential that the defective
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  Speaker 1: section be disconnected within a reasonable time. Tests have been
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  Speaker 1: made on an eleven thousand volt system which prove that
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  Speaker 1: the voltage across the arc between the two conductors amounts
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  Speaker 1: to two or three percent of normal voltage, the value
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  Speaker 1: being practically independent of the current. This is easily explained
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  Speaker 1: if we assume that the resistance of the arc decreases
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  Speaker 1: as the current through it is increased. These tests were
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  Speaker 1: made by sawing off the cable and allowing the arc
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  Speaker 1: to form between the conductors at its end. It is
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  Speaker 1: not only possible, but also probable that when the arc
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  Speaker 1: is combined in the cable, its resistance will increase as
• 10:48
  Speaker 1: the pressure increases due to the generated gases, and this
• 10:52
  Speaker 1: naturally will increase the voltage across the arc. This voltage
• 10:57
  Speaker 1: will ensure the correct operation of directional relays even when
• 11:00
  Speaker 1: a fault occurs close to the switching station. Most faults
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  Speaker 1: start as a ground between one conductor and the lead sheath,
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  Speaker 1: usually due to the entrance of moisture into the cable.
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  Speaker 1: By means of a suitable relay installation, it is often
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  Speaker 1: possible to disconnect any of these faulty cables before the
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  Speaker 1: arc has involved the other two conductors. One operating company
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  Speaker 1: is found by an analysis of defective sections of the
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  Speaker 1: cable that about fifty percent of the cable troubles are
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  Speaker 1: cleared without developing short circuits. Power factor of short circuits.
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  Speaker 1: It is the general impression that short circuits are of
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  Speaker 1: low power factor, but this is usually not so. It
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  Speaker 1: is true, of course, that the total reactance of the
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  Speaker 1: circuit involved in the short circuit is high compared to
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  Speaker 1: the resistance, but most of this reactance is in the
• 11:49
  Speaker 1: apparatus and is not usually of importance in relay work.
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  Speaker 1: We are interested usually in the power factor of the
• 11:57
  Speaker 1: line itself during times of short circuit, and this of
• 12:00
  Speaker 1: course depends on the relative resistance and reactants of the
• 12:04
  Speaker 1: line conductor, and is therefore independent of its length. High
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  Speaker 1: voltage sixty cycle overhead transmission lines sometimes have a power
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  Speaker 1: factor of less than fifty percent, but that is not
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  Speaker 1: the rule, and twenty five cycle lines in all cable
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  Speaker 1: circuits have a much higher power factor. It should be remembered, however,
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  Speaker 1: that the power factor of a short circuit or ground
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  Speaker 1: fault may vary widely according to conditions, and this fact
• 12:29
  Speaker 1: should always receive consideration when applying relays, especially those dependent
• 12:34
  Speaker 1: for their operation upon both voltage and current distortion of phases.
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  Speaker 1: In faults involving all three wires of a three phase system,
• 12:45
  Speaker 1: the only abnormal conditions which ordinarily will exist are the
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  Speaker 1: excess flow of current into the fault and the low
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  Speaker 1: voltage in its vicinity. But these present no difficulties in
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  Speaker 1: relay protection. Any other fault which may occur, such as
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  Speaker 1: a two wire short circuit or a ground on one
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  Speaker 1: or two wires three wire three phase system considered, will
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  Speaker 1: not only cause an abnormal current flow, but will also
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  Speaker 1: necessarily result in an unbalanced or distorted condition of phases
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  Speaker 1: on account of the abnormal current flow being in only
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  Speaker 1: one or two of the three lines. This distortion is
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  Speaker 1: greatest in the vicinity of the fault and gradually lessens
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  Speaker 1: as the distance from the fault increases. Exact conditions existing
• 13:26
  Speaker 1: after a short circuit or ground has occurred are hard
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  Speaker 1: to determine because so many conditions and characteristics are present
• 13:32
  Speaker 1: in every system, which vary widely in different systems. Some
• 13:37
  Speaker 1: few years ago, mister G. H. Gray and mister L. N.
• 13:40
  Speaker 1: Crichton made extensive tests respecting such conditions, using an artificial
• 13:45
  Speaker 1: transmission line for the purpose. Figures twelve, thirteen, fourteen, fifteen, sixteen,
• 13:51
  Speaker 1: and seventeen shows some of the results of these tests,
• 13:54
  Speaker 1: plotted vectorially. The values plotted were taken from the selograms
• 13:59
  Speaker 1: and thus re present actual conditions existing after the fault occurred.
• 14:04
  Speaker 1: Effect of phase distortion on relay operation phase distortion has
• 14:09
  Speaker 1: no effect on the operation of straight over current relays.
• 14:13
  Speaker 1: In the case of directional relays, however, where the correct
• 14:16
  Speaker 1: operation of the relay is dependent on the operation of
• 14:18
  Speaker 1: a watt meter element, phase distortion must be considered. A
• 14:22
  Speaker 1: true watt meter will reverse if the angle between the
• 14:25
  Speaker 1: current andly applied voltage becomes greater than ninety degrees. Therefore,
• 14:30
  Speaker 1: the problem in the application of directional relays is to
• 14:33
  Speaker 1: make the voltage in current phase relations such that the
• 14:35
  Speaker 1: worst conditions of phase distortion will never cause the angle
• 14:39
  Speaker 1: between the current and the voltage to exceed ninety degrees.
• 14:42
  Speaker 1: The vector diagrams Figures eighteen and nineteen show the conditions
• 14:46
  Speaker 1: which may exist. In Figure eighteen, the current and voltage
• 14:50
  Speaker 1: vectors of only one phase are shown. Ao represents the
• 14:54
  Speaker 1: voltage from A to neutral and isob one represents the
• 14:57
  Speaker 1: current in leg A with a slightly lagging power factor.
• 15:01
  Speaker 1: Figure nineteen shows a possible distortion of phases when a
• 15:04
  Speaker 1: short circuit occurs from line A to C. A short
• 15:08
  Speaker 1: circuit of unity power factor would mean a heavy current
• 15:10
  Speaker 1: in phase with voltage AC, but due to the reactants
• 15:14
  Speaker 1: of the circuit, the current would probably lag considerably more
• 15:17
  Speaker 1: and would take a direction as shown by the vector
• 15:19
  Speaker 1: ISoP two AO. The voltage to neutral changes its direction
• 15:24
  Speaker 1: only slightly, and as is evident, the angle between AO
• 15:28
  Speaker 1: and isob two may easily be greater than ninety degrees. Now,
• 15:33
  Speaker 1: if a directional relay with a wat meter winding has
• 15:35
  Speaker 1: its voltage coil connected to AO and its current coil
• 15:39
  Speaker 1: to phase A, it will tend to reverse as soon
• 15:41
  Speaker 1: as the angle OAI exceeds ninety degrees. It will thus
• 15:45
  Speaker 1: close its contacts and trip the circuit breaker with current
• 15:49
  Speaker 1: flowing in the normal direction opposite to that for which
• 15:52
  Speaker 1: the relay is supposed to trip. Actual tests have shown
• 15:56
  Speaker 1: that the angle OA ISoP two may be as great
• 15:59
  Speaker 1: as a hunder twenty degrees with a short circuit between
• 16:02
  Speaker 1: A and C. This will cause directional relays to close
• 16:05
  Speaker 1: their contacts on normal direction of power flow, when the
• 16:08
  Speaker 1: directional contacts should be held positively open. The most satisfactory
• 16:14
  Speaker 1: solution of this problem is to connect the potential coil
• 16:17
  Speaker 1: of the relay to a voltage which is thirty degrees
• 16:19
  Speaker 1: behind the current at one hundred percent power factor, referring
• 16:23
  Speaker 1: to Figures eighteen and nineteen. If the voltage were taken
• 16:26
  Speaker 1: from phase A to C, then under short circuit conditions,
• 16:30
  Speaker 1: the angle CAI always would be less than ninety degrees
• 16:34
  Speaker 1: and the directional relay contacts would never close on normal
• 16:37
  Speaker 1: direction of power flow phase relations with Delta Star connected transformers.
• 16:44
  Speaker 1: Where directional relays are to be applied to substations having
• 16:47
  Speaker 1: Delta Star connected power transformers, it is often desirable to
• 16:51
  Speaker 1: connect the voltage transformers on the low voltage side, while
• 16:54
  Speaker 1: it is necessary to have the current transformers connected in
• 16:57
  Speaker 1: the high voltage leads in order to get the true
• 17:00
  Speaker 1: current flowing. Special care is necessary in such cases in
• 17:04
  Speaker 1: order to secure the connections which will give the proper
• 17:07
  Speaker 1: phase relations between the voltage and current for the relay.
• 17:10
  Speaker 1: As already stated, in order to assure proper operation of
• 17:14
  Speaker 1: the relay with the phase distortion accompanying a short circuit
• 17:17
  Speaker 1: or ground, the relay current should lead the relay voltage
• 17:20
  Speaker 1: by thirty degrees under normal conditions with one hundred percent
• 17:24
  Speaker 1: power factor on the system. Figures twenty twenty one, twenty two,
• 17:29
  Speaker 1: and twenty three show the correct connections for such combinations
• 17:32
  Speaker 1: to be used in applying Type CR directional relays. The
• 17:36
  Speaker 1: vectors show the relative phase relations for each step of
• 17:38
  Speaker 1: the transformation. Section two Methods of clearing short circuits. The
• 17:46
  Speaker 1: simplest system of distribution is one having a single source
• 17:49
  Speaker 1: of power with a number of feeders leaving the generator
• 17:52
  Speaker 1: bus bar, each feeder in turn being subdivided into a
• 17:56
  Speaker 1: number of small feeders. This is termed the radial distributions system.
• 18:00
  Speaker 1: The protection of such a system against short circuit may
• 18:03
  Speaker 1: be secured by overcurrent protective devices. The smaller branches may
• 18:08
  Speaker 1: be disconnected automatically from the remainder of the system by
• 18:11
  Speaker 1: the blowing of a fuse or the operation of instantaneous
• 18:14
  Speaker 1: circuit breakers. The circuit breakers near the generator should be
• 18:18
  Speaker 1: equipped with definite time relays. A time interval between the
• 18:22
  Speaker 1: successive relays long enough to assure a reasonable margin of
• 18:25
  Speaker 1: safety above that required for the circuit breaker to operate
• 18:29
  Speaker 1: is allowed. Relays at the generator bus are given the
• 18:32
  Speaker 1: highest time settings. The settings of the other relays are
• 18:36
  Speaker 1: decreased as their distance away from the generator increases, until
• 18:39
  Speaker 1: the most remote relay has an instantaneous setting. Fig. Twenty
• 18:44
  Speaker 1: four shows a typical radial system. The smallest branches, such
• 18:48
  Speaker 1: as those leading out from substation C and D may
• 18:52
  Speaker 1: be sufficiently protected by fuses, while other remote feeders carrying
• 18:56
  Speaker 1: greater power may require a circuit breaker, which may be
• 18:59
  Speaker 1: equiped with an overcurrent trip coil, so that the breaker
• 19:02
  Speaker 1: will be opened almost instantly upon the occurrence of a
• 19:05
  Speaker 1: short circuit on the feeder as we proceed back towards
• 19:09
  Speaker 1: the generating station. A over current relays such as the
• 19:13
  Speaker 1: co relays, are necessary in conjunction with the circuit breakers
• 19:17
  Speaker 1: to give a definite time for tripping. Assuming that there
• 19:21
  Speaker 1: is a short circuit at M, the excess current will
• 19:23
  Speaker 1: flow through relays numbers one and four, but number four
• 19:26
  Speaker 1: will trip in one second, thus cutting off the short
• 19:29
  Speaker 1: circuited line before number one has had time to trip. Likewise,
• 19:34
  Speaker 1: with a faulted n both relay numbers two and five
• 19:38
  Speaker 1: will be affected, but number five will trip its breaker first.
• 19:42
  Speaker 1: It is common practice to allow approximately zero point five
• 19:46
  Speaker 1: second between the successive relays and two point zero seconds
• 19:49
  Speaker 1: as the maximum permissible setting. In addition to securing discrimination
• 19:55
  Speaker 1: on the part of the relays by means of a
• 19:56
  Speaker 1: definite time feature, it is also possible to discrimin eight
• 20:00
  Speaker 1: by the current setting, because trouble which occurs at the
• 20:03
  Speaker 1: far end of one of the branch lines will not
• 20:06
  Speaker 1: draw so heavy a current as if it were near
• 20:08
  Speaker 1: the generating station. Relays such as number seven and eight Fig.
• 20:13
  Speaker 1: Twenty four on the remote lines may therefore be given
• 20:17
  Speaker 1: a setting which will cause them to operate on a
• 20:19
  Speaker 1: much lower current than will the relays near the generator.
• 20:23
  Speaker 1: The Westinghouse COO relay is especially adapted for such installations
• 20:27
  Speaker 1: because it combines the inverse and definite minimum time characteristics,
• 20:31
  Speaker 1: and because it also may be set to operate on
• 20:33
  Speaker 1: various values of current. In radial systems in which the
• 20:38
  Speaker 1: number of branches becomes so great that it is difficult
• 20:41
  Speaker 1: to allow the necessary time setting between successive relays without
• 20:44
  Speaker 1: making the time setting on the relays at the generating
• 20:47
  Speaker 1: station extremely high, the CZ distance or impedance relay may
• 20:51
  Speaker 1: be used instead of the COEO relay. When this relay
• 20:55
  Speaker 1: is used, the relay nearest the fault operates the quickest,
• 20:58
  Speaker 1: thus making time settings unnecessary. This type of relay is
• 21:02
  Speaker 1: slightly more expensive than the COO and requires voltage connections
• 21:06
  Speaker 1: in addition to the current connections. The ring or a
• 21:11
  Speaker 1: loop system The ring or loop system as indicated by
• 21:15
  Speaker 1: the name is simply a continuous transmission line running through
• 21:19
  Speaker 1: a series of substations and finally terminating at its starting point.
• 21:23
  Speaker 1: This forms one of the best ways of securing uninterrupted
• 21:26
  Speaker 1: service with a minimum expense for feeders. The number of
• 21:30
  Speaker 1: substations which may be thus tied together depends upon the
• 21:33
  Speaker 1: geographical location and the relay protection that is desired. Where
• 21:37
  Speaker 1: the CEO and CR relays are used, the number of
• 21:40
  Speaker 1: stations is limited due to the fact that the required
• 21:43
  Speaker 1: time intervals necessary between successive relays add up to an
• 21:47
  Speaker 1: unsafe value. With the advent of the CZ relay, however,
• 21:51
  Speaker 1: this difficulty is largely removed on a simple loop system
• 21:56
  Speaker 1: having only one source of power. Directional over current relays
• 22:00
  Speaker 1: with definite time limit characteristics may be used for protection
• 22:03
  Speaker 1: against short circuits. The time setting of each successive relay
• 22:07
  Speaker 1: must be increased by an amount sufficient usually zero point
• 22:10
  Speaker 1: five second in practice, to allow time for the circuit
• 22:13
  Speaker 1: breaker and the preceding substation to open. The directional relays
• 22:17
  Speaker 1: applied at each substation are fixed so that they will
• 22:20
  Speaker 1: trip only when excess current is flowing away from the
• 22:22
  Speaker 1: substation bus. It is therefore evident that with successive time settings.
• 22:28
  Speaker 1: The relays at each end of any faulty section will
• 22:31
  Speaker 1: be the first to operate, serving to isolate this section
• 22:33
  Speaker 1: of the loop. Straight over current relays may be used
• 22:37
  Speaker 1: at the generating station or source of power, as any
• 22:40
  Speaker 1: fault on the system can cause power flow in only
• 22:42
  Speaker 1: one direction from this point. Fig. Twenty six shows a
• 22:45
  Speaker 1: typical loop system tying in four substations with the generating station.
• 22:50
  Speaker 1: It shows the application of directional overcurrent relays on each
• 22:53
  Speaker 1: side of the substations and straight over current relays at
• 22:56
  Speaker 1: the generating station. The arrows indicate the direction in which
• 23:01
  Speaker 1: the overcurrent must flow to trip the relay, and the
• 23:04
  Speaker 1: figures show the definite minimum time setting of the relay.
• 23:07
  Speaker 1: Considering a fault at M, current will flow to it
• 23:10
  Speaker 1: from substations B and C, the zero point six second
• 23:14
  Speaker 1: relay will trip out the substation C end, and the
• 23:17
  Speaker 1: one point six second relay at the substation B end.
• 23:20
  Speaker 1: It should be noted that the same fault current flows
• 23:23
  Speaker 1: through the one point one second relay at substation D
• 23:26
  Speaker 1: in the proper direction and the one point six second
• 23:29
  Speaker 1: relay at D. The relay at C, however, trips first,
• 23:33
  Speaker 1: clearing the fault. The same current flows through the zero
• 23:36
  Speaker 1: point six second relay at D and the zero point
• 23:38
  Speaker 1: one second relay at E, but in the wrong direction
• 23:41
  Speaker 1: to cause them to close their contacts. A fault at
• 23:44
  Speaker 1: any point may be similarly checked out. Where a loop
• 23:47
  Speaker 1: system has more than one source of power, the application
• 23:51
  Speaker 1: of relays become somewhat more complicated. The same type of
• 23:54
  Speaker 1: relay may be applied with the same system of successive
• 23:57
  Speaker 1: time settings. One difficulty that is encountered in such a
• 24:01
  Speaker 1: system is that if the relays are set for a
• 24:03
  Speaker 1: condition with both sources of power connected, such settings must
• 24:07
  Speaker 1: be changed somewhat when one source of power is disconnected.
• 24:10
  Speaker 1: In this case, the entire relay system has to be readjusted.
• 24:14
  Speaker 1: This illustrates the necessity for using relays whose adjustment can
• 24:18
  Speaker 1: be changed quickly type CO and CR relays. The Westinghouse
• 24:25
  Speaker 1: CO and CR relays are specially adaptable for use on
• 24:28
  Speaker 1: loop systems, as they embody the definite time limit characteristics
• 24:32
  Speaker 1: with a very flexible adjustment for values of operating current.
• 24:36
  Speaker 1: In addition to the dependable accuracy of the relay settings
• 24:39
  Speaker 1: and the speed of action, Any necessary changes in the
• 24:42
  Speaker 1: settings or adjustments may be made quickly and conveniently type
• 24:47
  Speaker 1: CZ impedance relay. Sometimes it is very difficult to arrive
• 24:52
  Speaker 1: at a suitable method of protection for a loop system
• 24:55
  Speaker 1: due to the fact that when applying the standard time
• 24:57
  Speaker 1: element relay and allowing the necessary time intervals in order
• 25:01
  Speaker 1: to secure selectivity, the time settings of the relays at
• 25:04
  Speaker 1: the ends of the loop become necessarily higher than is
• 25:07
  Speaker 1: either safe or advisable. A more recent and flexible relay
• 25:11
  Speaker 1: application for such complicated systems is the use of the
• 25:15
  Speaker 1: impedance or distance relay developed by the Westinghouse Company. With
• 25:19
  Speaker 1: the proper application of the CZ relay, all time settings
• 25:23
  Speaker 1: are taken care of automatically. After each relay has been
• 25:26
  Speaker 1: set to protect its own section of the line, each
• 25:29
  Speaker 1: relay is set to trip out its own section of
• 25:31
  Speaker 1: the line in a time corresponding to the distance to
• 25:34
  Speaker 1: the fault, the maximum time required being zero point seven
• 25:37
  Speaker 1: to five second, which corresponds to a short at the
• 25:40
  Speaker 1: far end of the line. Regardless of on what section
• 25:44
  Speaker 1: of the system the fault takes place, the proper relay
• 25:46
  Speaker 1: will trip out and at no time will the isolation
• 25:49
  Speaker 1: of the faulty line require more than zero point seventy
• 25:52
  Speaker 1: five to one second to accomplish. The other CZ relays
• 25:56
  Speaker 1: on other sections of the line will start the function,
• 25:58
  Speaker 1: but by reason of their greater distance from the fault,
• 26:01
  Speaker 1: they will not trip out their breakers, but will return
• 26:04
  Speaker 1: to their normal position as soon as the relays on
• 26:06
  Speaker 1: the faulty line have isolated the fault. This automatic increase
• 26:10
  Speaker 1: in timing with increase in distance to the fault is
• 26:13
  Speaker 1: the unique feature of a CZ installation. With the addition
• 26:17
  Speaker 1: of a directional element to the relay main element, this
• 26:20
  Speaker 1: relay may be applied on loop systems in the same
• 26:22
  Speaker 1: way as CO and CR relays. The choice between the
• 26:26
  Speaker 1: use of the two applications is dependent mainly on the cost.
• 26:30
  Speaker 1: The CO CR application is preferable whenever it can be
• 26:34
  Speaker 1: supplied satisfactorily, inasmuch as the relays themselves are less expensive
• 26:38
  Speaker 1: and the COO relays do not require a voltage connection.
• 26:42
  Speaker 1: Where the system is complicated and the sections of sufficient
• 26:45
  Speaker 1: length long enough to give approximately three percent voltage drop
• 26:49
  Speaker 1: between ends of the section, the CZ relay is applicable
• 26:52
  Speaker 1: and should be used. This voltage, which is a measure
• 26:56
  Speaker 1: of the impedance voltage drop to the point of short circuit,
• 27:00
  Speaker 1: is supplied to the voltage element of the relay by
• 27:02
  Speaker 1: potential transformers connected either to the high or the low
• 27:05
  Speaker 1: side of the power transformers. If connected to the high
• 27:09
  Speaker 1: side of the power transformers, the potential transformers will supply
• 27:13
  Speaker 1: to the relay elements voltages corresponding exactly to the line voltage,
• 27:17
  Speaker 1: and accordingly, the relays will function properly. In case only
• 27:21
  Speaker 1: potential transformers connected to the low side of the power
• 27:24
  Speaker 1: transformers are available, it sometimes becomes necessary to use voltage
• 27:28
  Speaker 1: compensators similar to those used with voltage regulators, to compensate
• 27:33
  Speaker 1: for the drop through the transformer which has been inserted
• 27:36
  Speaker 1: between the relay and the line to be protected. Compensators
• 27:40
  Speaker 1: are provided one to compensate for the impedance drop in
• 27:43
  Speaker 1: the power transformer, which drop is present when short circuit
• 27:47
  Speaker 1: current is flowing through the transformer into the line that
• 27:50
  Speaker 1: is shorted. Two to compensate for the shift in the
• 27:54
  Speaker 1: neutral at the unngrounded substation end of a solidly grounded system,
• 27:58
  Speaker 1: which shift occurs in case of an accidental ground on
• 28:01
  Speaker 1: the high side. Figures twenty seven, twenty eight, and twenty
• 28:05
  Speaker 1: nine show typical CZ applications with and without compensation as
• 28:11
  Speaker 1: required protection of parallel feeders. Where two or more identical
• 28:17
  Speaker 1: or similar lines connect a generating station and a substation,
• 28:21
  Speaker 1: two substations, or two generator stations, they are termed parallel feeders.
• 28:26
  Speaker 1: Such an arrangement of feeders may be applied to either
• 28:29
  Speaker 1: radial systems or loop systems, and it has come into
• 28:32
  Speaker 1: very common use too, not only to the fact that
• 28:35
  Speaker 1: it is an economical and convenient means of increasing transmission capacity,
• 28:39
  Speaker 1: but also to the fact that it forms a most
• 28:42
  Speaker 1: flexible system for economical operation of lines and lends itself
• 28:46
  Speaker 1: very readily to relay protection CO and CR relay scheme.
• 28:53
  Speaker 1: In general, the standard application of CO overcurrent and CR
• 28:57
  Speaker 1: directional overcurrent relays may be applied to parallel feeders in
• 29:01
  Speaker 1: the same way as in the case of single lines
• 29:03
  Speaker 1: of loop systems. Where the system is large, however, the
• 29:07
  Speaker 1: time settings on some of the relays must necessarily be
• 29:09
  Speaker 1: high in order to secure selectivity, and this in itself
• 29:13
  Speaker 1: is often a difficulty which cannot be overcome. Balance power scheme.
• 29:20
  Speaker 1: A general solution of this problem is to use a
• 29:22
  Speaker 1: balance system of relays. Separate parallel circuits which normally carry
• 29:27
  Speaker 1: equal currents, are balanced against one another. If the currents
• 29:31
  Speaker 1: maintain their equality in spite of their magnitude, it is
• 29:34
  Speaker 1: a good indication that no fault exists in that particular group.
• 29:38
  Speaker 1: On the other hand, a certain degree of unbalancing would
• 29:41
  Speaker 1: be a safe indication of trouble. The problem, therefore, is
• 29:46
  Speaker 1: in general to separate a current which is representative of
• 29:50
  Speaker 1: the actual unbalancing and pass it through proper relays so
• 29:53
  Speaker 1: that the faulty feeder may be disconnected. One method of
• 29:57
  Speaker 1: solving this problem is by balancing the current fel lowing
• 30:00
  Speaker 1: in the two feeders by the cross connection of directional relays.
• 30:04
  Speaker 1: This scheme may be applied to any system, no matter
• 30:07
  Speaker 1: how complex. If its feeders are run parallel between the
• 30:10
  Speaker 1: switching points, the relays may be set to act almost instantaneously.
• 30:14
  Speaker 1: With this system, the relays can be set to operate
• 30:17
  Speaker 1: on currents smaller than the full load value of each feeder.
• 30:20
  Speaker 1: This arrangement enables the clearing of trouble on a system
• 30:24
  Speaker 1: having the neutral grounded through a high resistance when the
• 30:27
  Speaker 1: total trouble and load current flowing through a single cable
• 30:30
  Speaker 1: may be less than the maximum load current of that cable.
• 30:34
  Speaker 1: The schematic diagram Figure thirty two shows the scheme of
• 30:38
  Speaker 1: cross connection of directional relays applied to a system of
• 30:41
  Speaker 1: four parallel feeders. Figure thirty three shows a complete diagram
• 30:46
  Speaker 1: of standard connections for a pair of three phase feeders,
• 30:49
  Speaker 1: except that the tripping circuit is omitted. By reference to
• 30:53
  Speaker 1: these two figures, it will be seen that all the
• 30:55
  Speaker 1: current transformers in the generating station are connected in series,
• 30:59
  Speaker 1: likewise those in the substation. Each relay is a unidirectional
• 31:04
  Speaker 1: relay and is shunted across its own current transformer. Under
• 31:08
  Speaker 1: normal conditions, the load in each of the cables will
• 31:11
  Speaker 1: be the same, and since the relays are of a
• 31:13
  Speaker 1: higher impedance than the current transformers, the current from the
• 31:17
  Speaker 1: latter will circulate through all of the transformers and series
• 31:20
  Speaker 1: without any flowing through the relays. If the trouble occurs
• 31:23
  Speaker 1: at any point outside the section protected by these cross
• 31:26
  Speaker 1: connected relays, the current through the protected lines will still
• 31:29
  Speaker 1: be balanced, and consequently there will be no unbalanced current
• 31:33
  Speaker 1: tending to operate the relays. On the other hand, if
• 31:36
  Speaker 1: trouble occurs on a line within the section, the current
• 31:40
  Speaker 1: through the defective cable will be higher than that in
• 31:42
  Speaker 1: the other, and the excess current from its current transformer
• 31:45
  Speaker 1: must therefore pass through the relays. While the system is
• 31:49
  Speaker 1: in this unbalanced condition, current will flow through all the relays,
• 31:53
  Speaker 1: but in the proper direction to cause only the relay
• 31:55
  Speaker 1: at each end of the defective cable to act. Fig.
• 31:59
  Speaker 1: Thirty three shows auxiliary pallette switches connected in the transformer
• 32:03
  Speaker 1: secondary circuit. These switches are also connected mechanically to the
• 32:07
  Speaker 1: operating mechanism of the breakers, so that when the braker opens,
• 32:11
  Speaker 1: the current transformers on the feeder will be short circuited.
• 32:15
  Speaker 1: By this method, a line can be cut out of
• 32:17
  Speaker 1: service without interfering with the electrical balance in the remaining
• 32:21
  Speaker 1: transformer circuit. However, these pallet switches are not needed in
• 32:26
  Speaker 1: the transformer circuits except where low ratio bushing type current
• 32:30
  Speaker 1: transformers are used. Ordinarily, the impedance of the current transformer
• 32:34
  Speaker 1: secondaries is sufficient when the primaries are open circuited to
• 32:38
  Speaker 1: prevent any appreciable current from passing through them. By referring
• 32:43
  Speaker 1: to Figures thirty two and thirty three, it will be
• 32:46
  Speaker 1: observed that under balance conditions, no current is flowing in
• 32:49
  Speaker 1: the relay current coil. When a fault develops on one line,
• 32:53
  Speaker 1: the current voltage and that group of relays are in
• 32:56
  Speaker 1: such a relation as to close the directional element contacts,
• 33:00
  Speaker 1: while in the other group of relays the voltage is
• 33:02
  Speaker 1: in the opposite direction and so opens the directional element contacts.
• 33:06
  Speaker 1: It is therefore apparent that if the directional element were
• 33:09
  Speaker 1: made double throw, one set of relays could be made
• 33:12
  Speaker 1: to protect two parallel feeders by cross connecting the current transformers.
• 33:17
  Speaker 1: Such a relay is available under the name of the
• 33:20
  Speaker 1: duodirectional type CR relay. Its chief disadvantage is that either
• 33:25
  Speaker 1: abnormal phase distortion or mechanical vibration may displace the relatively
• 33:29
  Speaker 1: light directional element and trip the circuit breaker before the
• 33:32
  Speaker 1: fault has been isolated, thereby interrupting service over the good line.
• 33:37
  Speaker 1: A further adaptation of the cross connected CR relay protective
• 33:40
  Speaker 1: scheme gives both short time low current protection when two
• 33:44
  Speaker 1: feeders are operating in parallel and longtime directional over current
• 33:48
  Speaker 1: protection when one of the feeders is operating singly. As
• 33:51
  Speaker 1: a part of a transmission loop. Fig. Thirty four shows
• 33:56
  Speaker 1: the complete connections for two incoming feeders to a substation.
• 34:00
  Speaker 1: Volage and current connections to the relays are so made
• 34:03
  Speaker 1: that the relays of each set close their directional element
• 34:06
  Speaker 1: contacts when unbalanced power is flowing out over the line
• 34:09
  Speaker 1: which those relays protect. Each relay is provided with a
• 34:13
  Speaker 1: tap between the directional element contacts and the overcurrent element contacts,
• 34:18
  Speaker 1: and these taps are joined to a common bus for
• 34:20
  Speaker 1: each pair of lines, so that any overcurrent element can
• 34:23
  Speaker 1: trip either circuit breaker by acting through the directional element
• 34:26
  Speaker 1: contacts of the corresponding relays. With the feeders carrying equal loads,
• 34:31
  Speaker 1: there is no current flowing through the current windings of
• 34:33
  Speaker 1: the relays. As soon as a fault develops on one line,
• 34:36
  Speaker 1: an unbalanced circuit circulates through the relays, causing the directional
• 34:40
  Speaker 1: element contacts on the faulty line group to close and
• 34:43
  Speaker 1: those of the good line group to open. The faulty
• 34:46
  Speaker 1: line circuit breakers tripped by the closing of the contacts
• 34:48
  Speaker 1: of the overcurrent element having the short time setting. As
• 34:52
  Speaker 1: soon as the faulty line circuit breaker is open, the
• 34:55
  Speaker 1: control circuit to the short time overcurrent element is broken
• 34:58
  Speaker 1: by the series connection through the circuit breaker auxiliary switches.
• 35:02
  Speaker 1: The remaining line then has directional overcurrent protection, only the
• 35:05
  Speaker 1: time and current settings of the overcurrent element being determined
• 35:08
  Speaker 1: by operating conditions on the loop. It is general practice
• 35:12
  Speaker 1: to base such settings on the assumption that with a
• 35:15
  Speaker 1: loop composed of normally balanced lines, not more than two
• 35:18
  Speaker 1: sections will be operating as single lines. It is interesting
• 35:22
  Speaker 1: to note that this scheme of connection obviates the use
• 35:25
  Speaker 1: of a circuit opening definite time delay direct current relay
• 35:29
  Speaker 1: to prevent improper operation of the good line circuit breaker.
• 35:32
  Speaker 1: It has been found necessary in many instances to employ
• 35:35
  Speaker 1: such relays with cross connected schemes in order to prevent
• 35:38
  Speaker 1: unnecessary tripping. Balance power relays are satisfactory, particularly where there
• 35:44
  Speaker 1: is no source of power available at one end of
• 35:46
  Speaker 1: the parallel lines, but the use of differential current relays
• 35:49
  Speaker 1: is simpler and is usually preferred where power can be
• 35:52
  Speaker 1: supplied from both ends of the group of lines. Type
• 35:57
  Speaker 1: CD selective differential relay scheme. Another scheme which is coming
• 36:02
  Speaker 1: into common use for parallel line protection is a scheme
• 36:05
  Speaker 1: using the balance current relay, known as the type CD relay.
• 36:09
  Speaker 1: It may be used at the transmitting end of any
• 36:12
  Speaker 1: number of parallel lines and at the receiving end of
• 36:15
  Speaker 1: any number of parallel lines in a system having a
• 36:17
  Speaker 1: source of power at both ends or at the receiving
• 36:20
  Speaker 1: end of three or more parallel feeders. In a radial system,
• 36:24
  Speaker 1: it cannot be used on a single pair of radial
• 36:26
  Speaker 1: parallel feeders. For in case of a fault on one line,
• 36:30
  Speaker 1: there would be an equal and opposite current in the
• 36:32
  Speaker 1: two elements and the relay would not operate. The same
• 36:36
  Speaker 1: condition will exist after one of three parallel lines has
• 36:39
  Speaker 1: been disconnected at the receiving end. For this reason, cross
• 36:43
  Speaker 1: connected power directional relays are usually used at such points.
• 36:47
  Speaker 1: With three PAARA lines protected by these relays, number one
• 36:50
  Speaker 1: line is balanced against number two, number two line is
• 36:53
  Speaker 1: balanced against number three, and number three line is balanced
• 36:56
  Speaker 1: against number one. In this way, complete balance protection is
• 36:59
  Speaker 1: secured until only one line remains in service, after which
• 37:04
  Speaker 1: overload protection is left on this line. If four or
• 37:07
  Speaker 1: more lines are thus protected, complete balance protection cannot be
• 37:11
  Speaker 1: assured after more than two lines have been removed from service. Thus,
• 37:15
  Speaker 1: with four parallel lines, number one being balanced against number two,
• 37:19
  Speaker 1: number two against number three, number three against number four,
• 37:23
  Speaker 1: and number four against number one, any one line can
• 37:26
  Speaker 1: be disconnected and the remaining lines will still be balanced. If, however,
• 37:31
  Speaker 1: numbers one and three lines both have been disconnected, numbers
• 37:35
  Speaker 1: two and four will be left with only over current protection.
• 37:38
  Speaker 1: On the other hand, if number two or number four
• 37:41
  Speaker 1: lines were disconnected with number one, remaining two lines would
• 37:45
  Speaker 1: still be balanced. Inasmuch as the action of the balance
• 37:49
  Speaker 1: current relay is practically instantaneous in clearing faulty lines, it
• 37:53
  Speaker 1: makes an ideal application for the protection of parallel lines
• 37:56
  Speaker 1: between generating stations. On other parallel feeders, it is often
• 38:00
  Speaker 1: very advisable to use overcurrent relays in connection with balanced
• 38:04
  Speaker 1: current relays, thus securing single line protection in addition to
• 38:08
  Speaker 1: quick balance protection when both lines are in service. The
• 38:12
  Speaker 1: chief advantages of the CD balanced current relay scheme are
• 38:15
  Speaker 1: as follows. One, the relay operates on current a loan,
• 38:20
  Speaker 1: no source of potential being required. Two. The differential current
• 38:25
  Speaker 1: setting is automatically doubled when one line is opened at
• 38:28
  Speaker 1: one end. This permits the use of a minimum differential
• 38:31
  Speaker 1: current setting and eliminates the necessity of additional apparatus in
• 38:35
  Speaker 1: the trip circuit in order to render it non automatic
• 38:38
  Speaker 1: until the two lines are again in service. Three, each
• 38:42
  Speaker 1: current transformer may be grounded, thus giving maximum protection to
• 38:46
  Speaker 1: the apparatus. This is not possible ordinarily with balance protection.
• 38:52
  Speaker 1: Four only one relay is required for two lines five
• 38:57
  Speaker 1: relay action is practically instantaneous. Type CZ impedance relay. The
• 39:05
  Speaker 1: CZ impedance relay may be used for parallel line protection
• 39:08
  Speaker 1: in the same way as the CO and CR relays.
• 39:12
  Speaker 1: The CZ directional relay is applicable in places where the
• 39:15
  Speaker 1: CR is used. The advantages of the impedance relay have
• 39:18
  Speaker 1: been explained in detail on page twenty. Network systems. The
• 39:25
  Speaker 1: simple radial and loop or ring systems, very often involving
• 39:29
  Speaker 1: parallel lines between stations, lend themselves very readily to a
• 39:32
  Speaker 1: scheme of relay protection, as has been outlined. However, when
• 39:37
  Speaker 1: additional tie lines are added, connecting substations and several sources
• 39:41
  Speaker 1: of power into the system at different points, it becomes
• 39:44
  Speaker 1: what may be termed a network, and then the relay
• 39:47
  Speaker 1: protection problem becomes more difficult. Type CO and CR scheme.
• 39:54
  Speaker 1: The CO overcurrent and CR directional overcurrent relays are sometimes
• 39:58
  Speaker 1: applied to such a system, vision being made to have
• 40:01
  Speaker 1: certain lines trip out almost instantly with a short circuit
• 40:04
  Speaker 1: in the vicinity. With such an arrangement, the network is
• 40:08
  Speaker 1: often converted into a simple ring. After the tie lines
• 40:11
  Speaker 1: have been disconnected and the usual scheme of time and
• 40:14
  Speaker 1: current settings clears up the trouble. The cutting out of
• 40:17
  Speaker 1: the tie line or lines in such cases does not
• 40:20
  Speaker 1: in any way jeopardize the service and serves to make
• 40:23
  Speaker 1: selective relay protection possible. For example, in Fig. Thirty eight,
• 40:28
  Speaker 1: the tie line between substations E and B may be
• 40:31
  Speaker 1: disconnected without disturbing the service supplied by the network type
• 40:36
  Speaker 1: CZ impedance relay scheme. A more recent and flexible relay
• 40:42
  Speaker 1: application for such complicated systems is the use of the
• 40:45
  Speaker 1: impedance or distance relay. With proper application of this type
• 40:48
  Speaker 1: of relay, all time settings are taken care of automatically,
• 40:52
  Speaker 1: the inherent characteristics of the relay being such that the
• 40:55
  Speaker 1: one nearest to the fault will operate first. Pilot wise
• 41:00
  Speaker 1: scheme of relay protection on systems where definite time relay
• 41:04
  Speaker 1: protection is desired. The number of substations which may be
• 41:08
  Speaker 1: connected successively in a system is limited by the sum
• 41:11
  Speaker 1: of the time intervals which it is necessary to allow
• 41:13
  Speaker 1: between successive relays. When such a difficulty is encountered and
• 41:17
  Speaker 1: the distance between some of the substations is small, a
• 41:20
  Speaker 1: pilot wire scheme of protection may be used on the
• 41:23
  Speaker 1: shorter sections. As this scheme acts almost instantaneously upon the
• 41:27
  Speaker 1: occurrence of a fault within the section and is unaffected
• 41:30
  Speaker 1: by the faults outside of the section, the section may
• 41:33
  Speaker 1: be disregarded entirely. When applying other relays, the system may
• 41:37
  Speaker 1: be considered as if that particular section were entirely cut out.
• 41:42
  Speaker 1: The pilot wire scheme has current transformers at each end
• 41:44
  Speaker 1: of the conductor short circuited upon each other through the
• 41:47
  Speaker 1: pilot wire see Fig. Forty. The relays are connected between
• 41:51
  Speaker 1: the pilot wire for each respective transformer and the common return.
• 41:55
  Speaker 1: When a short circuit or fault occurs on the line
• 41:58
  Speaker 1: between the two substations, the current transformers at the two
• 42:02
  Speaker 1: ends are no longer short circuited upon each other, but
• 42:05
  Speaker 1: the currents which they produce are opposed to each other,
• 42:08
  Speaker 1: so the current must flow through the relays, and the
• 42:10
  Speaker 1: faulty section will thus be tripped out. Inasmuch as the
• 42:14
  Speaker 1: current flowing through the pilot wire must be kept at
• 42:17
  Speaker 1: the minimum, special current transformers and co relays with special
• 42:21
  Speaker 1: windings to operate on low current are used for the
• 42:23
  Speaker 1: pilot wire protection scheme. Split conductor scheme. The split conductor
• 42:31
  Speaker 1: scheme is a modification of the current balance scheme of
• 42:33
  Speaker 1: parallel line protection, in which a special cable is used
• 42:36
  Speaker 1: to obtain a division of the current which is susceptible
• 42:39
  Speaker 1: to being balanced as ordinarily installed. The split conductor cable
• 42:44
  Speaker 1: is a three strand cable in which the transforming each
• 42:47
  Speaker 1: conductor are separated into two parts, a central portion and
• 42:51
  Speaker 1: a circumferential portion, the two being separated from each other
• 42:55
  Speaker 1: by the necessary installation. Thus, by the use of current
• 42:59
  Speaker 1: transforms a differential connection between the two halves of each conductor,
• 43:03
  Speaker 1: and overcurrent relays connected in the circuit, a very effective
• 43:07
  Speaker 1: system of protection is obtained. This system is not widely
• 43:11
  Speaker 1: used in the United States, due largely, perhaps to the
• 43:14
  Speaker 1: high cost of the special cable. End of Chapter three,
• 43:19
  Speaker 1: Part one