Latest / Silent Sentinels- Protective Relays for A-C and D-C Systems / 005 - Alternating-Current Systems part 2

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005 - Alternating-Current Systems part 2 · Silent Sentinels- Protective Relays for A-C and D-C Systems

• 0:00
  Speaker 1: Chapter three, Part two of Silent Sentinels by Westinghouse Electric
• 0:05
  Speaker 1: and Manufacturing Company. This LibriVox recording is in the public domain.
• 0:12
  Speaker 1: Section three Methods of clearing ground faults solidly in low
• 0:18
  Speaker 1: resistance grounded neutral systems. These systems, when protected by types CO,
• 0:24
  Speaker 1: CR and CD relays, require no additional relays. For wired
• 0:28
  Speaker 1: to ground faults, three current transformers and three relays are
• 0:32
  Speaker 1: necessary for the complete protection of each circuit breaker. However,
• 0:36
  Speaker 1: where type CZ impedance relays are used in order to
• 0:39
  Speaker 1: obtain the minimum time for clearing faults, two sets of
• 0:42
  Speaker 1: impedance elements are required for phase to phase and phase
• 0:45
  Speaker 1: to ground faults. The reason for this is that a
• 0:48
  Speaker 1: ground fault on one wire results in fifty seven point
• 0:51
  Speaker 1: seven percent voltage being maintained across two sides of the
• 0:54
  Speaker 1: voltage triangle and one hundred percent voltage across the third side.
• 0:58
  Speaker 1: The voltage between the grounded wires and neutral is, of
• 1:01
  Speaker 1: course zero or nearly so. If only three elements having
• 1:06
  Speaker 1: their directional element potential coils connected in STAR are used,
• 1:10
  Speaker 1: a wired to wire short circuit will reduce the voltage
• 1:13
  Speaker 1: across each relay coil from fifty seven point seven per
• 1:16
  Speaker 1: cent of the line voltage to twenty eight point eight
• 1:18
  Speaker 1: percent line voltage, so that each relay still has fifty
• 1:22
  Speaker 1: percent normal voltage impressed across it. With fifty percent normal
• 1:26
  Speaker 1: voltage acting on the potential coil of the CZ relay,
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  Speaker 1: The time required to close the contacts is entirely too
• 1:32
  Speaker 1: long for satisfactory operation, and two sets of elements must
• 1:36
  Speaker 1: be employed for complete protection, medium resistance, grounded neutral systems.
• 1:44
  Speaker 1: Relay equipment, and additional to that required for line short circuits,
• 1:48
  Speaker 1: is required to care for phase to ground failures. For
• 1:51
  Speaker 1: those installations where the magnitude of the ground current in
• 1:55
  Speaker 1: the circuit and the ratios of the current transformers are
• 1:58
  Speaker 1: such as to produce a single phase curve cur of
• 2:00
  Speaker 1: at least two ampers in the secondary of one current
• 2:02
  Speaker 1: transformer for a fault from that wired to ground, a
• 2:06
  Speaker 1: type co relay having a low current winding is inserted
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  Speaker 1: in the neutral connection between the neutral point of the
• 2:12
  Speaker 1: line relays and the neutral point of the current transformer secondaries.
• 2:16
  Speaker 1: The low current type COO relay has taps for operation
• 2:19
  Speaker 1: at zero point five, zero point six, zero point eight,
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  Speaker 1: one point zero, one point five, two point zero, and
• 2:27
  Speaker 1: two point five ampers. It can generally be operated on
• 2:31
  Speaker 1: the zero point five amper tap without danger of tripping improperly.
• 2:35
  Speaker 1: As under normal operation, there is no current flowing in
• 2:37
  Speaker 1: the neutral connection. Fig. Forty one shows the connections for
• 2:41
  Speaker 1: the ground relay. When the circuit breaker is controlled by
• 2:44
  Speaker 1: type CO over current relays, it will be observed that
• 2:48
  Speaker 1: the tripping contacts of the ground relay parallel to the
• 2:50
  Speaker 1: tripping contacts of the line relays. The same scheme of
• 2:54
  Speaker 1: current and tripping connections is employed when non directional single
• 2:57
  Speaker 1: impedance element type CZ impedance relay are used to protect
• 3:01
  Speaker 1: the feeder against line faults. When the circuit breaker is
• 3:05
  Speaker 1: controlled by type CR directional overcurrent or type CZ single
• 3:09
  Speaker 1: impedance element relays for phase to phase faults, the low
• 3:13
  Speaker 1: current relay is inserted in the neutral circuit. For ground
• 3:16
  Speaker 1: fault protection. The tripping contacts do not, however, go directly
• 3:21
  Speaker 1: to the circuit breaker trip coil, but are connected in
• 3:23
  Speaker 1: series with the directional contacts of the line relays. To
• 3:27
  Speaker 1: secure proper directional indication on ground faults as well as
• 3:30
  Speaker 1: line faults, a tap must therefore be provided on the
• 3:34
  Speaker 1: line relays between the directional element contacts and the overcurrent
• 3:38
  Speaker 1: or impedance element contacts. Fig. Forty two shows the connections
• 3:43
  Speaker 1: using three type CR line relays and one type CO
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  Speaker 1: low energy ground relay. The same tripping circuit connections hold
• 3:50
  Speaker 1: when Type CZ impedance relays are used for line faults.
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  Speaker 1: This arrangement should be used with care. It has been
• 3:58
  Speaker 1: found that a high resistance at the point point where
• 4:00
  Speaker 1: an accidental ground occurs will limit the current to such
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  Speaker 1: a small value that the load current flowing through the
• 4:05
  Speaker 1: CR relays will be greater than the ground current and
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  Speaker 1: the CR relays will not operate in the proper direction.
• 4:12
  Speaker 1: A low current Type CD selective differential current relay is
• 4:15
  Speaker 1: available for ground protection at those locations protected against line
• 4:19
  Speaker 1: faults by type CD relays. Each winding is provided with
• 4:24
  Speaker 1: taps for zero point five, zero point six, one point
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  Speaker 1: two and two point zero ampers operating current. One winding
• 4:31
  Speaker 1: is connected in each neutral connection of the balance pair
• 4:34
  Speaker 1: of lines. Fig. Forty three shows the complete connections for
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  Speaker 1: line and ground protection on two balanced lines for both
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  Speaker 1: balanced and single line operation. High resistance grounded neutral system
• 4:49
  Speaker 1: protection against wired to ground faults on high resistance grounded
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  Speaker 1: neutral systems cannot be readily provided in less the magnitude
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  Speaker 1: of the ground current in the circuit and the ratio
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  Speaker 1: of the current transformers are such says to provide one
• 5:01
  Speaker 1: and a half to two ampers in the secondary of
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  Speaker 1: one current transformer with a ground fault on that wire.
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  Speaker 1: Since the ground current is less than the load current
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  Speaker 1: in the circuit to be protected, a directional element which
• 5:13
  Speaker 1: shall be independent of load is required. Fig. Forty four
• 5:17
  Speaker 1: shows complete line and ground protection for two parallel lines
• 5:21
  Speaker 1: on a high resistance grounded neutral system. The line potential
• 5:25
  Speaker 1: transformers are connected star star with the star points grounded.
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  Speaker 1: The line relay potential coils are energized by the delta
• 5:32
  Speaker 1: voltages of this bank. If the potential transformers are chosen
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  Speaker 1: as to give one hundred and fifteen volt single phase
• 5:39
  Speaker 1: secondary when energized by a single phase voltage of line
• 5:42
  Speaker 1: to line value, the line relay should be provided with
• 5:45
  Speaker 1: one hundred and twenty five vault potential coils. The secondaries
• 5:49
  Speaker 1: of the line potential transformers also energize an auxiliary bank
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  Speaker 1: of three two hundred to one hundred voult potential transformers
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  Speaker 1: connected star on the primary side. The star point being
• 5:59
  Speaker 1: can acted to the two star points to the line
• 6:01
  Speaker 1: potential transformers. The secondaries of the auxiliary bank of transformers
• 6:07
  Speaker 1: form a closed delta through the potential coil of the
• 6:10
  Speaker 1: ground relay. A low current low energy directional type CR
• 6:15
  Speaker 1: relay having the same current winding as the low current
• 6:18
  Speaker 1: type co previously mentioned, is required for ground protection. The
• 6:23
  Speaker 1: current coil of this relay is connected between the neutral
• 6:26
  Speaker 1: point of the current transformer secondaries and the neutral point
• 6:29
  Speaker 1: of the line relays. When a fault from one wire
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  Speaker 1: to ground occurs, the distortion of the voltage triangle caused
• 6:36
  Speaker 1: by a current being fed to the ground fault by
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  Speaker 1: the potential transformer develops sufficient voltage across the potential coil
• 6:42
  Speaker 1: of the directional relay to obtain in conjunction with the
• 6:46
  Speaker 1: residual current in the relay the proper directional indication. The
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  Speaker 1: proper time of operation for the relay is secured by
• 6:53
  Speaker 1: an adjustment of the time index lever of the over
• 6:56
  Speaker 1: current element. When power transformer banks are can connected star
• 7:00
  Speaker 1: delta and the star point is grounded through a resistor,
• 7:04
  Speaker 1: the potential for the directional ground relays can be secured
• 7:07
  Speaker 1: by connecting the primary of a two hundred to one
• 7:09
  Speaker 1: hundred volt potential transformer between ground and a point on
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  Speaker 1: the resistor which is two hundred volts above ground. When
• 7:17
  Speaker 1: carrying the maximum ground current. The secondary of the potential
• 7:20
  Speaker 1: transformer energizes the potential coil of the relay. Section four.
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  Speaker 1: Calculation of short circuit current. In applying any protective scheme,
• 7:33
  Speaker 1: it is necessary to determine the short circuit currents which
• 7:35
  Speaker 1: may develop under all conditions. It is unfortunate that the
• 7:39
  Speaker 1: term overload has ever come into use in connection with
• 7:42
  Speaker 1: sectionalizing distribution systems, because it implies that the relays should
• 7:47
  Speaker 1: be set to operate at a value determined by the
• 7:49
  Speaker 1: normal load on the feeder. Such a setting is possible
• 7:53
  Speaker 1: if definite time limit relays are used, but where a
• 7:56
  Speaker 1: relay having inverse time characteristics is used, is necessary to
• 8:00
  Speaker 1: consider the current that occurs during times of trouble, and
• 8:03
  Speaker 1: that may be tens or even hundreds of times greater
• 8:06
  Speaker 1: than the normal current. An approximate method of determining the
• 8:10
  Speaker 1: possible short circuit current is to observe the voltage drop
• 8:13
  Speaker 1: between two stations at normal load. Short circuit current equals
• 8:17
  Speaker 1: normal voltage divided by voltage drop times load current. For example,
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  Speaker 1: if a certain load current causes a drop of five
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  Speaker 1: percent in voltage between the generating station and the substation,
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  Speaker 1: the maximum short circuit current would be twenty times the
• 8:32
  Speaker 1: load current. Results obtained in this way are likely to
• 8:36
  Speaker 1: be too large, particularly in lines having high inductance. In
• 8:41
  Speaker 1: order to determine more accurately the RMS current, an analysis
• 8:45
  Speaker 1: of short circuit phenomena on an alternating current network is necessary,
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  Speaker 1: and it may be desirable to call attention to some
• 8:51
  Speaker 1: of the conditions under which relays may be called upon
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  Speaker 1: to function. A short circuit is a transient condition, and
• 8:59
  Speaker 1: this current at any point in an electrical system is
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  Speaker 1: a variable depending on the resistance, inductance, electrostatic capacity, magnetic capacity,
• 9:08
  Speaker 1: point on the voltage wave at which the disturbance takes place,
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  Speaker 1: and current conditions of the circuit immediately preceding any change
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  Speaker 1: in the steady condition of that circuit. It represents a
• 9:18
  Speaker 1: readjustment of the stored electromagnetic and electrostatic energy in the
• 9:22
  Speaker 1: system from an initial steady state condition until a final
• 9:26
  Speaker 1: steady condition is reached. It is manifested during every change
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  Speaker 1: in the condition of the circuit, although its effects on
• 9:33
  Speaker 1: the circuit are usually not appreciable under certain conditions, such
• 9:38
  Speaker 1: as short circuits. The effects of the transient current, however,
• 9:41
  Speaker 1: are of prime importance. Short circuiting a system at any
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  Speaker 1: point immediately causes an abnormal current rush to occur on
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  Speaker 1: that system. During the first complete cycle following the short circuit,
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  Speaker 1: the current usually rises to maximum value. If now the
• 9:58
  Speaker 1: generator field circuit is assumed to remain unchanged by automatic
• 10:01
  Speaker 1: generator voltage regulators or similar devices than during succeeding cycles,
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  Speaker 1: the current will rise to lesser and lesser values until
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  Speaker 1: a steady condition again exists on the system. The current
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  Speaker 1: wave during the transient period, as shown by assilograms, may
• 10:18
  Speaker 1: be symmetrical or asymmetrical with respect to the line representing
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  Speaker 1: zero current, depending on the point on the voltage pressure
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  Speaker 1: wave at which the short circuit occurred. The transient current
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  Speaker 1: is said to be symmetrical when the assumed line connecting
• 10:34
  Speaker 1: points on the current wave midway between the peaks coincides
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  Speaker 1: with the line of zero current. Similarly, a transient current
• 10:41
  Speaker 1: is said to be symmetrical when the assumed line connecting
• 10:45
  Speaker 1: points on the current wave midway between the peaks does
• 10:48
  Speaker 1: not coincide with the line of the zero current. The
• 10:52
  Speaker 1: value of the current at any point on the current
• 10:54
  Speaker 1: wave during the transient condition, if referred to the line
• 10:57
  Speaker 1: representing zero current, is a measure of the effects produced
• 11:01
  Speaker 1: by the current at that point. Some of these effects
• 11:04
  Speaker 1: are indicated by the breaking of insulators supporting buses, the
• 11:07
  Speaker 1: displacing of windings of transformers and generators, the distortion of
• 11:12
  Speaker 1: oil circuit breaker connections, and the fusing of conductors. In
• 11:15
  Speaker 1: various parts of electrical systems, the value of the current
• 11:20
  Speaker 1: at any point on the current wave during the transient
• 11:22
  Speaker 1: condition is frequently referred to an assumed zero line between
• 11:26
  Speaker 1: the peaks. This values used to calculate the electrical constants
• 11:29
  Speaker 1: of a circuit, such as impedance, reactants, etc. The maximum
• 11:34
  Speaker 1: asymmetrical value of a transient current in RMS ampers during
• 11:38
  Speaker 1: a short circuit on an ordinary alternating current generator is
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  Speaker 1: approximately equal to one point eight times the generated voltage
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  Speaker 1: pressure of the generator in volts immediately preceding the short circuit,
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  Speaker 1: divided by the impedance of the circuit in omes between
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  Speaker 1: the point of power supply and the point of short circuit.
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  Speaker 1: The maximum symmetrical value of the transient current is a
• 11:59
  Speaker 1: proximately equal to zero point five to five times the
• 12:02
  Speaker 1: maximum asymmetrical value. The transient of an electrical system at
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  Speaker 1: a point remote from the source of power supply will
• 12:10
  Speaker 1: be different from that at the terminals of an alternating
• 12:13
  Speaker 1: current generator. Its amplitude and duration will depend upon the
• 12:17
  Speaker 1: electrical constants of the system and the distance between the
• 12:20
  Speaker 1: point of short circuit and the source of power supply.
• 12:24
  Speaker 1: In general, the greater the distance between the source of
• 12:26
  Speaker 1: power and the point of short circuit, the less the
• 12:29
  Speaker 1: amplitude of the transient, and the shorter its duration. Automatic
• 12:34
  Speaker 1: generator voltage regulators introduce system transients differing from those which
• 12:38
  Speaker 1: occur in systems not so equipped. They tend to maintain
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  Speaker 1: constant the system pressure, but on account of the inherent
• 12:44
  Speaker 1: time lag of the iron portions of the generator and
• 12:48
  Speaker 1: exciter armatures and fields, their effect is not felt immediately.
• 12:52
  Speaker 1: The effect of automatic voltage regulators on system transience may
• 12:56
  Speaker 1: be predicted for any known set of conditions, and account
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  Speaker 1: taken as such effects in the application of relays. Fig.
• 13:03
  Speaker 1: Forty five is a typical acilogram showing the current flowing
• 13:07
  Speaker 1: in one phase of the external circuit when a system
• 13:10
  Speaker 1: is short circuited under ordinary operating conditions. In this diagram,
• 13:15
  Speaker 1: O is the origin of the coordinates and is taken
• 13:18
  Speaker 1: at the instant at which the short circuit occurs. Ox
• 13:22
  Speaker 1: is the axis of absissa, and the absissa represent time.
• 13:27
  Speaker 1: O Y is the axis of ordinates and the ordinance
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  Speaker 1: represent current. CD is a curve passing through the maxima
• 13:34
  Speaker 1: of the wave of total current, and EF is a
• 13:37
  Speaker 1: current passing through the minima. AB is a curve which
• 13:41
  Speaker 1: cuts the vertical everywhere midway between CD and EF. The
• 13:47
  Speaker 1: wave of total current, which the crests lie along curve
• 13:50
  Speaker 1: CD and EF, and of which ordinates are measured from
• 13:54
  Speaker 1: the axis OX, may be regarded as having two components,
• 13:58
  Speaker 1: namely one a direct component, two an alternating component. The
• 14:05
  Speaker 1: direct component is represented at any time by the ordinate
• 14:08
  Speaker 1: to the curve AB, or at the time X by
• 14:12
  Speaker 1: the coordinate GH. The alternating component is a wave whose
• 14:16
  Speaker 1: cress value at any time is the difference between the
• 14:20
  Speaker 1: ordinance to the curve CD and AB. This difference at
• 14:24
  Speaker 1: the time X has the value HJ. The RMS values
• 14:29
  Speaker 1: of this alternating component are shown on curve st At
• 14:33
  Speaker 1: any instant, this component is considered to have the same
• 14:36
  Speaker 1: RMS value as an alternating wave of constant amplitude whose
• 14:41
  Speaker 1: CRESS value is represented by one half the distance between
• 14:44
  Speaker 1: curve CD and EF at that instant. The RMS value
• 14:49
  Speaker 1: of the total current wave under short circuit at any
• 14:52
  Speaker 1: instant is the square root of the sum of the
• 14:54
  Speaker 1: squares of the value of the direct component and the
• 14:57
  Speaker 1: effective value of the alternating component at that instant. The
• 15:01
  Speaker 1: correct application of relays to an electrical system requires that
• 15:04
  Speaker 1: certain characteristics of the system be known or assumed. These
• 15:08
  Speaker 1: system characteristics are one normal voltage, two maximum current carried
• 15:15
  Speaker 1: by the circuit in which the relay is connected, three
• 15:20
  Speaker 1: normal frequency section, four maximum CAVIA capacity of synchronous apparatus, transformers, reactors,
• 15:28
  Speaker 1: and lines. Five transind current characteristics during short circuit or overload.
• 15:35
  Speaker 1: The RMS current at any point of a system under
• 15:38
  Speaker 1: short circuit conditions is affected by the following factors. One
• 15:43
  Speaker 1: the total CAVA, reactants and transient characteristics of the synchronous
• 15:48
  Speaker 1: machines connected to the system. Two number reactance, resistance, capacitance,
• 15:56
  Speaker 1: and arrangement of all circuits over which power can be
• 16:00
  Speaker 1: supplied to the point of short circuit. Three CAVA arrangement
• 16:06
  Speaker 1: resistance reactants and capacitance of all reactors and transformers through
• 16:11
  Speaker 1: which power can be supplied to the point of short circuit. Four,
• 16:16
  Speaker 1: contact resistance at the short circuit. Five, the nature of
• 16:20
  Speaker 1: the short circuit, whether single phase or polyphase. Six, the
• 16:26
  Speaker 1: CAVA and power factor of the load being carried at
• 16:28
  Speaker 1: the time of short circuit. Seven, the point of the
• 16:32
  Speaker 1: voltage pressure wave at which the short circuit was established.
• 16:38
  Speaker 1: Eight the use of automatic voltage regulators. The short circuit
• 16:43
  Speaker 1: transient for systems may be determined by tests, by calculation,
• 16:47
  Speaker 1: or less closely by assumption. Obviously, the determination by test
• 16:53
  Speaker 1: for all circuits of a large system is expensive and
• 16:56
  Speaker 1: involves considerable time and interruption to service. This will be
• 17:00
  Speaker 1: practical in but a few cases. The determination by calculation
• 17:04
  Speaker 1: is also a matter of considerable labour, but is feasible
• 17:08
  Speaker 1: if only the important factors listed above are considered practical.
• 17:12
  Speaker 1: Approximate selection sufficiently accurate for most cases can be made
• 17:17
  Speaker 1: by using only reactants and an accepted group of time.
• 17:20
  Speaker 1: Current decrement curves table IE gives decrement factors for three
• 17:25
  Speaker 1: phase short circuits. These factors are intended primarily for use
• 17:29
  Speaker 1: in relay application and setting where minimum short circuit values
• 17:33
  Speaker 1: are important. The effect of using RMS values instead of
• 17:37
  Speaker 1: peak values is to appreciably reduce the ratio between short
• 17:41
  Speaker 1: circuit and rated ampers. For example, assuming ten per cent
• 17:45
  Speaker 1: reactants and a short circuit current containing the maximum possible
• 17:49
  Speaker 1: direct component, the ratio of the peak value of the
• 17:53
  Speaker 1: first alternation of the short circuit current to the peak
• 17:56
  Speaker 1: value of rated current is roughly twenty. Under the same condition,
• 18:00
  Speaker 1: the ratio of the RMS value of the first alternation
• 18:03
  Speaker 1: of the short circuit current to the RMS value of
• 18:06
  Speaker 1: the sinusoidal rated current is roughly seventeen. The effect of
• 18:12
  Speaker 1: using the flux at rated voltage and the assumed load
• 18:15
  Speaker 1: instead of at rated voltage and no load is to
• 18:18
  Speaker 1: increase the short circuit current by a somewhat lower percentage
• 18:21
  Speaker 1: than the alternator reactance percentage. This effect in alternating current
• 18:26
  Speaker 1: generators of low reactants is relatively unimportant, but becomes of
• 18:30
  Speaker 1: increasing importance as the generator reactants increases. The characteristic shapes
• 18:36
  Speaker 1: of the time current decrement curves have been reached by
• 18:39
  Speaker 1: analysis of generator tests, including a celigraph studies of short
• 18:43
  Speaker 1: circuits occurring when the generators were excited to full voltage
• 18:46
  Speaker 1: and were carrying various loads at various power factors. In
• 18:51
  Speaker 1: the curse or total reactancies up to including fifteen percent,
• 18:55
  Speaker 1: the reactance is assumed to be wholly within the alternating
• 18:57
  Speaker 1: current generators, and for higher values of reactants, the generators
• 19:03
  Speaker 1: were taken at fifteen percent, and due allowance was made
• 19:06
  Speaker 1: by calculation for the effect of the external reactancies. In
• 19:10
  Speaker 1: the latter case, if alternating current generators of the other
• 19:14
  Speaker 1: reactants had been used, the results would have been somewhat different,
• 19:17
  Speaker 1: but the error is not large enough to be of
• 19:19
  Speaker 1: practical importance. The final values of current ie the sustained
• 19:25
  Speaker 1: short circuit current have been assumed in accordance with experience
• 19:28
  Speaker 1: and tests, and are based on the behavior of machines
• 19:32
  Speaker 1: of predominating design. The study of representative assilograms of short
• 19:36
  Speaker 1: circuit tests showed that in most cases, the direct component
• 19:40
  Speaker 1: disappeared within zero point five second and that the transient
• 19:43
  Speaker 1: portion of the alternating component disappeared within three seconds. These
• 19:48
  Speaker 1: time values have therefore been used in constructing the characteristic
• 19:51
  Speaker 1: decrement curves. Several alternating current generators with the same reactants
• 19:57
  Speaker 1: and synchronous impedance will not necessarily they have the same
• 20:00
  Speaker 1: rate of RMS current decay. This has been considered in
• 20:04
  Speaker 1: constructing the characteristic curves, and the decrement curves may therefore
• 20:08
  Speaker 1: safely be taken as representing the greatest RMS current that
• 20:12
  Speaker 1: will be given by modern alternating current generators of normal design.
• 20:18
  Speaker 1: The percentage reactance in any leg of a circuit is
• 20:21
  Speaker 1: a reactance drop in that leg of the circuit at
• 20:23
  Speaker 1: normal rated current, expressed as a percent of the voltage
• 20:26
  Speaker 1: to neutral of that circuit. For single and two phase systems,
• 20:31
  Speaker 1: this is the voltage between phase wires divided by two.
• 20:34
  Speaker 1: For three phase systems, this is the voltage between phase
• 20:37
  Speaker 1: wires divided by one point seventy three. The percentage values
• 20:41
  Speaker 1: in Table I are initial values based on a symmetrical
• 20:44
  Speaker 1: sin wave and on the maximum rating of the machines
• 20:47
  Speaker 1: connected to the bus. The percentage of reactants of generators
• 20:52
  Speaker 1: varies from about eight percent to thirty percent. The percentage
• 20:56
  Speaker 1: reactance of transformers varies from about three percent to fifteen percent.
• 21:02
  Speaker 1: Nature of short circuits. When making current calculations, it should
• 21:06
  Speaker 1: always be assumed that a short circuit is due to
• 21:09
  Speaker 1: a metallic connection between the conductors In a high voltage
• 21:13
  Speaker 1: aerial line using wooden pins and cross arms. It sometimes
• 21:16
  Speaker 1: happens that an insulator is broken, with the result that
• 21:19
  Speaker 1: the wood is gradually heated by the passage of the
• 21:21
  Speaker 1: current through it, until it finally bursts into flame, thus
• 21:25
  Speaker 1: causing an arc between conductors. A little consideration shows that
• 21:29
  Speaker 1: the flow of current is small until the arc is established,
• 21:33
  Speaker 1: and that it is wrong to speak of automatically disconnecting
• 21:35
  Speaker 1: a section of line which has such a high resistance
• 21:38
  Speaker 1: short circuit. It has sometimes been assumed that an arc
• 21:42
  Speaker 1: has a high resistance, but this is not the case,
• 21:45
  Speaker 1: and in general the presence of an arc at the
• 21:47
  Speaker 1: point of short circuit will not decrease the short circuit
• 21:50
  Speaker 1: current by more than a few percent. Incidentally, it may
• 21:54
  Speaker 1: be of interest to note that on a high voltage
• 21:56
  Speaker 1: ungrounded neutral system, the capacity current to ground through an
• 22:00
  Speaker 1: arc is greater than it is through a direct ground.
• 22:05
  Speaker 1: There is only one case where a short circuit is
• 22:08
  Speaker 1: likely to increase in intensity as it develops, and that
• 22:11
  Speaker 1: is on a system where the neutral is grounded through
• 22:13
  Speaker 1: a resistance A cable breakdown, for instance, frequently occurs first
• 22:18
  Speaker 1: between one conductor and the sheath, and the current flow
• 22:21
  Speaker 1: may be limited by the neutral resistance. The trouble will
• 22:24
  Speaker 1: quickly involve all the conductors in the cable, resulting in
• 22:27
  Speaker 1: a heavy short circuit, but it is possible that it
• 22:30
  Speaker 1: will require an appreciable time to do this, in which
• 22:33
  Speaker 1: case the relay operation may be unsatisfactory. This is particularly
• 22:37
  Speaker 1: liable to happen if the neutral is not grounded at
• 22:40
  Speaker 1: every substation. Section five. The protection of AC apparatus overload protection.
• 22:51
  Speaker 1: Overload protection of the larger type of electrical apparatus, as
• 22:55
  Speaker 1: a general rule, receives little special attention inasmuch as the
• 22:58
  Speaker 1: relays provided for short circuit protection serve also for overload
• 23:02
  Speaker 1: protection of the apparatus of the system in addition to this,
• 23:06
  Speaker 1: in generating. In substations where there are operators in attendance,
• 23:09
  Speaker 1: the load on the apparatus is usually under their supervision
• 23:12
  Speaker 1: and is governed by them accordingly, except in cases of
• 23:15
  Speaker 1: short circuit trouble on the system. In small installations where
• 23:19
  Speaker 1: some scheme of overcurrent protection is necessary, the CO overcurrent
• 23:23
  Speaker 1: relay or CW overpower relay can generally be used to
• 23:27
  Speaker 1: give satisfactory protection protection against internal faults. Protection against internal
• 23:36
  Speaker 1: faults in the windings of electrical apparatus is receiving much attention.
• 23:40
  Speaker 1: In present day practice, these internal faults might be roughly
• 23:44
  Speaker 1: classified as phase to phase short circuits, short circuited turns,
• 23:48
  Speaker 1: open circuits, and grounds. Experience has shown that most of
• 23:52
  Speaker 1: the internal faults developed in larger types of rotating electrical
• 23:55
  Speaker 1: apparatus develop originally as a ground from the phase windings
• 23:59
  Speaker 1: to the frame. If the neutral is not grounded, a
• 24:02
  Speaker 1: fault of this sort will not unbalance the phase current,
• 24:05
  Speaker 1: and it is therefore rather difficult to locate until it
• 24:08
  Speaker 1: is developed into a phase to phase short circuit. Thus,
• 24:12
  Speaker 1: from the consideration of protection, it is best, wherever possible
• 24:17
  Speaker 1: to have the neutral grounded. Grounds are more likely to
• 24:19
  Speaker 1: occur near the line end rather than the neutral end
• 24:22
  Speaker 1: of a winding, but in the event of the latter,
• 24:25
  Speaker 1: the current which flows may be relatively small, especially if
• 24:28
  Speaker 1: a high resistance ground is used. The fact that the
• 24:31
  Speaker 1: fault current is often very small at the beginning, and
• 24:34
  Speaker 1: that it is highly desirable to detect such a fault
• 24:37
  Speaker 1: and disconnect the apparatus from the line before a phase
• 24:39
  Speaker 1: to phase short circuit can develop, makes the protection of
• 24:43
  Speaker 1: apparatus against internal faults rather a difficult problem. A differential
• 24:48
  Speaker 1: scheme of protection, making use of either standard over current
• 24:52
  Speaker 1: relays or special over current relays operating on very small current,
• 24:56
  Speaker 1: is the most general scheme for protection against internal faults.
• 25:00
  Speaker 1: This form of protection consists of balancing the current entering
• 25:03
  Speaker 1: the apparatus against the current flowing from it. This scheme
• 25:07
  Speaker 1: of protection is usually accomplished by connecting a current transformer
• 25:11
  Speaker 1: at each end of each phase winding, connecting their secondaries
• 25:14
  Speaker 1: in series, and then connecting a current relay across these secondaries.
• 25:19
  Speaker 1: As long as the secondary currents are equal, no current
• 25:22
  Speaker 1: will flow through the relay windings. Any leakage of current, however,
• 25:28
  Speaker 1: to other phases or to ground, will upset this balance
• 25:31
  Speaker 1: and send current through the relay. The relay, in turn
• 25:34
  Speaker 1: operates to trip the circuit breaker and disconnect the faulty apparatus.
• 25:39
  Speaker 1: The principle of operation used in this scheme allows relatively
• 25:42
  Speaker 1: low current settings on the over current relays because obviously
• 25:46
  Speaker 1: no current will flow under normal conditions. It is also
• 25:50
  Speaker 1: free from trouble due to circulating current when synchronizing or
• 25:53
  Speaker 1: switching on any part of the system is taking place,
• 25:56
  Speaker 1: inasmuch as such current should be balanced in each phase.
• 26:00
  Speaker 1: This scheme is very flexible in its application and might
• 26:03
  Speaker 1: be applied to machines of any size either star or
• 26:06
  Speaker 1: delta connection, provided that both ends of each phase winding
• 26:09
  Speaker 1: are capable of being brought out and connected to current transformers.
• 26:14
  Speaker 1: No definite rule can be set down in regard to
• 26:17
  Speaker 1: the size of apparatus to which relay protective scheme should
• 26:20
  Speaker 1: be applied. It is largely a question to be decided
• 26:23
  Speaker 1: from a consideration of the importance of limiting the amount
• 26:26
  Speaker 1: of damage that can be done to the machine. In
• 26:28
  Speaker 1: other words, it resolves into an economic consideration in which
• 26:32
  Speaker 1: the cost of relay protection must be balanced against the
• 26:35
  Speaker 1: value of service rendered and then compared with the cost
• 26:38
  Speaker 1: of possible damage to the apparatus. In the case of
• 26:42
  Speaker 1: large generators and sometimes motors, where the neutral is grounded
• 26:46
  Speaker 1: directly or through resistance, and the faulty currents may or
• 26:49
  Speaker 1: may not be of any great value, a relay sensitive
• 26:52
  Speaker 1: to very small current is required. The low energy COO
• 26:57
  Speaker 1: relay is especially adapted for the differential protection of such applications.
• 27:03
  Speaker 1: An improvement on this method has been made by designing
• 27:05
  Speaker 1: a relay the type CAA that trips on a percentage
• 27:09
  Speaker 1: of the line current rather than on a given value
• 27:11
  Speaker 1: of current. This makes it possible to get more sensitive
• 27:14
  Speaker 1: protection without danger of having the relay operate due to
• 27:17
  Speaker 1: the circulating currents which usually occur at times of through
• 27:20
  Speaker 1: short circuits. In the differential protection of transformers, highly sensitive
• 27:26
  Speaker 1: settings are not so necessary and in many cases are undesirable,
• 27:30
  Speaker 1: as they would only serve to disconnect the transformers upon
• 27:33
  Speaker 1: some transient condition which accompanies a momentary unbalance but not
• 27:37
  Speaker 1: a fault. The standard over current co relay is therefore
• 27:42
  Speaker 1: applicable for the differential protection of transformers. A special form
• 27:48
  Speaker 1: of the type caa relay with taps provided so that
• 27:50
  Speaker 1: unequal currents can be balanced without the use of auto transformers,
• 27:54
  Speaker 1: has been developed for transformer protection. This tap arrangement, combined
• 27:58
  Speaker 1: with its adaptability to cases where circulating current appears under
• 28:02
  Speaker 1: transient conditions, makes it of particular value, especially when bushing
• 28:06
  Speaker 1: type transformers are to be used. Open or reverse phase protection.
• 28:13
  Speaker 1: In many installations of polyphase motors and rotary converters of
• 28:17
  Speaker 1: any size, it is becoming a common practice to provide
• 28:21
  Speaker 1: protection against running in case of an open phase or
• 28:24
  Speaker 1: in case of reversal phases. A reversal of two phases,
• 28:27
  Speaker 1: of course, reverses the direction of rotation of any motors
• 28:30
  Speaker 1: on the line, and so it is especially desirable to
• 28:33
  Speaker 1: have this condition protected against. In cases where a reversal
• 28:37
  Speaker 1: of the direction of rotation of any driving motors, such
• 28:40
  Speaker 1: as one in an elevator service, may be disastrous, the
• 28:44
  Speaker 1: CQ phase balance current relay gives ample protection against the
• 28:48
  Speaker 1: occurrence of such conditions on a system. Temperature protection of
• 28:53
  Speaker 1: windings inasmuch as the overheating of the windings and electrical
• 28:58
  Speaker 1: apparatus is closely realid to the overcurrent protection, the overcurrent
• 29:02
  Speaker 1: protection is often adequate to provide the necessary temperature protection.
• 29:07
  Speaker 1: In some cases, however, it is more desirable to furnish
• 29:10
  Speaker 1: protective apparatus which will include in its operation not only
• 29:14
  Speaker 1: the load on the apparatus, but also the existing temperature
• 29:17
  Speaker 1: of the windings. In other cases, the temperature of the
• 29:21
  Speaker 1: winding alone is taken as the determining factor in the
• 29:24
  Speaker 1: operation of a temperature protective scheme. In most of the
• 29:29
  Speaker 1: larger generating stations or substations, where constant supervision of the
• 29:33
  Speaker 1: operation of the various machines as possible, temperature indicating devices
• 29:37
  Speaker 1: are used to show the attendant when the temperature of
• 29:39
  Speaker 1: the windings in questions becomes such that some means should
• 29:42
  Speaker 1: be used to reduce it. Such indications are usually provided
• 29:46
  Speaker 1: by means of exploring coils embedded in the windings of
• 29:49
  Speaker 1: the apparatus, serving either to actuate a signal when the
• 29:52
  Speaker 1: temperature reaches some predetermined point or to operate some temperature
• 29:56
  Speaker 1: indicating instrument. Thus, when the critical degree of heat is indicated,
• 30:01
  Speaker 1: the operator may relieve the condition, either by reducing the
• 30:04
  Speaker 1: load on the apparatus or by increasing any existing cooling means,
• 30:08
  Speaker 1: such as the flow of air or water, as the
• 30:10
  Speaker 1: case may be oil filled thermal relays where manual supervision
• 30:17
  Speaker 1: is not possible, such as in automatic substation equipment and
• 30:21
  Speaker 1: in plants using numerous motors, it is necessary to have
• 30:24
  Speaker 1: temperature protection that will disconnect the apparatus when any predetermined
• 30:28
  Speaker 1: conditions exist. A common scheme for providing such protection is
• 30:32
  Speaker 1: to apply in a protective relay a heating element with
• 30:35
  Speaker 1: thermal characteristics similar to those of the machine. To be protected,
• 30:40
  Speaker 1: a current proportional to the load current in the apparatus
• 30:43
  Speaker 1: itself flows through the heating element of the relay. The
• 30:48
  Speaker 1: characteristics of this element are such that its temperature will
• 30:51
  Speaker 1: be approximately the same as that of the protected apparatus
• 30:54
  Speaker 1: under all loads and at all times. When the temperature
• 30:57
  Speaker 1: of this element reaches the critical points, it serves to
• 31:00
  Speaker 1: trip the apparatus offline. The Westinghouse BA and BD oil
• 31:05
  Speaker 1: filled thermal relays serve to provide this class of protection
• 31:10
  Speaker 1: type CT temperature relay. Another scheme used for the temperature
• 31:15
  Speaker 1: protection of windings is to have the protective apparatus depended
• 31:18
  Speaker 1: on the current flowing at any given time, and also
• 31:21
  Speaker 1: at the same time, on the temperature within the windings
• 31:24
  Speaker 1: of the apparatus to be protected. In such a scheme,
• 31:28
  Speaker 1: the temperature is usually determined by means of copper exploring
• 31:31
  Speaker 1: coils buried in the windings and connected in some sort
• 31:34
  Speaker 1: of bridge arrangement, so that the change in resistance of
• 31:37
  Speaker 1: the exploring coils due to any change in the temperature
• 31:40
  Speaker 1: of the windings will give the proper temperature indication to
• 31:43
  Speaker 1: the protective relay by using the change in the resistance
• 31:46
  Speaker 1: values to cause a flow of current. In addition to this,
• 31:50
  Speaker 1: a current proportional to the current flowing in the apparatus
• 31:54
  Speaker 1: flows through another winding of the relay, and thus the
• 31:57
  Speaker 1: following three conditions are necessary before the apparatus is disconnected
• 32:00
  Speaker 1: from the source of power. First, the temperature of the
• 32:04
  Speaker 1: windings of the apparatus must be above the value for
• 32:07
  Speaker 1: which the relay is set to operate. Second, the current
• 32:11
  Speaker 1: flowing in the apparatus must be above the normal value,
• 32:14
  Speaker 1: and third, these two conditions must have existed for a
• 32:18
  Speaker 1: given period of time. Thus, the liability of the machines
• 32:22
  Speaker 1: being disconnected upon the occurrence of a transient overload is
• 32:25
  Speaker 1: guarded against. The CT temperature relay provides such protection and
• 32:30
  Speaker 1: embodies the necessary characteristics to assure maximum continuity as service.
• 32:35
  Speaker 1: The simplest form of over temperature protection is that in
• 32:38
  Speaker 1: which the temperature of the surrounding material is alone used
• 32:41
  Speaker 1: as an indication of the temperature of the winding and
• 32:44
  Speaker 1: is taken as the determining factor in the operation of
• 32:47
  Speaker 1: any protective relays used. Such a scheme is used largely
• 32:51
  Speaker 1: in the protection of transformers, where the exploring coils, expansion bulbs,
• 32:56
  Speaker 1: or whatever means are used for measuring the temperature of
• 32:59
  Speaker 1: the surrounding material may be embedded between the coils of
• 33:02
  Speaker 1: the transformers in such a way that an approximate indication
• 33:05
  Speaker 1: of the temperature of the windings is secured. The HM
• 33:09
  Speaker 1: thermostat relay is sometimes used for this purpose. Temperature protection
• 33:14
  Speaker 1: of bearings. In the case of rotating machinery, it is
• 33:19
  Speaker 1: often necessary to provide some scheme of protection whereby any
• 33:22
  Speaker 1: overheating in the bearings will either cause a signal to
• 33:25
  Speaker 1: operate or else cause the machine to be disconnected entirely.
• 33:29
  Speaker 1: Where actual supervision is possible, such protection may provide only
• 33:33
  Speaker 1: for the operation of a signal to call the attendant's
• 33:36
  Speaker 1: attention to the fact that all is not well with
• 33:38
  Speaker 1: this machinery. In cases such as automatic substations, it is
• 33:43
  Speaker 1: usually necessary to have the protective apparatus disconnect the machine entirely,
• 33:47
  Speaker 1: either shutting down the station or else disconnecting the individual
• 33:51
  Speaker 1: piece of apparatus. The HM thermostat relay is used largely
• 33:55
  Speaker 1: for this application. End of Chapter three, Parts two