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006 - Direct-Current Systems · Silent Sentinels- Protective Relays for A-C and D-C Systems

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  Speaker 1: Chapter four of Silent Sentinels by Westinghouse Electric and Manufacturing Company.
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  Speaker 1: This LibriVox recording is in the public domain. Direct current systems.
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  Speaker 1: The direct current systems of today, though small in comparison
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  Speaker 1: with the alternating current systems, offer relay problems. Equally important,
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  Speaker 1: comparable to the service demanded of other public utilities, is
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  Speaker 1: at which congested cities demand from street railway companies. The
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  Speaker 1: growth and interconnection of some of these systems have created
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  Speaker 1: a demand for adequate direct current relay protection. This demand
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  Speaker 1: for service brought about both the development of service restoring
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  Speaker 1: equipment and the supervision of feeder apparatus by supervisory control.
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  Speaker 1: Protection against overcurrents caused either by accidental faults on feeders
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  Speaker 1: or lines, or by excessive overloads onlines can be affected
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  Speaker 1: by the use of overcurrent relays. The short circuit detector
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  Speaker 1: type of equipment meeting the demands of railway systems, permits
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  Speaker 1: heavy overloads due to acceleration, but gives protection upon the
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  Speaker 1: occurrence of a short circuit on the line. Service restoring
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  Speaker 1: system as in the case of alternating current service, the
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  Speaker 1: question of continuity of service often makes necessary some sort
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  Speaker 1: of device to restore service after it has been interrupted.
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  Speaker 1: Where a feeder has been tripped out because of a fault,
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  Speaker 1: it may sometimes be necessary to provide means for automatically
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  Speaker 1: reclosing the breaker several times, allowing a definite interval of
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  Speaker 1: time between each reclosing. Thus, if the fault is of
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  Speaker 1: a transient nature and clears itself quickly, the breaker will
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  Speaker 1: remain closed and service will be continued on the line. However,
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  Speaker 1: if the fault is of a permanent nature, the breaker
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  Speaker 1: will open each time it is reclosed until the specified
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  Speaker 1: number of reclosures have occurred, and then it will remain
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  Speaker 1: open permanently, indicating that the line requires attention. Another method
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  Speaker 1: used for service restoring is to have a relay which
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  Speaker 1: has included an it designed characteristics, whereby it discriminates between
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  Speaker 1: a good and a bad feeder. In this case, the
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  Speaker 1: feeder is opened under overcurrent or short circuit conditions and
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  Speaker 1: the breakers reclose automatically as soon as the excessive load
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  Speaker 1: is removed. After the breaker has been opened, it may
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  Speaker 1: be reclosed by a resistance measuring relay. This relay determines
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  Speaker 1: when the fault has been cleared sufficiently to allow the
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  Speaker 1: circuit breaker to be closed with safety. When continuity of
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  Speaker 1: services of great importance, and when the capacity behind the
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  Speaker 1: feeder is large, it is desirable to have the breaker
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  Speaker 1: remain closed, except in cases of actual short circuits or
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  Speaker 1: violent overloads. Operators of large urban railway systems demand this
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  Speaker 1: class of service as it permits heavy peaks for acceleration
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  Speaker 1: of the cars and prevents untimely interruption of the service. Nevertheless,
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  Speaker 1: in meeting this demand, the feeder circuits must be protected
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  Speaker 1: against short circuits and heavy overcurrents. The short circuit detector
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  Speaker 1: type of equipment provides this protection without interrupting the circuit
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  Speaker 1: on ordinary overloads. Except for heavy railway service, it is
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  Speaker 1: usually desirable to open a circuit on a definite overcurrent.
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  Speaker 1: For this application, the overcurrent type of equipment with an
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  Speaker 1: overcurrent relay is used instead of the short circuit detector.
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  Speaker 1: The overcurrent setting is usually for instantaneous tripping, but occasionally
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  Speaker 1: a time element is provided protection of direct current apparatus.
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  Speaker 1: The protection of the direct current apparatus might be classified
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  Speaker 1: as protection against overcurrent, reversed current, and over temperature. In
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  Speaker 1: most cases, such protection on motors and generators is secured
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  Speaker 1: by means of circuit breakers equipped with necessary overcurrent or
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  Speaker 1: reverse current coils or by fuses of suitable capacity. In
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  Speaker 1: many installations, however, it is found necessary to use a
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  Speaker 1: protective relay in order to secure the needed accuracy and
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  Speaker 1: reliability of protection. In such cases, the D or to
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  Speaker 1: overcurrent relay is used, and in cases where protection is
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  Speaker 1: really required against reverse current, the d reverse current relay
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  Speaker 1: is applicable. Over temperature protection of direct current apparatus is
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  Speaker 1: obtained in a manner similar to that described under alternating
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  Speaker 1: current apparatus. The BD oil filled thermal relay is used
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  Speaker 1: largely for motor protection, while in some cases where it
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  Speaker 1: is possible to have exploring coils in the windings, the
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  Speaker 1: DT temperature relay is used electrochemical work when a line
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  Speaker 1: supplies a load. For such work as electrochemical processes where
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  Speaker 1: a reversal of polarity is extremely dangerous, it is sometimes
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  Speaker 1: necessary to supply a highly sensitive relay which serves to
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  Speaker 1: disconnect the circuit when any condition shows that the polarity
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  Speaker 1: is liable to reverse the A polarity directional relay has
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  Speaker 1: been used very successfully for this purpose, fulfilling the protective
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  Speaker 1: requirements and passing the underwriter's inspection. The sensitivity and design
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  Speaker 1: of this relay are such that the contacts will be
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  Speaker 1: closed upon any reversal of the polarity of the curve
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  Speaker 1: to which it is connected, or upon a fall in
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  Speaker 1: the voltage to below twenty percent of normal value. End
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  Speaker 1: of Chapter four