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013 - Instrument Transformers · Silent Sentinels- Protective Relays for A-C and D-C Systems

• 0:01
  Speaker 1: Chapter ten of Silent Sentinels by Westinghouse Electric and Manufacturing Company.
• 0:10
  Speaker 1: This LibriVox recording is in the public domain. Instrument transformers.
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  Speaker 1: Instrument transformers are used for two reasons, first to protect
• 0:26
  Speaker 1: station operators from contact with high voltage circuits, and second
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  Speaker 1: to permit the use of instruments with a reasonable amount
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  Speaker 1: of insulation and a reasonable current carrying capacity. The function
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  Speaker 1: of instrument transformers is to deliver to the instruments a
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  Speaker 1: current or voltage which shall be always proportional to the
• 0:54
  Speaker 1: primary current or voltage, and which shall not exceed as
• 0:58
  Speaker 1: safe potential above ground. Generally, the secondary of a voltage
• 1:04
  Speaker 1: transformer is designed for about one hundred and fifteen vaults,
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  Speaker 1: and the secondary of a current transformer for five ampiers,
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  Speaker 1: and both these secondary circuits are grounded together with the
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  Speaker 1: cases of the relays, instruments or meters to which they
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  Speaker 1: are connected. Types and ratings. There are two general classes
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  Speaker 1: of instrument transformers, dry and oil insulated. Dry type voltage
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  Speaker 1: transformers are listed for primary voltages up to the six
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  Speaker 1: nine hundred volt class, and the oil insulated type up
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  Speaker 1: to the sixty six thousand vult class. Current transformers are
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  Speaker 1: listed for primary currents up to ten thousand ampiers voltage
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  Speaker 1: classes up to twenty three thousand volts, the dry type
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  Speaker 1: of current transformer is used, and for the higher voltage
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  Speaker 1: is the oil insulated type. The demand for transformers of
• 2:13
  Speaker 1: higher ratings than those given is so small that they
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  Speaker 1: are not listed, although they can be built on special order.
• 2:23
  Speaker 1: Oil is used in oil insulated instrument transformers, chiefly for
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  Speaker 1: its insulating properties. It is extremely important, therefore, that it
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  Speaker 1: be of the proper quality and that it be kept
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  Speaker 1: free from dirt and moisture polarity. In connecting instrument transformers
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  Speaker 1: to watt meters, what hour meters, power factor meters, et cetera,
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  Speaker 1: it is necessary to know the relative instantaneous direction of
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  Speaker 1: currents in the leads. For this reason, one primary and
• 3:01
  Speaker 1: one secondary lead of each Westinghouse transformer are marked with
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  Speaker 1: a white polarity marker. The relation of the marked leads
• 3:11
  Speaker 1: is such that the instantaneous direction of the current in
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  Speaker 1: them is the same that is toward the transformer in
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  Speaker 1: the marked primary lead and from the transformer in the
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  Speaker 1: marked secondary lead, or vice versa. This marking of the
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  Speaker 1: leads is carefully checked by a polarity test grounding of secondary.
• 3:36
  Speaker 1: All instrument transformers should be grounded on the secondary side
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  Speaker 1: as an extra precaution against danger from the high voltage
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  Speaker 1: in case the insulation should be punctured by lightning or
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  Speaker 1: other abnormal process. In polyphase groups, any point of the
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  Speaker 1: secondary may be grounded, but it is prof to use
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  Speaker 1: a neutral point or a common wire between two transformers. Insulation.
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  Speaker 1: The insulation of instrument transformers protects the meters and control apparatus,
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  Speaker 1: as well as the power station operators from the high
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  Speaker 1: voltage circuits. It is highly important, therefore, that it be
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  Speaker 1: able to withstand the strains of service. For this reason,
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  Speaker 1: the insulation of Westinghouse instrument transformers is designed with special care.
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  Speaker 1: Dry type transformer coils are impregnated with a compound impervious
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  Speaker 1: to moisture. Coils. For oil insulated transformers are dipped in,
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  Speaker 1: varnished or impregnated to seal them against moisture insulation tests.
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  Speaker 1: The insulation tests conform to the standardization rules of the
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  Speaker 1: American Institute of Electrical Engineers. Voltage transformers uses. Voltage transformers
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  Speaker 1: are used with volt meters, what meters, what hour meters,
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  Speaker 1: power factor meters, frequency meters, synchronoscopes, and synchronizing apparatus, protective
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  Speaker 1: and regulating relays, and the no voltage in over voltage
• 5:24
  Speaker 1: trip coils of automatic circuit breakers. One transformer can be
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  Speaker 1: used for a number of instruments at the same time
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  Speaker 1: if the total current taken by the instruments does not
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  Speaker 1: exceed that for which the transformer is designed and compensated.
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  Speaker 1: Voltage transformers have a capacity of two hundred volt ampiers,
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  Speaker 1: but are compensated to give correct ratio at forty volt ampiers,
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  Speaker 1: as this is the average blow demanded by a voltage transformer.
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  Speaker 1: Special transformers may be compensated for correct ratio at any
• 6:03
  Speaker 1: load up to the full capacity of two hundred volt ampiers.
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  Speaker 1: The standard secondary voltage is one hundred and fifteen volts
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  Speaker 1: nominal to suit standard instruments. Instrument transformers with special secondary
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  Speaker 1: voltages can be made if necessary. Principle of operation, the
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  Speaker 1: voltage transformer is in principle an ordinary constant potential transformer,
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  Speaker 1: especially designed for close regulation, so that the secondary voltage
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  Speaker 1: under any condition will be as nearly as possible a
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  Speaker 1: fixed percentage of the primary voltage. The secondary voltage can
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  Speaker 1: never be exactly proportional to the primary voltage or exactly
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  Speaker 1: opposite in phase to the primary voltage on account of
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  Speaker 1: the losses in the transformer and the magnetic leakage between coils.
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  Speaker 1: There are two classes of errors inherent in voltage transformers,
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  Speaker 1: ratio error and phase angle error. The part of these
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  Speaker 1: errors due to the exciting current is constant for any
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  Speaker 1: particular voltage. It can be reduced to a minimum by
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  Speaker 1: choosing the best quality of iron and working it at
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  Speaker 1: a low magnetic density. The part of the errors due
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  Speaker 1: to the load current varies directly with the load and
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  Speaker 1: can be minimized by making the resistance and reactance of
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  Speaker 1: the windings very low. Current transformers general, the current transformer
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  Speaker 1: is a special development of the transformer principle. The object
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  Speaker 1: is to maintain a constant ratio between the currents in
• 8:01
  Speaker 1: the primary and secondary windings, instead of a constant ratio
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  Speaker 1: between the voltages, which is the usual requirement. Uses. Current
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  Speaker 1: transformers are used with ammeters, watt meters, power factor meters
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  Speaker 1: wt hour meters, compensators, protective and regulating relays, and the
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  Speaker 1: trip coils of circuit breakers. One current transformer can be
• 8:28
  Speaker 1: used to operate several instruments, provided that the combined burden
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  Speaker 1: does not exceed that for which the transformer is designed
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  Speaker 1: and compensated. Principle of operation. The ordinary voltage transformer or
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  Speaker 1: distribution transformer is connected across the line, and the magnetic
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  Speaker 1: flux in the core depends upon the primary voltage. For
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  Speaker 1: a given voltage, the flux is therefore fixed, while a
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  Speaker 1: current crent in the winding rises and falls as the
• 9:03
  Speaker 1: load of the secondary winding changes. The current transformer is
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  Speaker 1: connected directly in series with the line for a fixed
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  Speaker 1: number of instruments in the secondary, which is the usual condition.
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  Speaker 1: A rise or fall in the line current requires a
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  Speaker 1: corresponding rise or fall in the secondary voltage to force
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  Speaker 1: the secondary current through the impedance of the meter load.
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  Speaker 1: The magnetic flux in the iron which supplies this voltage
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  Speaker 1: thus follows the rise and fall of the primary or
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  Speaker 1: line current. In any transformer, the primary ampere turns may
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  Speaker 1: be considered as made up of two parts, one small
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  Speaker 1: element which supplies the magnetizing and core loss current, and
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  Speaker 1: another element which supplies the working current. The working current
• 10:01
  Speaker 1: ampier turns are always exactly equal to the secondary ampire urns.
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  Speaker 1: End of Chapter ten and of silent sentinels by Westinghouse
• 10:16
  Speaker 1: Electric and Manufacturing Company,