Download Current Transformers

Current Transformers
Why Current Transformers are Used. — A current transformer is
an instrument transformer for the transformation of current from one value
to another, usually a lower one, or for the transformation of current at a
high voltage into a proportionate current at a low voltage with respect to
earth potential. Current transformers are used in conjunction with
alternating-current meters or instruments where the current to be measured
is of such magnitude that the meter or instrument current coil cannot
conveniently be made' of sufficient carrying capacity. They are also used
wherever high-voltage current has to be metered, because of the difficulty of
providing adequate insulation in the meter- itself. In this connection-supply
voltages exceeding 660 volts are considered to be high voltage. I n meter
practice current transformers are used wherever the current to be metered
exceeds 100 amperes, and in some instances a lower value than this is
regarded as the desirable maximum for direct measurement.
Construction of Current Transformers. — A current transformer
comprises a magnetic circuit, usually in the form of iron stampings
assembled together to form a core, on which are wound two electric circuits
called the primary winding and secondary winding respectively. The primary
winding carries the current to be measured and is con nected in the main
circuit. The secondary winding carries a current proportional to the current to
be measured and the secondary terminals are connected to the current
winding of the meter or instrument. Both windings are insulated from the
core and from each other. The secondary insulation is arranged to withstand
a test pressure of 2,000 volts applied between the winding and the core for
one minute. The insulation of the primary is arranged to withstand for
one minute a test pressure applied between the primary and secondary
windings approximately equal to four times the voltage existing under
working conditions. During this test the core and the secondary winding
are connected together.
The primary circuit of a current transformer may consist of a sin gle
conductor in the form of a bar or cable instead of a winding, when the
current to be measured is of the order of 600 amperes or more. In lowvoltage circuits the current to be measured may be so heavy that it is not
convenient to provide a primary integral with the transformer and the latter
then consists of an iron core of appropriate shape with a secondary
winding thereon, the whole being mounted
on the busbar or cable. The nominal full-load current of a transformer is termed
the "rated primary current" and is the value in amperes of the primary
current marked on the rating plate.
The secondary winding of a current
transformer is usually constructed to
deliver five amperes to the meter or
instrument ' when rated primary
current flows in the main circuit.
This is referred to as the "rated
secondary current" and five amperes is
the standard value adopted in most
countries. In power-station practice
it is not unusual for the meter to be separated from its current
transformers by a distance of several hundred feet. The PR loss in the
connecting leads together with the loss in the meter current coils may
impose a burden in excess of the transformer rating if a five-ampere
secondary current is adhered to. By adopting a lower value for the rated
secondary current the loss in the leads can be substantially reduced and
one ampere or 0.5 ampere values are permissible alternatives. Since the
loss varies as the square of the current the adoption of one of these
alternatives will reduce the loss in the leads to one -twenty-fifth or onehundredth of the original value respectively.
The magnetic and electric circuits of a current transformer are
represented diagrammatically in Fig. 14; the prima ry winding is shown
surrounding one limb of the core and the secondary winding surrounding
another. In actual practice the two windings would not be separated in this
manner as the primary would be superimposed on the secondary, but they
are shown thus for the sake of clarity in the diagram. The primary
terminals are indicated by the letters M and L, and the secondary terminals
by the same letters enclosed in a circle.
The cores of current transformers are usually built up with lami nations of
silicon-steel but where a high degree of accuracy is desired a high-permeability
nickel-steel such as Mumetal or Permalloy may be used. Three types of
magnetic circuit are in common use, namely, "ri ng-type", "core-type" and
"shell-type" and are illustrated in Fig. 15.
М.А. Беляева и др. «Сборник технических текстов на англ. языке»