Техническое чтение для энергетиков. Бухарова Г.П. - 64 стр.

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66
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. In 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 connected 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 single 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 low-voltage 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 one-hundredth of the original value respectively.
The magnetic and electric circuits of a current transformer are represented
diagrammatically in Fig. 14; the primary winding is shown surrounding one limb of
Fig. 14. Magnetic and electric
circuits of a current transformer

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