As a protection CT may be required to deliver
high values of secondary current, the secondary winding resistance must be made
as low as practicable. Secondary leakage reactance also occurs, particularly in
wound primary current transformers, although its precise measurement is difficult.
The non-linear nature
of the CT
magnetic circuit makes it
difficult to assess
the definite ohmic value representing secondary leakage reactance. It
is, however, normally accepted that a current transformer is of the low
reactance type provided that the following conditions prevail:
a. The core is of the joint less ring type
(including spirally wound cores)
b. The secondary turns are substantially evenly
distributed along the whole length of the magnetic circuit
c. The
primary conductor(s) passes
through the approximate centre
of the core
aperture or, if wound, is
approximately evenly distributed along the whole length of the magnetic circuit
d. Flux equalizing windings, where fitted to the
requirements of the design, consist of at least four parallel-connected coils, evenly distributed along the whole
length of the magnetic circuit, each coil occupying one quadrant
Alternatively, when a current transformer does
not obviously comply with all of the above requirements, it may be proved to be
of low-reactance where:
e. The composite error, as measured in the
accepted way, does not exceed by a factor of 1.3 that error obtained directly
from the V-I
excitation characteristic of the secondary winding
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