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Wednesday 3 April 2019

CT Secondary Winding Impedance

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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