GUERRA, F. C. F.; http://lattes.cnpq.br/1912783247913427; GUERRA, Francisco das Chagas Fernandes.
Résumé:
The current transformers (CT's) have magnetic cores that present nonlinear properties, such as
saturation, hysteresis and dynamic losses. As a consequence, the currents in the secondary
winding can be distorted. This fact causes improper operation of the measurement and protection
devices, or no operation of these last ones. The problem becomes worse in the case of occurrence
of short-circuits in the primary system, when asymmetric faults currents reach high peak values,
producing deep saturation in the magnetic core. This fact can cause improper operation or nooperation
of protection devices.
This work presents current transformer models that consider saturation, hysteresis and
dynamic losses, which are able to provide simulations with excellent results, concerned the
steady state and transient behavior for low frequencies (25 to 400 Hz, according to ANSI and
IEC).
As a specific contribution, a hysteresis model is proposed, which is able to reproduce the
closing and removal rules of minor loops, as well as remanence. The obtained results by
simulations are validated by comparisons with experimental results as well as the Jiles-Atherton
and Preisach models, recognized as the ones that present a higher degree of importance in the
representation of the magnetic hysteresis.
The classic eddy-current losses and the anomalous losses are represented by resistive
components, determined from measurement of the total magnetic losses, with variable frequency.
Based on the proposed CT model, a correction method of distorted currents in the secondary is
presented, which is applicable to steady and transient states.
Besides, a method of calculation of the time between the instant of the fault incidence and the
instant in which saturation occurs is presented, which presents good results.
