Modelos de transformador de corrente para estudos em baixas freqüências

Abstract

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.