Investigação sobre os efeitos da saturação em transformadores de corrente de proteção

Abstract

This work is dedicated to the evaluation of current transformer (CT) saturation and of factors involved in this phenomenon, taking as reference the CT performance in asymmetrical short-circuit conditions. Initially, the system conditions where the CT is applied are outlined. The parameters necessary for evaluating the CT performance are identified, namely system reactance and resistance, absolute value and angle of short-circuit current, and primary and secondary time constants. Other important parameters refer to the characteristics of the load seen from the CTs secondary terminal and the CTs saturation voltage. Together with the asymmetry of the primary current and its representation in the CT secondary, these parameters play an important role on the saturation of the CT core. For evaluating the performance of CTs of classes 10B200 and 10B400, both a spreadsheet proposed by IEEE and an equivalent model implemented in ATPDraw are considered. In both cases, three different load conditions are assumed in the CTs secondary terminal. The results indicate that the two evaluated CT classes lead almost to the same results if their respective standard loads are considered, the same happening if the default impedance value is maintained except for a change in its power factor to a value close to 1. If a load with low-ohmic value is connected at the secondary of both CTs, the CT class 10B200 leads to a less satisfactory result, with higher initial saturation in relation to TC class 10B400. Although this result does not hinder the use of CT class 10B200 in the particular condition investigated in this work, it is indicative of the need to assess whether the relay is able to perform the compensation of the CT saturation and actuate in the first cycles of the fault.