Transformadores de corrente com núcleo de liga nanocristalina utilizados para serviços de medição e proteção em sistemas elétricos.

Abstract

The current transformers (CT) used in electrical systems can be divided into two mainly types: CT for measurement and CT for protection. The main difference in the constitution of the CT types is the material used in the magnetic core. In CT for measuring service are employed cores with high magnetic permeability, and in CT for protection purposes are used cores with high saturation flux density. In this thesis is presented a theoretical and experimental study about the application of nanocrystalline alloys in CT toroidal cores applied to electrical systems. Based on the electromagnetic properties of nanocrystalline alloys, i.e. high relative magnetic permeability, magnetic flux saturation density, high resistivity and low hysteresis losses, are presented and discussed the influence of these characteristics in the CT performance. From the obtained results, it was verified that the use of nanocrystalline alloys in CT cores for measurement services, due to its high permeability, contributes to the reduction of phase errors, thereby improving the accuracy class. The high permeability of nanocrystalline alloys decreases the CT exciting current, and presents a smaller variation of the phase error when changing the load connected to the secondary winding and when the primary current presents harmonic components. In order to use a CT which can be used both for measuring and protection services it was evaluated a CT with composite core, which is composed of a nanocrystalline alloy and FeSi materials. The CT with composite core when tested with a low saturation flux density, the predominant characteristics are the nanocrystalline alloy one, therefore ideal for use as CT for measurement. When CT with composite core is applied with a high saturation density, characteristics of the core FeSi become apparent, being ideal for protection applications. Therefore, the CT with composite core can be used for metering and protection services.