Comparative Analysis of ANSI and IEC Current Transformers in Protection Applications: Copyright Material IEEE, Paper No. PCIC-2018-24

Current transformers (CT) produce scaled values of primary currents suitable for protective relays and measuring devices. For correct implementation of protective relays and proper use of their functions, it is important to understand standards to specify the current transformers and how selection of current transformer parameters influences performance of the relays. The same protection relays manufactured by the international manufacturers or by the US manufacturers are used on both international and US projects. However, the current transformers used on international and US projects are different and standards for current transformers, international (IEC) and in the United States (ANSI/IEEE), are different. This paper discusses methods for protective current transformer selection per IEC and per ANSI/IEEE and provides comparison between the two methods. The intention of this paper is to help engineers familiar with one set of standards to better understand definition of protective current transformer parameters in the other set of standards and how current transformer parameter definitions per different standards may influence performance of relays and relaying schemes.