In 2020 – 2021, the saturation time (t sat) of P class current transformers (CTs) without a non-magnetic air gap in the magnetic core was calculated for power facilities in the Unified Energy System of Russia (UES) to address the risks of improper operation of relay protection (RP) devices during transient modes of short circuits (SC) with the presence of an aperiodic component in the current, leading to CT saturation. It was frequently concluded that replacing these CTs with PR class CTs featuring a non-magnetic gap was necessary. However, several important factors were overlooked in these calculations, including the allowance in certified values for accuracy limit factors, the overestimation of calculated residual magnetic induction values in CT cores, the potential load reduction on CTs through fiber-optic communication between CTs and RP devices, and the use of new algorithmic solutions in modern RP devices. This paper introduces a methodology for calculating the primary current limit ratio of P class CTs based on their current-voltage characteristics. The calculated values of residual magnetic induction in CT cores influenced by currents in load modes were determined. Furthermore, it was suggested to mitigate the impact of residual magnetic induction in CTs by employing synchronized breaker closure in the automatic reclosing cycle. Feasible values for the time constant of the secondary circuit for 10PR class CTs were established when used as current information sources by fast-acting protection devices. It was demonstrated that when using these values, it is acceptable to employ universal CT characteristics provided in GOST R 8669–2019 for calculating the saturation time. Additionally, it was emphasized that compliance verification of CT characteristics for the correct operation of relay protection devices in transient modes, as well as CT selection, should be carried out based on experimentally determined values of the reliable current measurement time, referred to as t rp. © 2023, Springer Science+Business Media, LLC, part of Springer Nature.
