Background. Since 2019, when designing and operating relay protection and automation systems (RPA), much attention has been paid to the operation of electromagnetic current transformers (CTs) in transient short-circuit (SC) modes. The calculated value of the time until the CT magnetic core is saturated is used as a characteristic of CT operation in such modes. This time is compared with the time of accurate current transformation required by RPA. Considering that the introduction of such calculations has occurred recently, with the introduction of GOST R 58669-2019, the volume of published research on this issue is relatively small. The existing studies often lack recommendations for the practical application of the proposed methods for special cases (for example, the connection diagram of the secondary windings of the CT “sum of currents of two stars”) and possible ways to improve their accuracy. A topical issue is the automation of calculations of the time to saturation of the CT, since design engineers note that performing calculations “manually” takes a lot of time and requires large labor costs. In addition, GOST R 58669-2019 provides some assumptions intended for “manual” calculations, which can be avoided with automation and at the same time increase the final accuracy.
Materials and methods. The authors have used the methods of mathematical analysis to determine the time to saturation of magnetic cores of CTs of various classes and expressions compiled in accordance with the fundamental laws of the theoretical foundations of electrical engineering for calculating the magnitude of the load connected to the CT with various connection schemes of secondary windings. The derived analytical formulas have been verified using simulation modeling in the MATLAB/Simulink software package.
Results. The authors have considered some issues that make it possible to mitigate the existing shortcomings of the calculation methods of GOST R 58669-2019. The authors have developed and verified analytical formulas to determine the initial phase of the periodic component of the SC current, at which the minimum time to saturation of the CT magnetic cores (classes P, PR, TPX, TPY, TPZ) is achieved during the first and second SC. Clarified analytical formulas are given to determine the value of the load connected to the CT at various connection schemes of their secondary windings for all possible types of SC. Cases of application of the developed formulas when performing calculations for individual branches feeding the SC location are considered. Software implementation of the developed analytical expressions and formulas in a single calculation service is performed.
Conclusions. The obtained analytical formulas can be used for implementation of design calculations in terms of selecting CT characteristics that provide the required time to saturation of their magnetic cores under specified operating conditions. The research results have been implemented in the cloud service for modeling electric power facilities “RZA Laboratory”. Future research is related to the development of methods for accurately determining the values of the attenuation time constant of the aperiodic component of SC currents in branches, considering intersystem connections, to improve the accuracy of calculations of the time to saturation of CT magnetic cores.