Study of the influence of transformer magnetizing current inrush on the operation stability of measuring elements of relay protection with the purpose of improving the methods of selecting their settings

Transformer magnetization current inrush (MCI) occurs in 0,4–750 kV electrical networks when connecting a transformer to the network (during operational switching, testing, etc.). MCI is accompanied by a sharp increase in the r.m.s. value of the current which flows through the transformer / other power facilities supplying it, which can be the cause of incorrect relay protection operation. To eliminate such incorrect operation when MCI occurs, a number of measures of a calculation, design or algorithmic nature are taken in relay protection devices. Measures for tuning out MCI which are associated with the use of calculation methods, however, are based on expressions from the Guidelines for Relay Protection obtained back in the 1970s for electromechanical relays. The main objective of this work is to check the functioning of digital measuring devices of various types in MCI and to justify the relevance/lack of relevance of the use of the mentioned calculation expressions. For study of transient and steady-state modes during MCI, and for assessment of the operation stability of relay protection measuring elements of various types, simulation study in Matlab software was used. The quantitative parameters and characteristics of transformer MCI are studied with the usage of a power transmission line with a transmission branch switched from one side. Diagrams which illustrate the nature of the change in various components of the MCI and the nature of the change in measurements of digital measuring devices of various types when the MCI appears are obtained. An assessment of the relevance of using known calculation expressions in the conditions of widespread use of microprocessor protection is given. The obtained results make it possible to draw conclusions about the advisability of using a number of calculation expressions for setting calculation for overcurrent and distance protection of power transmission lines/transformers in modern conditions and make it possible to simplify the process of parameter setting the specified protection devices.