Measurement of currents in systems of power-system protection from single-phase earth faults and in automated control by arc-suppression reactors.

The behavior of a null-string current transformer as an inertial element was analyzed, and the transfer function was derived in accordance with the equivalent circuit of current transformer. The oscillograms of real null-string currents and their mathematical description in the time and frequency domains were presented. The spectral characteristics of mathematical models of signals depending on the form and duration of the signal observation were shown. The signal passage through the measurement transformer in the transient mode was investigated. Mathematical expressions in the time and frequency domains that describe the output signals and errors of transform of the null-string current transformer for various input actions were obtained. The experimental results are presented, in particular, the oscillograms of real transient processes for various transformers, which confirm the reliability of the used mathematical models. The relations of the parameters influencing the transform error during the transformer operation in the dynamic mode were found. A suitability of the existing null-string current transformers for power protection purposes was estimated. Requirements for new null-string current transformers taking into account the features of modern digital power-system-protection devices were formulated. [ABSTRACT FROM AUTHOR]