Abstract
Filters for symmetrical current components can be designed with the use of not only ferromagnetic current transformers, but also with inductors and reed switches. Reed switches are preferable, but this design should include duplication and self-diagnostics to ensure reliability. A measurement unit on the basis of reed switches fixed near the coplanar phases of an electrical installation to be protected is suggested. It simultaneously functions as a current relay and a negative-sequence current filter. The reliability of its operation is ensured by the design simplicity, self-diagnosis of malfunctions, and duplication of elements. A technique for calculating the parameters of the current filter elements is developed. It consists of 29 formulas, most of which are used for the calculation of the coordinates of the reed switches and of the inductions of magnetic fields affecting them. The features of operation of protection with this measurement unit and self-diagnostics procedures are described.