Comprehensive Protection of Furnace Transformers on Magnetically Sensitive Elements

Abstract

A comprehensive protection of furnace transformers on magnetically sensitive elements is proposed, consisting of differential protection and protection against damages to individual phase conductors from low voltage side. In differential protection, a reed switch is installed on the lower side near the conductors of each phase. Its control winding is connected to the current transformer from the higher voltage side. The magnetic fluxes created by the currents in the mentioned conductor and in the winding are compared. If their difference exceeds the value required for the reed switch to operate, it gives a command to turn off the transformer switch. In the first protection against damages to the conductors, the EMF induced in them by the currents of the conductors at n (m, p) points near phase A (B, C) is measured using inductance coils, the average EMF values from the n (m, p) coils are determined, and if one of the values reaches the specified value, the mentioned command is given. In the second protection, three equal groups of phase conductors are each covered by a Rogowski coil; the EMF induced in them by the currents in the conductors are compared with each other, and if the difference of any pair of EMF exceeds a certain value, a signal is given to turn off the switch. Schemes of all developed protections and detuning from the influence of voltage regulation under load of the protected transformer are given. The behavior of the protections in various modes is analyzed.