Overcurrent Protection Scheme Utilising Reed Switches Instead of Current Transformers

This paper states the need to create overcurrent protection without the use of current transformers due to their inherent disadvantages. Reed switches are proposed as current sensors, since they can simultaneously act as a current sensor, a current relay, and an analog-to-digital converter. The known overcurrent protections utilising reed switches are analyzed. The three closest alternatives of the proposed protection scheme are considered in more detail. A certain disadvantage of these protection schemes is failure to trip in case of a short circuit if reed switch contacts are stuck. Another disadvantage is the possibility of a false tripping if an interference occurs in the reed switch control winding due to a magnetic field. To eliminate the first disadvantage, a corresponding time relay is proposed to be started after each tripping of the reed switch with a delay shorter than the protection time for the interval between the first and the considered tripping of the reed switch. To eliminate the second disadvantage, the control winding and contacts of the specified reed switch are proposed to be connected in series to the control winding and contacts of the blocking reed switch. The latter has normally closed contacts, which open only in case of interference in the control winding circuit. The operation of the protection circuits and the measurement device has been analyzed in various modes of operation. A method for the protection scheme measurement device adjustment is proposed, which takes into account the inequality of the calculated and actual inductions of the magnetic field acting on the reed switch.