Differential current measurement using summing current ring-core transformer - has magnetoresistive sensor in first flux gap in core and provides pre-adjustment by flux from permanent magnet and yoke connected across second air gap

摘要

The three-phase and neutral lines (2) pass through an annular core (1) of soft magnetic material as in a summing current transformer, with a magnetoresistive sensor (3) introduced into the air gap (13) for fault current detection. The variation of resistance is measured using a reference voltage (Uref) applied through a feedback differential amplifier (7) to a FET (8). The working point is adjusted by means of the DC field of a permanent magnet (15) and a yoke (16) bridging a second gap (14) in the ring. ADVANTAGE - DC premagnetisation gives precise adjustment of working point of magnetoresistive sensor at esp. favourable cost.