2. CURRENT TRANSDUCERS
a) Bulk-Optics Current Transducer
ABB’s first generation magneto-optic current
transducer (MOCT) with more than 10 years of field
experience exploits the Faraday effect in a block of
fused silica glass with a central aperture for the
current conductor1. The polarization-rotation of the
transmitted light is detected after a single pass
around the conductor. The glass body is thermally
annealed to eliminate stress-induced birefringence.
Multimode fibers with a 200 μm core diameter guide
the light from the LED source to the sensor block
and back to the detector. The sensor is mainly used
for revenue metering over a primary current range
from 3150 A to less than 5 A and reaches accuracy
according to IEC class 0.2. For applications
requiring particular immunity to shock and vibration
a sensor version with two counter-propagating
beams has been developed. Subtracting the two
corresponding outputs doubles the current-induced
signal while any reciprocal modulation due to
mechanical effects is largely cancelled.