Beside inductive current transformers, oil or SF6 insulated, Hitachi ABB Power Grids has developed since 1990s optical current sensors, based on the Faraday effect principle, whereby light is used to deduce the precise magnitude of current that is creating the magnetic field. As a result, the design is inherently free of magnetic saturation, therefore suitable for capturing fast transient currents, short circuit currents, and alternating current (AC) with DC-offset.
FOCS-FS is a 3-phase sensor system consisting of:
- three hollow insulators filled with nitrogen at ambient pressure and supporting the sensor heads
- one outdoor kiosk, installed on the steel structure of the central phase and connected via optical fiber to the three HV columns and to the relay house via a duplex ethernet optical cable (IEC61850-9-2LE protocol).
Any optoelectronic module located in the kiosk does:
- send polarized light to the sensor
- receive the reflecting polarized light from the sensor
- compare in close-loop control the phase displacement in the polarized light in proportion to the magnetic field and the primary current
- convert the result into an optical IEC 61850 Ethernet output.
The level of redundancy that can be specified is given by the number of optoelectronics units integrated into the system.
All solutions can be equipped on request with merging units, which digitalize analog signals coming from voltage transformers, synchronize those signals with the digital signal coming from FOCS-FS and consolidate both signals into a digital output stream complying with IEC61850-9-2 LE protocol.