# Fiber-optic current sensor

A fiber-optic current sensor (FOCS) is a device designed to measure direct current. Utilizing a single-ended optical fiber wrapped around the current conductor, FOCS exploits the magneto-optic effect (Faraday effect). The FOCS can measure uni- or bi-directional DC currents up to 600 kA, with an accuracy within ±0.1% of the measured value.$^{:2}$

## Design

The fiber-optic current sensor uses an interferometer to measure the phase change in the light produced by a magnetic field. As it does not require a magnetic yoke, the FOCS is smaller and lighter than Hall effect current sensors, and its accuracy is not reduced by saturation effects. The inherent insulating properties of the optical fiber make it easier to maintain electrical isolation. It also does not need recalibration after installation or during its service life.
The optical phase detection circuit, light source and digital signal processor are contained within the sensor electronics; this technology has been proven in highly demanding applications such as navigation systems in the air, on land and at sea.$^{:5}$
Interferometric fiber optic current sensors (FOCS) employ circularly polarized light traversing a closed loop path around an electrical conductor's current-generated magnetic flux, which reflects off a mirror and experiences a reciprocal phase shift as the refractive index, and effective path length, is modulated by the presence of a magnetic field which optically induces circular birefringence, and where the interference pattern relative to a reference wave form is a transduced optical intensity value corresponding to the current magnitude.
Such configurations are vulnerable to acoustic perturbations in the fiber optical cables, as changes to the linear birefringence of the fiber cable causes additional phase shifts between the orthogonally polarized modes which must be of equal magnitude to generate circular polarization, as an exact quarter wave displacement between the fast and slow axis modes is required for a circular polarization state, and additional phase shifts in the sensing network cause the circularly polarized measurement photons, which experience a phase shift in the fiber optic current sensing coil in proportion to magnetic field density, to degenerate to a random form of elliptical polarization, which degrades interference measurement abilities as the measurement and reference photon wave forms become non-coherent at the analyzer.

## Applications

As FOCS are resistant to effects from magnetic or electrical field interferences, they are ideal for the measurement of electrical currents and high voltages in electrical power stations.
In 2013, ABB introduced a 420 kV Disconnecting Circuit Breaker (DCB) that integrates FOCS technology replacing many conventional current transformers, thereby simplifying the engineering and design of the substation. By reducing the materials needed (including insulation), a 420 kV DCB with integrated FOCS can reduce a substation's footprint by over 50% by minimizing the need for materials, in comparison to conventional solutions involving live tank breakers with disconnectors and current transformers.