Fiber optic current sensor

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A fiber optic current sensor (FOCS) is a current sensor for measuring direct current. By using a single-ended optical fiber around the current conductor that utilizes the magneto-optic effect (Faraday effect), FOCS measures uni- or bidirectional DC currents of up to 600 kA within +/- 0.1 % of the measured value.[1][2]

The benefits of the FOCS in comparison with conventional current sensors is that it doesn’t need recalibration after installation or at any point throughout its lifetime, and it’s unaffected by stray magnetic fields due to its optical measuring. Because of the latter there’s no need for magnetic centering, and there’s also no reduction in accuracy due to saturation effects.[1][2]

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.[3]


A disconnecting circuit breaker with integrated FOCS

In 2013, ABB launched a 420kV Disconnecting Circuit Breaker with integrated FOCS. Since one FOCS replaces many conventional current transformers, engineering and design of the substation is simplified. By reducing the material needed, including insulation, a 420kV DCB with integrated FOCS can reduce a substation’s footprint by over 50% compared to the conventional solution of live tank breakers with disconnectors and current transformers.[4]