A metrology grade fibre optical current sensor
This work presents the design and experimental evaluation of a traceable, wide
bandwidth, fibre optic based sensor system for measuring electrical current up to 10 kA.
The proposed system aims at surpassing the performance of the existing fibre optic
current sensing systems to provide tools for performing non-intrusive calibration of
conventional current sensors on the power grid. The sensor system is designed for a
nominal current of 10 kA with a target measurement uncertainty better than 0.01% for 0.1
to 120% of the rated current. The target sampling rate of the system is 10 kS/s.
The key challenges are to achieve traceable calibration with a measurement uncertainty
of better than 0.01 % for 0.1 % to 120 % of a rated current of 10 kA under field conditions.
A measurement uncertainty of 0.02 % is needed for calibration of the sensors in the grid.
This range is three orders of magnitude compared to the 1 % to 120 % defined in the
standards dealing with instrument transformers for protection and metering applications.
Three different signal detection schemes have been studied, one system using
interferometric detection and two systems using phase sensitive detection. The
interferometer design offer highly sensitive detection, but is sensitive to perturbations as
acoustics and temperature. The phase detection design has given a robust and sensitive
solution but need further development to be used for metrology and possible future onsite
calibration. All systems offer a sampling rate of at least 100 kS/s. The final all fibreoptical
system, mounted in a reflective mode configuration and based on phase detection
is presented here. A lock-in technique for phase modulation has been used to increase
the sensitivity and noise immunity. In parallel to experimental work a theoretical model of
the complete system is now available.
A concept has been developed for producing High Birefringence Polarisation Maintaining
spun fibre, which is crucial for separating the Faraday Effect from contributions by
birefringence. The sensor systems were designed for a wavelength around 635 nm, in
contrast to commercial systems running at 1310 or 1550 nm, in order to increase the
sensitivity. Doping with Terbium nanoparticles of the fibre material was attempted to
increase the Faraday Effect, but was not successful probably owing to absorption of light
by dopant itself.
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