Development and characterisation of precision optical current sensors

For new sub-stations and future power grids fibre optic current sensors represent a class of sensors with high immunity to electric interference,

Coordinator: Jari Hällström

 

For new sub-stations and future power grids fibre optic current sensors represent a class of sensors with high immunity to electric interference, offer compact and cheap solutions, have the potential to achieve a much higher bandwidth than conventional current measurement systems (reaching MHz rather than kHz) and are easily interrogated over long distances. Optical current sensors are now relatively well established and have been developed by a range of manufacturers for the measurement of electrical current. Examples of fibre optical current measurement systems state ranges up to 500 kA, an accuracy and linearity of ±0.1 % for 1 % to 120 % of the rated current, a repeatability of ±0.02 % and a temperature sensitivity of < ± 0.002 %/°C. The sampling rate for this system is typically a few kS/s. 

 

De-regulation of the electricity market has forced the introduction of metering at tie-points that  

were formerly part of a monopoly utility that did not require intermediate metering. Today there is a lack of installed metering class instrument transformers and interest has surfaced to qualify – if possible – existing protection class transformers. Transient phenomena in the high voltage grid can be transmitted to the secondary circuit and possibly cause damage to the energy meter. Methods to perform this type of measurement are not yet fully consolidated and there is a need for a non-intrusive on-site calibration of current sensors in the power grid. The advantage of using an optical sensor technique lies in its non‑intrusiveness and robustness and that it cannot be damaged by overcurrent or transients. With proper design it can also be used for the detection and measurement of transients. It also has a wide bandwidth and can be designed for a wider dynamic range than traditional sensors.

This leads to a need to improve the fibre optic current sensing technique with the final goal of being able to perform non-intrusive calibration of current sensors in the power grids. The system will be designed for a nominal current of 10 kA, a target measurement uncertainty better than 100 ppm (over the range 0.1 % to 120 % of the rated current), and a sampling rate of at least 10 kS/s.

The key challenges are to achieve traceable calibration with a measurement uncertainty of better than 100 ppm for 0.1 % to 120 % of a rated current of 10 kA under field conditions. This range covers 3 orders of magnitude compared to the 1 % to 120 % defined in the documentary standards. Typical fibres in existing systems have either been spun or annealed, to reduce shape induced birefringence, however using new fibre materials with nanoparticles to tailor the desired optical properties has opened possibilities to fine tune the Faraday-effect and suppress unwanted effects. 

For more information, see the project webpage >>