It has been known to measure voltages using optical means.
A known technique for measuring high voltages is described in EP 0 682 261. The technique described in EP 0 682 261 relies on the electro-optical effect, in which an electrical field changes the refractive index or birefringence of a light-transmitting element, namely a bulk single crystal. The crystal is a Pockels-type electro-optical material that changes its refractive index or birefringence linearly with the applied electric field. The method described in EP 0 682 261 allows the voltage to be measured over the crystal. For this purpose, two orthogonally polarized light beams are passed through the crystal twice, and the returned light beams are brought to interference, e.g. in a polarizer or an interferometer, for measuring their mutual phase shift.
Optical voltage sensors of this type for applications in air-insulated high-voltage substations commonly involve expensive dielectric insulation. The sensor of EP 0 682 261 makes use of the electro-optical effect in a bulk Bi4Ge3O12 (BOO) crystal. The voltage of a power line (up to several 100 kV) is applied to the crystal having a length typically between 100 mm and 250 mm. This results in very high electric field strengths near the crystal. To avoid dielectric breakdown, the crystal is mounted in a high-voltage insulator tube filled with SF6 gas under pressure.
The voltage sensor described in Ref. [2] as identified below uses several small electro-optical crystals which are mounted in a high-voltage insulator. The crystals measure the local electric fields. The sum of these local fields serves as an approximation of the line integral of the field. Here, the field strengths are lower and insulation with nitrogen at atmospheric pressure is sufficient. However, this technique involves such as extra measures permittivity-shielding to stabilize the electric field distribution and avoid excessive approximation errors. The present disclosure identifies particular references by numerical designation. The references are specifically identified herein following the detailed description of the exemplary embodiments.