Application of optical sensors for measurement of high frequency overvoltages in power transformers

For power utilities and the manufacturers of power transformers, the knowledge of very fast transient overvoltages (VFTO) in the network is of great interest. Indeed, the VFTOs contain frequencies far above those arising in conventional open-air substations and may, for example, be associated with faults or resonance in GIS insulation or in the insulation of other equipment connected to it such as the transformer. The knowledge of the generated VFTOs is strategic to avoid damage on the insulation system of the connected equipment. Until today no device really exists industrially capable of measuring such high frequencies with a suitable bandwidth. Alstom has developed a special "on-line" sensor, based on the optical sensor technology. A fraction of the high voltage is given by a geometrical capacitive divider near the base of the high voltage bushing. This fraction of voltage is applied to a Pockels cell specially studied to measure high frequencies. The use of optical transmission to the electronics provides good protection against electromagnetic disturbances and guarantees correct representation of the VFTO. The measurement allows validations of mathematical simulations of the generated VFTOs. This information is now available in the design stage of the substation.