Direct Monitoring of Axial Clamping Forces in Power Transformers Revisited

Transformers are the most complex and costly components of the electric power grid and that is why there are monitoring systems for most operating parameters except for the winding axial clamping forces. It has been demonstrated that they vary in time as a result of stresses to which the transformer is subjected. An objective assessment of the lifetime mechanical stability can be made only by the direct measurement of the static winding clamping forces which, irrespective of the operating regime, must be greater than the associated electro-dynamic forces. The measuring systems described in the paper have evolved over time, thus fiber optic sensors for local or global sensing are used exclusively nowadays. It has been proved that the mechanical stresses resulting from the transformers transport to the putting into operation place can be objectively identified and precisely located using the method described. An application program is being proposed, which is developed in order to relate the measured clamping force to its initial value based on models included in a global correction factor. The system is already used for highly demanded transformers such as those used for internal services in power plants or for short-circuit tests in high power laboratories but can be used both in new large power transformers or in on-site refurbished/repaired ones.