APPLICATION OF MODERN TECHNIQUES FOR THE CONDITION ASSESSMENT OF POWER TRANSFORMERS

1. Summary The outstanding technical and economical importance of power transformers in the electrical transmission and distribution networks does not need to be discussed. The continuous awareness of the evolution of the conditions of the insulation system and of the internal mechanical sturdiness is of great value for the system operator as it allows to optimise the lifecycle management of the machine as well as the scheduling of the maintenance operations. Power transformers are designed to withstand dielectric stresses linked with the operating voltage and overvoltages as calculated in the insulation coordination process, thermal stresses linked with their loading guide and electrodynamical stresses linked with external short-circuits. Internal displacements or damages may be generated during the transportation of the machines if the internal mechanical structure of the windings and core are not correctly tight together. When a transformer has undergone abnormal stresses, it is necessary to check its response to the stresses and consequently its conditions, to be aware of its residual ability to withstand further abnormal forces. The Sweep Frequency Response Analysis method (SFRA) has been found to be adequate for the evaluation of possible displacements and deformation of the windings as well as for the pointing out of short circuits among coils. Furthermore, the level of humidity in the paper-oil insulation system may cause a premature ageing and, in presence of abrupt variation of the temperature, formation of bubbles leading to potential internal discharges in the machine. The Polarization and Depolarization Current (PDC) method has been deeply investigated and analyzed on machines in service and in the workshop and has appeared to be effective in pointing out defects in this respect. The paper reports and discusses the results of part of the activity in the frame of an extensive research program carried out in the laboratory, in the field and in the manufacturers workshop; the acquisition and elaboration techniques are dealt with and some hints for the correct interpretation of test results, based on the experience, are given.