On-site applications of advanced diagnosis methods for quality assessment of insulation of power transformers

On-site diagnosis methods for quality assessment of large power transformers rise in importance, as deregulation of the energy market demands an improved life time management for expensive equipment. Here, the three complementing methods in use in Switzerland are discussed: Off-line partial discharge diagnosis (PD), frequency response analysis (FRA) and polarisation / depolarisation current measurements (PDC). For PD and for FRA noteworthy experience exists already. However, the PDC method became only recently available as an user-friendly method; its aptitude is described in detail and it is shown how to obtain information on the integral condition of the oil-cellulose insulation system from PDC measurements.