The PDC-measurement (polarization / depolarization current measurement) is a new modern
measuring procedure for the evaluation of the insulation system (power transformers and
cables). The procedure uses DC voltage to test the specimen. From the DC voltage step a
polarization / depolarization current results in the insulating medium, which decays
exponentially. The PD current is typical in the nA- range. The temporal process, the size and
the shape of the PD current give information about condition and characteristic values (oil
conductivity, polarization time constants).
At a transformer, a DC voltage step is applied between OS and US coil during a certain time
TL the so-called polarization duration (Fig. 3.5). Thus, a charging current of the insulation
system capacitance, the so-called polarization current, flows.