The first results of investigations having the purpose of evaluating the Partial Discharge (PD) behavior at constant voltage, under different current and temperatures levels, in Medium Voltage (MV) underground cables, are reported and discussed in this paper. Twenty MV cables equipped with different types of joints were connected together to form a ring in short circuit. The ring was energized at the rated voltage of 13.3 kV (phase-to-ground) and different currents were circulated to simulate different loads. PD measurements were performed when the temperature reached a steady-state condition in correspondence of the different current levels. An innovative PD instrument capable of acquiring the PD amplitudes and to identify the different defect typologies, has been used in this investigations. The experimental results show that there is no direct correlation between the temperature increase and the PD amplitude. Moreover, the PD-patterns can change their shape thus providing variable defect identification. These results are very important, particularly to interpret on-line PD test results.
