A device (1) for detecting and processing signals relating to partial electrical discharges in an electrical apparatus (2) powered by a square-wave (3) voltage modulated by a modulating wave (4) in such a way as to form an alternating voltage of predetermined frequency, comprises: a sensor (5) connectable to the apparatus (2) for detecting a discharge signal (6) representing the electrical pulses generated by the partial discharges; a processing unit (7) connected to the sensor (5) to receive the discharge signal (6) and derive the detection instants of each of the pulses detected with respect to a predetermined time reference, a synchronization module (8) designed to receive an electrical signal (9) representing the power supply voltage and equipped with a filter (10) configured for extracting from said power supply signal (9) a signal (11) representing the modulating wave (4), the processing unit (7) being configured for associating with the pulses detected the values of a phase parameter (f) representing the phase of the modulating wave (4) at the pulse detection instants.