A method and device for determining partial discharge in an electrical insulation system of a medium- or high voltage apparatus, the method and apparatus including: detecting, with a sensing unit, an electrical signal that comprises partial discharge pulses due to dielectric breakdowns in the electrical insulation system of the medium- or high voltage apparatus; generating an in-phase and a quadrature-signal from the electrical signal comprising the partial discharge pulses in an I/Q-mixer unit by mixing the electrical signal including the partial discharge pulses with two local oscillator signals having the same frequency and a phase difference of 90°; filtering each of the in-phase and the quadrature signals with a low-pass filter; processing the filtered in-phase and the quadrature signals in a processing unit with a low-frequency Analog-Digital converter to obtain digital signals representing an in-phase and a quadrature value.