Partial discharges are generated in the insulation system of stator of rotating machines, where the local electric field strength exceeds break value. The measurement of partial discharges in rotating machinery can be made online (i.e. during normal operation with load or idle) or offline (i.e. during the machine stop, energizing each winding one after another). Insulation materials used in rotating machines are resistant to a certain level of partial discharge.
Therefore, in normal operating conditions a lot of pulses of discharge partially in each of the phases might expect. However, certain partial discharges can be indicative of defects in aging in isolation or damage to the structure of the winding: loose coils or damaged semiconductor coatings can give rise to partial discharges in slots; the hollows in the insulation or delamination may create internal partial discharges; moisture or contamination of the wound superficial partial discharges will be created. There are several different sources of partial discharges simultaneously activated in the stator winding insulation. Therefore, the key to the correct measurement of partial discharges in the stator windings is the separation of sources of partial discharges which are active in parallel and the distinction between harmful partial discharges, cases of normal partial discharges and noise outside, inevitably present in the industrial environment. Engineers commonly used nowadays digital partial discharge measurement system MPD 600 system synchronous multi-channel. It works with three or more channels synchronized to separate sources of internal partial discharges between itself and also to separate signal from noise, and individually assesses each source of partial discharges in the stator winding.