This paper presents new developed noise reduction methods and results of partial discharge detection and measurements of electric power apparatus using digital signal processing techniques. We have examined noise diminution by the algorithm in laboratory based on measurements of PD signals in SF6 gas and real noise as well as interference in practical HV and EHV substations. Consequently, the techniques, enabled us to reduce sinusoidal continuous noise considerably, so that they are promising for future application in actual HV substations.