Comparison of acoustical partial discharge signals under AC and DC stress

It is well known that partial discharges (PD) can generate chemical, acoustical, electrical and UHF signals. Acoustical detection methods have an advantage of immunity to electromagnetic noises, which makes their use very common for PD localization. Some new investigations showed that not all of the PDs during AC stresses could generate acoustic emission (AE) signals and were therefore called silent PDs [2]. Investigations under AC stresses have been done to find the effect of different configurations and situations on silent PDs, but still the exact reason of creation this kind of PDs is not clear. Therefore in this paper, investigations on the effect of DC stresses on this kind of PDs will be described, in order to better understand and explain their nature and to determine some characteristics of silent PDs. Special configurations under AC stress have been applied to create silent PDs inside an oil tank. After that, positive and negative DC voltages have been applied to investigate this phenomenon under different DC stresses. AE signals have been recorded by an oscilloscope and electrical PD signals via a standard PD measurement device. The results show different discharges at AC and DC stresses, thus the probable reason of this fact is discussed in details in the contribution.