1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (K.W. ); [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (K.W. ); [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (K.W. ); [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (K.W. ); [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (C.S.) 1  
[PERSON]  
1 Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland; [EMAIL_ADDRESS] (K.W. ); [EMAIL_ADDRESS] (C.S. transformers in Europe [1]. The dynamics of the defect development caused by the activity of partial discharges depends on a variety of factors, including the type of the discharge itself and its energy, location of the defect, degree of aging and moisture content of the insulation system, varying load affecting the oil temperature changes as well as moisture migration process. In some cases, the defect may develop over many years of power transformer operation not leading it to damage. In other cases, the defect may develop extremely rapidly, mostly as a consequence of a random event, such as overvoltage or a sudden increase in oil temperature caused by overloading of a unit or malfunction of the cooling system [1; 5; 6; 7]. Very often the dynamics of the defect development grow in its final stage, shortly before the damage itself [8]. For that reason, currently, the monitoring online systems have gained popularity, since they have the advantage over periodic tests allowing for instant detection of sudden growth of partial discharges activity [9; 10; 11; 12; 13]. Additionally, in combination with the SCADA (supervisory control and data acquisition) system, which is installed on the majority of substations, they allow to find the correlation between PD intensity and power transformer parameters, e.g., voltage or oil temperature [14; 15]. It increases the credibility of insulation system condition assessment and enables the introduction of current limitations of power transformer exploitation, assuming that its condition is not critical, and it does not require an immediate shutdown. At present, the acoustic emission method (AE), electromagnetic methods (high-frequency (HF), very-high frequency (VHF), and ultra-high frequency (UHF)), and dissolved gas analysis (DGA) are some of the most important PD detection methods adjusted to operating in online monitoring mode [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. The last technique is indirect and because of its limitations, such as long time intervals between successive measurements, the sensitivity depending on the dimensions of transformer tank and distance of the gas sensor from the PD source as well as complex and non-standardized methods of DGA results interpretation, it is not capable to detect very quickly developing defects.