The acoustic emission method and electromagnetic methods have both strengths and weaknesses, therefore none of them may be recognized as more efficient or universal. For this reason, in the context of power transformers diagnostics, the possibility of elaborating a hybrid system combining at least two measurement methods is suggested more and more often. The main advantage of such a system application is increase in PD detection reliability. This is because the individual methods are based on different physical phenomena (detection of electromagnetic and acoustic waves) and different coupling methods (inductive, capacitive, or acoustic), therefore each of them is sensitive to another type of disturbance. For instance, in the AE method, an ultrasonic transducer may register core magnetostriction noise (Barkhausen effect), pumped oil and fans noise, environmental noises (thunderstorms, rain, hail, etc. ), tap changer operation, and other random signals [26]. In the high-frequency method, sensor HFCT is installed outside the transformer tank, therefore, it may register external electromagnetic interferences (EMI) occurring on the substation, such as corona discharge from HV transmission lines and signals accompanying switching operations. In turn, in the UHF method, the antenna may receive a radio signal emitted by FM radio stations, aerial navigation systems, civilian and military aviation, transmitters of civil services, DVB-T digital television, and GSM operators [27]. Therefore, when a hybrid monitoring system, e.g., registers numerous UHF pulses and none of AE signals, it may be assumed that the source of the signal is external EMI. In turn, when the system registers pulses simultaneously by all methods and the fluctuations of their intensity are similar, then with high probability one may assume that they are generated by partial discharges. As it was mentioned before, the credibility of the diagnosis may be increased by finding a correlation between PD parameters and power transformer parameters registered by the SCADA system. Those parameters that may affect the partial discharge occurrence are voltage and oil temperature levels. Other researchers also point to the benefits of combining two PD detection methods. [PERSON] et al. [28] believe that a combination of AE and UHF sensors can be successfully used as a tool to highlight the presence of PD phenomena and can be used conveniently for monitoring and trending. Similar conclusions were formulated by [PERSON] and [PERSON] [29], who indicated that simultaneous registration of PDs by two diagnostic techniques gives the possibility to elaborate new PD descriptors, which increase the efficiency of defects recognition. [PERSON] et al. [30] proved that a combination of the AE and HF methods allows eliminating the diagnostic errors and simultaneously indicate if the problem is caused by partial discharge, mechanical faults, arcing, or loose parts inside the transformer. Combining PD detection methods allows obtaining of additional monitoring system functionalities. For instance, simultaneous registration of UHF and AE improves the accuracy of PD sources localization with trilateration technique [31; 32; 33]. The presented in the article novel on-line partial discharge (PD) monitoring system is based on simultaneous UHF, HF, and AE signal registration. It is a world first system equipped in active dielectric windows, which are a new concept of combined acoustic emission and electromagnetic partial discharge detector for power transformers. Each elaborated prototype PD detector, i.e., meandered planar inverted-F antenna, active dielectric window with built-in ultrasonic transducer as well as high-frequency current transformer (HFCT) were specially optimized to the detection of the partial discharges occurring in oil-paper insulation. Thanks to the implementation of new PD sensor concepts, a significant improvement in sensitivity was achieved for individual partial discharge detection methods.