## 1 Introduction
In the UHF method, the source of interference may be transmitters of radio stations and civil services, air navigation systems and transmitters of DVB-T digital television operators, and GSM mobile telephony operators. Therefore, if the measuring system records only pulses of one type (UHF or AE), their source may be both interference and PDs. In turn, if the system simultaneously records UHF and AE pulses, with correlated changes in their parameters (such as amplitude, energy, and frequency), it can be assumed with a high probability that these signals originate from PDs. Additionally, the reliability of this diagnosis can be increased by finding a correlation between PD parameters and basic transformer parameters recorded by the superior SCADA system. In the last decade, research has intensified on the simultaneous use of two or more PD detection methods to improve the reliability of the evaluation of the condition of the transformer insulation system. A large part of the publications concerns the issue of the localization of PD sources using the TDoA technique, which involves the analysis of the differences in the arrival times of acoustic signals to AE sensors in relation to the radio signal recorded by a UHF antenna or a high-frequency current transformer. The research results have shown that the accuracy of the location of the PD source using the combined electromagnetic-acoustic method is higher than that of a fully acoustic system with four or more ultrasonic transducers [20]. A relatively new area of scientific research is related to the development of combined PD detectors. [PERSON] et al. proposed a detector consisting of an antenna and an AE sensor, which is introduced into the transformer tank through an oil valve [21]. A similar design of the in-oil PD sensor was also presented by [PERSON] et al. [22]. In turn, [PERSON] et al. proposed a sensor that comprises a conformal L-shaped ultrasonic array with 13 elements and an ultra-high-frequency electromagnetic sensor [23]. This L-shaped array creates 97 virtual elements through advanced high-order cumulant processing technology, which improves the aperture and direction sharpness of the array. In [24], [PERSON] presented an active dielectric window (ADW), which is a new concept of a combined acoustic emission (AE) and an electromagnetic PD detector intended for assembly in a transformer inspection hatch. The ADW has been optimized for operation in mineral oil, which ensures higher PD detection sensitivity than the universal AE sensor mounted outside the transformer tank. In [25], [PERSON] et al. presented a novel approach to monitoring power transformers using PD measurements simultaneously according to the IEC 60270 standard [26] and radio techniques. The authors propose a method that allows one to distinguish between internal sources of discharges and external disturbances, as well as to identify different internal sources of PD. Online PD monitoring using the AE and UHF methods simultaneously is such a new issue that there is a lack of publications that present in detail the practical experience collected in this area. In the work of [27] published in 2020, the authors presented the first online PD monitoring system, whose operation was based on three diagnostic methods (HF, UHF, and AE). The prototype of the multi-sensor monitoring system was first validated on a large 330 MVA power transformer during an induced voltage test with partial discharge (IVPD) measurement. Then, it was mounted on a 31.5 MVA substation power transformer and integrated with the SCADA system that records the voltage, active power, and oil temperature of the monitored device. The results obtained confirmed the advantages of the simultaneous use of three PD detection techniques and the possibility of correlating discharge parameters with other parameters of the power transformer.