# Non-Contact High Voltage Measurement in the Online Partial Discharge Monitoring System
## 1 Introduction  
The electrical power system belongs to a special type of critical infrastructure, as it determines the security of society and the economy. The key devices that ensure the proper functioning of this system are large power transformers. Unfortunately, a common problem in most countries is that a significant part of these transformers either exceed the lifetime predicted by manufacturers (estimated at about 30-35 years) or are close to this limit. Numerous reports and statistical analyses indicate more and more frequently occurring transformer failures [1; 2; 3]. These failures not only cause electrical power cuts but can also lead to oil leaks and fires (environmental contamination) and damage to high voltage (HV) devices associated with them at a substation or power plant. Replacing a large power transformer is extremely costly and time-consuming, and the lead time is generally over a year [4]. The main causes of transformer failures are mechanical winding defects and defects in the insulation system [5; 6; 7; 8]. The mechanisms of the formation and development of these defects are accompanied by certain symptoms, which, if recognized quickly enough, can provide valuable information on the current technical condition, the expected life of the device, and the possibility of its failure. One of the main and unambiguous symptoms indicating the deteriorating technical condition of the transformer and the impending failure is the partial discharge (PD) phenomenon in its insulation system [1]. With such an advanced degree of transformer overexpression, relying only on periodic diagnostics isinsufficient and generates too high a risk. For this reason, in the last dozen or so years, the intensity of work on the development of online partial discharge monitoring systems has been increased [9; 10; 11; 12; 13]. These systems most often use the acoustic emission (AE) method or electromagnetic methods for recording signals in the high frequency (HF), very high frequency (VHF), and ultrahigh frequency (UHF) radio frequency bands [14; 15; 16; 17; 18; 19]. However, due to the high purchase and maintenance costs, monitoring systems are relatively rarely installed permanently. From an economic point of view, it is much more advantageous to use them in the short-term (temporary) monitoring mode, i.e., for several days or weeks. This is usually long enough to confirm the PD phenomenon's presence and assess its intensity and trend. A significant problem of the implemented PD monitoring systems is their diagnostic reliability related to the need to evaluate whether the source of the recorded signals is the occurrence of partial discharges or external disturbances. Unfortunately, both the acoustic emission method and electromagnetic methods are exposed to numerous external disturbances. In the case of the acoustic method, these can be environmental noises (e.g., rain, hail, thunderstorms, sandstorms, wind), oil circulating pumps and fan noises, noise generated by magnetostriction in the core steel laminations, load noise produced by electromagnetic forces in the windings, or acoustic signals produced by switching operations in a substation [20]. In electromagnetic methods, the sources of interference may be corona discharge in HV transmission lines, switching operations, FM radio stations, aerial navigation systems, civilian and military aviation, transmitters of civil services, transmitters of digital video broadcasting-terrestrial (DVB-T) digital television, or transmitters of the Global System for Mobile Communications (GSM) and Long-Term Evolution (LTE) wireless internet [21; 22]. One way to improve the diagnostic reliability is to use a technically advanced system capable of recording pulses simultaneously with at least two methods, e.g., UHF and AE. If the system registers acoustic and electromagnetic pulses simultaneously, it can be assumed with a high probability that their source is partial discharges. Figure 1 shows the result of an eight-day PD monitoring carried out on a 31.5 MVA power transformer [12].