# Non-Contact High Voltage Measurement in the Online Partial Discharge Monitoring System
## 1 Introduction
The Pearson correlation coefficient between the hourly distribution of the UHF and AE pulses was \(r\) = 0.87 with significance \(p\) = 0.001, which means a very strong positive correlation and confirms the presence of partial discharges. Unfortunately, portable short-term PD monitoring systems that rely on more than one method of PD detection are not yet commonly available on the market. For this reason, another approach may be used to increase the diagnostic reliability of the system. It consists of assessing the correlation between the voltage value on the transformer and the intensity of the partial discharges (or more precisely: the number of pulses registered by the PD detectors). This is due to the fact that the higher the voltage value, the greater the electrical stress in the insulating system, and thus the probability of partial discharges ignition increases. Figure 2 shows the average values of voltage and AE pulse rate (per minute) over the period of one month, which were recorded on the monitored 330 MVA  
Figure 1: The result of the eight-day monitoring of the 31.5 MVA power transformer with the simultaneous use of the acoustic emission (AE) and ultrahigh frequency (UHF) method. power transformer [23]. The Pearson correlation coefficient for these two parameters was \(r=0.90\) with significance \(p=0.001\), which allows us to assume with a high probability that the source of the recorded AE pulses was the phenomenon of partial discharges, the intensity of which strongly depends on the voltage level. For a monitoring system permanently installed on a power transformer, obtaining information about the instantaneous voltage value is not difficult. Such a stationary system is usually integrated with a superior substation supervisory control and data acquisition (SCADA) system that reads the voltage values directly from the voltage transformers. The communication and data transmission between devices is usually carried out using the IEC 61850 standard. In the case of portable systems for temporary (e.g., several days) PD monitoring, obtaining information about the voltage value from the SCADA system or directly from the substation voltage transformers is a much more complicated task, often time-consuming and economically unjustified. This is due to the fact that each new installation and even the smallest interference in the infrastructure of a power substation (e.g., laying new signal cables) requires the preparation of a construction design, which must then be approved by the owner of the substation. This generates additional costs and delays the launch of the PD monitoring system. In addition, the monitoring system, in order to integrate with a supervisory SCADA system, must be equipped with a communication module compliant with the IEC 61850 standard or another data transmission protocol used at the given substation. In addition, the system firmware should be compatible with older versions of communication protocols. In some cases, it may even be necessary to use an additional protocols conversion module. The solution that eliminates the above-mentioned problems is the non-contact method of measuring high voltage discussed in this article. Currently, non-contact measurement techniques are most often based on the use of a Hall effect voltage sensor [24], fiber-optic voltage sensor [25], or capacitive probe [26; 27; 28], the advantage of which is the simple construction and low production cost. For the first time, the concept of using a capacitive probe as a sensor for measuring high voltage was proposed by [PERSON] [29]. Initially, this method was recommended mainly for use in laboratory conditions. In recent years, attempts have been made to adapt this method to field conditions. The works focused mainly on the voltage measurement on the high-voltage overhead line [26; 27; 28] and the high-voltage direct current (HVDC) converter station components [30].