# Non-Contact High Voltage Measurement in the Online Partial Discharge Monitoring System
## 6 Linear Relationship between the Voltage \(U_{1}\) and \(U_{2}\)  
Equations (1) and (4) show that the relationship between the voltage \(U_{2}\) measured with the capacitive probe and the actual value of the voltage \(U_{1}\) present on the tested object  
Figure 12: Measurement setup for determining the dependence of the signal amplitude received by the capacitive probe on the distance \(r\) from the signal source (spherical HV electrode). Figure 13: Measured dependence of the amplitude of the voltage signal registered by the capacitive probe on the distance \(r\) from the signal source and its approximating function. is linear in the entire voltage range. To confirm this, tests were carried out with the use of the measurement setup previously used to study the distance characteristics (see Figure 12). The capacitive probe was placed 100 cm from the spherical electrode that was the source of the signal. The electrode was supplied with a voltage from 5 to 50 kV. The obtained measurement results are shown in Figure 14.  
For the obtained measurement results, a linear regression analysis was performed using the York algorithm. The high value of the coefficient of determination (\(R^{2}\) = 0.9998) confirms that the applied linear function describes well (in the tested voltage range and for a given system configuration) the dependence of the voltage \(U_{2}\) measured with a capacitive probe on the actual voltage \(U_{1}\) prevailing on the object. This allows the use of a capacitive probe for non-contact (remote) high voltage measurement. Application Example of the Non-Contact High Voltage Measurement Method in Online Partial Discharge Monitoring  
The high voltage non-contact measurement module was first tested in the field during one-day monitoring of a 250 MVA power transformer. Before installing the monitoring system, the location of PD sources was carried out using a standard ausculatory technique, the assumptions of which are described in [37]. The analysis of the measurement results presented in the form of an acoustic map superimposed on the photo of the tested transformer (Figure 15a) showed the existence of two areas in which AE pulses of increased amplitude were recorded. In the first area, which was located directly in front of the upper part of the 1V phase windings, AE pulses with an average amplitude of approximately 350 mV were recorded. In the second area, which covered almost the entire height of the 1W phase winding, AE pulses with an average amplitude of approximately 600 mV were recorded (with the background noise at the level of 30-40 mV). Two piezoelectric AE sensors (CH01 and CH02) of the PDtracker monitoring system were installed in places where an increased amplitude of acoustic signals was found. The CH00 sensor (acting as the reference sensor) was installed close to the 1U phase, where no AE signals had been recorded during previous tests. The location and marking of the AE sensors are shown in Figure 15b. The capacitive probe for high voltage measurement was magnetically attached to one of the sprinkler pipes of the fire protection system, approximately 8 m from the connection point of the 400 kV transmission line with the transformer bushing of phase 1V (Figure 15c). Figure 14: Linear relationship between the voltage \(U_{2}\) measured with the capacitive probe and the actual value of the voltage \(U_{1}\) present on the tested HV object. The installation of a portable PD monitoring system was caused by the alarming results of the analysis of gases dissolved in the oil. On the day that the PD localization was performed, the hydrogen level was 1116 ppm and the acetylene level was 216 ppm. Typical values for the concentrations of these gases given in the IEC 60599 standard are respectively 60-150 ppm for hydrogen and 3-50 ppm for acetylene. In order to avoid a failure, the owner of the transformer decided to turn it off. The transformer was restarted for one day only in order to perform online monitoring of partial discharges and confirm their presence in the insulation system.