# Non-Contact High Voltage Measurement in the Online Partial Discharge Monitoring System
## 3 System Architecture for Non-Contact High Voltage Measurement
### Integration of the High Voltage Measurement Module with the Online PD Monitoring System  
The high voltage contactless measurement module has been integrated with the PDtracker system designed for online partial discharge monitoring, developed at the Poznan University of Technology. The system has eight analog channels, each of them is equipped with an independent analog-to-digital converter, sampling the signal with a maximum frequency of 20 MS/s. The default partial discharge detectors are acoustic emission sensors optimized to detect discharges occurring in oil-paper insulation [33]. The system has also been adapted to work with high-frequency current transformers (HFCT), UHF antennas, and active dielectric windows [12]. The detailed structure of the system and examples of its implementation for monitoring power transformers are discussed, among others, in [23]. The electronic circuit of the signal conditioning module and the DAQ card of the voltage measurement module are placed in a separate, compact housing with dimensions of 6 \(\times\) 6 \(\times\) 4 cm. The power supply and data transmission between the DAQ card and the industrial computer is carried out using the universal serial bus (USB) 2.0, which provides a data transfer up to 60 MB/s. The PDtracker system and the components of the non-contact voltage measurement module are shown in Figure 8.  
Figure 6: Comparison of voltage waveforms recorded in the system with and without a voltage follower. Figure 7: Schematic diagram of non-contact high voltage measurement with the use of a capacitive probe for an autonomous online partial discharge monitoring system. The non-contact high voltage measurement module works under the control of an application developed--similarly to the PDtracker's software--in the NI LabVIEW environment. The program functions related to the data acquisition (DrDAQ card configuration), digital signal processing (filtering and reconstruction), calibration, and display of the voltage value and voltage waveform are available in the main panel of the user interface (Figure 9). The program also allows for the automatic recording of results with an appropriate time step (monitoring of voltage changes over time) and a quick calibration (i.e., determination of the value of coupling capacitance \(C_{1}\) following the principles described in Section 2). The calibration procedure consists of determining the voltage ratio \(v\) of the measuring system based on Equation (1) as the quotient of the reference high voltage \(U_{1}\) and the voltage \(U_{2}\) measured with the capacitive probe. For this purpose, the instantaneous voltage value \(U_{1}\) read from the substation SCADA system or from an analog voltmeter installed in the substation control room is entered into the program. During calibration, the voltage value \(U_{1}\) is usually given by phone by the substation staff. An important function of the program is a special, three-stage sequential filter, thanks to which it is possible to measure the voltage with a capacitive probe even in the presence of strong electromagnetic interference, such as corona discharge, lightning discharge, and substation switching operations. At each stage of the sequence filter, the user can select one of the available filtering options: averaging filter, wavelet filter, Butterworth low-pass filter, or turn off the filtering stage. The standard setting in the program is two-stage filtering,  
Figure 8: Photo of the online PD monitoring system and the components of the non-contact high voltage measurement module. Figure 9: Graphical user interface in the program for configuration, control, and visualization of the high voltage measurement process. whereby the signal is averaged first (with a minimum of 2 and a maximum of 128 voltage periods) and then the higher harmonics and transient disturbances are removed from the signal with a second-order Butterworth low-pass filter.