The measurement equipment and mobile power supply systems enable PD measurements to be made at the place of installation of the transformer with power supply from the mains or from an external mobile power supply system. Such measurements are usually performed when there are reasonable grounds, e.g. in the results of a chromatographic analysis, in the form of the content of gases dissolved in oil that indicate the presence of PD. Making PD measurements using a combination of two measurement methods (the electrical and UHF methods) makes it possible to determine the magnitude and intensity of PD as well as to indicate the affected winding. If we wish to determine the location of PD more accurately, we can also apply a simultaneous PD measurement using the EA method.

Another important reason for making PD measurements is the performance of repair works at the installation location. Even relatively simple repair works, such as a replacement of bushings or repairs of the on-load tap-changer with a selector cage, is not without risk of damage to the components of the insulation system, contamination and moisture in the elements of the active part of the transformer. In the operating practice, there were cases of transformer failures after repairs. These failures did not occur immediately after the voltage was applied, but usually in the first 72 hours after start-up. In these cases, the basic post-installation tests did not reveal any irregularities. Removal of such failures is additionally associated with very high costs and operating difficulties for the owner of the unit. There is therefore an economically reasonable and technically feasible need to verify repair works by means of systems that enable PD measurements on site.

Using the measuring function of the device we record signals from all measuring tracks simultaneously. The graphical window of the program allows us to observe the measured signals and read their values. After the measurements are completed, the saved files can be analysed in detail using the software.

Fig. 5. View of the measurement result analysis software window

Using the software, it is possible to analyse signals generated by PD while eliminating interference signals, the level of which is usually high at a substation. The simultaneous use of UHF measurements also has a very significant impact on the possibility of reducing interference. A UHF probe located inside the metal transformer tank is not exposed to the influence of factors generating interference outside of the unit (the metal transformer tank provides shielding from external interference in a similar way to a Faraday cage). The combination of these two methods makes it possible to effectively reduce the impact of external interference and obtain reliable measurement results. Moreover, it enables, on the basis of measurement results obtained simultaneously from all three phases of the transformer, the determination of the winding in which defects that generate PD are present.