PD measurements at the place of installation of the transformer
For many years, PD measurements in transformers installed at stations were made only using the AE method. These measurements are made during normal operation of the transformer. Due to the development of measurement technology, for the last several years it has also been possible to use under these conditions the electrical method and UHF. In measurement practice, very often these two methods are used simultaneously. A UHF probe is placed in the middle of the transformer using an oil evacuation valve. On the other hand, in the electrical method it is the measurement terminals of bushings that are usually the source of the measurement signal.
Energo-Complex makes PD measurements when transformers are supplied with rated voltage from a so-called rigid network or from an external source. A significantly lower level of interference is obtained when power is supplied from a system consisting of a generator and a transformer that increases voltage. In this case it is also possible to make measurements in a wider voltage range. However, the selection of the power source for measurement purposes is an individual matter and depends on the specific situation of the transformer and network conditions.
In the measurement practice, Energo-Complex uses Montesto 200 measuring and recording system, which enables a simultaneous analysis of electrical signals as well as signals recorded in the ultra-high frequency band by a UHF probe.
Off-line measurements of transformer PD
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.
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.
On-line PD monitoring
Energo-Complex also offers the possibility of continuous monitoring of a transformer with regard to PD detection in the insulation system of the unit. Discharges that occur in the insulation of power transformer windings are a highly stochastic phenomenon. They often change their amplitude or intensity of occurrence. In many cases a reliable assessment of the degree of damage to the insulation system requires observation of the phenomenon over a longer period of time. A device that works in the monitoring mode is permanently installed on the tested unit, while access to the measurement data is possible remotely using a computer or mobile devices.
During the configuration of the monitoring function it is possible to determine the time intervals in which recording is carried out and the duration as well as all metrological parameters of the measurement signal. Basic measurement values can be read out very easily using the cursor. After clicking on a given measurement point, a graphic window opens in which a full analysis can be performed.