Condition Assessment of Power Transformers by UHF PD Measurements

Partial discharge (PD) measurement has been established to detect local defects in the oil/paper insulation. Partial discharges can be measured by either electrical PD measurements according to IEC 60270 or by electromagnetic measurements in the ultra-high frequency range (UHF: 300 MHz -3 GHz). The electromagnetic emissions of PD are recorded using an UHF antenna which is installed into the transformer tank. The importance of PD measurements is reflected in the standard for electrical measurements (IEC 60270). Because of the standard, apparent charge, QIEC, is now an essential factor with regard to determining insulation quality in transformers. To also be an accepted factor for quality testing, UHF technology must be reliable and reproducible to complement electrical PD measurements in FAT and SAT. To achieve this, a standardized calibration procedure is needed that makes UHF measurement results from different systems comparable. Because this calibration procedure is lacking, current UHF technology is not yet used as an acceptance criterion for type or routine testing. However, it shows advantages for acceptance tests and diagnostic purposes, e.g. with regard to the signal/noise ratio and the localization of PD sources The contribution will present two different types of ultra-high frequency (UHF) PD sensors for power transformers and explains the installation process. One is for new transformers using dielectric windows and the other is for retrofit of transformers using drain valves. Also, the contribution will provide a description of the calibration process for electromagnetic UHF PD measurements consisting of two steps: 1) Calibration of the measurement system analogous to the calibration of the electrical PD measurement circuit using a defined reference signal. 2) Calibration of the used UHF sensor using its antenna factor (AF). Furthermore, it will provide a recommendation/strategy on how to place window type UHF sensors at new power transformers. Another important aspect of UHF condition monitoring is the three-dimensional localization of PD sources. A combination of electromagnetic simulations and AI was used to improve the localization of PD sources.