Betrachtung galvanisch eingekoppelter Gleichströme im Übertra- gungsnetz und deren Auswirkungen auf Leistungstransformatoren Galvanic DC coupling in transmission grids and its effects on power transformers

Power transformers are facing different stress factors during service. Usually, the effects of DC components superimposing AC phase currents are not taken into consideration. Nevertheless, DC can occur in transmission grids due to different sources. This contribution presents ongoing field measurements which show DC in the range of serval Amperes. Different superimposing DC sources are identified using the obtained data, the known grid layout and additional ambient information. Additionally, mechanical oscillations of the considered transformers are recorded using accelerometers on tank walls. By correlation of the mechanic oscillations and superimposed DC components its impact on transformer noises are determined and the suitability of vibration measurements as DC detector is evaluated. A large-scaled laboratory setup using two connected 380 kV / 110 kV, 350 MVA 5-limb power transformers is used to verify the findings from the flied measurements.