Galvanic coupling of direct currents in transmission grids and its effects on power transformers

Different stress factors impact power transformers during service. In most cases, the effects of additional direct current (DC) components are not taken into consideration. Nevertheless, this contribution considers DC or quasi-DC components which occur in the German transmission grid (400 kV level). Current measurements are therefore performed at several grounded star points of grid coupling power transformers using a customized current monitoring system. At a first glance, DC levels in the range of several Amperes can be observed. The subsequent detailed analysis allows discriminating different superimposing DC sources contributing to star points’ direct currents. The correlation of current measurements at different locations with the actual grid topology and switching operations enables the identification and localization of different sources. Additional comparisons to temporal changes within the earth’s magnetic field provide information about the impact of geomagnetically induced currents (GIC). The effects of observed direct currents on power transformers are determined by an evaluation of mechanical oscillations of affected transformers. The effects are linked to the transformer’s magnetic equivalent circuit and derived from half wave saturation effects of the magnetic hysteresis loop. A large-scaled laboratory setup which consists of two connected 380 kV / 110 kV, 350 MVA 5-limb power transformers is used to emulate DC effects and mechanical oscillations. Comparisons to field measurements are used to verify the model theory and the setup. In this context, the ability of low cost vibration measurement systems to detect direct currents in power transformers is also evaluated.