Influences of Direct Currents on Power Transformers Caused by AC- HVDC Interactions in Hybrid Grids

Summary

The implementation of HVDC systems into the German grid is an essential aspect in the transition from conventional coal and nuclear power to renewable sources. The country's large wind farms in the North of Germany could provide a significant share of electrical power. The main consumption is located in the middle and South. Considering given distances and required additional transmission capacities several studies have shown the need of HVDC corridors from North to South. Nevertheless, the installation of new overhead lines is a sensitive issue in public perception. E.g. the changes in the landscape that come along grid expansion are widely rejected. Therefore, alternatives are considered to improve transmission capacities without the installation of additional line systems. One option is to replace one of two existing AC-systems on an overhead line with a high voltage direct current (HVDC) system. The close proximity of AC and DC on the same tower implicates interactions between the systems which have to be considered in terms of system safety and reliability. Recent field tests published at CIGRE have shown that ions generated by HVDC corona can couple into AC systems. The result is a DC component in the AC system. This can lead to critical operational conditions for affected power transformers. These effects are determined in this contribution. Therefore, two 3-limb transformers for 110 kV are determined in a back-to-back setup. The total amount of DC as well as the distribution on each phase can be varied. The transformers' electric response is determined by the occurring hysteresis effects and the resulting non-active power ratings. Changes in the mechanical oscillation given by the DC dependent magnetostriction are determined by solid borne sound sensors on transformer tank walls. The results of these field tests provide an overview of the severity of DC scenarios.

Additional informations

Authors

Adamczyk Bartlomiej, Wu Junliang, Mizutani Yoshinobu