Active Damping Method for Harmonic Current Mitigation in Grid-Connected MMC-HVdc Applications

This paper presents an active damping method to mitigate specific harmonics by means of an enhanced control strategy for Modular Multilevel Converters (MMC). Compared to low level Voltage Source Converters (VSC), the complexity of MMCs reveals additional degrees of freedom and therefore enables implementation of advanced control features. As penetration of power electronic devices within the European ac grid rises, MMC control strategies including network services seem beneficial. Hence, active harmonic damping is implemented in this paper. This measure additionally reduces stresses of high voltage resources.While common active damping methods for HVdc applications based on signal filters use combinations of lead-lag and notch filters to eliminate various resonant frequencies, this paper focusses on the extraction of grid specific harmonics by means of multiple complex coefficient filters. Thus, mitigation of specific harmonics is implemented in a closed control loop, whereby negative interactions with other damping facilities in the grid are avoided and system stability is improved.