Onshore AC Fault Ride-Through Control in Multi-Terminal HVDC Systems

This paper proposes an AC Fault Ride-Through (AC-FRT) control for onshore Modular Multilevel Converter (MMC) stations in High Voltage Direct Current (HVDC) systems in accordance with modern grid codes. The presented strategy also provides a backup energy controller for AC faults for cross-control MMCs where the converter's total energy is normally regulated through the AC side current. The proposed AC-FRT control is capable of positive and negative sequence voltage support and properly limits current references to the specified safe limits accordingly with a tunable response time. Furthermore, the backup energy control allows for a distinct response for AC-FRT operation, independent of the energy controller employed during normal operation. The effectiveness of the proposed methods is validated using time-domain simulations in a Multi Terminal Direct Current (MTDC) grid. Furthermore, an analysis is carried out to determine the effects of the MMC energy controller and the AC-FRT controls on the AC and MTDC grids interlinked by the converter for different fault scenarios. The analyses above are used to attain a design process for the converter's AC-FRT control response time and backup energy controller in order to attain a reasonable balance between AC fault current response time and MTDC grid dynamic variations.