Blocking Converter Criteria Considering Protection Coordination Strategies of VSC-MTDC

Multiterminal high-voltage direct current (MTDC) systems are anticipated to enable the massive integration of renewable energies into modern grids. However, the protection of MTDC systems is a major challenge for their application in connection with offshore wind farms (OWF). This is due to the extremely high short circuit currents, present for very short timeframes, and characterised by the absence of zero-crossing currents. Additionally, economic and technological issues further challenge widespread implementation of MTDC systems. The main constraints of MTDC protection coordination systems are generally associated with fault detection, DC reactor (DCR), DC circuit breaker (DCCB) operation time, and converter blocking time. This paper presents strategies for achieving full-selectivity (F-S) and partial-selectivity (P-S) in the protection of voltage source converter (VSC)-MTDC systems. A case study is setup to design both the F-S and P-S protection schemes. Initially, suitable DCRs are calculated for the case study, followed by selecting DCCB operation times with offshore converter blocking for both cases. Finally, an inverse time-current curve is proposed for different scenarios, considering sensitivity to DCR ratings to enhance converter blocking performance. The case study is conducted in PSCAD/EMTDC software and the simulation results demonstrate the impact of the different scenarios on the MTDC systems.