This paper presents an assessment methodology of protection strategies for meshed grids. It also proposes the computation of two performance indicators to evaluate protection strategies through a reliability and speed perspective. The Monte Carlo method is used to calculate the two indicators proposed. These two indexes can be used as criteria for comparison between protection strategies. Due to the increasing debate around the protection for HVDC grids, three proposals of protection of HVDC grids were chosen as application cases.
This study proposes a non-selective protection strategy for multi-terminal high voltage direct current grids based on resistive-type superconducting fault current limiters (SFCLs). Located at the output of AC/DC converters, the SFCL limits the current contribution from the AC grid in case of DC fault. With this approach, the fault clearing time constraint is relieved allowing the use of mechanical DC circuit breakers for fault current interruption. Furthermore, the breaking capability and energy dissipation requirements of the breaker are greatly reduced. To achieve a fast restoration of the DC grid, a redundant SFCL is introduced in parallel to those in operation, bypassing them when the fault is effectively suppressed. In addition, primary and back-up protection schemes are described and tested using a three-terminal bipolar HVDC grid based on half-bridge modular multilevel converters and cable transmission. Simulations are implemented in EMTP-RV® to analyse and discuss performances of the proposed fault clearing strategy.