Low-Energy Protection System for DC Grids Based on Full-Bridge MMC Converters

This paper studies protection and control methods for a large dc grid based solely on full-bridge modular multilevel converters (MMCs). An initial theoretical study concludes that dc circuit breakers (CB) energy dissipation will depend on inductance and square of fault current, but not on protection operating time. Using differential protection is proposed because of robust selectivity and since it operates well with small series inductors. The analysis of dc CB dissipated energy leads to new protection logic design that delays tripping signals until local current reduces to low values. Low-speed dc CB with very small inductors are adequate. The design of controllers for MMC converters should be coordinated with dc grid protection and the study derives values for current controller references. The fault recovery time is found to depend on current reference settings at MMC terminals, and optimal values are derived. The conclusions are confirmed using Electromagnetic Transients Program (EMTP) simulation on a 400-kV, 4-MMC dc grid considering two topologies: with five overhead lines and with five dc cables.