Short-circuit protection in DC ship grids based on MMC with full-bridge modules

Abstract

DC grids offer highly efficient distribution of electric energy, eliminating components and optimizing the use of cables. Efficient generation, however, is still based on AC generators. Power-electronic devices link generators and grid and customize energy flow to the loads. Efficient distribution with low losses demands high voltage, e.g. in the range of 10 kV for ship-size grids. Such voltages challenge power electronics as well as protective devices: Short-circuit protection with fast auto-reclosing in a context where fuses are not applicable has to be solved. This paper proposes modular multilevel converters (MMC) with full-bride (4QC) modules and demonstrates fault mitigation in controlled MMC operation by measurement results. The associated control separates asset characteristic defining the steady-state and transient behaviour and converter-near control operating in the subtransient and transient regime. The feasibility of the approach is demonstrated by selected measurement results.