RT23 |Real-Time Simulation of Three-Phase Dual-Active Bridge

The use of dc voltage in various applications is becoming increasingly popular due to the need for efficient and sustainable energy supply, especially with the integration of renewable energies and energy storage systems. Although dc grids offer several advantages such as improved controllability, system efficiency, and economical feasibility, protection strategies for dc grids are more challenging than those for ac grids. This is because dc grids lack the zero crossing of the current. As a result, it requires specially designed dc circuit breakers or breaker-less dc grids that use output current limiting modulation schemes to clear faults. As part of the FEN medium-voltage dc research grid, a three-phase Dual-Active Bridge (DAB) designed for 5 kV is being investigated. The converter operates at a switching frequency of 1 kHz, and three single-phase high-frequency transformers provide the galvanic isolation. To use this dc/dc converter in breaker-less dc grids, a fault current limiting control has been developed, which takes several aspects of high-power converters into account. The most critical factor is the small leakage inductance of the converter transformers, which results in high power dynamics during normal operation and excessive fault currents when the dc link voltage collapses. The presentation will demonstrate the necessity for high sampling frequencies and concise time steps when performing real-time simulations of a three-phase DAB, comparing the results with an actual prototype. Furthermore, it will showcase fault-current limiting modulations using the novel electric hardware solver, contrasting these results with offline simulations. Finally, the conclusion will feature a controller-hardware-in-the-loop simulation, illustrating a fault-ride through within a single converter to use in a breaker-less dc grid.