RT22 | Fault Current Limiting Control for Three-Phase Dual-Active Bridge (DAB)

The use of DC technology in various applications is increasingly coming to the fore with a view to efficient and sustainable energy supply with the integration of renewable energies and energy storage systems. Besides advantages in improved controllability, system efficiency, and economic feasibility, protection strategies of dc grids are more challenging compared to ac grids due to the missing natural zero crossing of the current. Therefore, specially designed dc circuit breakers or the conception of breaker-less dc grids, using current limiting modulation schemes need to be developed. As part of the Flexible Electrical Networks (FEN) Medium Voltage Direct Current (MVDC) research grid, a 7 MVA three-phase Dual-Active Bridge (DAB) is built with a nominal voltage of 5 kV. The switching frequency of this converter is 1 kHz, and three single-phase high-frequency transformers provide galvanic isolation and the necessary series inductance. For high power converters in breaker-less dc grids, a fault current limiting control is developed, taking several aspects into account. The most critical factor, thereby, is the small leakage inductance of the converter transformers, resulting in high power dynamics during normal operation and excessive fault currents when the DC link voltage collapses. Another issue is the large minimum turn-on time of the used IGCT semiconductors, which permit the use of conventional modulation schemes. This presentation shows a control-hardware-in-the-loop simulation to test the actual converter control platform in case of a grid fault. Therefore, the control platform is connected via adapter boards to an OPAL-RT OP4510 real-time simulator. One adapter board convert fiber optical signals into electrical while the other one is feeding back measurement signals for the closed loop control. The converter waveforms of the real-time simulation are compared to offline simulation and hardware measurements at 2 KV. Moreover, the fault current limiting modulation in case of pole-to-pole and pole-to-ground fault is implemented on the control platform and tested using the real-time simulation. The results are compared to offline simulation.