Blackout Protection of HVDC Supergrids

The increased global demand for renewable energy is pushing existing transmission networks to their limits. High voltage direct current (HVDC) is an interesting technology, and resilient HVDC supergrids, capable of preventing cascading blackouts, may be the answer. This study extends an existing Matlab/Simulink point-to-point HVDC model into a three-terminal supergrid equipped with three modular multilevel converters (MMCs) and two transmission links of 400 km each. 5 pole-to-ground faults are introduced per link, and a protection method is implemented based on direct current circuit breakers (DCCBs) and a single-ended voltage derivative algorithm for fault detection. The goal is to isolate the faulty cable while keeping the remaining cables unaffected. Two parameters, the line inductance and breaker trip threshold, are tuned to increase system performance. With an inductance of 50 mH, traveling waves from a neighboring cable cause unintended trips, especially for faults farther from the measurement point. Increasing the inductance to 100 mH reduced the traveling waves and results in a window where the breaker trip threshold can be placed. A value of 6 kV/s is chosen for the threshold. The results show that the current on the faulty cable goes to zero, while keeping other cables unaffected. These results confirm that single-ended detection with properly tuned parameters and DCCBs can protect HVDC grids from cascading failures.