Communication-Free Protection Scheme With Improved Selectivity and Backup Reliability for Ring DC Networks

This paper proposes a novel protection method for low- and medium-voltage DC networks. It is an easy-to-implement, low-cost, and communication-free protection scheme, applicable in ring configurations. The relays in this method calculate the difference between the post-fault and pre-fault currents, and the tripping time is determined using a time-current curve based on this difference. The core principle of the proposed scheme leverages the reduction in post-fault load current, caused by the significant voltage drop, and the controllable increase in inverter-based distributed generation current relative to their pre-fault values. As a result, relays located closer to the fault detect a more significant current variation, making them trip first. Specifically, in the simulations, relays located near the fault detect a current variation from 100 A to 300 A due to the fault, which can be effectively leveraged for fault detection. This behavior enhances selectivity and ensures reliable backup operation. Simulations conducted in MATLAB/Simulink validate the method speed, selectivity, and reliability. Moreover, the method demonstrates strong noise immunity, as confirmed by simulations, even for signal-to-noise ratios (SNR) as high as 10 dB. A brief comparison with state-of-the-art protection schemes underscores the advantages of the proposed method.