Abstract:

Grid-forming renewable energy sources have attracted widespread attention due to their excellent damping and inertia support characteristics. However, their complex fault response characteristics lead to compromised effectiveness of conventional protection schemes. To address this issue, this paper proposes a novel differential protection scheme combining fault current limiting and waveform similarity. First, based on an analytical model of fault current in grid-forming sources, the failure mechanism of traditional protection is revealed. Further considering inverter current tolerance and grid code requirements, an adaptive virtual impedance calculation module is designed to form an auxiliary control strategy for grid-forming sources, effectively suppressing inverter overcurrent during system short circuits. Finally, based on waveform similarity theory, cosine similarity is employed to quantify current waveforms at both ends of transmission lines. By analyzing significant differences in similarity between internal and external faults, a new differential protection criterion is established. A simulation model built in PSCAD/EMTDC verifies the proposed scheme under various fault scenarios. Results demonstrate that the scheme achieves simultaneous short-circuit current suppression and accurate fault zone identification, maintaining high reliability, sensitivity, and rapidness even under high-resistance grounding conditions.

Key words: grid-forming control, transmission lines, virtual impedance, cosine similarity, pilot protection