Line protection scheme for four-terminal flexible DC ring grids based on integrated characteristics of two-terminal transient currents

To address the difficulty of balancing selectivity and sensitivity in existing two-terminal transient protection for four-terminal VSC-HVDC ring networks with centralized current-limiting reactors—caused by the lack of physical boundaries and the multi-path propagation of traveling waves—a protection method based on integrated characteristics of two-terminal transient currents is proposed. Analysis based on the fault superimposed network reveals that internal faults exhibit consistent polarities of two-terminal components along with local energy dominance. In contrast, disturbed non-faulted lines display false co-directionality due to multi-path interference but lack energy superiority. The proposed method utilizes cosine similarity to characterize the direction for initial screening, and employs the local energy ratio, incorporating the energy of adjacent lines, to determine the attribution of the response energy for intensity verification. These two aspects form a coordinated criterion via a logical AND operation. Furthermore, fault pole selection is achieved using the transient energy ratio between the positive and negative poles. PSCAD/EMTDC simulations demonstrate that this method can reliably identify internal and external faults, block disturbed lines, effectively improve protection selectivity under the simulated conditions, and exhibit a certain degree of robustness against fault resistance and noise.