T-connected lines are increasingly applied in hybrid DC systems due to their excellent flexibility and scalability. However, their asymmetric boundaries and the unclear physical boundaries at both ends of the LCC-side boundary element pose challenges for relay protection. To address the inability of conventional DC line protection to identify internal and external faults on the LCC side, this paper proposes an identification method based on the attenuation characteristics of electromagnetic wave energy. On this basis, a complete protection scheme for T-connected lines is proposed. The protection is initiated by the rate of voltage change of the T-connected bus; faults inside and outside the T-zone are identified by the direction of the line-mode current on the T-zone outgoing lines; and internal and external faults on the LCC side are identified by the line-mode energy ratio of electromagnetic waves at both ends of the boundary element. Additionally, the fault pole is selected by the electromagnetic wave energy change of the positive and negative poles. A simulation model of a hybrid DC system containing a T-connected line is constructed on PSCAD/EMTDC, and the effectiveness of the method for identifying internal and external faults on the LCC side and the protection scheme are verified.
Zhang et al. (Thu,) studied this question.