Zone-1 Protection of Transmission Lines Connecting Battery Energy Storage System

Large-scale battery energy storage systems are increasingly integrated into the grid through transmission lines. Protecting these lines using distance relays is challenging due to the bidirectional nature of battery current flow and the current control scheme of the interfacing inverter, which affects the magnitude and phase angle of the relay current during faults. As a result, the impedance calculated by the distance relay at the battery end often remains outside the tripping zone for various faults. This paper presents a Zone-1 protection algorithm for transmission lines connected to converter-interfaced battery storage. The method estimates the positive sequence line impedance up to the fault point by determining the phase angle of the faulted loop current and computing the equivalent grid impedance and its voltage. Using the substitution theorem, the fault path current angle is resolved to address nonhomogeneity in the system. The proposed algorithm is validated using PSCAD/EMTDC simulations on a modified New England 39-bus system with battery storage. The method accurately detects faults under various charging/discharging modes and reactive power support scenarios, in accordance with IEEE Std P2800-2022. A comparative assessment demonstrates its reliability across different fault types, resistances, and locations.