Interpolation-Free Hybrid Bergeron–π Line Model with Accurate Zero-Sequence Impedance

Fixed-step real-time electromagnetic-transient (EMT) simulation of large power networks typically relies on parallel partitioning, where transmission-line elements serve as step-synchronous decoupling boundaries between subsystems. In distribution and subtransmission studies, however, many line sections are electrically short and have propagation delays smaller than the simulation step. Classical Bergeron models then lose their pure one-step delay structure and require interpolation or sub-stepping, which undermines step determinism and limits the availability of decoupling boundaries, thereby constraining partition quality and scalability. This paper proposes an interpolation-free hybrid Bergeron–π boundary-line model with zero-sequence impedance modification (HB-π-ZIM). A one-step uncoupled per-phase Bergeron section enforces a delay equal to the simulation step to provide a strictly step-synchronous interface. Shunt compensation removes the artificial shunt susceptance introduced by the enforced delay, and a passive RL two-port synthesis reconstructs the residual series impedance so that, at the fundamental frequency, the terminal positive- and zero-sequence series impedances and shunt admittances match the conventional lumped-pi model. Case studies show close agreement with the lumped-pi benchmark under representative balanced and unbalanced transients, while parallel tests on a 327-node network demonstrate near-linear speedup (9.31 times on 10 cores) when HB-π-ZIM is applied only to cut-set lines. The proposed model therefore enlarges the feasible set of decoupling boundaries in short-line-dominated networks and enables scalable fixed-step real-time EMT simulation.