Using virtual buses and optimal multipliers to converge the sequential AC/DC power flow under high load cases

•The in-depth convergence properties of sequential AC/DC power flow are analyzed.•A modified sequential method using virtual buses and optimal multipliers is proposed.•The decoupled AC grid is augmented with virtual buses to enhance robustness.•Optimal multipliers are applied to the inner loop of NR method to ensure convergence.•It is accurate and efficient for AC/DC system with highly loaded HVDC network. The sequential AC/DC power flow method has been practically employed to solve power flow for systems with HVDC lines, because the sequential method is well aligned with existing Newton–Raphson method for AC systems. However, it is known to be difficult to converge when the HVDC network is highly loaded. In this paper, an in-depth analysis of the divergence of the sequential method under heavy DC power is given. Then, an enhanced approach based on the sequential AC/DC power flow method is proposed to improve power flow convergence under high load cases without compromised accuracy. Two critical techniques, virtual buses and optimal multipliers, are proposed to address the challenge. First, the decoupled AC grid is augmented with virtual buses which enhance the robustness of calculation for inner loops. Then, the inner Newton–Raphson method is combined with optimal multipliers to ensure convergence. Test results with the modified IEEE 14-bus and 300-bus systems demonstrate the effectiveness of the proposed approach and the successful convergence under high load cases which cannot be achieved by conventional sequential AC/DC power flow.