A Fast Method for Estimating Lower Voltage Bounds in Power Distribution Systems*

Power flow analysis is essential for system operation and planning. As distribution networks expand with urbanization, electrification, and distributed resources, computational challenges increase. Traditional nonlinear methods face efficiency and convergence issues with large-scale, complex networks. Establishing voltage lower bounds before detailed calculations prevents wasteful computation on infeasible solutions, reduces resource consumption, defines operational boundaries, and improves decision-making. This paper proposes a method for rapid determination of voltage lower bounds in distribution systems through a unified hybrid AC/DC network modeling framework. The approach extends the LinDistFlow model and theoretically proves its voltage solutions consistently remain below Newton- Raphson solutions. Validation on modified IEEE test cases demonstrates that the proposed model provides strict lower bounds while significantly improving computational efficiency. This research offers a reliable voltage constraint assessment tool for distribution network planning and operation, with important implications for large-scale hybrid AC/DC systems.