Iteratively-Coupled Co-simulation Framework for Unbalanced Transmission-Distribution System

The need for integrated transmission and distribution system analysis (T & D) has been recently identified by research and industry alike. This paper presents an integrated unbalanced T & D analysis framework using an iteratively coupled co-simulation approach. The unbalanced T & D systems are solved separately using dedicated solvers. An iterative approach is developed for T & D interface coupling and to ensure convergence of the boundary variables. To do so, analytical expressions governing the T & D interface are obtained and first-order convergent methods using Fixed-point iteration (FPI) is proposed to solve the system of nonlinear T & D interface equations. The proposed framework is tested using an integrated T & D system model comprised of 9-bus IEEE transmission test system integrated with a real-world 6000-bus distribution test system. It is demonstrated that the proposed framework can model the impacts of system unbalance and increased demand variability on integrated T & D systems and converges during stressed system conditions.