Transmission-Distribution Co-Simulation: Analytical Methods for Iterative Coupling

TL;DR

This paper introduces an iterative co-simulation framework for integrated transmission and distribution system analysis, effectively modeling unbalance and demand variability with proven convergence and improved computational efficiency.

Contribution

It develops analytical expressions for T&D interface coupling and compares fixed-point and Newton's methods for nonlinear system convergence, enhancing integrated system analysis.

Findings

Newton's method converges faster than FPI.

The framework models unbalance effects accurately.

Convergence is achieved under stressed system conditions.

Abstract

With the increased penetrations of distributed energy resources (DERs), the need for integrated transmission and distribution system analysis (T&D) is imperative. 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. First-order and second-order convergent methods using the Fixed-point iteration (FPI) method and Newton's method, respectively are 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 a 9-bus IEEE transmission test system integrated with a real-world 6000-bus distribution test system. The results show 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. As expected, Newton's method converges faster with a fewer number of iterations as compared to the FPI method and the improvements are more pronounced during high levels of system unbalance and high loading conditions.