Distributed PV Penetration Impact Analysis on Transmission System Voltages using Co-Simulation

With the growing penetrations of distributed energy resources (DERs), it is imperative to evaluate their impacts on transmission system operations. In this paper, an iteratively coupled transmission and distribution (T&D) co-simulation framework is employed to study the impacts of increasing penetrations of distribution-connected photovoltaic (PV) systems on transmission system voltages. The co-simulation framework introduces iterative coupling and unbalanced transmission system analysis that help accurately replicate the standalone T&D system results without resorting to the computational challenge of developing large-scale standalone T\&D models. The integrated T&D systems are evaluated for multiple PV deployment scenarios based on randomly generated PV locations and sizes. A test system is simulated using the IEEE-9 bus transmission system model integrated at each load point with three EPRI's Ckt-24 distribution feeder models. The results are thoroughly validated using a standalone T&D system model simulated in OpenDSS

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

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