A Framework to Analyze Interactions between Transmission and Distribution Systems

Analyzing the interactions between transmission and distribution (T&D) systems is becoming imperative with the increased penetrations of distributed energy resources (DERs) on electric power distribution networks. An assessment of the impacts of DERs connected to distribution system on the transmission system operations is required especially for the conditions leading to increased level of system unbalance. In this paper, a coupled T&D analysis framework is developed through co-simulation approach. The framework separately solves the decoupled T&D models. The T&D interactions are captured by exchanging the power flow solutions at the point of common coupling (PCC). The proposed co-simulation framework adopts an iterative coupling approach that results in a co-simulation model close to the stand-alone T&D system model. The proposed framework is tested using IEEE-9 bus transmission system model and EPRI's Ckt-24 test distribution feeder model. A case study in which the IEEE-9 bus model interfaced with three ckt24 models is also presented to demonstrate the scalability of the approach. The conditions of convergence by exchanging the boundary variables at the PCC are examined in detail using several case studies with varying levels of load unbalance.