An Iterative Co-simulation approach for integrated transmission and Distribution system analysis

Gayathri Krishnamoorthy

Analyzing the interactions between electric power transmission and distribution (T&D) systems is becoming imperative with increased penetrations of distributed energy resources (DERs). An assessment of the impacts of distribution system connected DERs on the transmission system operations is required especially when it may result in system unbalance and increased levels of variability in load demands. This thesis aims at addressing the future requirement of the integrated grid with increasing DER penetrations by developing an iteratively coupled T&D analysis framework through co-simulation approach. The framework utilizes dedicated software modules to separately solve the decoupled T&D models. The T&D interactions are then captured by exchanging network solutions at the point of common coupling (PCC). The proposed co-simulation framework adopts an iterative coupling approach resulting in a co-simulation model that closely approximates the behavior of a stand-alone uniﬁed T&D simulation platform. The integrated T&D framework includes a three-sequence transmission power ﬂow module in MATLAB, a three-phase distribution module in OpenDSS, and a co-simulation interface module developed using MATLAB. Furthermore, an integration of transmission system operational framework is demonstrated along with the integrated co-simulation approach by interfacing a transmission system economic dispatch program. In addition to this, a mathematical model that represents the coupled T&D system with interface constraint equations is developed. This analytical approach helps characterize the convergence of proposed iterative co-simulation framework as a function of T&D system conditions. The developed non-linear interface constraint equations are solved using the ﬁrst order Fixed-Point Iteration (FPI) technique. The proposed framework is tested using IEEE 9-bus transmission test system model and EPRI's Ckt-24 distribution circuit model. A case study in which multiple distribution circuit models coupled to transmission load points 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 distribution load unbalance. It is found that the iteratively coupled co-simulation framework provides accurate convergence characteristics at the boundary compared to the existing loosely coupled co-simulation models.

An Iterative Co-simulation approach for integrated transmission and Distribution system analysis

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