| Summary: | The critical need to model and analyze transmission and distribution (T&D) systems together, given the increasing levels of distributed energy resource (DER) penetrations, has led to the development of several T&D co-simulation platforms, both commercial and open-source. The strength of coupling between the T&D system dictates the accuracy of the co-simulation studies; however, higher accuracy comes at the cost of the increased computational burden. The objective of this paper is to (1) systematically model the different coupling protocols, viz. decoupled (DC), loosely coupled (LC), and tightly coupled (TC), for quasi-static T&D co-simulation studies; and (2) thoroughly compare the three T&D coupling protocols for their accuracy and computational efficiency. The T&D coupling protocols are evaluated for varying system parameters such as DER variability, load unbalances, DER penetration, and size of T&D network. It is observed that the accuracy of both DC and LC models deteriorate with increasing the: (1) system unbalance, (2) DER penetration and variability, and (3) number of T&D coupling points. The results further highlight the need for a tightly coupled (TC) protocol as the T&D system gets more stressed due to the influx of DERs.
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