Accurate Representation of Distribution System Dynamics With DERs for Bulk System Studies

While there has been significant progress in recent years to incorporate the dynamics of distribution systems and industrial loads into the transmission using aggregated composite load models (CLM); there is no study that evaluates the accuracy of a well-tuned CLM model versus the true dynamics of a distribution system (DS) with 3-φ, 1-φ motors, ZIP loads, and distributed energy resources (DERs) connected to a transmission system (TS). This paper aims to address this gap by analyzing the limitations of the CLM and proposing physics-based approaches to address these drawbacks. We show that a single CLM is not flexible enough to represent the aggregated DS dynamics in situations when only a few motors in the DS stall or a few DERs trip. To address the drawback of the single CLM, we propose a DS structure preserving reduced order model (DS-ROM) that clusters nodes in the DS and represents each cluster using a CLM. We then present a Python-based automated approach to determine the parameters of the DS-ROM. Results on IEEE-14-bus TS connected to IEEE-37-node DS and a practical 240-node DS demonstrate that DS-ROM can capture the DS dynamics for various kinds of TS faults with an accuracy of ∼98% .