Dynamic Transmission and Distribution Analysis for Asymmetrical Networks in the Presence of Distribution-Connected DER Units

This paper aims to study the dynamic performance of the asymmetrical power systems in the presence of distribution network and inverter-based distributed energy resources (DERs). An asymmetrical transmission-distribution (T&D) co-simulation algorithms is developed to simulate the dynamics of transmission and distribution networks together. The proposed co-simulation framework simulates the transmission system through sequence networks whereas a three-phase solver is used to solve the detailed distribution network. The three-phase DER-A control model introduced by the North American Electric Reliability Council (NERC) is applied to the DER unit, which is placed at a distribution substation. The effects of the DER control parameters on the transmission system under balanced and unbalanced system models are investigated. The results show that the positive sequence modeling and simulation cannot accurately predict the synchronous generator and DER dynamic frequency and voltage responses as they can be significantly different under symmetrical and asymmetrical faults. In addition, dynamic overvoltage is different using detailed distribution system model compared to when a symmetrical lumped load represents the distribution network especially for faults within the distribution system. When DER operates in reactive power (Q-Priority) priority control, reactive power injection can help maintain the voltage fluctuations due to load variations; however, higher control gains can lead to large voltage spikes. Limited results of this work have been published in TPEC2023. This paper has significantly more theoretical results and case studies.