A combined transmission-distribution system dynamic model with grid-connected DG inverter

With increasing deployment of DG inverters, simulating transmission and distribution systems together is gaining great attention. In this work a dynamic model of a Combined Transmission-Distribution System (CoTDS) is presented. The CoTDS model includes a Distribution System (DS) with static loads, induction motor loads, reactive shunts and DGs, all represented with dynamic details, and is integrated with a comprehensive time-domain Transmission System (TS) model with detailed dynamic characteristics of generator components. The DG inverter model is based on a power-electronic inverter using a Proportional-Integral (PI) controller to control the active and reactive power into the distribution system. A linearized small-signal model is derived from the elaborate non-linear system and is employed to identify the system eigen values. Timedomain simulations and eigen values of the combined system are compared with results from standalone transmission and distribution system models and results indicate the need for a combined simulation platform for conducting further studies on distribution systems with increasing DG penetration.