Positive-Sequence Modeling of Droop-Controlled Grid-Forming Inverters for Transient Stability Simulation of Transmission Systems

This article describes a positive-sequence model to represent two widely reported droop-controlled grid-forming inverters for transmission system transient stability simulation studies. Methods of how to develop the equivalent voltage source behind impedance to represent inverters with and without inner control loops, modeling of P-f and Q-V droop controls, active and reactive power limiting modeling, and modeling of fault current limiting controls have been described in detail. The model was implemented in commercially available positive-sequence simulation tools and compared with detailed electromagnetic transient models in both a single-grid-forming-inverter infinite-bus system and a modified IEEE 39-bus system. Finally, the model was tested on the U.S. Western Interconnection system. The study results showed that the model has a good level of accuracy compared to the detailed EMT models, and at the same time also achieves a high computational efficiency, making it suitable for large-scale transmission system simulation studies.