The Equivalent Models of Grid-Forming Inverters in the Sequence Domain for the Steady-State Analysis of Power Systems

The modeling of electric sources in the sequence domain helps understand their response during faults when they are integrated into power systems. Since the fault response of inverters is significantly different from that of synchronous generators, their equivalent sequence models must differ as well. While the sequence models of synchronous generators are well-known and verified, practical sequence models of inverters remain an area of research. This manuscript addresses the modeling of grid-forming inverters in the sequence domain where the focus is on three-leg three-phase inverters. To generalize the analysis, two different control strategies used with grid-forming inverters are considered. The equivalent sequence models of these inverters with both strategies are discussed in detail and numerical simulations are used to further clarify the analysis. It is observed that grid-forming inverters with different types of control strategies show a consistent behavior in the positive-sequence circuit during faults for a given voltage drop. Conversely, the negative-sequence model of grid-forming inverters is heavily influenced by the control strategy. The analysis and results are useful to develop reliable protection schemes in inverter-based grids, as understanding the characteristics of electrical elements in the sequence domain is essential for any protective function.