Modeling inverters with volt-var functions in grid-connected mode and droop control method in islanded mode

New control strategies for distributed generation have emerged in recent years. Since 2009, EPRI and others have been defining standard functions for inverters operating in grid-connected mode. In islanded mode, droop control method is one of the strategies commonly used for distributed generation in microgrids. This paper develops steady-state models for inverters operating with advanced functions such as “intelligent volt-var” or with droop control for simulations with backward-forward sweep power flow-based methods. The models consider changes in system frequency and reference node voltage, fulfilling the zero external power condition, as the case may require. A linear approximation of the system is used to avoid convergence issues in the iterative adjustment of reactive power. The numerical results in the grid-connected mode were consistent with OpenDSS. In the islanded mode, the inverter model faithfully represents the droop control, and the proposed method for power flow based on backward-forward sweep set zero power from the outside, adjusting both system frequency and the reference voltage efficiently. The models proposed will help researchers and companies to update power flow tools, including advanced operating modes of inverters.