Comparative Analysis of Negative Sequence Behavior in Grid-Following and Grid-Forming Inverters: Modeling, Control, and Protection

Grid-forming (GFM) controls are expected to enhance the stability of power systems with high penetration of inverter-based resources (IBRs). However, during unbalanced grid conditions, GFM controls alter the magnitude and angle of the negative sequence current contributed by IBRs, differing from grid-following (GFL) controls and synchronous generators. This behavior may not be in line with fault ride-through (FRT) requirements and cause maloperation of certain protection elements. This paper clarifies the fundamentals and impacting factors of GFL and GFM inverters in the negative sequence system using phasor-domain analytical models. A simplified impedance computation method for GFM inverters is proposed, and FRT control solutions for GFM and GFL inverters are implemented for balanced positive sequence control (BPSC) and positive and negative sequence control (PNSC) schemes. Comparative studies using the IEEE PSRC D29 system reveal that the magnitude and angle of negative sequence impedance are primarily determined by the inner voltage control parameters under the GFM BPSC scheme. Depending on the settings, GFM inverters can provide significant negative sequence reactive current under the BPSC scheme as opposed to GFL inverters. On the other hand, IBRs should still rely on PNSC strategies to regulate the negative sequence system response appropriately and in coordination.