PNNL

Abstract

Historically, two similar grid-forming droop controls are widely reported in literature—the single- and multi-loop droop controls. Although being very similar, the authors find that their dynamic performance and stability characteristic are very different in a microgrid. Compared with single-loop droop control, the multi-loop droop control is prone to be less damped and easier to lose stability under some circumstances. This paper provides a novel insight into the different dynamic responses of these two basic controls. It points out that these two similar controls adjust the angular frequency and voltage magnitude at different locations within the inverter, impacting dynamic response and stability due to the difference in equivalent coupling reactance. The use of single-loop droop control results in a larger coupling reactance, which helps to improve dynamic response and stability. This novel insight is verified through full-order small signal analysis, offline electromagnetic transient simulation, and real-time hardware-in-the-loop simulation experiments. Results show that the microgrid has a larger small signal stability boundary when using single-loop droop control, and this difference increases as the value of an inverter’s inner filter inductance increases.