A Comparative Study of Two Widely Used Grid-Forming Droop Controls on Microgrid Small-Signal Stability

Historically, two similar grid-forming droop controls are widely reported in literature-the single-loop and multi-loop droop controls. Although being very similar, the authors find that the dynamic performance and stability characteristics of each control method are very different in a microgrid. Compared with the single-loop droop control, the multi-loop droop control is prone to be less damped and loses stability more easily under some circumstances. This article provides a novel insight into the different dynamic responses of the two basic controls. It points out that the two similar controls adjust the angular frequency and voltage magnitude at different locations within the inverter, resulting in different coupling reactances that impact the dynamic response and stability of microgrids differently. The use of the single-loop droop control results in a larger coupling reactance, which helps improve the 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. The 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.