Small-Signal Stability Analysis of Droop-Controlled Point-to-Point Low-Frequency AC Transmission Lines

Due to better and more efficient forms of AC/AC conversion, Low-Frequency AC Transmission (LFAC) has been proposed in the literature as an alternative for long-distance AC transmission and offshore wind farm interconnections. The idea of LFAC is the formation of a transmission sub-grid at the desired voltage and frequency using a grid-forming control strategy such as Droop Control. For this reason, this work evaluates the small-signal stability of a point-to-point LFAC line controlled by Droop Control to establish safe limits in selecting the droop gains. In this sense, the Eigenvalue Analysis is used to determine how the droop gains and the system’s parameters affect the stability. The results show that the overall system’s dynamic is governed by the sum of the converters’ droop gains. Additionally, the droop gain at the marginally stable operating point is affected by the line’s length, the line’s R/X ratio, the operating frequency, and voltage. Also, the slow dynamics in the power measurement filter increases the droop gain range over which the system is stable. This work also proposes useful symbolic expressions that allow us to visualize how the system’s parameters affect the marginally stable operating point. Finally, simulation and experimental results are presented to validate the analytical findings.