Islanded Microgrid Voltage Control Structure Small-Signal Stability Analysis

Voltage and frequency stability is one major concern within inverter-based microgrid operating in islanded mode. Control of those interfaced inverters in grid-forming mode is one of the proper solutions, which has been widely discussed to improve the islanded microgrid stability. However, impacts of voltage control structure based on conventional proportional-integral controllers (PI) as suitable control technique, on islanded microgrid dynamics and stability, have not been distinctly investigated. Therefore, much control structures, with or without additional loops such as feedforward and decoupling loops, can be found in existing literature when the d-axis and q-axis synchronous reference frame is adopted. This paper proposes a thorough control structure analysis by considering the influence of each supplementary loop on inverter dynamics and stability, and on microgrid. System stability is investigated through computation and trajectory analysis of inverter small-signal model eigenvalues plotted in the complex plane. The obtained modelling results are compared with two model results reported in literature. These show that the effects of feedforward and decoupling loops strongly influence the microgrid dynamics and these help identify robust voltage control structure.