Low voltage microgrids are autonomous subsystems, in which generation, storage and power and electrical energy consumption appear. In the paper the main attention has been paid to the voltage stability issue in low voltage microgrid for different variants of its operation. In the introduction a notion of microgrid has been presented, and also the issue of influence of active and reactive power balance on node voltage level has been described. Then description of voltage stability issue has been presented. The conditions of voltage stability and indicators used to determine voltage stability margin in the microgrid have been described. Description of the low voltage test microgrid, as well as research methodology along with definition of considered variants of its operation have been presented further. The results of exemplary calculations carried out for the daily changes in node load of the active and reactive power, i.e. the voltage and the voltage stability margin indexes in nodes have been presented. Furthermore, the changes of voltage stability margin indexes depending on the variant of the microgrid operation have been presented. Summary and formulation of conclusions related to the issue of voltage stability in microgrids have been included at the end of the paper.

Analytical design of the PID-type controllers for linear plants based on the magnitude optimum criterion usually results in very good control quality and can be applied directly for high-order linear models with dead time, without need of any model reduction. This paper brings an analysis of properties of this tuning method in the case of the PI controller, which shows that it guarantees closed-loop stability and a large stability margin for stable linear plants without zeros, although there are limitations in the case of oscillating plants. In spite of the fact that the magnitude optimum criterion prescribes the closed-loop response only for low frequencies and the stability margin requirements are not explicitly included in the design objective, it reveals that proper open-loop behavior in the middle and high frequency ranges, decisive for the closed-loop stability and robustness, is ensured automatically for the considered class of linear systems if all damping ratios corresponding to poles of the plant transfer function without the dead-time term are sufficiently high.

In recent years, the high frequency oscillation (HFO) accidents caused by long link delay in modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission projects have posed new challenges to the safety and stability of power system operation. This paper adopts delay stability margin to measure the high frequency stability of the MMC-HVDC system and derives the state space model of the MMC-HVDC time-delay system considering the link delay. The Lyapunov direct method is extended to the stability analysis of the MMC-HVDC time-delay system and the delay stability margin of the system is solved based on the linear matrix inequality (LMI). Then the influence of the controller parameters on the delay stability margin of the MMC-HVDC system is analyzed. Based on improved Smith predictive compensation control, an HFO suppression strategy of the MMC-HVDC system is proposed to improve the high frequency stability of the system by equivalently reducing and eliminating the total link delay. The effectiveness of the Lyapunov direct method for solving the delay stability margin of the MMC-HVDC system and the superiority of the proposed HFO suppression strategy are verified by the time-domain simulation in PSCAD/EMTDC. The research provides a novel viewpoint for the study of the HFO and suppression strategy of the MMC-HVDC system.