On the Loadability and Voltage Stability of Islanded AC-DC Hybrid Microgrids During Contingencies

The future smart grid encompasses ac-dc clusters known as ac-dc microgrids. For reliability and security purposes, each microgrid hosts a mix of synchronous-based and converter-based distributed resources. However, synchronous-based generators, in particular, are characterized by their limited reactive power capabilities because of the limitation of their excitation systems, which could lead to voltage instability problems when the hosting microgrid is islanded. AC-DC microgrids also comprise controllable thermal and electrical loads. Most modern ac-dc loads have power electronics interfaces that control the voltage at their ends, exhibiting constant power characteristics. AC-DC microgrids with high penetrations of constant power ac-dc loads are vulnerable to voltage collapse. As with other power system architectures, microgrids are subject to contingencies, e.g., a line circuit outage, during which their loadability can be jeopardized. For this reason, the work reported in this paper involved the investigation of the steady-state voltage stability of islanded ac-dc hybrid microgrids (HMGs) during contingencies. Voltage stability analysis was carried out on a 12-bus ac-dc HMG. Several case studies were designed to reveal the likelihood of voltage instability/collapse in microgrids under severe contingencies. The voltage stability analysis also shows that interfacing ac and dc microgrids does not always enhance loadability. The analysis was carried with the goal of raising awareness among microgrid planners and operators of the possibility that a voltage collapse phenomenon can develop in islanded ac-dc microgrids under extreme events. Contingency analysis should therefore be incorporated into planning and operations criteria for islanded ac-dc microgrids.