Modern/distributed electric energy systems, with ever larger penetration of renewable (photovoltaic, wind, wave, and hydro) energy sources and time-variable outputs, are in need of stronger/higher frequency and alternating current (AC) (direct current (DC)) voltage control. In fact, faster and more stable active and reactive power in the presence of frequency and voltage sags and swells is needed. Power electronics-controlled variable speed generators do not have enough energy storage (inertia) for the scope (static synchronous compensators (STATCOMs) included). This is because power electronics tends to decouple the generator from the power system. While virtual inertia control in doubly fed induction generators (DFIGs) offers a partial solution to these problems, a more robust and comprehensive framework is required for advanced grid support. This is how, by extending the dual-excitation principles, the dualaxis excited electric synchronous generators (DE-SG) provide superior flexibility in two variants summarized here: as a multifunctional DFIG and dual-axis vs. single-axis excited synchronous generator (SG), and as a synchronous condenser (SC), with dual DC and AC excitation (as a no-load DFIG with inertia wheel), where variable speed is used to accelerate/decelerate the SC and thus provide additional assistance in frequency stabilization. These solutions, good for short-time transients, are not meant, however, to replace the large bidirectional energy storage systems (pump-hydro, hydrogen, batteries, etc.) which are crucial for the daily inherent variations of output energy in modern power systems with multiple power sources. The present paper offers a summary of techniques used in the dual-axis excited vs. single-axis excited SGs (SE-SGs), and SCs topologies, modeling, and control for better stability in modern multiple-source energy systems. This survey includes multiple case studies to shed light on prominent methods.

Doubly fed induction generators (DFIG)  /  Dualaxis excited electric synchronous generator (DE-SG)  /  Dual-axis excited electric synchronous condenser (DE-SC)  /  Grid stability  /  Virtual inertia