Hierarchical frequency control strategy of hybrid droop/VSG-based islanded microgrids

•A frequency control strategy in a hybrid droop/VSG-based islanded microgrid is investigated.•The interactions between droop control and VSG control are detailedly discussed.•A secondary frequency control based on the internal model control is proposed when considering the communication delay. Compared to the conventional centralized power system, in which synchronous generators with speed control offering favorable dynamic behaviors, the islanded microgrid dominated by distributed generators may encounter severe frequency instability. Thus droop control and virtual synchronous generator control have been proposed to design the primary frequency level of the islanded microgrids. In this context, both of these two control strategies will coexist and interact with each other in a microgrid due to their different reaction speed. This paper focuses on the frequency stability of islanded microgrids. The interactions between virtual synchronous generator-based and droop-based parallel inverters are firstly investigated. The small-signal model is used to study the effects of variation of important control parameters. Then the secondary level is also established to compensate the frequency deviation. The internal model control based strategy is used to improve robustness for communication delays of the secondary level. Furthermore, a traditional PI controller is also proposed based on robust H∞ method for comparison. An islanded microgrid test system including four distributed generators dominated by different control strategies is built in PSCAD/EMTDC to verify the proposed control structure.