A Review on Primary and Secondary Controls of Inverter-interfaced Microgrid

The demand of electricity and environmental issues associated with conventional power generation plants are increasing significantly. Modern technology has transformed the conventional power system through the integration of distributed generation (DG). With the help of modern power electronic technology, the conventional power system is able to support the integration of DGs based on renewable energy sources (RESs). The systematic combination of DGs with energy storage system forms a microgrid (MG), which can operate in islanded mode or grid-connected mode. The intermittent nature of RES and varying load pose substantial obstacles such as voltage and frequency instability, and the unreliability of RES. Unequal feeder impedances and non-linear loads are considered as present challenges in MG control. Hierarchical control has been useful in undertaking solutions to these issues. This paper covers the deep insight of different control methods applied at the primary and secondary control levels in hierarchical control. In primary control, the classification based on droop and non-droop controls is discussed. The virtual synchronization machine (VSM) based control method is reviewed. Voltage and frequency restoration control and economical operations at decentralized and centralized secondary control are analyzed in detail. Based on the existing literature, critical discussion on MG control and future trends are also presented to provide future research perspectives.