Communication infrastructure (CI) in microgrids (MGs) allows for the application of different control architectures for the secondary control (SC) layer. The use of new SC architectures involving CI is motivated by the need to increase MG resilience and handle the intermittent nature of distributed generation units (DGUs). The structure of secondary control is classified into three main categories including centralized SC (CSC) with a CI, distributed SC (DISC) generally with a low data rate CI, and decentralized SC (DESC) with communicationfree infrastructure. To meet the MGs operational constraints and optimize performance, control and communication must be utilized simultaneously in different control layers. In this survey, we review and classify all types of SC policies from CI based methods to communication-free policies, including: CSC, averaging based DISC, consensus-based DISC methods, containment pinning consensus, event-triggered DISC, washoutfilter based DESC, and state-estimation based DESC. Each structure is scrutinized from the view point of the relevant literature. Challenges such as clock drifts, cyber-security threats, and the advantage of event-triggered approaches are presented. Fully decentralized approaches based on state-estimation and observation methods are also addressed. Although these approaches eliminate the need of any CI for the voltage and frequency restoration, during black start process or other functionalities related to the tertiary layer a CI is required. Power hardwarein-the-loop (PHiL) experimental tests are carried out to compare the merits and applicability of the different SC structures.

The absence of rotational masses from synchronous generators in converter-interfaced microgrids leads to a lack of inertia. Consequently, the system exhibits steeper frequency variations and higher frequency nadir, which may degrade the dynamic performance and challenge the operation of sensitive equipment such as protective relays in the grid. Virtual synchronous generator is introduced as an effective solution to increase the inertial response of converter interfaced renewable energy sources. This paper proposed a fuzzy controller, which is augmented on the virtual synchronous generator topology to damp the perturbation during transients by increasing the inertia of the system. The proposed fuzzy control adds a correction term to the the governor's output power that increases the system inertia during transients. In order to compare the inertial response improvement, a comparison between proposed fuzzy control technique and cost function based inertia and damping coefficient optimization is done on a virtual synchronous generator platform. It is shown that online measurement based adaptive methods have a better inertial response against other time-consuming techniques. To further verification, a number of experiments are done, which confirm the merits of the proposed fuzzy based virtual synchronous generator control method.

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In this paper, a novel decentralized control structure is proposed to compensate voltage and frequency deviations of an ac microgrid (MG) with higher bandwidth compared to the conventional control structure with no need for a communication network. This approach is realized by firstly employing finite control set model predictive control of voltage source converter at the primary control level. Then, an adaptive droop control is presented to keep the voltage and frequency of the MG stable in steady state and serve as a secondary level of hierarchical control. Therefore, the MG voltage and frequency are restored to the nominal value with a decentralized communicationfree control structure. Simulation results verify the accurate frequency and voltage restoration as well as fast power-sharing during the transient and steady-state performance with no need for communication infrastructure.