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.

An attempt is made to design a fault-tolerant control system using a nonlinear technique, called the state-dependent Riccati equation (SDRE) method. The proposed mechanism consists of a master controller, an observer-based fault detection and isolation system, and three emergency controllers, designed using the SDRE methods. The master controller, as a suboptimal nonlinear regulator, is designed to return DC microgrid (MG) to its desired equilibrium point in normal operation condition. On the other hand, an SDRE observer is designed to establish a unified framework for fault detection and isolation in the DC MG. Indeed, in the case of unexpected fault occurrence, the proposed mechanism can isolate the faulty DG, and then, the emergency controllers are triggered immediately to stabilize the MG. Simulation results indicate that the proposed method is so useful to achieve the control objectives and online protection of the considered MG.

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