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The use of renewable energies, such as wind and solar, is rising in microgrids. However, because of their intermittent nature, achieving a stable system in spite of the fluctuations of generation and demand is very difficult. Aiming to increase the stability margin of AC islanded microgrids comprising nondeterministic resources, properly share real power (P), and keep the network frequency (f) at nominal value, this study exploits the presence of energy storage units (ESUs) along with a load-shedding scheme. To this end, this study proposes a multitier decentralized control strategy that is based on frequency and includes three control schemes, distributed generation unit (DGU) control, ESU control, and local loads control (LLC). These controllers operate simultaneously. The DGU controller shares the loads among DGUs in accordance with the frequency droop method. On the other hand, the ESU controller controls the production of ESUs and also the LLCs manage the loads and dump-load of the microgrid. This method ensures the proper operation of AC microgrids that comprise intermittent resources under stand-alone conditions and despite numerous events. The time responses of P and f prove the high performance of the method. The simulations are performed and coded in m-file of MATLAB. [ABSTRACT FROM AUTHOR]

This paper reports a new control strategy to improve sharing of unbalanced currents in islanded LV microgrids. This technique provides fast and effective sharing of positive-, negative- and zero-sequence currents, and is the first example of zero-sequence current sharing in the literature. The controllers are designed in the stationary frame. The control structure consists of four loops: (1) the current controller; (2) the voltage controller; (3) the droop controller and the (4) negative and zero sequence current controllers. The output current is considered unknown for the controller and is added to the control system as a disturbance. The proposed controller features a high gain in fundamental and harmonic frequencies, hence a good voltage quality is obtained in the presence of unbalanced and nonlinear loads. To this aim, a proportional-resonant (PR) controller is adopted as the current controller. By using a multi-resonant controller as current controller, a unified control structure is obtained which is suitable for both grid-connected and islanded modes. The voltage controller is designed using a resonant controller so that the voltage can have low VUF and THD in the presence of unbalanced and nonlinear loads. Furthermore, in this paper, the droop method is applied to the control structure to share real and reactive powers. Simulation studies show that the conventional droop method cannot share the oscillatory part of the output power that is due to the presence of unbalanced loads in the microgrid. This paper relies on using zero and negative sequence virtual impedance controller to share the oscillatory part of output power. By using zero-sequence virtual impedance controller (ZSVIC) and negative-sequence virtual impedance controller (NSVIC), the zero and negative sequence currents in the microgrid are controlled and shared effectively. By compensating zero- and negative-sequence currents locally, the flow of these currents in the microgrid is minimized, and the overall power quality of the islanded LV microgrid is improved. [ABSTRACT FROM AUTHOR]