-It is obvious that power quality is an important characteristic of today's distribution power systems as loadsbecome more sensitive on the other hand nonlinear loads are increasing in the electrical distribution system. Considering the distributed nature of harmonic loads, the need for distributed power quality improvement (PQI) is inevitable. From years ago, researchers have been working on various kinds of filters and devices to enhance the overall power quality of power system, but today the nature of distribution system has been changed and power electronic based DGs play an important role in distribution grids. In this paper, a thorough survey is done on power quality enhancement devices with emphasis on ancillary services of multi-functional DGs. A literature review is also done on microgrids concept, testbeds and related control methods. Although there were some applications of DGs for PQI improvement these applications were not defined multi-functional DGs. Various control methods are studied and categorized regarding different viewpoints in the literature. Finally, a couple of thorough comparisons are done between the available techniques considering the nature, capabilities, advantages and implementation costs.

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This study proposes a new method to improve low-voltage ride through of the doubly-fed induction generator (DFIG) wind turbines (WTs) using gate-controlled series capacitor (GCSC) in series with the generator rotor. Using GCSC, as soon as a voltage dip occurs at the generator's terminals, the GCSC inserts its capacitor in series with the rotor, increasing the voltage seen by the rotor winding. Consequently, the rotor over-current is limited and transmission of the inrush energy to the rotor side converter is reduced. Moreover, by this method, the chopper is not required for protecting the DC-link capacitor. Unlike the Crowbar method, which control of the generator is lost during the fault, the proposed method in this study avoids the loss of control during the grid faults. To validate the effectiveness of the proposed method, extensive time-domain simulation is performed using MATLAB/SIMULINK on a 1.5 MW DFIG-based WT.

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In this study, the real-time energy management system (RT-EMS) of a microgrid (MG) is proposed to deal with different uncertainties due to the errors in the prediction of renewable generation, load and market price. In the day-ahead EMS, the error in prediction of data; thus, the uncertainties in the scheduling are dealt with using different scheduling methods. Nonetheless, utilising the online RT measurements is an advanced solution to eliminate the uncertainties because there would be no prediction error in employing the RT information. In this study, a RT-EMS of a MG is designed using the Lyapunov optimisation method. In RT-EMS, satisfying the time-coupled constraints such as the battery energy limit and provision of load quality of service is a demanding challenge. This problem is addressed in Lyapunov optimisation by defining distinct virtual queues for satisfaction of time-coupled constraints. Moreover, the variable V algorithm is employed to provide a better compromise between stabilising the virtual queues and the total operation cost. A test MG system consisting of combined heat and power units, renewable energy sources, energy storage systems and flexible loads is used for evaluation. The underlying distribution network and power distribution loss are further considered satisfying the voltage limits.

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