The environmental issues, declining fossil fuel reserves, and stresses in the supply have led to the genesis of small-scale sources of electricity generation and the return to clean energy. Microgrids (MGs) are unit control systems that enable the supply of electrical energy on a small and medium scale locally. MGs have many advantages such as more reliability, power loss reduction, local voltage support, improved power quality and high-speed communication system. However, there exist numerous problems to provide reliable operation for MGs either in the grid-connected or islanded mode that needs to be resolved. Power quality problems, operation and protection challenges of MGs are the areas that require more investigation in MGs. The power quality problems include harmonic components, voltage unbalance, flicker, over/under voltage, sags and swells voltage etc. For example, harmonics distortions are one of the main challenges which need to be resolved before MGs become commonplace. Nonlinear loads and high penetration level of intermittent renewable energy sources (RESs) are the main cause of harmonics distortions in MGs. Therefore, it is necessary for MGs to propose improved harmonic compensators to avoid a threat to the distribution network in terms of power quality and stability issues.

This special issue aims to solve the mentioned problems, and various equipment and methods can be applied. Typically, the modules required to solve power quality problems are based on power electronic. These modules are divided into several categories, and the most important of these elements are power filter compensators, unified power quality conditioner (UPQC), a flexible alternating current transmission system (FACTS) devises, etc. The methods and techniques used to increase the quality components of power can be used as a complement to compensatory elements. Among these methods is the topic of optimizing the control systems of each module based on meta-heuristic algorithms, using other methods based on machine learning, etc.