# Smart Inverters for Microgrid Applications: A Review
## 1 Introduction  
Microgrids are a form of small-scale grids that contain DGs, energy storage units and linear or nonlinear loads that can operate in grid-connected or islanded mode. In microgrids, DGs can be of renewable or non-renewable nature, and the components of such grids are interfaced by power converters [1; 2; 3]. The CIGRE working group C6.22 Microgrid Evolution Roadmap (WG6.22) provides a standard definition for microgrids: Microgrids are electricity distribution systems containing loads and distributed energy resources, (such as distributed generators, storage devices, or controllable loads) that can be operated in a controlled, coordinated way either while connected to the main power network or while islanded [4]. In the early years of introducing renewable energy sources (RES), the power generated by them was not substantial in comparison to the large conventional generators powering up the grid, therefore, their impact on the performance of the grid was almost unnoticeable. At those years, the scheme was to allow them to produce as much power as possible and inject it into the grid by using their own algorithms. The main conventional generators could regulate the small unbalances and fluctuations caused by these DGs [5; 6]. However, due to the recent development and expansion of renewable energy technologies, RESs have become a major energy source in some countries. Therefore, the quality and parameters of their injected power into the grid must be carefully monitored. In other words, their operation needs to be accompanied by power management schemes centered on power sharing or load sharing to be fully controllable [7]. The concept of power sharing between parallel DGs was initially introduced for synchronous generators in large-scale grids, and recently some novel approaches were proposed to incorporate the idea of droop control algorithms into microgrids [8; 9; 10; 11; 12; 13]. Although using droop-based methods can be very beneficial, some drawbacks can be identified. For example, the trade-off between voltage regulation and load sharing [14], the ineffective management of harmonics introduced by nonlinear loads [15] and the slow dynamic response due to the incorporation of low pass filters for calculating average values of active (\(P\)) and reactive (\(Q\)) power [16]. Because of the problems of conventional droop control, its principles have undergone numerous modifications and improvements. In [17] a new droop scheme aiming to regulate the voltages of each inverter and improve current sharing was proposed. When nonlinear loads such as rectifiers are present, they introduce harmonics in the system, and prevent droop control from working efficiently. In [18], this problem was addressed and regulated by adding harmonic droop characteristics. This issue can also be compensated by using a virtual variable impedance in series with the load. As its name indicates, this impedance is purely simulated and no physical components are added to the circuit [19; 20; 21; 22]. In [13] a hierarchical control method was proposed to control microgrids. This hierarchy consists of three layers: the primary level is droop control; the secondary level compensates for the unbalances and deviations resulted by the first level by sending voltage and frequency references; and the tertiary level (energy management level) which sends the droop coefficients is where the connection with the grid. In this method, only the first level works as communication free, since it is droop control, while the rest of them require communication between inverters. Although most of the existing microgrids are supplied in AC to simplify their interface with utility grids, DC microgrids are attracting more attention in recent years. This is not only because of the DC nature of several RESs such as solar energy, fuel cells and energy storage units like batteries and supercapacitors, but also because of some additional benefits. In a DC microgrid, the problem of harmonic current sharing no longer exists. Neither is there reactive power sharing.