**1. Introduction**

A renewable hybrid energy system comprises of a couple of energy sources, a power conditioning device, a controller and sometimes with an energy storage system. When such renewable energy sources (RES) are integrated to the grid, variable output due to the stochastic nature of input may lead to instability and power quality issues [1]. In this changing scenario, micro-grids (MGs) have come up as a solution to maintain power supply in small scale as an autonomous entity in the event of grid failure. It has complementing resources or different DG sources in combination with storage with power electronic interface. Distributed energy resource (DER) can be either a distributed generator or distributed energy storage. Under its spectrum, it can be PV, wind, heat pumps, combined heat and power (CHP) generation, energy storage (ES), fuel cells (FCs), electric vehicles (EVs), energy efficiency (EE) and demand response (DR). The behavior of the resources, such as EE, DR, heat pumps, and EVs, is user dependent. Further, the PV source has no inertia. So ES and FCs can provide more reliability and flexibility to the grid if operated in a manner coincident with grid needs that respect storage limitations. These DGs have made the grid more resilient, efficient, environment-friendly, flexible, less vulnerable, easier to control, immune to issues at some other location, slow gradual capital investment, integrating to grid with minimal disturbance to existing loads during commencing. Participation of DERs in operation is profitable in respect of load shifting without grid up-gradation curtails peak demand, grid support by storage responding to demand thereby improving frequency response reducing spinning reserve. EVs and MGs can provide ancillary services. Under normal operation of the grid, varying capabilities of the DERs support voltage and reactive power whereas under fault voltage and frequency ride through capability is expected. Under such fault, the inverter must respond as per requirement. With the coordination of inverter-based resources in a group, it is possible that the DERs counteract to grid contingencies such as voltage and frequency deviations, and assist in fast recovery. So they are termed virtual inertia. But, at the same time, some issues are of concern and have drawn the attention of researchers. They are mainly due to stochastic nature such as load following, power vs. energy profile in storage, stability, reliability, cost, control architecture, autonomous control, power quality issues and grid interconnection. Considering these issues, in [2] the feasibility study, the unit commitment for reliable power supply and modeling of energy systems of PV, wind and diesel generator are focused. In the past decade, more significant development has taken place with various combinations of sources and storage. Optimization in all respect of wind energy for grid integration has been thoroughly reviewed [3] and observed to have good success. The control topology and the objectives have also changed in recent years. In addition to other reviews, control aspects and reliability issues with such sources are discussed [4]. The application of evolutionary technique and game theory in hybrid renewable energy is also presented. The chapter revisits the changing requirements due to DG, summarizing the research works done in the last 10 years with some vision of future trends.
