Journal of critical reviews 376 Journal of Critical Reviews ISSN- 2394-5125 Vol 7, Issue 1, 2020 Review Article A REVIEW ON HYBRID AC/DC MICROGRIDS: OPTIMAL SIZING, STABILITY CONTROL AND ENERGY MANAGEMENT APPROACHES Bhavana Pabbuleti, Jarapula Somlal Koneru Lakshmaiah Education Foundation, Vaddeswaram, AP, India Koneru Lakshmaiah Education Foundation, Vaddeswaram, AP, India Received:09.11.2019 Revised: 08.12.2019 Accepted: 20.01.2020 Abstract Electricity is the greatest gift of science to humanity reached for civilization where electricity is used for all purposes. However, in recent times a paradigm shift is evolving in the generation of electrical energy from the concept of using major generating plants to minor generating units allied to the distribution systems in the form of microgrids with alternative energy sources called renewables. Around the world renewable energy use is on the rise and these alternate energy sources can generate pollution-free electrical energy to the society. Although these are new centers and units with diminishing cost, there are still many challenges in operation and control of islanded and grid-connected microgrids configured in both AC and DC. Uniting the benefits of microgrids of AC and DC, Hybrid AC- DC Microgrids (HACDC) were developed.Thus, it is relatively imperative to investigate the optimal size, stability control, and strategies of economic efficiency operation of HACDC microgrid. Hence a great review on optimal sizing methods, stability control, and energy management strategies using various iterative and intelligence techniques published in different articles proposed by many authors were presented in this paper. Keywords: MRenewable Energy Sources, Hybrid AC/DC Microgrid, Inter-Allied Converter, Energy Management System. © 2019 by Advance Scientific Research. This is an open-access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) DOI: http://dx.doi.org/10.31838/jcr.07.02.72 INTRODUCTION Recently From the survey, it has been identified that an average Indian consumes 1075kwh of electricity annually, and 85% of electricity is generated from fossil fuels as principal resources of energy, which causes large amount of CO2 emissions to lead to global warming. But due to the growth of demand for electric power, the inadequate reserve and lifting worth of conservative sources such as firewood and petroleum, etc. renewable energy sources become a gifted substitute, available free of cost, atmosphere friendly and has a reduced amount of operational and maintenance cost. The higher penetration of various technologies of renewable energy sources such as solar, wind, tidal, biomass, and geothermal forms a distribution generation (DG). The vast scale incorporation of DG’s will bring operational confronts to the power system network, and a vital solution to this predicament is a microgrid achieved much concentration globally [1]. At present, due to generation of electrical energy in both (AC and DC) forms with the use of various renewables; microgrids are classified as AC Microgrids, DC Microgrids, and HACDC Microgrids [2]. In AC microgrids, the DC generating sources such as PV, Fuel Cells are converted to AC with the use of DC/AC converters, while AC generating sources are directly allied using power electronic interfaces [3]. Whereas, in case of DC microgrids AC generating sources are converted to DC using AC/DC converters. However, these multiple conversions results in losses. An immediate solution to the problems mentioned above is hybrid microgrid, which minimizes multiple conversions and reduction in losses [4]. The two notable and key aspects bracketed together concerning hybrid systems are worth of electrical power and consistency of the system. The systems' best possible blueprint must be cost-effective and consistent, and it can be accomplished with the rally round of appropriate choice of apparatus of the system. Thus, an optimal sizing method is obligatory to propose a proficient and cost- effective HACDC microgrid system. The structure of hybrid microgrid consisting of renewables Photo-Voltaic (PV), Wind Turbine (WT) with Battery Energy Storage, and loads allied to utility grid is shown in Fig 1. Solar Irradiation Temperature DC/DC DC Loads DC/DC Battery AC/DC Bidirectional Inter-Allied Converter AC/AC AC Loads AC Bus Utility Grid DC Bus PV Array Wind Figure.1 Layout of HACDC Microgrid From Fig.1, the HACDC microgrid is allied to the service grid through an isolator switch [5], which helps to isolate the HACDC microgrid during faulty times. Under steady-state conditions, the HACDC microgrid can be functioned in two modes, such as grid allied mode and island mode. Maintenance of power balance between hybrid and utility grid is moderately easy in grid-connected mode [6] as compared to islanded mode because of infinite bus behaviour of utility grid [7] and be able to absorb or supply to the HACDC microgrid. In the case of islanded mode, the HACDC microgrid is no longer allied to the utility grid. Hence, the HACDC itself must supply the total load demand through Inter-Allied Converter (IAC). Therefore, a communication link is allied among the sources. A centralized method of control is applied between the sources, but it is a single point failure. A commonly known method called droop method, used for sensing of load demand by each source and regulates its production according to its rated capacity [8]. During transition from grid allied mode to islanded mode or vice-versa, unstable harmonic currents and voltages are of crucial concern. In addition to this, proper synchronization of voltage and phase between HACDC microgrid and utility grid is necessary. To increase the generation and to meet the load demand, renewable sources, together with storage devices, are