Decentralized Autonomous Hybrid Renewable Power Generation

Power extension of grid to isolated regions is associated with technical and economical issues. It has encouraged exploration and exploitation of decentralized power generation using renewable energy sources (RES). RES based power generation involves uncertain availability of power source round the clock. This problem has been overcome to certain extent by installing appropriate integrated energy storage unit (ESU). This paper presents technical review of hybrid wind and photovoltaic (PV) generation in standalone mode. Associated components like converters, storage unit, controllers, and optimization techniques affect overall generation. Wind and PV energy are readily available, omnipresent, and expected to contribute major future energy market. It can serve to overcome global warming problem arising due to emissions in fossil fuel based thermal generation units. This paper includes the study of progressive development of standalone renewable generation units based on wind and PV microgrids.


Introduction
Development of innovative power solution, capable of minimizing environmental concerns, has been key point of interest for power system researchers. These sources have gained attention since the oil crisis faced in early 1970. Depleting fossil fuel reserves need to be replaced by alternate economic and environment friendly power generation sources. Wind energy conversion system (WECS) and PV generation have proven to be potential power generation sources but its nonpromising nature has been of major concern. As these sources are climate and environment dependent, it may not efficiently meet load demand for specified time duration. Overall delivery cost of centralized electricity generation and grid distribution counts up to four times the cost of generation by stand-alone and minigrid options for "minimum threshold" demand scenario.
Most common sources of energy currently utilized worldwide for generating electricity include coal (39.3%), petroleum (0.7%), natural gas (27.6%), nuclear power (19.5%), hydro power (6.7%), wind (4.2%), and other renewable power (2.1%) that covers mainly geothermal, biomass, and PV energy [1]. PV generation has been found more promising than wind generation for small scale generation [2] but it is sure to remain unavailable during night time. So, in wind rich regions, standalone minigrid WECS is preferred more than PV generation. WECS-PV integrated storage unit causes accountable power loss in conversion equipment during conversion process [3][4][5][6][7]. Complementary characteristics of PV and wind reduce overall requirement of storage unit. In specific locations, hybrid wind-PV generation with storage unit can provide highly reliable power [8] to isolated loads like satellite earth station, broadcasting stations, hill top load stations, and so forth. Overall power reliability depends on optimal sizing of conversion equipment, optimization techniques, meteorological data, and load forecast. RES involves efficiency and economical issues. Among reported solutions, ensuring spinning reserve [9][10][11][12] and suitable storage unit facilities [13][14][15][16] have been considered as the most effective ones.
Due to variable nature of PV and wind round the clock and throughout the year, it becomes a herculean task to obtain regulated power supply [17]. Generation gets affected by weather and climatic condition. Tsoutsos et al. discussed 2 Journal of Renewable Energy impact of environment on PV generation and proposed necessary measures for proper project designing so as to ensure public acceptance [18]. Haruni et al. proposed a novel operation and control strategy for hybrid wind-fuel cellelectrolyzer-battery and a set of loads as standalone unit. Overall control strategy is based on a two-level structure, namely, energy management and power regulation system, to avoid system blackout. Depending on reference dynamic operating points of individual subsystems, local controllers control wind turbine, fuel cell, electrolyzer, and battery storage units [19]. Zhou et al. proposed an autonomous unified var controller to address system voltage issues and unintentional islanding problems associated with distributed PV generation systems. The controller consisted of features of both voltage regulation (VR) and islanding detection (ID) functions in a PV inverter based on reactive power control to ensure (1) fast VR due to autonomous control, (2) enhanced system reliability because of capability to distinguish between temporary grid disturbances and islanding events, (3) negligible nondetection zone (NDZ) and no adverse impact on system power quality for ID, and (4) no interferences among multiple PV systems during ID [20]. Hong et al. proposed a novel multiobjective nonlinear programming to determine shed loads of UFLS 81L relays in a hybrid wind-PV-gas turbine microgrid. Method incorporates GA to ensure decrease in load shed [21]. Koutroulis and Kolokotsa presented a methodology for optimal sizing of hybrid wind-PV power generation as standalone unit. He suggested a list of commercially available system devices, optimal number, and type of units ensuring that total system cost for about 20 years is minimized subject to constraint that load energy requirements are completely covered, resulting in zero load rejection implementing cost (objective) function minimization using gas [22]. Eftekharnejad et al. investigated impact of increased penetration of PV systems on static performance and transient stability of large power system. Advantages and problems associated with utility scale and residential rooftop PVs have also been identified for steady state stability and transient stability performance [23]. Kadda et al. discussed optimal sizing issue of a hybrid wind-PVbattery as standalone unit along with diesel generator, located at Oujda/Angad, in order to minimize overall generation cost [24]. Lund evaluated problems, associated with hybrid wind-PV-tidal power generation as standalone unit, in terms of excess or scarce electricity production due to fluctuating RES [25]. Kim et al. presented hybrid wind-PV-SMES (superconducting magnetic energy storage) system to operate under abnormal conditions, such as reactive power or current fluctuations. SMES can significantly enhance dynamic security of such distributed power systems due to its high energy density and quick response characteristics during fault or surge conditions [26].
PV power industry has gained attention due to its easy installation at domestic and commercial level. Associated problems have been minimized to great extent in consequent researches, yet a lot more problems need to be shorted out [27][28][29][30].  Figure 1: Progressive load demand dependent step by step RES standalone system development.

Decentralized Power Generation
Decentralized mode of power generation or distributed generation that depends on locally available resources, mostly RES, is either in standalone mode or connected to utility grid. This paper investigates standalone mode of power generation. The concept of standalone or grid isolated system has been revised from time to time, as shown in Figure 1. Initially RES based power generation only aimed to obtain an alternate source of electrical energy; but with increasing load demand, hybrid RES power generation gained attention to satisfy load suitability and ensure regulated electrical power. Presently, researchers have been trying to obtain regulated power from hybrid RES-ESU and meet liner as well as nonlinear load demand economically.

Decentralized WECS.
Wind energy is supposed to be a major contributor in future world energy scenario and continues to be one of the fastest growing energy resources round the globe. Major challenge associated with WECS is uneven distribution of wind energy. Fluctuating wind brings voltage and frequency regulation problem. These problems can be dealt by dividing wind study into three time frames, namely, regulation, load following, and unit commitment [31]. The regulation time frame includes the period during which generation automatically compensates minute-byminute deviations in load. Load following time frame is generally longer than regulation and refers to time required to obtain different set points of capacity to cope up with the load. Dedicated peak load generating units either have ready to use power or can be started quickly. Load following time frame generally ranges from 10 minutes to a few hours, depending upon time required to move generating unit to different set points of capacity and involved cost constraints. Unit commitment ranges from several hours to several days depending on scheduling dedicated generation to meet required electric demand. At present, standalone wind system is more economical than standalone PV system for off-grid regions due to continuous ongoing research. Main components required for wind power generation is turbine, gearbox, generator, stepup transformer, nacelle, and tower. Santoso et al. described design and construction of wind power in terms of steadystate and dynamic operation of induction machines (IMs), speed of alternator, and modeling of aerodynamic, mechanical, and electrical components [32].
Journal of Renewable Energy 3 For WECS commonly employed alternators include selfexited induction generators, doubly fed induction generator (DFIG), permanent magnet (PM) brushless generators [33], PM synchronous generator (PMSG), switched reluctance generators, and doubly salient PM generators [34]. These alternators are not concerned with maximum power generation. Liu et al. proposed doubly excited PM brushless generator to tap maximum wind energy using online flux control [35].
Abu-Elhaija and Muetze discussed effects of fluctuating wind speed on minimum capacitance requirement to selfexcite single phase self-excited reluctance generators by analyzing overall system damping and amplifying the components of Eigen values with lower and upper natural frequencies [36]. Singh and Sharma presented design, development, and analysis of voltage and frequency controllers (VFCs) for standalone WECS. An isolated asynchronous generator, a synchronous generator (SG), and a PMSG are used with these WECS [37]. These VFCs are developed with threephase generators driven through a wind turbine to feed threephase and single-phase loads. A battery energy storage system is used invariably with each system configuration to facilitate load leveling during change in wind speeds and/or consumer loads [38]. Performance of VFCs has been demonstrated to validate their operation as a load leveler, load balancer, phase balancer, neutral current compensator and an active filter along with a VFC.

Decentralized PV Generation.
In 1839, French physicist named Edmund Bacquerel discovered PV effect [39]. Modern PV module consists of PV cells, mounting structure, MPPT mechanism, converters, storage unit, and electrical and mechanical connections to regulate and utilize electrical output [40]. Electrical power output of PV module depends on electrical, thermal, PV spectral, and optical property of PV cell array, PV angle, and irradiance [41]. Technical aspects and environmental factors affect optimal PV generation [42]. Zhao et al. discussed PV energy conversion standards and processes involved [4]. Main role player countries like Germany, Italy, Japan, Spain, USA, and South Korea contribute to about 30 to 50 percent of PV annual growth rate [43,44]. Technical problems such as islanding detection, harmonic distortion, electromagnetic interference, and low efficiency of PV cells are major bottleneck for widespread application of PV systems [45].
Conventionally, PV cell designs are based on band gap energy (eV). Low band gap energy has high current ( = ) but low voltage ( = / ), and vice versa. Here, is electron charge, is number of photons, is energy gap, and is surface area of solar cell. It is preferred to use materials with energy gap between 1 and 1.8 eV like crystal silicon (1.12 eV), amorphous silicon (1.75 eV), copper indium diselenide (1.05 eV), cadmium telluride (1.45 eV), gallium arsenide (1.42 eV), and indium phosphate (1.34 eV) [46][47][48]. These modules are rated in terms of peak kilowatts (kWp), that is, amount of expected electrical power output when sun is directly overhead on a clear day. Kosten et al. improved efficiency of silicon PV cell by restricting light escape angle. Restricting light escape angle to 2.767 ∘ in silicon PV cell of 3 m thickness improved light trapping and efficiency by 3% [49].
There has been progressive growth in PV cell material [50][51][52][53][54]. Adamian et al. investigated possibility of porous silicon layers application as antireflection coating in common silicon PV cells (ZnS) [50]. Hanoka discussed a silicon ribbon growth method by comparing string Ribbon with two other vertical ribbon technologies and discussed characteristics of this ribbon, specially dislocation distribution, and explained growth progress of 100 m ribbon [51]. Yang et al. explored amorphous-Si PV technology and achieved an AM 1.5, 13% stable cell efficiency for splitting triple-junction spectrum made with roll-to-roll continuous deposition process [52]. Fave et al. compared epitaxial growth of silicon thin film on double porous sacrificial layers obtained by liquid or vapor phase epitaxy and found that mobility and diffusion length are slightly higher with vapor phase epitaxy compared to liquid phase epitaxy. Fabricating PV cells using a detached film obtained with vapor phase epitaxy and without any surface passivation treatment or antireflective coating gives an efficiency of 4.2% with a fill factor of 0.69 [53]. Dobrzański and Drygała explored laser texturization for PV cells made of multicrystalline silicon to improve interaction between laser light and test PV cell [54].

Decentralized Hybrid WECS-PV Generation.
Wind and PV are complement to a certain extent. Due to individual merits and demerits of PV and WECS, hybrid PV-wind generation system with storage backup unit has proved to be reliable power source [55,56] to feed electrical loads that need high reliability [57] and uninterrupted power supply [58]. Hybrid generation has been considered preferred choice for remote systems like radio telecommunication, satellite earth stations, or sites isolated from conventional power system [59,60].
Hybrid system has also been an optimal choice for locations where grid connection has been farfetched idea due to economical and technical reasons [61][62][63]. Daniel and Gounden presented an isolated hybrid system consisting of a three-phase square wave inverter integrated with PV array and a wind-driven induction generator. They developed mathematical model for hybrid scheme consisting of variables in terms of synchronous reference frame [64]. Chen et al. proposed a multi-input inverter for hybrid PV-wind power system to obtain regulated supply and reduce overall power cost. Multi-input inverter consisted of a buck-boost fused multi-input dc-dc converter and a full bridge dc-ac inverter [65]. Kim et al. discussed power-control strategies for hybrid PV-wind generation with versatile power transfer. Hybrid system consisted of PV array, wind turbine, and battery storage connected to a common dc bus. Versatile power transfer has been defined as multimode operation, including normal operation without use of battery, power dispatching, and power averaging, which enables grid-or user-friendly operation [66]. Further Chiang et al. presented a hybrid regenerative power system consisting of PV-WECS hybrid generation with grid-tie system and uninterruptible power supply (UPS) for critical load applications. System included six-arm converter topology with three arms for rectifierinverter, one arm each for battery charging/discharging and third arm for power conversion of the PV module and WECS alternator [58]. Liu et al. discussed PV-wind hybrid generation in standalone mode employing doubly excited PM brushless machine used for maximum electrical power extraction by using online flux control [35].

Maximum Power Point Tracking (MPPT) Strategies
Maximum power extraction, commonly known as maximum power point tracking (MPPT), includes maximum mechanical and electrical power extraction from wind. Mechanical power extraction is obtained by regulating tip speed ratio of the wind turbine, whereas electrical power extraction is associated with voltage and frequency regulation. In order to track maximum power, suitable control strategy, depending on site, needs to be implanted. MPPT's estimated usable efficiency (EUE) should be as high as possible. Table 1 shows some commonly used control strategies for WECS-MPPT and EUE obtained. In order to track maximum power, useful MPPT techniques for PV applications have been developed and applied [67]. Commonly used methods for MPPT includes perturb and observe (PO), power matching, incremental conductance, fractional open-circuit voltage/short-circuit current, power differential feedback control, curve fitting, dc-link capacitor droop control, intelligent control, and some other special control methods [68,69]. Due to simplicity and robust nature, PO and curve fitting techniques are widely used. MPPT techniques and algorithms have gained attention of power system researchers [70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87] due to its dominant advantages over conventional techniques. Table 2 shows some commonly used control strategies for PV-MPPT and EUE obtained.

Modeling of Hybrid Generation
Optimal sizing of conversion equipment is necessary to meet load demand. A number of useful simulations and optimization techniques have been used to evaluate performance of hybrid PV-wind systems [88,89]. Commonly employed software tools include hybrid optimization model for electric renewables (HOMER), HYBRID2, hybrid optimization by GA (HOGA), HYBRIDS, hydrogen energy models (HYDROGEMS), transient systems simulation program (TRNSYS), village power optimization for renewable (ViPOR), Dymola, and matrix laboratory (MATLAB) simulink tool, which are employed for cost and performance analysis [90,91]. These optimization tools have helped in optimizing simulation configuration in terms of production cost and reliability. Table 3 shows year of development, developer, merits, and demerits of some commonly used optimization tools used to simulate hybrid RES generation.
HOMER is public domain software used for hourly simulations to obtain optimum target. It is a time-step simulator using hourly load and environmental data inputs for RES assessment; it facilitates optimization of RES based on net present cost for a given set of constraints and sensitivity variables. HOMER has been used extensively in previous renewable energy system case studies [92,93] and in renewable energy system validation tests [94]. Although simulations can take a long time, depending on number of variables used, their operation is simple and straightforward. Program's limitation is that it does not enable user to intuitively select appropriate components for a system, as algorithms and calculations are not visible or accessible. HYBRID2 is hybrid system simulation software with precise simulation, as it can define time intervals from 10 min to 1 h. NREL recommends optimizing system with HOMER and then once optimum system is obtained, designing is improved by using HY-BRID2. HOGA is a hybrid system optimization program. Optimization is carried out by means of GA and can be single objective or multiobjective. Simulation is carried out using 1 h intervals, during which all of the parameters remained constant. Control strategies can also be optimized using GA. HYBRIDS assesses technical potential of RES for a given configuration to determine potential renewable fraction and evaluate economic viability based on net present cost. HYBRIDS is a Microsoft Excel spreadsheet based RES assessment application and design tool, requiring daily average load and environmental data estimated for each month of the year. Unlike HOMER, HYBRIDS can only simulate one configuration at a time and is not designed to provide an optimized configuration. HYBRIDS is comprehensive in terms of RES variables, level of detail required, and need of higher level of knowledge of RES configurations as compared to HOMER. It is designed so as to improve renewable energy system design skills through its application.

Energy Storage Technology
Renewable hybrid generation is incorporated with ESU to ensure better reliability and meet energy gap between generation and load demand.
Depending on application, classification of ESU technology has been shown in In large-scale nonregulated hybrid renewable electricity networks, energy storage (mostly bulk systems) is required to absorb the shock of energy overproduction and compensate Journal of Renewable Energy 5 [ 161] energy gap during low generation or blackouts [95]. This is also applicable renewable energy based distributed generation system (either isolated or connected to any distribution network), to deal with power quality issues and efficient power flow management [96,97]. Optimal sizing of storage unit is important to obtain reliable power generation [98,99].
In domestic and microgrid renewable generation, energy storage units are employed for satisfaction of electricity needs  Investigates fuzzy-logic based MPPT controller for PV systems under fluctuating weather conditions. Hill climbing MPPT has been improved by fuzzifying its rules. This provides less oscillations and fast convergence.
[78]     components, namely, PV arrays, wind turbine, electrolyzer, hydrogen storage tanks, and fuel cell. They considered a twolevel control system consisting of a supervisory controller to ensure power balance between intermittent renewable generation, energy storage, and dynamic load demand, as well as local controllers for PV, wind, electrolyzer, and fuel cell unit [104]. Therefore depending upon the storage need of respective RES, suitable storage technology is put into application. While considering suitable storage technology, optimal size, economical, and technical specifications are the dominant factors. The specifications and data enlisted in Tables 4 and  5 are among the key specifications which can be considered for different RES integration with suitable storage unit.

Barriers and Market Challenges
Due to high investment and low efficiency of renewable energy resources, generation unit should generate sufficiently high enough power to ensure economic power generation. This can be assured by using optimization techniques in hybrid generation system [105]. Optimization techniques include optimization scenario based on different meteorological data [106], graphic construction method [107], probabilistic approach [108], iterative technique [109], AI methods [110,111], genetic algorithm [112], system control for energy flow and management [113][114][115], and multiobjective design [116][117][118] to obtain optimum size of generation unit to guarantee lowest investment with full use of system component. In order to obtain reliable optimum system configurations quickly and accurately, feasible optimization technique should be incorporated [119].
Nishioka et al. discussed variation in electrical characteristics due to temperature in InGaP/InGaAs/Ge triple junction PV cells under concentration and found that conversion efficiency decreases with increase in temperature and increased with increase in concentration ratio resulting in an increase in open-circuit voltage for these PV cells [106]. Hernández et al. presented a systematic algorithm to determine optimal allocation and sizing of PV grid-connected systems in feeders. It could efficiently compromise technical and economical aspect of multiobjective optimization approach and is robust  with moderate computer requirements [120]. Shatter et al. investigated a hybrid generation system consisting of PV, wind, and fuel cells incorporating fuzzy based controller to ensure maximum power tracking for both PV and wind energies in order to obtain maximum power at fixed dc voltage bus [121]. Koutroulis and Kolokotsa proposed methodology to obtain optimum number of commercially available system devices and units so that overall system cost is minimized using GA with constraint like load energy requirements causing zero load rejection [122].

Policy Development
Consumers prefer low cost and reliable electricity irrespective of environmental concerns. Policy should ensure basic needs of users and reduce burden of fossil fuel based energy sources. Therefore respective policy should foot higher subsidy on renewable energy generating equipment to encourage common users to use green energy. Furthermore, large subsidies need to be offered to rural users to meet their economy. Rural users use very low efficient fuel like kerosene (about 6%), causing loss of a large amount of kerosene. Renewable energy is not only cost saving but also reduces carbon emission. Chaurey and Kandpal estimated in rural household of India that 373 kg carbon dioxide emission per year can be avoided by installing PV panels of 20-53 W [123]. For promoting decentralized rural electrification projects, India has provided capital subsidy up to a 90% for installation of new plants in some regions.
Power system researchers have continuously been working to make electricity market user friendly. Jia et al. evaluated a joint schedule problem for PV power, wind power, combined cooling, heating, power generation, high temperature chiller, liquid desiccant fresh air unit, battery, and power grid in order to satisfy electricity load in buildings with minimal expected cost. They concluded with two important results; that is, (1) simulation-based policy improvement (SBPI) methods are developed to improve from given base policies and (2) performance of these methods is systematically analyzed through numerical experiments. For sufficient computing budget, SBPI methods improve given base policies [124]. Shaahid and Elhadidy assessed technoeconomic feasibility of hybrid PV-diesel-battery power systems for a typical residential building at Dhahran (East-Coast, KSA) and evaluated that hybrid PV-diesel model configuration with battery storage decreases overall cost of diesel with increase in PV capacity [125]. Sarkar and Ajjarapu studied a stochastic planning approach for assessing MW resource of three wind and PV hybrid models by fixing varying penetration ratio level at 10%, 20%, and 30%. Method found applicability for different parameters such as cut-in speed, rated speed, furling speed, power rating of wind turbines, efficiencies of heat exchanger, steam turbine and electric generator, and maximum load [126]. Sun et al. studied a joint schedule problem to schedule PV power, wind power, combined cooling, heating, power generation, high temperature chiller, liquid desiccant fresh air unit, battery, and power grid in order to satisfy electricity load, sensible heat load, and latent heat load in buildings with minimal expected cost. Two major contributions have been presented; that is, three simulation-based policy improvement methods are developed to improve from given base policies and performance of these methods is systematically analyzed through numerical experiments [127].
Algorithm based policy approach has helped in maximizing efficiency and reducing cost function [128][129][130][131]. Arabali et al. proposed strategy to meet controllable heating, ventilation, and air conditioning (HVAC) load with a hybridrenewable generation and energy storage system. GA based optimization approach is incorporated with a two-point estimate method to minimize cost and increase efficiency. Minimized cost function ensured minimum PV and wind generation installation as well as storage capacity selection to supply HVAC load [128]. Lannoye et al. proposed insufficient ramping resource expectation (IRRE) metrics to measure power system flexibility for long-term planning and derive adequacy metrics from traditional generation. A flexibility metrics can identify time intervals over which a system is most likely to face a shortage of flexible resources and can measure relative impact of changing operational policies and addition of flexible resources [129]. Mei et al. proposed game approaches for hybrid power system planning to model planning of a grid-connected hybrid power system consisting of wind turbines, PV panels, and storage batteries [130]. El-Tamaly and Mohammed investigated a fuzzy logic technique to calculate and study reliability index of PV-wind hybrid power system to determine impact of interconnecting system with utility grid [131].

Financial Approaches
Several economic criteria exist in providing useful power to utility grid. The costing structure should be simple so as to make user understand it when levied upon them. Until overall cost is user friendly, users may not prefer to adopt the system. Marí and Nabona splitted wind-PV hybrid generation cost into five parts, namely, initial investment cost, operating and maintenance cost, replacement cost of equipment, cost of power exchange between hybrid power generation unit and grid, and regulation cost of utility grid [107]. Different approaches have been made to encourage dependence on renewable energy and reduce fossil fuel based energy dependence [132][133][134]. Tezuka et al. suggested method to reduce amount of CO 2 emission by imposing carbontax revenue and give subsidy on PV-system installations and concluded that amount of CO 2 emission reduces by advertising PV system with subsidy policy even under the same tax-rate and CO 2 payback time [135]. Nelson et al. discussed unit sizing and made an economical evaluation of hybrid wind-PV-fuel cell generation system. They obtained a clear economic advantage of hybrid wind-PV-fuel cellelectrolyzer system over traditional hybrid wind-PV-battery system for a typical home in US Pacific Northwest [136]. Bilal et al. proposed a methodology of optimal sizing of hybrid systems PV/wind/battery in order to minimize annual cost system (ACS) and loss of power supply probability (LPSP) using multiobjective GA. The obtained results show that cost of optimal configuration strongly depends on LPSP. For example, cost of optimal configuration decreases by 25% when LPSP grows to 1% from 0% [116].

Microgrid and Equipment
9.1. Converters. In order to obtain regulated power supply from fluctuating power supply, converters need to be designed to meet frequency and voltage standards. Commonly employed converters include AC/DC (rectifiers), DC/DC (choppers), and DC/AC (inverters). Inverters are commonly employed at point of common coupling and need to be designed optimally. Depending upon output waveform, inverters are classified as square wave, modified square wave (quasi square wave or modified sin wave), and multilevel (multistep) and sin wave (high frequency PWM) inverters. Daniel and Gounden proposed three-phase square wave inverter for an isolated wind-PV hybrid scheme for the first time. They presented a dynamic mathematical model of the hybrid scheme in terms of synchronous reference frame and verified it for transient load conditions [64]. But this converter had high harmonic components and was not fully controlled. Park et al. presented five-level PWM inverter employing dead beat control for minimizing harmonic components of output voltage and load current [137]. Park et al. described an assembly of multilevel PWM inverter and cascaded transformer scheme for standalone generation to obtain high quality output voltage waveform. They validated the proposed system for 11-level and 29-level PWM output [137]. Nasrudin A. Rahim et al. presented five-level and seven-level single-phase multilevel PWM scheme for fluctuating reference input. They employed fluctuating output voltage as reference and compared it with triangular carrier signal to generate desired PWM signals for the switches of converter [138,139]. Kumaravel and Ashok studied a diode-clamped multilevel inverter using bidirectional buck-boost choppers using single-pulse, multipulse, and hysteresis band current control schemes. Single-pulse scheme involves slow switching actions but needs high current rated chopper devices whereas multipulse scheme involves faster switching actions and low current rated chopper devices but has slower response. The hysteresis band current control scheme has faster switching action and lower current rating of the chopper devices and can nullify the initial voltage imbalance as well [16]. Further Gautam and Gupta discussed cascaded H-bridge multilevel PWM Inverter using multiband hysteresis modulation employing current control scheme [140]. Liu et al. proposed control scheme for ZS and qZS cascaded multilevel inverter (CMI). A multilevel space vector modulation integrated with shoot through states for single-phase qZS-CMI synthesizes staircase type multilevel voltage waveforms having low harmonic contents [141]. Fatu et al. discussed a variable-speed motion sensorless duel converter PI current controlled control scheme PM synchronous generator for WECS. They presented a voltage control scheme with selective harmonic compensation for standalone mode operation [142].

Power Flow Controllers (PFCs).
PFCs are essential for promising reliable and economical power supply to connected load in microgrid for standalone operation [32]. Fluctuating variation in source causes stability and power quality problems in terms of voltage and frequency regulation. Situation becomes worse for reactive power demand due to limitation of reactive capability of wind generating system. Mendis et al. proposed a standalone hybrid system consisting of a PMSG, hybrid energy storage (battery storage and a supercapacitor), a dump load and a mains load for obtaining voltage and frequency regulation. Energy management algorithm has been used to improve performance of battery storage and active-reactive power flows [143].
Therefore, an efficient and intelligent PFC is necessary to ensure balance between load and source of generation. This can be assured by forecasting load demand and scheduling regulated power [144]. Depending on power flow, Chauhan Journal of Renewable Energy 13 and Saini divided PFCs into three categories: centralized control arrangement, distributed control arrangement, and hybrid control arrangement [145]. In centralized control arrangement, system consists of one master controller (centralized controller) and several slave controllers for various individual power sources and energy storage unit. Master controller operates in close coordination with all sources and slave controllers. In distributed control arrangement, each power source sends measurement signals to its local controller. Local controllers communicate with one another to take appropriate decision for global optimal solution [146,147]. Hybrid control arrangement is combination centralized and distributed control schemes. In hybrid control scheme, RES are grouped within integrated system [148,149]. Centralized control scheme is applied within each group and distributed control scheme is used to coordinate each group. In such hybrid control scheme, local optimization is achieved through centralized control within group and global optimization among different groups of energy sources is achieved by distributed control [145]. In this quest, Santoso et al. proposed a hierarchical control including a master and slave controllers for hybrid generation. Master controller selects power generation source and slave controller maintained constant DC bus voltage regulating duty cycle of DC/DC converters [32]. Zeng et al. presented a reduced switch count multiport dc-dc converter for standalone PVwind hybrid generation. Converter has been applied for simultaneous MPPT control of a wind/PV hybrid generation system consisting of one wind turbine generator (WTG) and two different PV panels [150]. Botterón et al. proposed a high reliability DC/DC converter and a single-phase PWM inverter for standalone microgrids. It aims at designing two different controllers for two converters for reliable operation of microgrid and results have been validated for a 5kVA PWM modulated single-phase inverter, fully controlled by DSP TMS320F2407 [151].

Conclusion
In order to meet pollution-free future power demands, dependence on renewable energy should be encouraged by providing subsidy on installation products. Energy policymakers need to encourage RES based usage and research to facilitate latest technology for power extraction. Narula et al. investigated probability of achieving 100% electricity demand in South Asia region by year 2030 by increasing dependence on RES based distributed generation [152]. Timilsina et al. estimated to achieve 1845 GW, 1330 GW, and 2033 GW power from PV by year 2030, 2040, and 2050, respectively [153]. More research activity needs to be carried out to improve reduced generation cost, improve storage unit facility, and load forecasting and efficiency of conversion equipment. By increasing dependence on DC microgrids, conversion losses can be minimized and exploitation of power could be optimized.
Hybrid PV-wind integrated with storage unit has been a feasible solution to meet off-grid load demand. A comprehensive review of PV-WECS standalone generation, converter topologies, storage facility options, hybrid simulation tools, and challenges associated with RES generation has been investigated in this paper. It has been found that hybrid generation is supposed to be major contributor in electrification of isolated regions to feed loads which need reliable power source. Suitable optimization techniques using GA and AI can optimize global optimal generation.
High cost of installation has been major issue in widespread RES based power generation. Hence power policy needs to be made liberal to encourage power dependence on RES. More research work needs to be carried out to improve overall durability and performance of storage facility and power conversion equipment.