Saad Dilshad

Received: 1 June 2019 Revised: 2 November 2019 Accepted: 18 November 2019 DOI: 10.1002/er.5033 REVIEW PAPER Limitations, challenges, and solution approaches in grid-connected renewable energy systems Muhammad Abdul Basit | Saad Dilshad Syed Muhammad Sami ur Rehman Department of Electrical and Computer Engineering, COMSATS University, Islamabad, Pakistan Correspondence Muhammad Abdul Basit, Department of Electrical and Computer Engineering, COMSATS University, Islamabad, Pakistan. Email:- | Rabiah Badar | Summary In the modern world, only conventional energy resources cannot fulfil the growing energy demand. Electricity is a fundamental building block of a technological revolution. Today, most of the electricity demand is met by the burning of fossil fuels but at the cost of adverse environmental impact. In order to bridge the gap between electricity demand and supply, nonconventional and eco-friendly means of energy generation are considered. Renewable energy systems (RESs) offer an adequate solution to mitigate the challenges originated Funding information Higher Education Commission, Pakistan, Grant/Award Number: 2EG2-034 due to greenhouse gasses (GHG). However, they have an unpredictable power generation with specific site requirements. Grid integration of RESs may lead to new challenges related to power quality, reliability, power system stability, harmonics, subsynchronous oscillations (SSOs), power quality, and reactive power compensation. The integration with energy storage systems (ESSs) can reduce these complexities that arise due to the intermittent nature of RESs. In this paper, a comprehensive review of renewable energy sources has been presented. Application of ESSs in RESs and their development phase has been discussed. Role of ESSs in increasing lifetime, efficiency, and energy density of power system having RESs has been reviewed. Moreover, different techniques to solve the critical issues like low efficiency, harmonics, and inertia reduction in photovoltaic (PV) systems have been presented. Unlike most of the available review papers, this article also investigates the impact of FACTS technology in RESs-based power system using multitype flexible AC transmission system (FACTS) controllers. Three simulation models have been developed in MATLAB/Simulink. The results show that FACTS devices help to maintain the stability of RESs integrated power system. This review paper is believed to be of potential benefit for researchers from both the industry and academia to develop better understanding of challenges and solution techniques for REsbased power systems and future research dimensions in this area. KEYWORDS climate change, CO2 emissions, energy storage, FACTS, grid-connected system, hydropower, renewable energy systems, SMES Int J Energy Res. 2020;1–31. wileyonlinelibrary.com/journal/er © 2020 John Wiley & Sons Ltd 1 2 1 | INTRODUCTION Since the inception of human life on earth, the socioeconomic development has remained an integral part of human nature. Humans have the exceptional ability to harness natural energy resources for daily life improvement. In medieval, human and animal muscles were used to fulfil the energy requirements; however, the first industrial revolution in the 18th century introduced coal as fuel. After the second industrial revolution in the 19th century, petroleum resources were much exploited to produce massive energy far beyond what could ever be achieved by the animal or human muscles. Industrialisation utilised nonrenewable sources for 200 years, having undesired side effects on natural environment. Extreme use of petroleum and coal led to the uncontrolled emission of carbon dioxide (CO2) and other toxic gasses, responsible for global warming.1 Oil crises2 of 1973 and Middle East war3 in 1991 depleted the geographic occurrence of petroleum products due to which scientist and researchers started to think about alternative sources of energy. One of the most significant transitions of global energy system is the decarbonisation and development in energy quality. A significant shift in energy usage has been observed in the history of humankind from wood to coal, coal to oil, and then towards harnessing the green energy using renewables.4 In the third quarter of the 20th century, energy extraction from atom gained the attention of scientists. Meanwhile, political and nuclear safety problems were raised. Referring the British Petroleum (BP) statistical report of 2019, the global primary energy demand increased to 2.9%. It was the fastest growth since 2010 led by natural gas and renewable energy.5 Now, the critical challenge is to meet increasing global energy demand to sustain industrialisation and fulfil domestic energy requirements without using traditional resources. It is projected that the demand for electricity in 2050 will be more than double, and by the end of this century, it will become threefold.6 Gielen et al7 have presented a road map and analysed the economic and technical aspects to accelerate energy transitions by 2050. They proposed that the share of renewable energy systems (RESs) will rise from 15% in 2015 to 63% in 2050. The burning of fossil fuels has resulted in environmental concerns for humankind. Thanks to RESs for their environment-friendly nature. By definition renewable energy sources, often called alternative sources, are those that can be used again and again to produce energy, eg, wind, solar, geothermal, or biomass.8 The total installed power generation using RESs is almost 2378 GW by the end of 2018.9 However, the integration of RESs with conventional power system brings several BASIT ET AL. new challenges due to their intermittent nature. Some of them, eg, biomass, hydropower, and geothermal operate in the same way as traditional power plants do, but most of the available RESs like solar and wind are less predictable and more time variant.10 Electricity is generated and consumed at the same time. Therefore, the surplus electricity must always be generated to meet varying demand. Power system stability and power quality of both AC and in high-voltage DC (HVDC) transmission systems may get affected in unbalanced situations between demand and supply.11-13 Moreover, seasonal and daily fluctuations of RESs further entangle the situation. In order to encounter these difficulties, the use of long- or short-term energy storage is considered as one of the prominent solutions. There are many types of energy storage systems (ESSs). It reduces the demand of system upgradation and enhances the plant efficiency by allowing the distribution and transmission networks to operate near their thermal limits. It may also help the network to withstand peak demand. They are also useful in smoothing out voltage sags and swells.14 RESs pose significant challenges to conventional grids such as frequency variation, voltage fluctuation, and harmonics. It is because of the stochastic and random nature of RESs and may cause problems in future if left unaddressed. Therefore, this survey also reviews the literature about RESs integration in the conventional grid. In literature, many researchers have focused on the challenges and problems associated with the RESs integration and presented different solutions for it.15-22 Johnson et al15 presented an investigation of rotational inertia of RESs integrated power system. They proposed that rotational inertia can be directly related to grid reliability. Manditereza and Bansal18 gave an overview of hidden challenges of distributed renewable generation from protection perspective. The main technical issues arising in the power system are the system stability, distributed generation, interfacing requirements, protection, and islanding. They suggested changes in protection strategies for faults occurrence due to nontraditional sources like new relay characteristics and identification of faults and their location. Stram20 has discussed the key challenges in expanding renewable energy in context of power quality issues of solar and wind integrated grids. The author has also presented an overview of recent trends in photovoltaic (PV) systems with power electronics equipment. Sen and Ganguly19 presented a discussion on opportunities, issues, and barriers in renewable energy development. They pointed out issues in terms of the development of RESs like market failures, information and awareness concerns, and socio-cultural barriers. This study also BASIT ET AL. presents the development stage of RESs technology in the continents and emphasises on global investment needs and strategies for RESs. The critical issues related to PV systems are their low efficiency and unreliability. The future opportunities to establish high-efficiency thin-film PV cells, use of ESSs, and enhancement in PV inverter technology with wide band-gap devices are essential for fast development and commercialisation of PV systems. The grid integration is also being explored in developed and developing countries; however, some issues like low inertia, harmonicsrelated issues, and the placement of different protections (short-circuit, lightning protection) to integrate it with the distribution grid need to be resolved. Bajaj and Singh23 presented a review of power quality issues with a grid-connected distributed generation. They have simulated a PV-based distributed generation system to demonstrate power quality challenges and presented some mitigation techniques for such issues. Olowu et al24 studied the penetration issues of PV integration with the grid. They concluded that problems arise due to complexity in protection coordination, feeder losses, thermal limits of the grid, and at the point of interconnection. Neetzow et al25 presented modelling coordination between grid and renewables. Authors have discussed the policies to mitigate distribution grid constraints using residential PV battery systems. Sinsel et al21 reviewed the challenges and solution for the integration of variable RESs. Nazir et al26 presented the impact of RESs on environment in terms of atmospheric pollution-related problems. Power quality problems in wind integrated power system were analysed by Sugirtha and Latha.16 They used grid-connected dynamic voltage restorer (DVR)-based wind farm to study voltage issues (sag, swell, and harmonics) and reactive power compensation. DVR is mostly utilised in distribution systems at the point of common coupling between the supply and critical load feeder. The results revealed that the introduction of DVR has improved power factor and stabilised the power system voltage. It has also decreased the feeder losses and current on the line. The intermittent wind speed affects power generation by wind generator. The availability of the economically feasible wind speed is mostly in a remote area which adds extra transmission infrastructures. The power quality issues that appear at grid due to the connection of wind electric generators are voltage transients, frequency and voltage variations, voltage imbalance, and voltage harmonics. Similarly, reactive power compensation, current harmonics, and injection of fluctuating power are some of the issues related to squirrel cage induction generators (SCIGs) and doubly fed induction generators (DFIGs), normally used in wind power generation.16 3 Abdelwahab et al27 discussed the improvement in penetration of renewable energy and its efficiency in the smart and green community. Blaabjerg and Ionel17 presented a comprehensive review of RESs, opportunities, research, future trends, and their challenges. They found that the challenges related to offshore wind farms are foundation issues, high maintenance cost, and the interconnection between onshore and offshore wind farms. Harmonics, uncertainty due to dependency on nature, and stability are the key concerns with both onshore and offshore wind farms. The major disadvantage of self-excited induction generators used in wind farms is that they absorb the reactive power for their regular operation.16 This creates a voltage stability issue in the system. The most common types of induction generators used in wind farms are SCIGs and DFIGs. The reactive power compensation is supplied from the grid by the source of capacitor banks in parallel with the stator terminal of the generator. Hence, there is always a need for dynamic reactive power compensation devices to stabilise the voltages and supply reactive power at SCIGs/DFIGs interface bus. Power electronics-based flexible AC transmission systems (FACTS) devices are considered an effective solution for reactive power compensation and bus voltage regulation. In literature, researchers are applying various approaches to enhance the performance of RESs integrated power system. Researchers have investigated different FACTS devices for improvement of power quality and stability of RESs integrated power systems like static VAR compensator (SVC) for PV-based power system stability,28 SVC for transient stability and power quality improvement,29 voltage source converters (VSCs) to improve grid reliability,30 static synchronous compensator (STATCOM) for increasing the grid transmission limit,31 STATCOM to facilitate the wind farm,32 and unified power flow controller (UPFC) to attenuate subsynchronous resonance (SSR) in wind farm.33 This paper presents a detailed literature review on extensive utilisation of main RESs along with the limitations and challenges originated due to their integration with the grid and possible solutions. Moreover, a discussion on the advantages and importance of ESSs with RESs technology is presented precisely. The impact of FACTS controllers on RESs integrated power system has also been investigated using MATLAB-based simulations. Three test systems with occurrence of various fault events have been simulated to test multitype FACTS controllers. The results depict that FACTS devices are efficient in providing reactive power support and enhance the stability of the power system undergoing any contingency conditions. 4 The paper is mainly divided into four parts. The first part (Section 2) presents comprehensive review of RESs and prominent technologies used to harness renewable energy including their current statistics and upgrading potential from all around the world. In the second part (Section 3) of the paper, limitations and constraints of large-scale integration of RESs with the conventional grid and their proposed solutions in literature are reviewed. In the third part of the paper (Section 4), the impact of ESSs with RESs is discussed. In the last part (Section 5), a simulation study on RESs integrated power systems is done to show the effectiveness of FACTS controllers in RESs integrated power system. BASIT ET AL. TABLE 1 RES technologies and their applications RES Application of RES Hydro Electrical power generation Wind Windmill, water pump, and electrical power generation Solar Solar PV, solar thermal (flat plate, evacuated glass tube, parabolic, etc), solar-based cookers, heat pumps, dryers, and cooling Biomass Gasification, pyrolysis, and electrical power generation Geothermal Hydrothermal, urban heating, and electrical power generation Abbreviations: PV, photovoltaic; RES, renewable energy system. 2 | R ENEWABLE ENERG Y I N THE WORLD Hydroelectric power, wind, solar, geothermal, biomass, and marine energies are most prominent RES.34 Table 1 presents main RESs along with their usage. During last three decades, a considerable percentage growth of RESs in comparison with lignite and coal energy has been observed; however, it is still behind the natural gas. The year 2017 was record breaking for RESs not only in terms of the most significant increase in power capacity but also for decrease in cost, enabling new technologies and investments.35 In 2018, almost 181 GW new RESs were installed in the world which was slightly more than the previous year installations. The rapid rise of RESs usage is due to increased access to finance, emerging economy, demand in electricity, and human and environmental health. Moreover, local governments in some countries are becoming leaders in RESs due to energy shortage issues. Many developed and underdeveloping countries have dedicated a suitable investment in the deployment of infrastructure for new RESs technologies. In 2017, the total share of fossil fuels, RESs, nuclear, and biomass in energy production was 79.7%, 10.6%, 2.2%, and 7.5%, respectively. About 4.2% of the total energy generation share was obtained from biomass, solar, and geothermal energy followed by the 3.6% of hydropower.9 In the years 2017 and 2018, worldwide installed RESs-based power capacity have seen an increase of more than 9% and 8%, respectively, as compared with previous years. In 2018, new RESs installations of solar, wind, and hydro were 55%, 28%, and 11%, respectively.9 The share of RESs in total power generation capacity grew to over 33% in 2018. In the year 2018, more solar PV power generation was added in the system than other RESs. Percentage of electricity generated through renewable and nonrenewable sources is around 73.85 and 26.2%, respectively.9 During last 10 years, RES power capacity has been almost doubled. Moreover, nonhydropower RE increased more than sixfold. Considering RESs generation, without hydropower, China is a leading country following the USA, Germany, and India.9 Carfora et al36 have studied the relationship between economic growth, energy consumption, and energy prices of Asian developing countries. In this regard, the Philippines, Indonesia, India, and Thailand were investigated broadly for their economic growth with the consumption and costs of energy. It covers data from more than 40 years of economic and energy history of these countries. Sahouane et al37 performed an investigation of the energy and economic efficiency of the PV system in Algerian desert conditions. RESs contribute almost 18.1% in total final energy consumption around the world as calculated in 2017. It includes 10.6% modern renewables and 7.5% traditional biomass. The share of fossil fuel is still high, causing an increase in greenhouse gasses (GHG) emissions.9,35,38 A turning point in efforts to restrict climate change by a global agreement was the Paris Agreement. It came into force on 4 November 2016 when 55 countries ratified this agreement. It was decided to limit the average global temperature to rise less than 2 C by the end of this century and to proceed further efforts to restrain it even below 1.5 C.39,40 In 2019, the United Nations (UN) climate change summit was held to check and further increase the effort for the climate. It was found that although parties are trying to fulfil their promises and accomplish their nationally determined contributions; however, achieving the temperature below 2 C or 1.5 C is not looking to happen with current efforts.41 Therefore, some more adequate efforts are necessary from developed countries in addition to the use of climate-friendly technology for developing countries to retain capacity. Meanwhile, efforts towards environmental improvements are BASIT ET AL. 5 becoming more famous and gained political acceptance internationally.42-45 2.1 | Types of renewable energy resources This section reviews the most prominent RESs technologies like hydropower, wind, solar, biomass, and geothermal energy used for the extraction of energy. These technologies are reviewed one by one with their latest statistics and countries having large infrastructure of these technologies. 2.1.1 | Hydropower Hydropower is one of the oldest technologies used for different purposes. Energy is harnessed by flowing water coming from higher elevation side to the lower. More than 2000 years ago, the kinetic energy of flowing water was used to grind wheat.46 Hydropower for electricity generation was introduced in the 19th century. Now, hydropower technology is used by more than 160 countries globally.47 Top countries producing electricity from hydropower are shown in Figure 1. In the 21st century, hydropower is technically a mature technology with its components used for multiple purposes, eg, electricity generation, irrigation, flood, and drought management.48 There are two basic configurations of hydropower electric generation. Dam with reservoirs and run-off-the-river plants without reservoirs. Dams are further categorised as large and small dams with storage, small dams with pumped storage.46 Mega dam projects required a large amount of investment, long lead time for development and construction.49,50 Run-offthe-river plants have relatively small-scale, eco-friendly, power generation as they do not interrupt the natural flow of the river. These projects are often distributed generation to feed far and remote areas.51 Pump storage projects have two reservoirs at different elevations. Water is allowed to flow from higher reservoir to the lower through the turbine to generate electricity during peak demand. During off-peak hours, excess electricity is used to operate the pump and water is pumped back to higher reservoir. The energy conversion efficiency of recently installed pump storage system is more than 80%.51 Hydel power electricity added in 2018 is shown in Figure 2. 2.1.2 | Wind exists everywhere on this earth, and harnessing of power from the kinetic energy of blowing wind is called wind power. Wind power is now considered as a mature technology used commercially for the generation of electricity since 1990s around the globe. The trend of wind technology is increasing in those countries where energy demand is fulfilled by burning of fossil fuels.52,53 Wind power has practical applications for power requirements in remote areas; however, it should be used in aggregation with the primary grid for continuity, maximum stability, and flexibility.54 There are two categories of wind farm depending on their location: onshore (land) and offshore (sea) wind farms. Onshore wind farms are usually grouped near grid stations on landscapes or mountains, mostly near coastal areas, whereas offshore wind farms are installed deep into the sea. Offshore technology is slowly adopted in the last few years due to higher uniform wind speed in deep oceans.55,56 Offshore wind farms usually have more nameplate capacity ratings than onshore farms. The maximum power coefficient (Cpmax) is 16/27 = 0.5926, known as Betz criterion. It states that one can only convert maximum of 59.26% energy from available kinetic energy of the wind through the wind turbine. Real-life wind turbines have an efficiency range of 40% to 50%, only.57,58 Moreover, the world's megacities are mostly located in coastal areas; therefore, long-distance transmission is avoided. Wind power added during- has linear behaviour. As shown in Figure 3, the global wind power electricity reached to 591 GW mark in 2018. Figure 4 shows the leading countries in wind power installation, where China added 21.1 GW wind power into the system in 2018 following USA (7.6 GW) and Germany (3.1 GW). 2.1.3 | Top nations producing hydel power9 [Colour figure can be viewed at wileyonlinelibrary.com] FIGURE 1 Wind power Solar power Sun is the most inexhaustible free source providing energy for billions of years. Sun provides nearly four 6 BASIT ET AL. F I G U R E 2 Top nations add hydel power in 2019 (Data Source: REN21 2019 Report) [Colour figure can be viewed at wileyonlinelibrary.com] million exajoules of energy to earth annually.59 Solar radiation received by the earth in 1 year is 10 000 times more than the annual global energy demand of modern life.60 Moreover, one-fourth of the total radiations that fall on the earth's paved area is enough for the global energy requirements. Solar energy has a remarkable potential to fulfil entire world energy demand if scientists develop technical equipment for the generation of electricity through it.61 Light and heat are two forms of solar energy which reach to the earth. PV technology uses sunlight to produce electricity. Electricity generation is dependent on the concentration of sunlight rather than any influence of heat,62 whereas concentrating solar thermal power (CSP) uses both the heat and light of the sun to generate electricity.63 There are subcategories of CSP including solar dish, solar central receiving systems, and parabolic solar through. Solar energy systems other than PV have some technical disadvantages due to rotating machinery and maintenance. Moreover, they are economically expansive. In coming years, it is expected that production and installation cost of CSP will be reduced up to 50% to 60% through technical research in this field.64 On the other hand, solar PV technology is well-established technology now and uses the photoelectric effect to generate electricity without any rotating components. Advanced power electronics equipment is involved in PV electricity generation which is now cheap and readily available. A lot of research is being carried out on this technology for further efficiency improvement since 2002.65 Nowadays, solar PV technology has an efficiency of 25.5%, claimed by the researchers of Hong Kong,59 whereas peak efficiency is nearly 30% and the average efficiencies of CSP are recorded as 20%.66 During-, solar PV addition has been raised exponentially all over the world as shown in Figure 5. Annual addition in 2018 was recorded as 100 GW in this field, out of which 45 GW was installed in China. USA (10.6 GW) and Japan (6.5 GW) has also introduced a considerable amount of solar PV in their systems, as depicted in Figure 6. 2.1.4 | Bioenergy Bioenergy is a vital source of energy extracted from agricultural waste and animal's husbandry residue which can be used as biodiesel for electricity generation.67 Bioenergy has all possible characteristics to face challenges related to increasing use of fossil fuel and minimising the emission of GHG.68 Bioenergy resources all over the world are 3 to 4 times more than the total human energy requirements.69 It is estimated that 35% of global energy (ie, 190 × 1018 Jy−1) can be extracted from bioenergy potential excluding dense forest, crop land, wilderness, and infrastructure.70 Currently, 9% of the world energy demand is fulfilled by bioenergy.71 Just like conventional fuel, biofuel also exists in solid, liquid, and gaseous states. Bioenergy is considered as a renewable energy source but not as cleaned energy. Most of the studies reveal that biofuel emits comparatively fewer GHG than conventional fuels, but some studies show that biofuel has a more negative impact on climate change.72 The efficiency of biomass electricity plant is around 20%.73 Figure 7 shows bioenergy produced in different regions of the world for the period-. Overall global bioenergy trend is increasing slowly; however, in some of the areas, bioenergy production is reduced than in previous years. 2.1.5 | Geothermal energy Geothermal energy means the energy naturally stored in the interior of the earth. Massive amount of geothermal energy reserves is present between earth crust and core BASIT ET AL. 7 F I G U R E 3 Wind power capacity and addition- (GW) (Data Source: REN21 2019 Report) [Colour figure can be viewed at wileyonlinelibrary.com] F I G U R E 4 Top nations producing wind power (GW) (Data Source: REN21 2019 Report) [Colour figure can be viewed at wileyonlinelibrary.com] F I G U R E 5 Solar photovoltaic (PV) capacity and addition- (Data Source: REN21 2019 Report) [Colour figure can be viewed at wileyonlinelibrary.com] in the form of heat or steam. These reserves are unevenly distributed, rarely concentrated, and at some places present at reasonable depth to exploit mechanically by drilling.48 History of geothermal power generation is over a century ago, and the first commercial power plant was installed in Italia in 1914.74 Among the renewable resources share of geothermal is about 1.5% and overall 0.3% of global power generation.75 Three types of power plants are used depending upon the temperature, state of fluid, and depth of well.76 Dry steam power generators are used in which the hydrothermal reservoir is vapour dominant with high temperature. Flash steam power generators are used where the hydrothermal reservoir is liquid dominant. Binary cycle plants are used mostly based on Rankine or Kalina cycle with temperature of the reservoir less than 150 C.77 Other possible 8 BASIT ET AL. F I G U R E 6 Top nations producing solar photovoltaic (PV) power (Data Source: REN21 2019 Report) [Colour figure can be viewed at wileyonlinelibrary.com] technologies to increase the percentage share of geothermal energy are discussed by Bruhn.78 Estimated geothermal energy is 84.8 TWh according to 2017 survey. The cumulative capacity has reached to 13.3 GW of geothermal electric power in 2018.9,79 Indonesia and Turkey are the most significant contributors of geothermal electricity followed by Iceland and USA, as shown in Figure 8. Enhancement of renewable resources is essential to meet the global climate goal without reducing economic growth and decelerating welfare. 3 | LIMITATIONS A ND CONSTRAINTS O F RENEWABLE INTEGRATION Apart from all advantages and worldwide availability of RESs, there are also some constraints and limitations for their use. Intrinsic characteristics of different renewable sources are responsible for slowing down their development. Insufficient geographic sites all over the world naturally limit the hydel power and geothermal generation.80 Biomass requires large space to store natural source. Under these constraints, wind and solar PV installation can be opted as better choice. On the other hand, these sources have intermittent nature. Electricity generated from wind is dependent on the nature of the turbine, density, and speed of the wind. If the wind speed is low (<2.5 m/s), it is unable to generate electricity, or if the wind speed is so high (>25 m/s), turbines are shut down for safety purpose. In contrast, solar PV generation depends on the intensity of light at that location which varies hour to hour and season to season and additionally affected by clouds. Thus, making uncertain these two mostly available primary sources. High penetration of solar PV and wind system can lead to critical and overall system stability challenges; the main reason behind it is the low inertia or non-existent inertial response of RESs. For example, solar PV units do not offer any inertia to the power system. Moreover, variable wind turbines are connected to power electronicsbased converter which effectively decouples the wind turbine inertia from mitigating system transients. Other RESs technologies are not installed on a large scale to provide considerable inertial support to power system.81 Therefore, the replacement of conventional power plants with RESs will reduce the inertia of the entire power system. It has been predicted that power system inertia would be reduced up to 70% in the United Kingdom between 2014 and 2034.82,83 The main technical issue that arises in these new RESs is frequency and overall system stability due to less generating units and small inertia constant. Another problem in the integration of RESs with the conventional grid is related to power quality issues. Power quality challenges include frequency fluctuation, flickers, unbalance voltage and current harmonics, and voltage variations.84 These challenges arise due to switching components (firing angle control) used in power electronics devices which are now an essential part of RESs technology.85 Strength of these issues in case of solar PV depends on the location of PV module, distributed system configuration, and solar PV penetration level. In case of wind, it depends on the type of turbine, density, and speed of wind.86 It has been found in literature that power quality issues can occur at distribution, generation, and transmission side in all RES integrated grids.55 Intermittent characteristics of solar PV appears due to rapid alterations between sunshine and clouds which cause voltage fluctuation and unbalance, whereas, varying wind speed affects the wind power generation.87 Voltage flickers caused by the impact of machines vary inversely with the fault level at point of common coupling which impose significant impact on grid. Voltage BASIT ET AL. 9 F I G U R E 7 Capacity and addition of bioenergy in different regions- (Data Source: REN21 2019 Report) [Colour figure can be viewed at wileyonlinelibrary.com] F I G U R E 8 Top nations producing geothermal energy (Data Source: REN21 2019 Report) [Colour figure can be viewed at wileyonlinelibrary.com] fluctuation affects the lifetime of sensitive electronics and electrical equipment.88 Harmonics generation is another unwanted phenomenon that occurs due to switching control in power electronics-based power systems. According to current techniques and practices, inverter inherits nonlinearity results in harmonics being injected in the main AC supply.89 Harmonics are considered as a cause of losses in the form of heat. In variable wind turbines, force commutated power converters not only produce harmonics but also produce interharmonics.84 In the solar PV system, DC power is converted into AC to inject the power to the grid which may cause harmonics in the plant.90 Generation of subsynchronous oscillations (SSOs) is one of the emerging problems that has been seen in wind power-connected grids. Power system stability problems are concerned with the generation of SSOs.91,92 This phenomenon in conventional plants is originated due to subsynchronous interaction of mechanical shaft of turbogenerator with active control of transmission lines or compensated transmission lines.93 Extensive literature has been studied and documented for mitigation of SSOs in conventional power systems.94 However, in wind powerconnected grids, SSOs are found because of induction generator with a series-compensated transmission line. Moreover, different types of wind controls play an essential role to make the issue of SSOs more complicated in wind connected system as compared with conventional plants.95 The problem of SSR occurs in PV parks instead of only in wind farms. Khayyatzaden and Kazemzadeh proposed a control algorithm to investigate the effect of SSR in IEEE second benchmark model aggregated with the wind farm. Mathematics for two different types of SSR in power system is given as an induction generator effect.96 Subsynchronous current is also included with a compensated series line with an electrical frequency: 10 BASIT ET AL. sffiffiffiffiffiffiffiffiffiffiffiffi X LðeqÞ fs =fn , Xc ð1Þ where fs and fn are the nominal and natural frequencies, respectively. XC and XL(eq) are the series capacitor and line reactance w.r.t. transformer and generator, respectively. fs =fr +fn ð2Þ At this frequency, the slip is given as s= f n −f r : fn ð3Þ The lower value of natural resonance frequency “fn” then the electrical frequency “fr” leads to negative slip values. Slip becomes negative since the natural resonance frequency “fn” is less than the electrical frequency corresponding to generator rotor speed. Therefore, at subsynchronous frequency, the magnitude of equivalent rotor resistance “(Rr/s)” becomes negative. The generator rotor torsional oscillations with frequency “fm” induces armature voltage components at the frequency given as f nðindÞ = f s f m : ð4Þ When the subsynchronous frequency component of “fe(ind)” comes in close vicinity or matches with any of the electric resonance frequency “fe,” the torsional oscillation and electrical resonance reinforce each other resulting SSR. 3.1 | Solution approaches In practice, various devices are being used to solve specific problems on the grid either caused by renewable integration or from other sources. Some solutions to tackle the problems caused by RESs integrations are explained below. VSMs is to correlate the modern dynamics of power electronics devices with static and dynamic properties of the conventional synchronous machine. The aim is to control grid-interface converter of distributed generation in such a way that it behaves like a real synchronous machine.98 Yu et al99 has discussed the detailed modelling of VSMs. As VSM behaves like a genuine synchronous machine, therefore, it can handle active and reactive power in both directions. Properties of the synchronous machine like rotating mass, transient reaction, and remote power dispatch are modelled in a new way for interaction between grid and generator.100 The critical aspect of VSMs is virtual inertia. Virtual rotational inertia can be produced by adding short-term energy storage at a distributed generating unit along with intelligent control interface to a conventional grid.101 The efficiency of VSMs depends on PI control-based current source inverter. Malla et al102 has proposed an adaptive dynamic programming-based supplementary controller for performance enhancement of the system. A VSM connected to solar- and wind-based hybrid power system is shown in Figure 9. 3.1.2 | Superconducting magnetic energy storage Superconducting magnetic energy storage (SMES) is a method other than VSM to control the fluctuating frequency of wind farm.103 DC current flowing through superconducting coil stores energy in the magnetic field which then released or absorbed in response of fluctuation of wind power. Other features of SMES include stability, output voltage, and power regulation. SMES offers an advantage of minimal actual specific energy (Wh/kg). The actual specific energy of SMES is around approximately 1 to 2 Wh/kg, whereas batteries have 10 to 200 Wh/kg, approximately. The specific power of SMES and battery energy storage is approximately 10 to 10 000 and approximately 0.001 to 10 kW/kg, respectively.104 They can store very large amount of power in small space as compared with conventional batteries. The schematic diagram of SMES is given in Figure 10. 3.1.3 | 3.1.1 | Variable synchronous machines Variable synchronous machines (VSMs) were proposed to counter the stability issues that arise due to frequency fluctuation and voltage reduction in RESs connected grids. This power electronics-based technique was introduced in 2007 by Beck and Hesse.97 The key idea behind Converter control for harmonics Kalair et al106 have reviewed the inverter harmonics, its modelling, and mitigating techniques. In research and practice, different harmonics mitigation techniques are widely adopted. These techniques are active power filter (APF),107 passive power filter (PPF),108 linear reactors, dynamic voltage regulators, isolation transformers (TFs), multilevel inverter, k-factor TFs,109 low and tuned filters, BASIT ET AL. 11 F I G U R E 9 A typical virtual synchronous machine (VSM) connected to renewable energy systems (RESs)-based hybrid power system [Colour figure can be viewed at wileyonlinelibrary.com] multiple-pulse converters, phase shifting TF,110 etc. Singlepulse half-wave, two-pulse full-wave, and three-pulse threephase half-wave rectifier produce all odd and even except triplen harmonics although the increase of converter pulses reduces harmonics. Hence, a multiple-pulse converter provides effective mitigation of harmonics.106 Prasad et al108 have used a passive harmonic filter for mitigation of current harmonics in PV integrated system. Different harmonics compensation and mitigation methods have been considered and proposed in previous studies.107-110 A comprehensive review is presented by Li and He111 for compensation of harmonics by using converter control in distributed RES units. Three control strategies for harmonics compensation are discussed including voltage control method (VCM), current control method (CCM), and hybrid control method (HCM). Summary of these control strategies is given in Table 2. 3.1.4 | Filter technologies (APF) Power system problems like harmonics and reactive power can effectively be solved by using APF.112,113 The APF is used in three different configurations: series APF, shunt APF, and hybrid APF. The schematic diagram of the APF is presented in Figure 11. The inverter parameters, control strategies, and current detection methods determine the execution of APF filter.114 In order to mitigate SSO, a control algorithm is proposed in addition to stator and rotor side control of DFIG-type wind farm.115,116 This control technique uses dq components of rotor and stator current as the input of proposed control. Another control technique based on the Schur method is proposed for mitigation of SSO.117 It uses state feedback design and a washout filter at rotor side converter to diminish the effects of SSO. Some other harmonics compensation and mitigation methods have been considered and proposed in previous studies.106,111,118 3.1.5 | Virtual impedance control Another method named virtual impedance control (VIC) is a smart way to shape a dynamic profile of converters used in RESs.119 The VIC loop can be embedded as an additional degree of freedom for disturbance reduction and active stabilisation. Furthermore, it is employed as a command reference generator for the converters to offer auxiliary services. Applications of VIC include power flow control, stabilisation, fault ride through, harmonic, and unbalance compensation. Figure 12 shows the classification of VIC based on their functions. He and Li120 have proposed and implemented a design of VIC for power electronics-based distributed generations. As discussed earlier, the immense integration of RESs with traditional grids may lead to transient or voltage stability issues. Furthermore, long-distance transmission from RESs integrated grid to load centre is more likely to become unstable due to lack of reactive power support.121 These issues are also addressed by incorporating FACTS devices such as static synchronous series compensator (SSSC), STATCOM, and UPFC with renewable integrated power grids. These FACTS devices provide reactive power support and voltage stability.22,122 The reactive power compensation in RESsbased grid is reviewed in Sarkar et al.123 Some other researchers also investigated the impact of FACTS controller for stability in conventional and RESs integrated grid.29,124 12 BASIT ET AL. TABLE 2 FIGURE 10 (SMES) schematic 3.1.6 | Superconducting magnetic energy storage 105 STATCOM in smart grid control STATCOM is a widely used FACTS controller, well known for its efficient reactive power compensation capabilities. Marine Current Turbine (MCT) driven marine current generators integrated with large offshore wind farms guarantee a large-scale renewable generation of electricity.125 Transients in this system disturb the stability of the main grid; therefore, efficient control system is required to cope with such conditions. In order to damp out the oscillations of one machine infinite bus system connected with this integrated offshore wind farm and marine current farm, STATCOM is proved to be an effective addition to the grid control.126,127 In this modern age of rapidly improving technologies, SCIGs wind turbines are in common use owing to their more straightforward design and lower cost. However, following a voltage drop in transient conditions, SCIGs slow down the voltage restoration leading to system voltage and rotor speed instability.128 STATCOM incorporation along with artificial neural network-based self-tuning PI controller resolves this issue. It can work effectively under extreme conditions, such as uneven loading, faults, and weakened grid support resulting in efficiently controlled voltage and rotor speed to maintain them within limits.129 DFIGs are modern edition of generators with wide application circle due to their higher efficiency.130 Moreover, DFIGs have an additional capability of decoupled control of active and reactive power to ensure better grid integration.126 However, DFIGs are extremely vulnerable and sensitive to the grid faults. Different studies have been carried out to find a potential solution to cater for this problem.131,132 STATCOM introduction into DFIGs-based wind power systems ensures smooth steady state and transient voltage control.133 The efficiency of STATCOM can be enhanced further by incorporating fuzzy logic into control system. Fuzzy controllers can be added as main or supplementary controllers of STATCOM. Main controller being used for AC bus voltage regulation and supplementary controller for DC capacitor voltage control.134 Type-II NeuroFuzzy-based controllers are widely used for enhancing the power system stability using STATCOM.135,136 These control schemes further enhance the transient stability of integrated smart or conventional grid. Harmonic compensation schemes111 VCM HCM VSM Local load harmonic compensation Yes Yes Yes (without good damping) DC line current harmonic rejection Yes Yes Yes (indirect current control) Grid impedance variation Insensitive Insensitive Sensitive Standalone operation with voltage control Yes Yes Yes Abbreviations: HCM, hybrid control method; VCM, voltage control method; VSM, virtual synchronous machine. 3.1.7 | SSSC in smart grid control SSSC is another FACTS controller being effectively utilised for stability enhancement of conventional and RESs integrated grid.137,138 As discussed earlier, large-scale integration of wind farms into electricity grid causes grid instability issues owing to the intermittent nature of wind.139 Moreover, it badly affects the grid during contingencies especially during sequential faults occurrence. Voltage and rotor angle instability of wind generators in transient conditions can even cause system shutdown resulting in severe blackouts. SSSC connection to the control system can effectively damp out system oscillations in such conditions through efficient reactive power support.140 Solar and wind farm inverters can be modelled and functioned as SSSC in grid-connected RESs. Operation of these inverters as SSSC can offer active and reactive power compensation. It can enhance the steady-state power transfer limit of the line in the event of increased load demand. Besides this, it can also be utilised to mitigate SSR and power oscillations.141,142 The capability of SSSC to work in both inductive and capacitive mode is very handy for power oscillation damping and rotor angle stability.143 Therefore, SSSC is gaining vast utilisation in smart grid applications installed with RESs. 3.1.8 | UPFC in smart grid control RESs mostly have an uneven and intermittent nature which disturbs the normal functionality of smart electricity grid. Power consumption management along with the mostly discussed stability issues of grid is a prominent issue to be effectively resolved.144,145 An efficient power consumption management model is introduced by Aghaebrahimi et al.146 This model proposes a technique BASIT ET AL. 13 FIGURE 11 Schematic diagram of active power filter107 FIGURE 12 Classification of virtual impedance control (VIC) to control the peak load reduction in smart grid using UPFC. This model decreases losses and controls overloading in the transmission line as well as reduces the generation costs during peak demand hours. This model works with maximum possible efficiency. PV integration with smart grid ensures a clean renewable addition to the grid, but it does have certain connection issues which need to be resolved. Grid synchronisation with PV and ensuring a stable connection between grid and PV system even during contingencies is a demanding task. Kumkratug147 has proposed a UPFC model for calculating transient stability of a multimachine system stocked with a grid-connected PV system. In addition to this, a simulation technique for evaluation of critical clearing time of a multimachine system is also provided. UPFC has also shown its effectiveness in controlling wind farm- 14 BASIT ET AL. T A B L E 3 Improvement of power quality in RES integrated power system by using FACTS FACTS Controllers Result/Conclusion SVC28 Power system stability improvement having a solar PV system SVC29 Transient stability and power system quality improvement VSC30 Improve the penetration of green energy and system reliability. STATCOM Increase in grid transmission limit for a PV solar farm integrated31 STATCOM facilitate the wind farm connection with conventional grid32 UPFC They are suitable to attenuate the SSR in wind farms Abbreviations: FACTS, flexible AC transmission system; PV, photovoltaic; RES, renewable energy system; SSR, subsynchronous resonance; STATCOM, static synchronous compensator; SVC, static VAR compensator; UPFC, unified power flow controller; VSC, voltage source converter. connected smart grid, even during fault occurrences. SSR often occurs in series-compensated lines, severely damaging the stability of the system even causing mechanical erosion of the generators. Offshore wind farm integration with the electricity grid often suffers from SSR.148 UPFC is found very effective and valuable to attenuate SSR in wind farm integrations.33 UPFC outperforms other FACTS devices in dealing with SSR due to its dual capability of series and shunt compensation. Power quality improvement by implementation of FACTS controller is summarised in Table 3.124 3.1.9 | D-FACTS controller Distributed FACTS (D-FACTS) is a new term used explicitly for distributed RESs power plants. It is cost-effective power flow solution.208 D-FACTS devices and their features are summarised in Table 4. Most of the problems like voltage fluctuation, voltage sag and swell, and harmonics associated with microgrids/smart grids are addressed by D-FACTS devices. The comparison of solution approaches due to problems associated with renewable integration are summarised in Table 5. 4 | S I GN IF IC AN CE O F ESS s U SED WITH RESs There are many indisputable advantages of RESs; however, they have few challenges. Unlike traditional power T A B L E 4 Features of distributed flexible AC transmission system (D-FACTS) controller D-FACTS Controllers Features Distributed TCSC (D-TCSC)149 System voltage control Reduction of real power (P) losses Development of a cyber-secure control scheme for the stability of the system Distributed Static Series Compensator (D-SSC)150 Reduction of real power (P) losses Cheaper and smaller than conventional FACTS controller Active power flow control of line Distributed Static Compensator (D-STATCOM)151,152 Reactive power control Current harmonic compensation Voltage regulation Uninterrupted power supply when using an energy storage device Distributed Switched Filter Compensator (D-SFC)152,153 Mitigation of total harmonic distortion (THD) at buses Power factor improvement Voltage stabilisation Distributed Green Plug SFC (D-GPSFC)154 DC bus voltage stabilisation Voltage regulation Fewer power losses plants, power extraction from different RESs is not adjustable according to the energy demand. Power is directly harnessed from RESs as their peak production time may not match the power requirement. In result, the system may face large fluctuations in output power in annual or even monthly cycles. Similarly, power demand varies from season to season. Therefore, energy storage is a crucial factor in making these sources fully reliable to use them as a primary source of energy. The idea is to store energy when it is abundant and release when energy production is less than demand. That is why energy storage becomes an integral part of reliable RESs technologies.14 In this section, some mature, developed, and commercially available energy storage techniques are overviewed. They are dissimilar in working principles and their underlying technologies. Energy storage is considered handy system and has better potential to improve the reliability of the power system.155 This activity provokes the intentions of RESs integration with power system to control the issue of energy deficiency while ESS is not another source of energy. Leading economic players in this field are China and the United States. They are investing a considerable amount in adding ESSs to their network. In China, 22.85 GW of ESSs were successfully installed in 2015. Meanwhile, the US ESS market grew BASIT ET AL. 15 TABLE 5 Comparison of solution approaches in grid-connected RESs Problems SMES VSM Low inertia ✓ ✓ Power flow control ✓ VIC D-FACTS ✓ ✓ APF Converter Control for Harmonics ✓ Fluctuating frequency ✓ ✓ ✓ ✓ Harmonics Reactive power compensation Fault ride through SSSC UPFC ✓ SSO Stability STATCOM ✓ ✓ ✓ ✓ ✓ Transient voltage control and problems due to DFIG ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Abbreviations: APF, active power filter; DFIG, doubly fed induction generator; D-FACTS, distributed flexible AC transmission system; SMES, superconducting magnetic energy storage; SSO, subsynchronous oscillation; SSSC, static series synchronous compensator; STATCOM, static synchronous compensator; UPFC, unified power flow controller; VIC, virtual impedance control; VSM, virtual synchronous machine. up to 243% in 2015 and predicted to reach 1.5 GW by- 4.1 | Types of ESSs Reliability of power system and energy efficiency are the critical aspects of electrical ESSs. Usually, electricity can be stored in different forms like kinetic, potential, or electromagnetic energy. Most prominent ESSs technologies with a brief description are gathered in Table 6. The exploitation of RESs can be utilised to its maximum by using ESSs which not only enhance the capability of RESs but also mitigate the issues in utility grids by reducing the effects caused by intermittent nature of RESs. ESSs have multiple applications ranging from transmission to distribution systems.158 4.1.1 | PHE storage Pumped hydel energy (PHE) is the mature technology among the currently available ESSs. According to Ibrahim et al,159 PHE has an efficiency range of 65% to 85%. Due to recent technological development, PHE can be hybridised with wind and PV systems to establish a hydel-wind pumped storage (HWPS) and hydel-PV pumped storage (HPVPS). During peak demand, electricity is produced by flowing water from higher potential to lower potential and pumped back to higher altitude by using wind or PV power.160 On the other hand, pumped storage needs a suitable site with significant capital cost. Table 7 summarises some of the operational PHE storages across the world.161-163 4.1.2 | Compressed air storage Like PHE, compressed air storage (CAS) is also a highly developed technology with low investment and operational cost. However, its installation is limited due to geographical constraints. It works on mechanical principles to store the compressed air using low-cost energy and release this compressed air during discharge to run a gas turbine.164 Table 8 summarises the information of two operating CAS. 4.1.3 | Batteries Electrochemical process is used to store chemical energy in the battery and release electrical energy when needed.166 Battery storage is widely used among the power system applications. Batteries power ranges from some kilowatts to megawatts. Moreover, the battery system is advantageous because of its power converters like those of solar PV and wind turbine. The advantages of batteries are high efficiency, fast response, and zero harmful emission.166 The significant disadvantages of batteries include environmental concerns like recycling and explosion danger and some other technical issues like protection, ageing, solid electrolyte interphase (SEI), memory effect, and lithiation.167 The lead-acid battery has the recycling problem due to toxic nature of lead and its compounds. A long-term exposure to these compounds may lead to kidney and brain problems. Nickel cadmium (NiCd) batteries also have a recycling problem due to toxic nature of cadmium. They also have a limitation of memory effect 16 BASIT ET AL. TABLE 6 Summary of different energy storage technologies157 Energy Storage Techniques Pumped hydel energy Thermal energy storage Description It works on the principle of potential energy stored in the water. Stored energy depends on a gravitational height between higher and lower reservoirs. Around 96% of the ESS installed capacity in the world.157 Heat energy from any source can be stored in the form of thermal energy stored in a medium with sensible, latent, and electrochemical reactions storage. Compressed air storage Electrical energy is stored in the form of compressed air in the caverns which is used in future as compressed air for a steam turbine. Batteries Chemical energy is stored in cells made up of different materials. Flow batteries Hydrogen storage and fuel cells Reverse electrochemical reaction is used to produce electricity. Stored H2, O2, and air are used as fuels to produce DC voltage through fuel cells. Flywheels Mechanical energy is stored as rotational kinetic energy in the flywheels. Supercapacitor energy There is no energy conversion in supercapacitor as electrical power is directly stored in the form of the charge stored between the two plates. Superconducting magnetic energy storage Energy is deposited in the form electromagnetic field in a superconducting magnet. Abbreviation: ESS, energy storage system. in which it only takes a full charge after series of full discharges. The gassing phenomenon, common in lead-acid batteries is the primary cause of their explosion. The low cycle life of some batteries is also a problem like 500 to 1000, 2000 to 2500, and 2500 cycles for lead-acid, NiCd, and sodium sulphur (NaS) batteries.157 Moreover, in the lithium-ion (Li-ion) battery, after few charging cycles, the SEI layer formed with the deposition of electrolyte causes cell degradation and lowers the capacity.168 Li-ion batteries also suffer from a loss of lithium during the first cycle due to SEI layer growth.169 This problem is solved by prelithiation of Li-ion batteries. Various prelithiation techniques are developed for addressing this issue.170,171 Prelithiation strategy is now Pumped hydel storage (PHS) operational sites161-163 TABLE 7 Plant Name Country Capacity, MW Established Year Bath country USA 3003 1978 Huizhou China 2448 2011 La Muela Spain 2000 2013 1800 1987 Grand'Maison France Ikaria Island Greece 2.655 Okutataragi Japan 1932 1998 Srisailam India 1670 1981 Mingtan Dam Taiwan 1602 1994 also applied in capacitors to reduce the initial capacity loss. Apart from these disadvantages, batteries offer a compact, portable and easy to use option for energy storage through RESs. The recent development and research on metal-air battery is underway. It has very high energy density (500 to 1000 Wh/kg) but low-efficiency range. These batteries with improved efficiency can be used in future for large-scale applications as they occupy minimal space.157 Another option is to reuse the batteries (capacity reduced by more than 20%) obtained from automotive industries. The automotive battery pack becomes unfit when it loses its capacity more than 20% or even 15% for some electric vehicles.172 The reduced cost of these reused batteries can present an excellent approach for energy storage of RESs like wind, PV, and pumped hydel storage (PHS). The largest display of repurposed batteries was installed in 2018 at Johan Cruyff Arena, Amsterdam. It consists of 3 MW power and 2.8 MWh capacity of ESSs having 340 new and 250 reused batteries. The reused cells are obtained from 24 kWh electric vehicle pack having a present capacity of less than 20 kWh.173 Ahmadi et al174 has discussed the environmental aspects of the reused batteries from electric vehicles. Some examples of ESSs are presented in Table 9 along with their applications. 4.1.4 | Flow batteries A flow battery is another type of electrochemical devices in which two electrolytes are used with one dissolved in the other.177 The significant advantage of these batteries lies in the fact that their capacity is not affected with time like the secondary cell batteries with the material depositing on the electrodes. The typical efficiency range of vanadium redox battery (VRB) and zinc-bromine battery (ZBB) is about 60% to 85% and 60% to 75%, respectively.10 Only VRB is a mature technology, and all other types of BASIT ET AL. 17 TABLE 8 Operational compressed air storage plants165 Efficiency, % Capacity, MW Kraftwerk Huntorf, Germany 42 290 McIntosh, Alabama, United States 54 110 Name, Country T A B L E 9 Applications and location of certain battery storage system162,175,176 Cites Application Capacity, MWh California Load levelling 40 Berlin Frequency control 8.5 Hawaii Power management 3.75 Puerto Rico Spinning reserve 1.4 flow batteries are still under research and development stage with few commercial applications. Different applications of flow batteries are surmised in Table 10. 4.1.5 | Fuel cells Fuel cells are quite similar to batteries but vary in their mode of operation. A fuel cell is an electrochemical device using hydrogen as the primary fuel to generate electricity.178 Hydrogen can be stored in tanks in the form of hydrides or using electrolyzes. It is becoming famous due to its clean, highly efficient and cost-effective energy. They also have a high dynamic response, longer storage duration, and longer life than batteries. However, similar to batteries, fuel cell has a high investment cost due to emerging technology.179 Fuel cells can be categorised in different types on the bases of used electrolyte. They include phosphoric acid (PA), alkaline, polymer electrolyte membrane, molten carbonate, and solid oxide-based electrolyte. The polymer, PA, alkaline, and direct methanol (DM) fuel cells work at low temperature. PA is a mature lowtemperature fuel cell technology used residentially and in distributed grids. The operating temperature of this fuel cell is around 150 C to 200 C.57,180 Alkaline, polymer electrolyte, and DM fuel cells are also low-temperature fuel cells with an operating range of 90 C, 40 C to 60 C, and 50 C to 150 C, respectively.57 The efficiency of these fuel cells is around 20% to 50%. DM fuel cell is used in transportation and portable devices as well. However, the catalyst (platinum) cost is a significant issue for its development. The major limitation in fuel cell development is its low round trip efficiency (50%) and high catalyst cost. It makes this device expensive in terms of USD/kW. However, molten carbonate and solid oxide-based fuel cells have operating temperature in the range of 600 C to 700 C and 600 C to 1000 C, respectively.57 The high-temperature solid oxide-based fuel cells are getting attention due to their application in micro combined heat and power systems to provide heat and power; however, their proper commercial availability lacks due to high operating temperature of the cell, and research is underway to reduce the operating temperature by investigating different materials to be used in micro combined power systems.181,182 They also have promising applications in aircraft and railway industries.183 4.2 | Role of ESSs in RESs applications Although RESs possess tremendous benefits of harnessing free energy from nature, however, its dependency on nature is another constraint for using this free energy.157 RESs mainly focused on getting power from nature with very high installation costs still lack to cope with the peak demands. Sometimes, fluctuations in power output are daily, and hence, they cannot be considered as primary energy resources. The dependency on nature can be somehow alleviated by employing the RESs with ESSs.167,184 A report published by International Energy Agency (IEA) revealed that global warming temperature can be reduced up to 2 C if the growth of ESSs rises from 140 000 MW in 2014 to 450 000 MW in- The heat energy from sun can be stored in the form of thermal energy. There are three types of storage techniques available based on their working principles: sensible, latent, and electrochemical reactions. They are mainly involved in storing the heat energy. Phase change materials (PCMs) are used in concentrated power plants,186 solar water heating,187-190 solar air heaters,191,192 hybrid electric vehicles,193,194 electronic cooling,195,196 solar thermal cooling, and air conditioning applications.187,197 PCMs are not directly used to store electrical energy or with the grid due to thermal energy storage. However, they greatly assist the electric utilities in heating and cooling applications and the form of concentrated power plants. Their integration with solar cooling and heating applications is common now, and research on finding new PCMs received great attention.198 Nazir et al199 have reviewed recent developments of PCMs for thermal energy storage. However, the selection of suitable technology to cope with the several issues of RESs using ESSs is another major area of research. 18 BASIT ET AL. TABLE 10 Applications and location of some flow battery storage system Application and Battery Type Capacity, MWh Astana, Kazakhstan ZBB 1000 Mississippi Polysulphide bromide (PSB) 120 Hokkaido, Japan Substation, all vanadium 60 Hokkaido, Japan Wind farm, all vanadium 6 Liaoning, China Wind farm, all vanadium 10 Cites Abbreviation: ZBB, zinc-bromine battery. 4.2.1 | Power quality improvement with ESSs The flywheel, batteries, SMES, and supercapacitors are used for power quality improvement in the RESs grid keeping in view the fact that power quality improvement requires very high cycle stability with short duration and high-power output. These devices have swift response time which can be divided into three types: (a) milliseconds, (b) seconds to minutes, and (c) several minutes. The applications related to improve electric power quality require very fast response time. The electrochemical batteries provide a power rating of up to 50 kW, and their energy densities are also high which makes them suitable for backup, primary regulation, and electric supply reserves. However, ESSs like SMES and supercapacitor offer high-power ratings up to 10 MW, yet their energy ratings are very low as 0.1 and 0.01 MWh for SMES and supercapacitor, respectively. This makes them unsuitable for long-duration storage and feasible only for short-term power quality improvement. Figure 13 shows the use of ESSs in grid with reference to their usage and storage time. 4.2.2 | Development status PHS is the only mature technology which works on both RE and energy storage principle. ESSs other than PHS include batteries like lead-acid and Lithium-ion batteries. They are suitable for providing backup power with vast grid-connected applications. Other batteries like sodium sulphur, nickel cadmium, vanadium redox, and ZEBRA batteries along with flywheel applications are also developed with few F I G U R E 1 3 Energy storage systems (ESSs) with their duration and time of use200 [Colour figure can be viewed at wileyonlinelibrary.com] grid-connected applications. Flow batteries like PSB and ZBB are still in their development stages. The metal-air battery has a promising feature of very high energy density among all other batteries and needs further research and development (R&D). The development of SMES technology is also in R&D phase with some commercial projects available up to 10 MW in the world. There is still a potential to increase up to 2000 MW in future.201 The supercapacitor is still in R&D stage with very few commercial grid-connected applications are present in the market. 4.2.3 | Efficiency, lifetime, and energy density of ESSs employed with RESs The typical characteristics of some ESSs operated with RESs are given in Table 11. PHS is ubiquitous in the world with around 169.557 GW of installed gridconnected capacity.157 Flywheels offer excellent efficiency of storing the energy at high costs (USD/kW) and low energy rating (MWh). The energy density of the flywheel system is high which makes them suitable for compact devices. The Ragone plot between energy density and power density of different energy storage devices is given in Figure 14. SMES and supercapacitor both have very high-efficiency range and similar costs. However, they lack due to their low energy density for high-power applications and can be used for very short duration to control the fluctuated frequency of wind farms. Batteries have wide range of applications from toys, mobile, computer, vehicles to distribution, and RESs around the world for backups. The electrical energy is converted in the form of hydrogen and oxygen gas through electrolyser, later stored and utilised in a fuel cell to produce electricity. Research is underway for solar hybrid hydrogen generation and combine micropower systems as they offer both heat and power as output.204,205 BASIT ET AL. 4.2.4 19 | Cost analysis of ESSs The PHS, flywheel, batteries, flow batteries, and fuel cells possess high cost (USD/kW). However, PHS, SMES, supercapacitor, CAES, and metal-air batteries are in low range in terms of cost ($/kW). The high energy density of batteries makes them compact in size TABLE 11 Name and more suitable to be used with PV and wind farms. The molten salt and PCMs are mostly used with thermal energy storage; however, they possess few environmental effects. The reuse of expired batteries from EVs can play a significant role in lowering the cost of batteries for making a battery ESSs to be used with RESs. Technical characteristics of energy storage systems (ESSs) suitable with renewable energy systems (RESs) Efficiency, % Lifetime, y Cost, USD/kW Power Ratting, MW Energy Ratting, MWh Power Density, W/L Energy Density, Wh/L References PHS 65-85 30-60 - - 1000+ 0.5-1.5 10,157,185,202,203 CAS 40-95 20-40 - 5-300 1000+ 3-12 157,185,202,203 Flywheel 75-95 15-20 - 0.1-10 0.01-5 - 20-80 157,185,202,203 SMES 90-95 15-20 200-350 0.01-10 - - 0.2-2.5 157,185,202,203 Supercapacitor 90-95 10-20 100-360 0.001-10 - 10 000+ 10-30 157,185,202,203 Batteriesa 75-95 5-20 - 0-50 100 ~500 157,185,202,203 Fuel cell ~50 5-15 10 000+ 0-50 - 57,157,185,202,203 Abbreviations: CAS, compressed air storage; PHS, pumped hydel storage; SMES, superconducting magnetic energy storage. a All batteries and fuel cell offer different characteristics; only overall ranges are presented in this table. F I G U R E 1 4 Power density vs energy density graph (Ragone plot) [Colour figure can be viewed at wileyonlinelibrary.com] FIGURE 15 Test system 1: Wind farm connected with grid [Colour figure can be viewed at wileyonlinelibrary.com] 20 BASIT ET AL. In the following section, the impact of FACTS controllers in RESs integrated grid has been demonstrated via simulations. Second-generation FACTS are considered for simulation studies with wind farms subjected to different large faults. 5 | SIMULATIONS STUDY ON RESS INTEGRATED POWER SYSTEMS In this section, a brief appraisal of the impact of FACTS controllers in a grid-connected wind farm is presented. For this purpose, simulations are performed on three different systems in MATLAB/Simulink. SCIG- and DFIGbased wind farms are used in simulation work. SCIG, known as first-generation induction generator, is easy to maintain and economically cheap. Mathematical model of SCIG is given below206: 2 vqs 3 2 r s + d=dt ðLs Þ 6v 7 6 0 6 ds 7 6 6 7=6 6 4 0 5 4 d=dt ðLms Þ 0 ωr Lms 0 vqs 2 3 r s + d=dt ðLs Þ The first system is composed of a 9 MW wind farm connected with 120 kV power grid through 25-km-long transmission line. STATCOM of 3 MVA rated capacity is connected in parallel to the wind farm. Single-line diagram of the system is shown in Figure 15. At time t = 15 seconds, a 0.1-second fault appears on one of the wind turbines. After fault clearance, system comes back to its original state. Result of grid side active and reactive power is shown in Figure 16. It is seen from the figures that wind farm as a significant part of power system underwrites to maintain the stability of the system. After fault clearance, wind turbine experiences a swing due to sudden drop in reactive power and does not come back to its actual state because of generator inertia. STATCOM installation provides it the required reactive power compensation during and after fault to d=dtðLms Þ r s + d=dt ðLs Þ 0 0 0 −ωr Lms r r + d=dt Lr d=dtðLms Þ DFIG is referred as second-generation induction generator for wind power generation. Its mathematical model is given below206: 2 5.1 | Test system 1 0 0 ωr Lr 0 0 32 iqs 3 76 7 d=dt ðLms Þ 76 ids- i 7 −ωr Lr 54 qr 5 0 0 0 r r + d=dt Lr idr ð5Þ maintain system stability. Due to non-linear control of STATCOM, active and reactive powers reach to their stable states. 32 3 iqs 76 7 d=dt ðLms cosθr Þ 76 ids 7 76 0 7, 76 i 7 0 54 qr 5 0 0 0 r r + d=dt Lr idr d=dt ðLms cosθr Þ − d=dtðLms sinθr Þ 6 7 6 0 r s + d=dtðLs Þ d=dtðLms sinθr Þ 6 vds 7 6 6 0 7=6 6 v 7 6 d=dt ðLms cosθr Þ d=dt ðLms sinθr Þ r 0 + d=dt L0 r r 4 qr 5 4 0 ð sinθ Þ d=dt ð L sinθ Þ 0 −d=dt L vdr ms r ms r Where rs, Ls, Lms, r r , Lr , and θr are stator resistance, selfinductance, magnetizing inductance, rotor resistance w.r.t. stator, self-inductance of rotor w.r.t. stator, and 0 0 rotor angle, respectively. vqs, vds, vqr , and vdr is the dq component of stator and rotor voltages, respectively. Detailed modelling of the SCIG and DFIG wind system is presented in Rashad et al.207 The modelling of test system 3, having two-machine system with STATCOM, can be found in Badar and Dilshad.126 0 ð6Þ 5.2 | Test system 2 In this test system, 9 MW wind farm is connected to 120 kV grid through 30-km-long transmission line. A 3 MVA rating SSSC is connected between buses 2 and 3, as shown in Figure 17. A three-phase fault is applied to the transmission line for half a second duration. In response of applied fault huge oscillations appear in the grid side active and reactive powers. Results for active and BASIT ET AL. reactive power flows are shown in Figure 18. It can be seen in the figure that the impact of fault without SSSC is significantly large. In case of presence of SSSC in the system, the system quickly restores its original state. 5.3 | Test system 3 In this test system, a wind farm of 9 MW is installed in two-machine system. Rating of machine M1 is 1400 MVA and machine M2 is 700 MVA. Transmission line between M1 and M2 is 500 km, whereas the length 21 of transmission line connecting M1 and wind farm is 10 km long. A STATCOM of 100 MVA rating is installed at the centre of the transmission line. Singleline diagram of the system is shown in Figure 19. This system is used to demonstrate the impact of STATCOM in suppressing low-frequency oscillations in the wind connected power system. At time t = 0.1 second, threephase fault is applied on a transmission line near bus 2. The system regains its original state after 12 cycles. Simulation results of rotor speed deviation, rotor angle deviation, and power on bus “m” are shown in Figure 20. The results reveal that power system without STATCOM becomes unbalance after the application of fault, whereas the system with STATCOM stabilises the operation efficiently. Huge chattering is seen in the power line having no STATCOM, but in the case where STATCOM is active, system becomes stable after experiencing some oscillations. Simulation results of all three cases show the stability of whole system when FACTS controller of any type is switched in the system. Dominancy of FACTS in power system is observed at different faulty conditions. This demonstrates that use of these controllers helps to maintain power system stability and provides better reactive power support in unusual scenarios. 6 | CONCLUSION F I G U R E 1 6 Test system 1: A, Active power and B, reactive power [Colour figure can be viewed at wileyonlinelibrary.com] Energy accessibility is considered as the basic block to sustain industrialisation and economic growth. Primary sources of energy fulfil the needs at the cost of low efficiency and environmental concerns. In order to meet the growing demand of energy, RESs are considered as alternative sources with almost zero environmental impacts. This article starts with an overview of different types of RESs and their adoption across the world. It has been found that investment and installation of RESs is increasing exponentially as federal and local governments are F I G U R E 1 7 Test system 2: Wind farm connected with grid with static series synchronous compensator (SSSC) [Colour figure can be viewed at wileyonlinelibrary.com] 22 seriously committed to limit the climate change according to Paris Agreement. China is a leading country in terms of investment and developing infrastructure in this emerging field. F I G U R E 1 8 Test system 2: A, Active power and B, reactive power [Colour figure can be viewed at wileyonlinelibrary.com] BASIT ET AL. On the other hand, RESs also have some constraints like suitable sites and vast area for dams and geothermal plants. Low inertial response and intermittent nature are the limiting factors for high penetration of RESs in large grids. Moreover, power electronics equipment used for RESs integration causes harmonics and SSOs in power system. These issues further lead to problems in power system like instability, voltage flickers, voltage fluctuation, and cascaded fault events. Based on the recent literature review, many solutions have been proposed in this paper to overcome these challenges. VSM, a power electronics-based machine having properties of synchronous machine, has been recommended to provide virtual inertia to the power system. SMES is proposed to avoid the frequency fluctuations in wind farms. The problem associated with harmonics, generated due to inclusion of power electronics-based converters, is resolved practically using converter control of harmonics, filter technologies (APF and PPF), line reactors, isolation, and k-factor TFs. APF is broadly used to improve the power quality. They are also used to mitigate the current harmonics in the system. The dynamic profile of the converter used in RESs can be managed by using VIC. VIC additionally provides power flow control, stability, and unbalance compensation. The transient voltage control in wind farms and problems associated with DFIGs can be solved using SMES and FACTS devices. The issues related to SSOs, stability, and reactive power support can be resolved by F I G U R E 1 9 Test system 3: Wind farm connected in two area systems with static synchronous compensator (STATCOM) [Colour figure can be viewed at wileyonlinelibrary.com] BASIT ET AL. 23 F I G U R E 2 0 Test system 3: A, Rotor speed deviation; B, rotor angle deviation; and C, power at bus Bm [Colour figure can be viewed at wileyonlinelibrary.com] incorporating VIC-based FACTS controllers. Secondgeneration FACTS controllers have broad applications in solar and wind farm integrated grids. Moreover, DFACTS are introduced for distributed networks. DFACTS are cheaper option to address voltage sag and swell issues and suppress the harmonics in the system. The impact of FACTS devices is also investigated for RESs integrated power system. A simulation study is performed to evaluate the performance of FACTS devices with grid integrated renewable applications using three test cases. It is observed that presence of FACTS provides reactive power support and improves overall system stability after the appearance of fault. Furthermore, it is seen that FACTS controllers are also capable of damping low-frequency oscillations in the system. 24 BASIT ET AL. Electricity demand and generation gap also produces many problems. Short-term and long-term energy storage is used to bridge the gap between demand and supply. The flywheel, batteries, SMES, and supercapacitors can support RESs not only in terms of storage capacity but also for power quality improvement of the grid. Power quality and low inertial issues of RESs connected grid can be resolved using these ESSs, thus maintaining the overall system stability. In future, the simulated RESs will be used along with adaptive NeuroFuzzy controllers to enhance the stability of the renewable integrated power systems. The stability analysis of wind integrated power system is an encouraging future work. The testing of PV system for harmonic analysis and investigation of new techniques to reduce the harmonics is also an interesting dimension of future work. Energy storage requirement puts a major constraint on utilisation RESs. R&D of new storage solutions and rectification of already existing technology will further broaden the application of horizon of RESs. NO MEN CLATU RE APF BP CAS CCM CO2 D-FACTS DFIG D-GPSFC D-SFC D-SSC D-STATCOM D-TCSC DVR ESSs FACTS GHG HCM HPVPS HVDC HWPS IEA MCT MDG PCM PHE PPF active power filter British Petroleum compressed air storage current control method carbon dioxide distributed flexible AC transmission system doubly fed induction generator distributed green plug switched filter compensator distributed switched filter compensator distributed static series compensator distributed static compensator distributed thyristor controlled series capacitor dynamic voltage restorer energy storage systems flexible AC transmission system greenhouse gasses hybrid control method hydel-PV pumped storage high-voltage DC hydel-wind pumped storage International Energy Agency Marine Current Turbine Millennium Development Goal phase change material pumped hydel energy passive power filter PSB RESs SCIG SMES SSO SSR SSSC STATCOM SVC TFs THD UPFC VCM VIC VRB VSC VSM ZBB polysulphide bromide renewable energy systems squirrel cage induction generators superconducting magnetic energy storage subsynchronous oscillations subsynchronous resonance static series synchronous compensator static synchronous compensator static VAR compensator transformers total harmonic distortion unified power flow controller voltage control method virtual impedance control vanadium redox battery voltage source converter virtual synchronous machine zinc-bromine battery ACKNOWLEDGEMENT The financial support from the Higher Education Commission (HEC) Pakistan, under PhD Fellowship for 5000 Scholars PIN No. 2EG2-034 is deeply acknowledged for conducting this research. 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