A comprehensive review on integrating renewable energy sources into smart grid : Challenges and Solutions

Smart grid is considered as future grid having numbers of advantages, not only for owners but customers also. One of the advantages of this future grid is, it is more reliable and environment-friendly grid as it helps to reduce greenhouse gases emission by integrating renewable energy sources with it. Renewable integration brings lots of benefits but before that numbers of challenges are needed to be solved and hence large-scale renewable penetration is considered as one of the biggest challenges to future smart grid. Researches are going on to deal with these hurdles and to make this integration obstacle-free. This paper presents a literature survey on these challenges and their possible solutions to give a brief view on current research scenario on this topic.

In [2], smart grid is explained with distributed generation, making distributed generation as a point of attraction. They simply mentioned that this dream grid will have features which we don't have currently hence called 'smart'. Authors also listed modern hardware as well as modern control systems which will be heart of future smart grid. They also provide brief description of Wide Area Monitoring Systems (WAMS), Supervisory Data Control and Acquisition (SCADA) system, Automatic Metering Infrastructure (AMU) and intelligent electronics devices (IEDs).
As smart grid is considered future grid with green energy, it necessitates penetration of large scale RES. For that different RES are available such as solar, wind, hydro, bio-mass, and geo-thermal and tidal. Every source is having its own specialty and place of availability. These all RES are explained in [3] which give a brief overview of smart grid. With some case studies and based on that studies the future requirements for smart grid is discussed in paper with current trend of researches of this field. As RES integration on large scale comes to picture or when it is considered as distributed generators (DGs), many challenges come to picture. Some of challenges such as voltage fluctuation, reactive power compensation, demand side management, harmonic distortion, forecasting, flexibility and stability of system and storage system are explained in [4] in very well manner. Authors used machine learning technique to develop a prediction model. Except RES integration, other challenges such as cyber security, data privacy, immature technology, lack of infrastructure, market uncertainty, lack of innovation in industries and regulatory framework are also areas of research on which many researchers are working currently [5][6][7][8]. [40,47,48] gives a brief view on challenges to be solved for SG integration with RES.
This paper gives a brief survey of literature for smart grid challenges and their solution for RES integration. Paper aims to gather maximum recent researches being conducted in the field. Second section of paper briefly describes literature related to smart grid, section III is explaining current status and future expectations from RES in brief, challenges and their solutions are discussed in section IV and section V respectively. At the end, whole paper is concluded in section VI.
II. SMART GRID The network of transmission and distribution lines which connect owner and consumer can be defined as electric grid. Here owner is supplying the energy and consumer is using that energy by traditional grid but what if end user contributes as an owner? How it is possible? The answer of such questions lead to a concept of some grid which gives platform to users to play both roles of user as well as owner by integrating distributed energy resources (DER) is known as 'smart grid'. One can find numbers of work being done on concept of smart grid from literature.
With the incidence of some F-15 plane crash of 1980's, [9] presented a revolutionary idea for our power system which can make them 'smart'. They provided the basic ideas that if we do so, we can make our transmission system smart, benefits of adopting intelligent processors to the system, introduction of SCADA system for diagnostic monitoring for transmission equipment, grid computing, making system self-healing and many more ideas-which are considered as the fundamental building blocks of smart grid today. This was one of the earlier papers (very first paper) giving a new concept of smart grid (SG).
As renewables are part of SG, reduction in total greenhouse gas emission along with demand management makes system more reliable and efficient [1]. This paper also provides information about development status of SG in America and Europe. A review of SG was made in [10] including brief architecture of SG, explaining effectiveness of dynamic optimal power flow (DOPF) for system efficiency of future SG by comparing computational efficiency results of DOPF and COPF (centralize optimal power flow). This paper is giving a brief discussion on power loss by comparing different algorithms, by comparing power loss at different loadings, comparing effects of introducing distributed generators to a system for power loss. As to mitigate the costumer's energy demand is another task for SG, authors also include storage system, demand side management, load shedding and generation scheduling. To mitigate Indian scenario of RES integration [49] presented a novel work on SG challenges for India.
If we talk about RES integration with SG, PV and wind are sources which are centre of attraction today for everyone because they are available almost everywhere. [11] Gives a basic introduction of smart grid and then connection of RES, mainly PV system with three types of inverters named string inverter, central inverter and micro inverter. Authors proposed a new topology of transformer less inverters for easy integration with SG. A review paper on smart grid energy security [12] gave the review on RES, digits of world energy consumption, SG and explained how integration of RES with SG can improve safety and security of electrical system. Paper also gave the brief review on different RES and their integration with SG and on other hand [58] presents a review for smart grid and RES integration which is supposed to support by means of Electrical vehicles.

III. RENEWABLE ENERGY RESOURCES -CURRENT SCENARIO
According to International Energy Agency (IEA) report [13] contribution of RES for electricity sector which is 23.9% in the year of 2017, will increase to 29.4% in 2023. According to [14] share of RES for global energy production will be more than 50% by 2050 from which solar PV and wind energy will contribute 35.8% and 36.4% respectively. The energy generation from RES within last decade [15] is expressed in figure 1 which shows exponential increase in utilization of RES. Figure 2 shows the contribution of different RES sources for total electricity generation. [40] Discuss about benefits of photovoltaic generation at too much large scale such as terra-watt level, also challenges are there and hence explains both benefits as well as challenges. The data above shows the importance of RES and their integration effect on future SG. [25] Describes the integration of RES with future SG, its applications and challenges in brief and [26] gives a review on challenges and its possible solutions for the same based on scenario of India. By utilization of RES and SG, concept of smart home in pudducherry is studied in [27].

IV. CHALLENGES OF INTEGRATING RES
As section II increase the attraction towards smart grid and section III describe the worth of RES, interest of integrating RES with future SG may reach to its highest level. Of course, SG will be intelligent enough to deal with its distributed generations means RES but today there are many challenges which we need to solve before penetrating RES in SG. Authors of [16] explained concept of SG and described challenges to SG such as protection of Future Renewable Electric Energy Delivery and Management (FREEDM) system, need of efficient communication system, operating the electric grid with high proportion of RES, monitoring and control, preparations of Electrical vehicles as a plug-in hybrid vehicles. Also, concerns with emerging RES to SG such as uncertainty, distance from load centres, voltage fluctuations and storage requirement are explained. Figure 3 show some of these challenges which are discussed in brief.

A. Voltage fluctuation
The undesirable random changes in voltage level which might be end up with impairment of device is called voltage fluctuation. Renewable sources mainly depend on weather conditions and hence change its potential according to weather situations with respect to time. Overvoltage cause reverse power flow and increased line losses which eventually affects operation of switch-gears and protection within different networks.This voltage fluctuation affects system stability and control because fluctuation in voltage level deteriorate the system stability. Hence, voltage fluctuation is one of the challenges for future SG to be connected with large scale with RES.

B. Energy Storage
As wind and solar resources are variable in nature, sometimes they generate more than sufficient required energy and sometimes are unable to produce enough energy to fulfill the demand. When RES generation is less than demand, we need some back-up to fulfill customer's demand and on other hand if generation is more than demand, the best option is to store the generated energy somehow to stop the wastage of that energy and to utilize it for future use. The appropriate solution for both cases is-Energy Storage System (ESS). ESS has vast area of its applications by both-technical point of view as well as economic point of view [16], but the main challenge with storage systems is their design. Electrical Energy Storage (EES) are designed for particular applications, means EES design as a solution of one problem may not be useful for another problem. So each case needs its special strategy of solution. As a result it consumes huge amount of time and also increase the level of cost [8]. Also if we talk about batteries for storage, the size of battery also comes to picture when large scale storage is required. So optimization of battery's size and modelling of battery is also a challenge for storage purpose.
As numbers of energy storage options are available in market today and some are at developing stage, each having its own characteristics and area of application-there is also a question of selection of correct option for given problem. Hence, selection of EES/ESS with respect to technical, economical and geographical aspects is also a challenge. One another big challenge which storage system is facing for SG integration is their desirable cost so that it can be economically beneficial for customer or utility.

C. Harmonic distortion
The RES integration with grid will be possible with the help of numbers of power electronic devices such as inverters and converters. This power electronic devices when operated with nonlinear appliances, injects harmonics to the system. This harmonics distort the voltage and current signals. Hence, create the problem of voltage distortion. This phenomenon supports excessive amount of heat which may adversely affect the power system. As SG design indicates SG will deal with thousands of power electronic devices-harmonic distortion is one the big challenges which SG is supposed to tackle with.

D. Synchronization
The ultimate goal of SG is to provide electricity with better power quality to end user. The uncertainty of RES produces power which is also variable in nature and hence needs to be synchronized with grid. There should be synchronization of frequency, phase and voltage level between RES generated power and grid power and if not done so, there might be chances of damage of equipment. Hence, synchronization with grid is one of the challenges.

E. Forecasting and scheduling
RES, especially wind and solar PV are uncertain by nature as it depends on climate conditions. Integration of such intermittent generation with grid will introduce variability in power and might be require more ancillary services. To deal with such prospects, accurate forecasting is necessary [38]. The forecasting is done by various methods. Numbers of algorithms and numbers of soft-wares are available for forecasting but the challenge is accurate forecasting with accurate timing. In addition with techniques of forecasting, weather information data, geographical data and past data of event should be available in sufficient quantity so now availability of data and security of data also comes to picture. Once forecasting is done, the demand side management starts playing its important role by scheduling the energy demand and generation. Hence, forecasting and scheduling is another challenge for RES integration Reactive power compensation.
While reduction in reactive power supports power factor improvement and system efficiency, certain amount of this power is a major need to control the voltage and tackle with transmission losses. Hence, to maintain stability of future SG, compensation of reactive power is necessary. As voltage stability itself is a challenge for SG, reactive power compensation is also a challenge to maintain system stability.
V. POSSIBLE SOLUTION OF CHALLENGES This paper is explaining some of the major challenges which SG is facing today to integrate RES. Many researches are going on to deal with these problems and make the dream of future SG with higher contribution of RES possible. Some of these current works is tried to showcase through this paper. During study of different literature for finding solution of these challenges, some of the solutions were able to solve more than one problem. Hence, this section focuses on numbers of solutions available for one or more problems.

A. Electrical Energy Storage(ESS)
As RES are uncertain in nature-they do not produce constant power due to which voltage fluctuation, synchronization and stability of system challenges the SG. If we introduce storage system to SG as a solution of these problems-it is very beneficial. According to [16], EES provides a critical link between owner and consumer, providing additional capacity to the grid to improve its stability. During explanation of applications of EES [18] one of the applications says that as EES supports active power for stable output, it increases the capability of grid for frequency regulation which is solution of another challenge to SG. Also, as EES provides additional power to the grid-it also helps to enhance system stability, reliability and flexibility along with providing relevance to congestion of transmission lines. The paper also classified energy storages according to the way they store energy, such as mechanical energy storage, heat storage, electrochemical energy storage, electromagnetic energy storage and chemical energy storage as shown in figure 4. Some of the literature also suggested a term named energy storage systems (ESS). Where lot of literature is available to support implementation of EES on large-scale, [59] presented electrical Vehicle as another option of storage supporting element which not only stores electrical energy but also supports voltage stability and reliability of system by giving power back to the grid when necessary. Along with these so called traditional storage systems, [19] represented hybrid energy storage systems (HESS) as a coupling of two or more energy storage systems. This paper begins with comparison of different storage elements and specialty explanation of some of them. The coupling can be done by many approaches from one of the simplest method is direct DC coupling of storage elements. Although this method has its own disadvantages, architecture of coupling with the help of one DC/DC converter (bidirectional) connected to high power storage. Concepts and applications of HESS are also explained in very well manner.
One of the most popular storage elements is a battery. Different types of batteries are available in market for different perspectives such as lead-acid batteries, lithium batteries and many more. Of course, some of the batteries are at research phase with much more advantages. With the vast information of some current installations of 'big batteries' (such as battery built by Tesla in Australia [60]), [20] discussed on battery energy storage systems (BESS) with well explanation of lithium-ion batteries as a better storage option against leadacid batteries. Also limitations of lithium-ion batteries, how its lifetime gets down and safety methods are expressed in brief along with large scale introduction of storage systems and related policies which can help to tackle with such problems are expressed.
Where [20] explain benefits of using lithium-ion batteries on small as well as large scale, a new idea of 'artificial photosynthesis' for energy is proposed in [21], this method presents an approach by which energy is produced and in addition it supports some biological processes for example making food and more. As discussed in section IV (B), the size and modelling of battery is also a big challenge to be solved. Because of by many reasons lithium-ion batteries are being more famous for energy storage, its modelling for cell life assessment is explained in very gentle way in [23] and to tackle with battery sizing issue, [24] proposed Particle Swarm Optimization as sizing algorithm for a hybrid storage system employing PV, battery and . Till now if the question of how many challenges we can solve by using storage systems wander in mind, the various fields of answer are frequency regulation, reactive power support, black start, capacity improvement and more [22].

B. STATCOM device
When it comes to system's voltage stability and harmonic impact from RES, another question of reactive power compansetion also plays an important role as a challenge and their possible solution might be Static Synchronous Compensator (STATCOM) device which is one of the members of FACTS family [28]. [29] presented a new methodology on voltage stability for by cooperating STATCOM at distribution side. Authors modelled a system of microgrid with the help of digital simulation and by comparing results of system without and with cooperation of STATCOM. They presented effectiveness of this methodology for voltage stabilization.
A novel work showing effectiveess of STATCOM for improvement in power quality by voltage stabilization was presented in [30]. Authors developed their results for wind connected system in MATLAB SIMULINK enviornment. The use of STATCOM for unbalaced power system by taking care of positive sequence voltage and negative sequence voltage at the point of common coupling (PCC) as a PCC controller was discussed in [31]. Another work of STATCOM utilization is presented in [32,36,43].
The nobel work for utilization of STATCOM as multipulse STATCOM was explained in [33], the results shows that implimentation of this device reduces the current harmonics from 13.44% to 1.37%. [34] also showed a work for STATCOM application of mitigating the harmonic challenge along with reactive power compansation. The model was analysed and THD (Total Harmonic Distortion) analyzation was also shown in this work. Not only voltage stability and harmonic distortion, STATCOM is also a solution for synchronization and grid control [35]. Furthermore, [46] explains selective harmonic elimination (SHE) system and its simulation results along with introduction of harmonics in system, its effects and different harmonic elimination techniques. This paper represents a new technique named multilevel selective harmonic elimination pulse-width modulation (MSHE-PWM) for transformer STATCOM system by using cascadle H-bridge inverter.

C. Forecasting and scheduling methods
From numbers of RES to be integrated with smart grid, major contributions are and will be from solar and wind sources as wind and solar capacity which are 398 and 515 GW in 2017, are supposed to contribute 2,033 and 1,198 GW respectively in the year of 2035 [37]. The biggest obstacle of integrating wind and solar with grid is their uncertain behavior. Hence, forecasting is the solution to deal with such hardels. Basic information of forecasting and importance of forecasting along with various types of forecastings is described in very well manner in [38]. From three vital methods of forecasting (differentiated such as classical stastistical technique, computational intelligent method and hybrid algorithms) time series method-most ususal classical method of forecasting is explained along with their mathematical expressions in [39]. The hybrid probabilistic forecasting model (HPFM) using one of the modified versions of PSO (DEEPSO) is introduced in [42]. Reults were analysed with the help of monte-carlo simulation model with the desire of finding the range of forecasted values without growing the forecast error.
As forecasting of RES very important for SG, the suppliers needs long term as well as short term forecasting for which they need some desirable information from customer side. [43] explains that providers need only some of the data from users and hence they presented a forecasting method which do not disturb user's privacy by their proposed method named group method of data handling (GMDH) with the help of one of the famous algorithms-artificial nuero network. On other hand, a model named Self Exciting Threshold Auto Regressive models with eXogenous regressors (SETARX) was offered by [45] as robust approach for spot prize forecasting of electricity. Paper presented modeling of SETAR, its simulation and prize forecasting for Italy's electricity market. Where a lot of literature is present explaining modeling and their results of forecasting, [50] presented power of optimization algorithms to improve overall forecasting of RES. They presented LSM (least square method) and EMM (energy minimization method) to enhance the forecasting by different case studies.The different time series methods for forecasting is shown in figure 5 [39].  [44] gives a review on challenges and solution for power quality for RES penetration with grid. Also, when comes to power quality, synchronization is also necessary for grid. [51] reviewed different methods of synchronization devided as PLL (Phase Locked Loop) based and non-PLL based techniques. The explained methods as PLL based methods [56] were PLL based on T/4 delay [52], PLL based on hilbert transform [53], PLL based on inverse Park's transform, Enhance PLL [54], Robust single-phase PLL [53], SOGI PLL with the help of SOGI-QSG filter [55] and non-PLL based methods were fourier analysis and inducverters [57].
VI. CONCLUSION In this paper, numbers of researches are surveyed to mitigate one of the major challenges of smart grid as large scale renewable integration. Numbers of challenges such voltage fluctuation, harmonic distortion, stability, storage system and forecasting of RES and related literature is explained in brief. The conclusion of this survey leads to the results that some of the presented work can become solution of more than one problem. STATCOM can be represented as a solution of voltage stability, reactive power compensation, synchronization and harmonic distortion. ESS can be solution of system stability, flexibility and voltage fluctuation. Numbers of PLL based and non-PLL based methods are available to solve the problem of synchronization. Forecasting has its own vast area of development to forecast weather conditions, energy demand and energy prizes.