Keywords

1 Introduction

The limited fossil fuel resources, global warming and environmental concerns, growth in the load demand, cyber-physical attacks, power shortage, and interconnection of new load types, such as Plug-in Hybrid Electric Vehicles (PHEVs), to power grids, have enforced the energy sector using Renewable Energy Sources (RESs) [1,2,3,4,5,6]. Conventional power systems are based on centralized and regulated power generation, transmission, and distribution systems and there is no flexibility for Distributed Generations (DGs) [1, 2]. The interconnection of DGs to power systems requires control, communication, and computation systems to ensure efficient, stable, and reliable operation [7]. Solar power plants, particularly Photovoltaic (PV) power plants, are one of the fast-growing types of DGs being integrated into power systems in recent years. Solar power plants reduce operational costs to generate electricity and provide added value to customers and utilities. The share of solar power plants capacities is increasing by roughly 40% annually [8]. The most favorable characteristics of solar power plants are the availability of solar irradiation in most of the world sites and the fact that solar power plants can be installed in a variety of sizes from small-scale to very large-scale systems [9].

Most of the solar power plants are integrated with the low-voltage distribution grids. While the increase in the solar power plants penetration into power systems leads to many challenges, which all depend on the point of interconnection of the solar power plants to power systems and the state and performance of equipment that are already installed on power systems [10]. To integrate solar power plants into power systems, advanced inverters, anti-islanding capability, protection systems, forecasting technology, and smart metering and control systems are required [11].

This paper comprehensively reviews the challenges with the integration of solar power plants, specifically PV power plants, into power systems and explains some possible technical solutions to mitigate such challenges and improve the security, reliability, and resiliency of power systems.

The rest of the paper is organized as follows. Section 11.2 describes the existing challenges of solar power plants integration into power grids. Possible solutions for solar power plants integration into power grids are presented in Sect. 11.3. A summary of the existing challenges and possible solutions for solar power plants integration into power grids is given in Sect. 11.4. Finally, some brief conclusions are indicated in Sect. 11.5.

2 Existing Challenges of Solar Power Plants Integration into Power Grids

The integration of solar power plants into power systems requires to take the characteristics of solar-generated electricity and their corresponding challenges into account. Such characteristics along with their challenges are presented in this section.

2.1 Variability

The output power of conventional power plants, including some hydro and geothermal power plants, that run on fuel can increase and decrease on command. Therefore, they are dispatchable. However, DGs, such as solar power plants, generate electricity when the Sun is shining. Power systems operators do not control DGs; they accommodate them, which need some agility. From the grid operator point of view, the output power of DGs, particularly solar power plants, is functionally equivalent to a reduction in the load demand. This can cause fluctuations in the demand for dispatchable power [12,13,14,15,16]. Therefore, there should be rules and regulations that govern existing power systems infrastructure while the integration of DGs into power systems.

2.2 Uncertainty

The output power of solar power plants in day-ahead and/or day of forecast cannot be accurately predicted [17,18,19]. Hence, power systems operators should ensure having excess reserve running to meet the demand.

2.3 Non-Synchronous Power Generation

Conventional power generation units provide voltage support and frequency control to power grids. Solar power plants do not currently help to maintain grid frequency [20]. In the case of doing so, additional capital investment is required.

2.4 Location-Specificity and Low Capacity Factor

The solar irradiation is good and efficient in some places than others. Such places may not be close to power grids. Thus, power transmission infrastructure is required to transfer the power to where it is needed. The area occupied by solar power plants is directly related to the size of the plant, solar irradiance at specific locations, and the technology and efficiency of solar cells. Such issues adversely affect the agriculture industry and the environment. In addition, solar power plants operate when there is enough solar irradiation [21,22,23,24]. Hence, the average capacity factor, which is defined as generation relative to potential, for large-scale solar power plants is approximately 30%. Comparing to the average capacity factor of a typical nuclear power plant, which is ~93%, DGs are generating electricity at low capacity factors.

2.5 Hazardous Materials and Life-Cycle Emissions

Solar cells contain heavy metals, such as lead and cadmium, which may be hazardous when they are decommissioned. Solar cells are mainly made with thin-films cells containing harmful materials, such as indium, gallium, and arsenic. Considering the manufacturing process of silicon-based solar cells, the silicon dust is released that causes breathing problems when inhaled. In addition, for cleaning and purifying the semiconductor surface of solar cells, hazardous chemicals are involved. Moreover, the solar cells manufacturing process, their transportation, and installation and decommissioning cause aggregate life-cycle emission [25,26,27]. It can be concluded that the increase in solar power plants installation can increase the risk of exposure to hazardous material, as well as an increase in the life-cycle emission.

2.6 Power Grids Flexibility

Proper operation of power systems depends on continuously balancing power generation from different conventional and non-conventional sources with the load demand. In order to balance the electricity supply with the load demand, when solar power plants cannot operate, thermal and hydroelectric power plants provide operating reserves and supply the loads. Also, there is a need for storing energy during periods of low demand and injecting the stored energy when it is needed, as well as (1) sharing energy and capacity across particular regions, which needs both power transmission capacity and power market participants to trade electricity and (2) changing the load demand in response to power grids conditions [28,29,30,31,32]. Solar power plants can cause overgeneration conditions. This refers to a condition during which the aggregated supply of conventional power plants and solar power plants exceeds the demand. To avoid this, solar power plants generation should be curtailed by either reducing the output from the inverter or disconnecting the entire power plants from power systems. To do so, the physical control systems of the generation sources are required.

2.7 Capacity Value to Meet the Peak Demand

Solar power plants can provide capacity value by reducing the load demand that must be supplied by the conventional generation units during periods of high demand. In other words, capacity value shows how much additional load can be added with the addition of solar power plants [32,33,34]. The major issue with this is related to the reliability of solar power plants to continuously and adequately supply the load demand.

2.8 Big Data and Cybersecurity

The high penetration of DGs, such as solar power plants, causes an increase in the volume of data. The data includes power consumption pattern data, smart metering and control devices data, and operational data [35,36,37,38,39,40]. The data is used for real-time monitoring and control of the entire grid. To successfully monitor and control the system, there is a need for a highly efficient communication infrastructure. At high penetration of solar power plants, the massive data intrusion and cyber-attacks can affect the quality of service. However, the latency, adequate bandwidth, efficiency, and reliability of communication should be considered.

3 Possible Solutions for Solar Power Plants Integration into Power Grids

To tackle the challenges of solar power plants integration into power systems, several solutions are further proposed.

3.1 Minimum Power Injection Limit

The minimum power injection limit can be applied when the output power of the grid-connected solar inverters falls below a certain threshold value and accordingly, the relay should disconnect the grid-connected solar inverters from the power grid. In the case of an overload (>1.20 of the nominal load level), the relay should also disconnect the grid-connected solar inverters from the power grid [41, 42].

3.2 Reverse Power Flow Index

When the power flow from the power grid becomes zero or changes to the opposite direction, the relay should disconnect the grid-connected solar inverters from the power grid [43, 44]. To do so, the relay should continuously monitor the direction of power flow and send the trip signal to the corresponding breaker, when the reverse power flow is detected.

3.3 Loading Condition

The output power of the solar inverter should be dynamically controlled by the Diode Clamped Inverters (DCIs) and smart inverters and varies based on the loading condition. This helps the solar inverters to efficiently operate and mitigate the impact of reverse power flow. In addition, solar inverters can participate in the voltage regulation on the feeders [45,46,47,48,49].

3.4 Utilization of Energy Storage Systems

As mentioned in Sect. 11.2.1, variability is one of the biggest challenges to integrate the large-scale solar power plants into power grids. To mitigate the impacts of the variability of the output power of solar power plants, Energy Storage Systems (ESSs), such as battery banks and supercapacitors, can be utilized to smooth the output power and prevent the sudden power outage [50, 51].

3.5 Utilization of Solid-State Transformers

Solid-state transformers are capable of improving the power quality and protecting the system against faulty conditions [52, 53]. However, the efficiency of the solid-state transformers is lower than electromagnetic induction-based transformers and their protection scheme both at the low voltage and high voltage is complicated, with their help, large-scale solar power plants can be integrated into power grids while providing real-time control and monitoring of energy dispatch and improving the power quality.

3.6 Optimal Energy Dispatch

A practical solution to supply the load demand is to generate electricity using different types of DGs. However, it is difficult to achieve efficient energy dispatch. To overcome this issue, optimal energy dispatch algorithms and power electronics topologies should be investigated [54,55,56,57,58,59]. By implementing optimal energy dispatch algorithms with high penetration of DGs, particularly solar power plants, (1) balancing the load demand in an efficient way can be guaranteed, (2) the feeders power losses can be minimized, and (3) the stability of the power grid can be improved.

3.7 Utilization of Advanced Control and Distribution Management Systems

Using the advanced control and distribution management systems can optimize the overall performance of power systems while solar power plants are integrated and provide an automated outage restoration procedure. In addition, the advanced control and distribution management systems enable smooth integration of DGs, voltage regulation, and resiliency improvement [60,61,62]. However, it is difficult to achieve an optimal solution by combining the advanced control and distribution management systems and the existing power grids infrastructure.

3.8 Utilization of Advanced Communication Infrastructure and Intelligent Protection Systems

The integration of DGs into power grids impacts the reliability, security, and resiliency of power systems. Using the advanced communication infrastructure and intelligent protection systems can improve the reliability and resiliency of power systems. Dynamic monitoring and control of the grid parameters, such as voltage, current, frequency, through a fast and reliable communication infrastructure can cause the operation of the entire grid more efficient [63,64,65,66,67,68,69,70,71,72,73,74]. To deal with the dynamic nature of power systems with the integration of solar power plants, more intelligent protection systems with self-awareness, self-reconfiguration, and self-healing capabilities should be deployed.

3.9 Determining the Optimal Size and Allocation of Distributed Generators

Determining the optimal size and allocation of DGs, especially solar power plants, is a function of the feeder power losses, voltage profile, operating costs, line load ability, and the existence of other previous DGs installations. For solar power plants, the average solar irradiation for the candidate locations is important. Therefore, the immense benefits with high integration of solar power plants can be achieved, if the size and location of solar power plants, subject to the technical and non-technical constraints, are optimally determined [74,75,76,77,78,79].

3.10 Utilization of Hybrid Grid-Connected Distributed Generators

In order to improve the efficiency and reliability of power systems, the grid-connected DGs can be used in a hybrid form. This means different types of DGs with different power capacities can be combined with each other. In this case, by properly selecting the type and size of DGs, such as wind, ESSs, etc., and integrating them with solar power plants, and also scheduling of such power generation units, the overall performance of the grid improves [80,81,82,83,84,85,86,87,88].

3.11 Utilization of Flexible Conventional Power Generation Units

Power systems planners always consider more flexible conventional power generation units, such as natural gas and small-scale Combined Heat and Power (CHP) plants to deal with the variable nature of power generation by non-conventional generation units [89, 90]. It should be noted that the operating costs of conventional power plants can be smaller than fuel savings from the increased DGs penetration.

3.12 Demand Response Program and Demand-Side Management

It was before mentioned that power generation units must be capable of supplying the aggregated load demand and power losses associated with transferring power from the generation units to the load centers. By dynamically control the loads to match the power generation in real-time through demand response program and demand-side management, i.e., load shifting and load shedding/curtailment, the DGs can be more dispatch-able within certain limits [91,92,93,94]. This also causes overcoming the variable nature of power generation by DGs. For instance, if the majority of the customers in a certain area participate in demand response program and demand-side management, the generated power by the concentrated solar power plants that have molten-salt storage becomes available for 24 h of the day. In addition, demand response program and demand-side management lead to a reduction in power generation and operating costs to balance the load demand, reduction in power market price variations, improving the reliability of the grid, managing the grid congestion, and improving power systems security [93, 94].

3.13 Interconnected Power Transmission Grids

If different DGs, e.g., solar power plants and wind farms, are aggregated across a broader region, the generated power by such DGs becomes less variable. In this case, if the geographical area linked up by power transmission grids is big enough, this is highly possible that the Sun is shining and the wind is blowing somewhere within that specific area [95].

3.14 Cloud Computing and Artificial Intelligence

To deal with big data and cybersecurity challenges, the collected data from all nodes in power systems should be stored in high capacity and high-speed data storage systems. One solution is to use big data streaming frameworks, such as Storm, Spark, Flink, Kafka, and Samza suggested. The other solution is to use cloud computing, which enables achieving more level of flexibility and efficiency in data management [96,97,98,99]. Some cloud computing services are Software as a Service (SaaS), Infrastructure as a Service (IaaS), Platform as a Service (PaaS), Data as a Service (DaaS), Communication as a Service (CaaS), and Monitoring as a Service (MaaS). However, the major challenges in cloud computing are data security and latency. In order to improve the security of cloud computing services, scalability, and visibility, and reducing the bandwidth and latency, the post-cloud computing frameworks, such as fog computing, Mobile Edge Computing (MEC), and dew computing, are proposed [100,101,102]. It should be noted that the deployment of artificial intelligence along with cloud computing and post-cloud computing helps to efficient integration of solar power plants into power systems by analyzing the new and historical data and weather forecasting, and optimal controlling of the system, estimating the state of the system, and diagnosing faults in power systems [103,104,105,106,107,108,109,110,111,112,113,114].

4 Summary of the Existing Challenges and Possible Solutions for Solar Power Plants Integration into Power Grids

Table 11.1 gives a summary of the existing challenges of solar power plants integration into power systems and possible solutions that can address such challenges. In addition, suggested future solutions are mentioned.

Table 11.1 Solar power plants penetration problems and suggested future solutions

According to Table 11.1, the integration of small-scale and large-scale solar power plants into power grids requires to develop more advanced control, protection and communication systems to improve the reliability, security, and resiliency of the power systems.

5 Conclusions

This paper presents different challenges with the integration of solar power plants into power systems and their consequences. In addition, considering the future of solar power plants integration, various existing and practical solutions are presented. As the existing solutions need to be further improved, some suggested future solutions are indicated in this paper. It should be noted that technical, operational, and environmental challenges with the integration of solar power plants into power grids are at the early stage and become more complicated with the increased level of penetration of solar power plants. Therefore, further investigations need to be performed to ensure the reliability, security, and resiliency of power systems.