Ansys Electronics Cooling Device

Energy Generation

This article provides the latest statistics and describes the newest developments in large-scale renewable power generation.

Grid Integration Of Renewable Energy (Ren)

requires efficient and reliable power conversion stages, par- ticularly with an increasing demand for high controllability and flexibility seen from the grid side. Underpinned by advanced control and information technologies, power electronics con- verters play an essential role in large-scale REN generation.

However, the use of power converters has also exposed sev- eral challenges in conventional power grids, e.g., reducing the system inertia. In this article, grid integration using power elec- tronics is presented for large-scale REN generation. Technical issues and requirements are discussed with a special focus on grid-connected wind, solar photovoltaic, and energy storage systems. In addition, the core of the energy generation and conversion—control for individual power converters (e.g., gen- eral current control) and for the system level (e.g., coordinated operation of large-scale energy systems)—is briefly discussed.

Future research perspectives are then presented, which further Manuscript received 21 May 2022; revised 5 December 2022 and 26 January 2023; accepted 1 March 2023. Date of publication 14 March 2023; date of current version 5 April 2023. This work was supported in part by the Reliable Power Electronics-Based Power Systems (REPEPS) Project from The VELUX Foundations through the Villum Investigator Program under Award 00016591, in part by the National Natural Science Foundation of China under Project 52107212 and in part by the Zhejiang Kunpeng Investigator Program, in part by the Taiwan Ministry of Science and Technology under Grant 109-2218-E-002-011-MY3, and in part by the Chilean National Agency for Research and Development (ANID) under Project FB0008, Project ACT192013, and Project 1210208. (Corresponding author: Yongheng Yang.) Yongheng Yang is with the College of Electrical Engineering, Zhejiang Jose Rodriguez is with the Faulty of Engineering, Universidad San Sebastian, Digital Object Identifier 10.1109/JPROC.2023.3253165 advance large-scale REN generation technologies by incorpo- rating more power electronics systems.

KEYWORDS | Control of large-scale renewable energy (REN); energy storage (ES); inverter-based resources; power convert- ers; REN generation; solar photovoltaic (PV) systems; wind power systems.

Fossil-

fuel energy, e.g., coal, oil, and natural gas, is not environmental-friendly and is a major contributor to cli- mate change. Furthermore, throughout several decades of intensive exploitation of fossil-based resources, an energy crisis has been foreseen across the globe , . However, energy consumption is still high and continues to increase as the global economy grows. It is, thus, imperative to develop and explore affordable and clean energy to enable the sustainability of the global society and to battle climate change . To achieve this, many efforts have been made to expand the use of renewable energy (REN) sources , as shown in Fig. 1, and various REN and alternate energy technologies are still emerging. Among these REN resources, wind and solar photovoltaic (PV) is currently the most favorable, which together make up more than half of the total globally installed REN capacity. As depicted in Fig. 2, the total capacity of wind and solar energy exceeded that of hydropower in 2020, and at the end of 2021, the total renewable capacity reached more than 3000 GW, more than 2/3 of which is from wind and solar generation.

It can be anticipated that more wind and solar PV capacity will be installed soon, as the cost of such technologies is still declining ; hence, large-scale wind and solar PV power generation is right around the corner.

This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation Fig. 1. Global direct primary energy consumption (relative) in the past two centuries, where the inefficiencies in fossil fuel production are not considered . It shows that the dependence on fossil fuels is declining, but it is still high, e.g., in 2019.

It is well known that the use of REN resources for elec- tricity generation is different from conventional fossil fuels. More specifically, conventional power generation uses syn- chronous generators (SGs) (using steam-based turbines), referred to as large-scale power plants, which govern the frequency and voltage of the grid. In this case, the gen- eration capacity is sized to meet the predicted demand, and it is directly dispatchable so that the generation and demand can easily be balanced. Conversely, REN genera- tion is highly intermittent such that the power generation from individual REN systems is nondispatchable and dif- ficult to predict. With the integration of large-scale REN resources, the balance between generation and demand

Is Still A Challenge To Maintain , , , ; Hence,

Fig. 2.

Global Accumulative Capacity Of Ren From 2000 To

2021 based on the data available from IRENA and the data represents the maximum net generating capacity of power plants and other installations using REN sources to produce electricity , where hydropower includes pumped storage and mixed plants; marine energy covers tide, wave, and ocean energy; solar energy includes solar PV and concentrated solar power; and wind energy covers onshore and offshore wind. As indicated, the sum of wind and solar capacity has exceeded the total hydropower capability in 2020.

Some data (from 2000 to 2011) are from previous reports (https://www.irena.org/publications/). power system-wide strategies should be developed to man- age the variable generation from multiple REN systems connected to the same grid. Moreover, controlling and conditioning the power generated from REN resources are achieved using (nonlinear) power electronics (power switching devices). It is, thus, expected that, as more REN power generation is deployed, the power electronics that control them will make up a larger portion of the grid.

Fig. 3 exemplifies the use of power electronics in modern power transmission systems while also looking toward

, , . In This Context, The Large-Scale Adoption

of power electronics for REN resources, being inverter- based, is making the utility grid more complicated, even to a point where the stability might be challenged ,

, , , . On The Other Hand, With The

advancements of power semiconductor devices (e.g., high- voltage blocking capability and high efficiency), the power converters, as the core of energy conversion from REN resources to electricity, provide more controllability and flexibility; however, the design and control should still

Be Further Advanced , , , , ,

toward systems that are more intelligent, lower cost, and more efficient. At the same time, the design and control of the employed power converters should be more open and/or standardized to address the challenge of modeling “black boxes” from different manufacturers that are often proprietary to the companies.

In addition, the operation of large-scale REN systems is mostly in harsh environments (e.g., offshore wind systems under high humidity). This imposes intense thermal load- ing on the power converters and challenges the reliability of the entire generation system. Reliability is an important index as REN systems are long-term investments that are often costly to repair. By improving the reliability, the over- all cost of REN resources can be (further) reduced, leading to higher competitiveness for the REN resources. Hence, many solutions for reliability, robustness, and resilience enhancements of inverter-based REN resources are emerg-

Ing , , , . For Instance, The Design For

reliability based on the physics of failures can improve the reliability of individual power converters and, in turn, the entire system. At the same time, the harsh operating environments for the REN systems can potentially induce large disturbances, which the power converters have to withstand. To guide the grid integration of REN resources, transmission system operators (TSOs) and/or distribution system operators (DSOs), together with other stakeholders or entities, have issued stringent interconnection require-

Ments And Grid Codes , For The Commissioning And

operation of large-scale REN power systems. Taking the that the distributed energy resources have to respond to abnormal voltages and ride-through of the disturbances properly. Relevant standards/grid codes can harmonize the work done by different entities to further increase the penetration of REN resources for a cleaner energy society.

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation Fig. 3. Power electronics in modern power transmission systems and its increasing applications in future energy systems (DFACTS—distributed flexible ac transmission system), which is anticipated to be more used. Notably, power electronics are more intensively used to process electrical energy in the right five kinds of applications (appliances, LED lighting, solar PV, wind, and electric vehicles) compared to the left three.

In other words, they can be used as the main design and planning benchmarks for large-scale REN power genera- tion. In all, the measures that should be taken to improve the integration of large-scale REN systems are related to the design, control, and operation of power electronic converters.

Furthermore, to enhance the grid integration of large-

Inverter-Based

resources can be improved by integrating large-scale energy storage (ES). That is, the development of advanced ES technologies is important , , , .

ES systems can be a buffer to balance and/or ease the management of the variable power generation from REN resources and the load demands. However, currently, large- scale ES systems are still highly costly, except for the geography-limited pumped storage hydropower systems.

Seen from the multifold benefits brought by ES systems, more ES will soon be seen in practice, as an essential asset to enhance the grid integration of REN resources.

Notably, power electronics (interlinking the REN resources, ES systems, and the grid or local loads) will again play a vital role in large-scale ES systems and in transforming energy paradigms. In particular, the coordinated operation

Can Be Achieved In A Cost-Effective Way , , ,

especially with the aid of artificial intelligence (AI) , to improve the economic performance of the entire system. The integration of ES is seen as a system-level solution to the performance enhancement of large-scale REN systems.

With the above, the challenges, solutions, and oppor- tunities for large-scale REN generation are discussed in this article, focusing on power electronics and advanced control. Compared to the existing survey papers and text- books, this article considers the most widely used REN sources, i.e., wind and solar PV energy, while considering ES (mainly, battery storage). A comprehensive review of key technologies for REN is provided, and the correspond- ing technology development prediction is also discussed.

Although this is a broad review article, it primarily focuses on wind, solar PV, and ES technologies seen from the power electronics perspective, as a timely summary of the key technologies for large-scale REN generation. The rest of this article is organized as follows. General requirements and demands for the grid integration of REN resources are presented in Section II. Advances in power electronics for wind, solar PV, and ES are overviewed in Section III, where power electronics for modern transmission systems is also discussed. Control techniques and system-level operation strategies for large-scale REN generation are briefly presented in Section IV, followed by challenges and future research perspectives in large-scale REN systems in Section V. Finally, concluding remarks are given in Section VI.

R E N E W A B L E E N E R G Y G E N E R At I O N

As pointed out in Section I, large-scale REN generation is different from conventional power generation systems. When integrating into the grid, various requirements and/or demands must be complied with at different lay- ers. In this section, the requirements and demands of large-scale REN systems are presented, including those for wind and solar PV systems (generator side), ES, and grid connection (grid side). The role of power electronics is highlighted.

Systems

As shown in Fig. 4, the wind and PV generation systems can be divided into generation and conversion stages. Fig. 4.

General configuration of (a) wind power generation system (in some cases, the gearbox is optional) and (b) PV power generation system, where the PE system is the power electronics system, including power converters and the associated control. Here, the PV power generation system is based on the photovoltaic effect.

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation More specifically, wind energy is captured through the mechanical system (including turbines), and ac electricity is generated. For solar PV systems, the energy is col- lected through many PV cells that statically convert the solar energy from sunlight based on the photovoltaic effect into dc electricity. Solar PV energy can also be collected through PV cells with a solar concentrator. In this article, we consider PV technology only. To comply with load characteristics at the point of common coupling (PCC), e.g., an ac grid, power electronics is used as the inter- mediate stage to condition the electricity generated from wind turbines (WTs) or solar PV cells. Dedicated strategies are implemented in the control of power converters for individual systems to better integrate the REN resources with the load (e.g., utility power grids). Requirements of wind and solar PV generation systems are summarized in Fig. 5, which indicates that different demands are imposed on the generation and conversion stages.

For WT systems, the electromagnetic torque is controlled by regulating the generator-side current that flows in the generator rotor or stator. By doing so, the power extracted from the WT can be optimized; this is a type of control known as maximum power point tracking (MPPT), which can be achieved by regulating the blade rotational velocity

And Pitch Angle As Well , . In Other Words, It

is to achieve an energy balance between the mechanical and electrical power conversion in the WT. As shown in Fig. 5, WT power generation systems also need to control the generator frequency. At present, for both WT and PV systems, the status of the generators (e.g., the turbines and the PV cells) should be monitored to enable timely maintenance or detect abnormalities. This can be further

Enhanced By Ai Technologies , , . Moreover,

either the voltage or the current (or both) should be controlled on the generator side. Due to the increasing capacity of an individual WT or solar PV array, many power semiconductors are used to assemble the power stage. Efficiency and power density are major considerations. For instance, for offshore WT systems, higher power density means lower installation costs, which further helps to reduce the cost of wind

Energy. Advanced Wide Bandgap (Wbg) Devices Bring

Fig. 5. Common requirements of individual wind and solar PV power generation systems (generator and converter), where MPPT represents the maximum power point tracking and WT stands for the wind turbine.

many opportunities to improve efficiency and power den- sity . Yet, power semiconductors may fail to operate due to sudden disturbances (e.g., short-circuit faults) or long-term fatigue. When this occurs, the shutdown of the power conversion system may be inevitable, which may challenge the entire grid stability, leading to huge economic losses and increased maintenance costs. Hence, power converters should have high reliability, which can be achieved through effective design and control for reliability . Alternatively, reliability and power density can be enhanced by better managing the thermal flow to reduce peak temperatures in the power electronic converters, as high temperatures accelerate component degradation . This is especially important for WT systems, where power converters are installed in the housing, called a nacelle, with limited space for heat dissipation. That is, cooling systems and thermal management should be prop- erly designed. Communication and status monitoring of power converters are increasingly required so that the per- formance of wind and PV power systems can be enhanced (e.g., reliability and efficiency improvement). In addition, the operation of large-scale systems can be coordinated via communication . This also helps to optimally dispatch the power flow.

As can be seen in Fig. 5, control of power converters is of importance. It helps the entire REN power genera- tion system to meet different demands, i.e., to make it multifunctional . The above indices (e.g., reliability, efficiency, and MPPT) can be improved through the con- trol of power converters. There have been many control technologies for PV and wind power systems developed in

The Literature , , , . Among Those, Linear

controllers, such as a proportional–integral (PI) controller, are the most widely used in practice, while nonlinear

Control, Such As Model Predictive Control (Mpc) ,

is also being advanced and implemented in recent years. Developments in information technologies are transform- ing the control capabilities of power converters. For exam- ple, AI-aided control has been used in power converters to improve their reliability and stability . In all, the control acts as the brain of REN systems to meet various requirements. Underpinned by communication, control is essential to the coordinated operation of large-scale REN systems.

Finally, the voltage level of individual wind or PV generators is still low compared to the voltage at the PCC. The low output voltage may need to be increased to facilitate power transmission. In other words, step-up transformers are typically adopted at the medium-voltage (MV) side. This imposes additional challenges on the power converters in terms of power converters topologies, passive filter design and integration, and control, espe- cially with the increasing adoption of WBG devices [e.g., silicon-carbide (SiC) devices] for efficiency improvements.

At the same time, it also provides opportunities to develop new technologies, e.g., solid-state transformers (SSTs) and substations , .

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation

B. Requirements Of Energy Storage Systems

In order to balance the ambient-condition-dependent power production (e.g., weather and time of day) from REN resources and varying consumption, ES is necessary.

To enable the increasing penetration of REN power genera- tion systems onto the grid, ES is an asset for grid regulation

With Many Benefits , , :

1) Conventional power generation systems can be oper- ated in an efficient and conventional way since the

Fluctuating Power Due To The Intermittency Of Ren

resources is mitigated by charging and discharging the ES units. 2) Power generation from the REN resources can be flex- ibly stored in ES units, and it can be used to improve the utilization and optimize the power flow, further reducing the need for processing peak power or trans- mission of large power capacity in short periods.

3) Emergency handling capability of the utility grid can

Be Enhanced By Es Units, Where Critical Operation

demands can be achieved, e.g., black starts. In addition, the integration of ES systems helps to achieve a more stable electricity price. In all, the ES becomes more important for the energy transition to meet the need for a more efficient and sustainable energy system.

Thus, Many Es Technologies (E.G., Pumped Storage,

Li-ion batteries, flow batteries, flywheels, and hydrogen- based systems) are being developed. Their characteristics differ, and thus, the corresponding control strategies are important, when being implemented in grid-scale systems.

For example, a pumped-hydro station is suitable for bulky load management applications due to its large energy and power capacities, while it is limited by its physical location. Moreover, one concern of the chemical-based ES systems is safety (risk of fire due to thermal runaway under unbalancing voltages among cells during operation) that can be enhanced by advanced energy management systems (EMSs). A qualitative comparison of the characteristics of selected ES technologies is given in Table 1. More

Detailed Benchmarks Can Be Found In , , ,

, and .

C. Interconnection And Integration Requirements

Power electronic converters should be multifunctional to enable the integration of REN systems. Different demands can be met through the control of power converters.

Among those, power quality is the foremost issue. For total harmonic distortion (THD) of PV systems should be below 5%. There are two reasons for large harmonic emis- sions: the inherent intermittency leading to power fluc- tuations and the continuous use of nonlinear power con- verters. When developing REN systems, harmonic inter- ference and interaction issues should be explored, which are strongly related to: 1) topologies and 2) controllers.

Power converters with WBG devices for REN applications Table 1 Comparison of Selected ES Technologies , , , , switching at a high frequency of up to a few hundreds of kHz may complicate the harmonic contents; harmonics from interconnected converters can interact with each other, possibly leading to resonance and instabilities. This requires the power converter to withstand severe condi- tions [e.g., unfavorable harmonic content and electromag- netic interference (EMI)].

To integrate more power electronic-based REN systems into the grid, it may become necessary for power con- verters to emulate the behaviors of conventional power plants irrespective of the operating states. This is ben- eficial to allow for stable operation even with the cut- in and cut-out of multiple generation units and the dynamic operation of the entire multiconverter REN systems. It is a means for power converters to effec- tively maintain the frequency and the voltage ampli-

Tude Of The Grid. Next-Generation Ren Systems Are

required to actively participate in grid regulation ,

. This Is A Measure To Maintain Robustness In

response to various disturbances apart from harmon- ics. For example, to ensure the safety of equipment Fig. 6.

LV ride-through requirements (voltage profile) for wind (solid lines) and PV (dashed lines) power generation systems in Spain and Denmark . Above the curves, Zone A and Zone B, the systems should remain connected; below the curves, it is allowed to disconnect the generation systems from the utility grid.

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation connected to the PCC, PV systems must have island- ing detection and protection. Under grid faults (e.g., voltage sags), both wind and PV power generation sys- tems should not only withstand temporary grid faults but also contribute to grid voltage recovery , .

Fig. 6 exemplifies the low-voltage (LV) ride-through requirements of REN generation systems. Other pro- tection schemes and strategies should be developed to ensure safe power generation from REN resources and its operation , , .

Beyond the above issues, as more SGs are being phased out and replaced by power electronic-based REN sys- tems, power converters may interact with each other , and additional functionality may be required. For instance, more and more grid operators demand wind

Damping And Black Start Capabilities . This Will

also enhance the transient stability and performance of large-scale REN generation systems, but, at the same time, the control strategies are becoming more advanced.

In all, in terms of interconnection and integration, large-scale REN power generation systems are becom- ing multifunctional, both statically and dynamically, to maintain the robustness and resiliency of the entire power grid.

P V, A N D E N E R G Y S T O R A G E S Y S T E M S

Power electronics plays an important role in the expansion of large-scale REN generation systems. In this section, the development of power electronics and converters is presented, followed by a detailed discussion of the power configuration architectures for wind, PV, and ES systems.

In addition, power electronics technologies for transmis- sion systems are discussed.

Converters

The history of power electronics goes back more than 100 years , as shown in Fig. 7. Since the invention of the mercury arc rectifier, power converters have been adopted to deal with the electrical conversion and con- trol of electrical power, especially after thyristors were invented in the 1950s. Since then, with the 65 years of development of power semiconductors, power electronics technology has been evolving, as presented in Fig. 7, from the basic search of topologies, control techniques, and efficiency to application-oriented technological improve- ments, such as packaging, thermal management, and mod- ularity. In other words, power electronics development was initially function-/mission-driven but has shifted to be more performance-driven. Along with this, information and communication technologies (ICTs) have been increas- ingly used in power electronics converters. Advancements in power electronics lay the foundation for the fast and effective use of REN generation systems.

In practice, as discussed in Section II, power electronics should achieve low losses, high reliability, and high-power density (e.g., using high switching frequencies). Accord- ingly, many power converter topologies have been pro- posed in the literature and used in industrial applications.

Among those, two- and three-level converters are the most used and commercialized topologies. Fig. 8 shows two- and three-level neutral-point clamped inverters that are voltage-source converters (VSCs). The multilevel technol- ogy, e.g., the three-level converter, is an effective method to Fig. 7.

Advancements in power electronics along with the evolution of power semiconductor devices technologies , being one of the major reasons for the fast development of REN power generation systems. Here, GTO: gate turn-off thyristor; GTR: giant transistor; MOSFET: metal–oxide–semiconductor field-effect transistor; IGBTL insulated-gate bipolar transistor; SiC: silicon carbide; GaN: gallium nitride; EMI: electromagnetic interference; EMC: electromagnetic compatibility; DT: digital twin; and AI: artificial intelligence.

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation Fig. 8. General power converters for REN systems: (a) two-level inverter and (b) three-level neutral-point clamped inverter, which can be used to build up large-scale REN generation systems. Here, vdc is the dc-link voltage; C, C1, and C2 are the dc-link capacitors; S represents an IGBT with a parallel diode; D indicates a diode; and va,b,c are the output voltages.

address limited voltage ratings of power devices, especially for large-scale REN generation systems. It can also help to reduce the filtering required at the output (e.g., using smaller and simpler filters but achieving a high power qual- ity). Modular multilevel converters (MMCs) are a represen- tative type that has been commercially used in high-voltage transmission systems. In addition to VSCs, current-source converters (CSCs) are seen in MV motor drive applications due to their strong fault-current withstanding capability.

To assemble large-scale REN generation systems, many general converters, such as those shown in Fig. 8, can be used to handle higher power levels and currents. Moreover, the concept of power electronics building blocks has become a general solution for power electronics develop- ment to scale up power.

Systems

Benefitting from the advancements of power semicon- ductor and power electronics technologies, wind power systems have been through a substantial technological transformation over the past several decades. This is also driven by the need for lowering the cost of energy. In the

1980S, A Wt System Of 50 Kw Was Considered Large,

where power electronics was barely utilized. Compared to the WT systems in today’s markets, where a WT power rating is typically at 2–3 MW, the 50-kW power level is relatively low. Now, the rating of a single WT system is approaching 15 MW , , as indicated in Fig. 9. It is, thus, becoming uneconomically viable to directly process such a large amount of power without power electronics.

As shown in Fig. 9, along with the increasing power capacity of a single WT, the utilization of power electronics is also increasing. At the same time, the functions of power converters are changing, e.g., from a single function (like soft start) to multitasking operation, as WT power systems are considered active contributors and stabilizers in the overall power grid. In terms of generator technologies, Table 2 Generator Technologies for WT Power Systems most of such systems were designed based on the doubly fed induction generators (DFIGs) (shared a larger portion of the market in the past). In this case, power electronics was partially used, i.e., only the rotor employed a power electronics converter with a reduced rating. To flexibly process the increasing power of the WTs, full-scale power converters were then increasingly used with induction, permanent magnet, and wound-field SGs, also due to the continuous cost reduction of power electronics. Table 2 summarizes the generator technologies from various WT manufacturers.

For large-scale wind power generation systems, the energy conversion should be as efficient as possible to further lower the cost of energy. This is also seen in the increasing utilization of power electronics in individual WT systems, as aforementioned (see Fig. 9). On the other hand, REN power generation should be more active in grid regulation, and it is required to comply with stringent grid codes, which have been presented in Section II and will be further discussed in Section III. Hence, the design and configurations of large-scale WT power systems, which involve a very large-scale adoption of power converters, are of importance. A better system configuration can con- tribute to high efficiency and easier compliance with grid requirements.

Many solutions have been proposed in the literature

, To Integrate Large-Scale Wind Energy Into

the grid. Among those, most wind farms are currently connected to the grid via medium-voltage ac (MVac) technology, as shown in Fig. 10. Both DFIG and permanent magnet SG (PMSG)-based WTs can be used, as exemplified in Fig. 10. Such a wind farm can easily have an accumu- lative power capacity of a few hundred megawatts, e.g., the Anholt offshore wind farm of 400 MW in operation in Denmark. It should be noted that power converters for the individual WT in such wind farms have to comply with and meet the demands outlined in Section II. More importantly, at the system level, additional measures may be taken to enhance grid integration. For instance, due to the limited reactive power capability, a centralized

341

Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation Fig. 9. Development of WT technologies, where “green area” indicates that the use of power electronics has been increasing significantly to process a large amount of power from WT systems. Here, D represents the diameter of the WT rotor; MW stands for megawatts.

reactive power compensation unit like a static synchronous compensator (STATCOM) may be installed to meet grid requirements, as shown in Fig. 10(a). By contrast, reactive power capability can be enhanced, especially when full- scale power electronics are adopted to assemble large-scale Fig. 10.

Configurations Of Large-Scale Wt Power Systems

connected to MVac grids: (a) DFIG-based wind farms and (b) PMSG-based wind farms, where the MVac grids are connected to the HVac grid through MV/HV transformers. Here, the voltage level of MVac grids is 11–33/34.5/46 kV while 60–245 kV for the HVac grid.

wind farms [see Fig. 10(b)]. In this, the generators can be PMSGs with full-scale power converters. Due to the ever-increasing need for flexibility and controllability of reactive power, the grid-side converter in each individual WT system can provide the required reactive power upon demand. As a result, additional reactive power compen- sators may be avoided, while, at the same time, the col- lector network should be properly considered. Notably, the individual power converters should be properly designed by following the requirements presented in Section II.

With respect to long-distance power transmission, which is common in wind farms (e.g., from an offshore wind farm to the grid), high-voltage dc (HVdc) transmission technol- ogy has been developed and implemented in recent years.

HVdc transmission is an important option, as it can achieve a high transmission efficiency, and there is no reactive

Power During Power Transmission , . In Addition,

HVdc technologies are used in wind applications, while high capacitance (due to longer cables) makes reactive power management very difficult if ac cables are adopted.

For large-scale WT power systems, HVdc transmission can be achieved through various configurations, as shown in Fig. 11. For example, the MVac voltage of the wind farm output can be converted to HVdc by a boosting transformer and high-voltage source rectifiers to achieve dc power transmission, as observed in Fig. 11(a). Alternatively, HVdc transmission can be attained by employing fully active- controllable power converters, e.g., SSTs, as presented in Fig. 11(b). In this case, the LV/MV of the WTs is converted to a medium/high dc voltage for transmission. In this configuration, a full dc power delivery in both the distri- bution and transmission lines can be realized and, thus,

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation Fig. 11. Configurations of large-scale WT power systems with HVdc power transmission: (a) full-scale converters with VSC rectifiers, (b) full-scale converters enabling both distribution and transmission dc grids, where the dc/dc station of the HVdc transmission system is optional, and (c) cascaded power converter structure. Here, G is the WT generator. SST: solid-state transformer. LV/MV: low voltage/medium voltage.

improve the overall efficiency of the power delivery and transmission , compared to the solution in Fig. 11(a). This is attributed to the removal of several power converter and transformer stages. What is more, SSTs (dc–dc power transformers) enable flexible power flow management in smart grids. In addition, another solution to power transmission is based on the cascaded structure, as shown in Fig. 11(c), whose control is more compli- cated than others to balance the power between each module and optimize the energy flow. Notably, considering the overall cost and limited space of offshore platforms, the active rectifiers using full-controllable power devices [e.g., insulated-gate bipolar transistors (IGBTs)] are not specifically feasible. Instead, multiple diode rectifiers can be employed to replace the VSC-based rectifier in the HVdc transmission system of Fig. 11(a). With this solution, a reduction of power losses and weight can be achieved for several-hundred-MW HVdc systems. Additional benefits, such as scalability, redundancy, and reliability, make this solution very cost-effective, while the controllability must be maintained at the LV or MV sides.

Systems

Compared to wind power systems, PV systems consist of a considerable number of solar PV cells/panels that are connected in series and in parallel, as discussed in Section II. The power level per panel of many solar PV cells is approaching 600–700 W , which is, however, still far lower than the capacity of an individual WT. For large-scale PV power systems, many series-connected PV panels (forming as PV strings) and then many PV strings in parallel (as arrays; see Fig. 12) are used for higher power generation. In such applications, inverters connected to a single PV string or multiple strings are called string inverters, and for converters interfacing PV arrays (i.e., many PV panels connected in series and in parallel), those are called central inverters. Both technologies are widely adopted in practice. Notably, the converter topologies can be the same for string and central inverters, e.g., the two- and three-level inverters shown in Fig. 8, while the power device ratings are usually different.

In addition, a large amount of PV arrays can occupy considerable land space, and thus, large-scale PV power plants are usually installed in areas with large open spaces.

Nonetheless, among the top in-operation PV plants, the central inverter technology (several central inverters) is the most widely adopted approach for such applications, as it is the simplest way to collect dc power from PV arrays with a low construction cost. In turn, it contributes to the reduction of the PV plant cost per watt of the nominal power, i.e., the cost of PV energy. In practice, multiple string inverters have also been used to maintain energy supply reliability. Fig. 12 shows the general configurations of large-scale PV power generation systems connected to the MV/high-voltage ac (HVac) grids, where single con- verters are exemplified.

It is worth noting that, for the central inverters with many PV panels connected in series and in parallel, which are typically installed in a large area, the MPPT efficiency is often low. This is because the PV panels spread over a large area may receive nonuniform irradiance (leading to mismatching characteristics that reduce power generation) compared to the string inverters installed in a smaller area, where each panel receives almost the same solar irradi- ance. On the other hand, as central inverters have fewer power conversion stages, the overall conversion efficiency is still generally higher than 95% . This is also another reason for large-scale PV generation systems to be installed in large open spaces (avoiding mismatched panel char- acteristics due to shadowing from buildings or trees) to increase the harvested energy. In practice, to address this and, thus, to further optimize the energy yield, many PV strings are connected to a single converter to ensure that the energy is supplied reliably, where a dc combiner box is adopted like the one shown in Fig. 12. Moreover, practical projects also employ several power converters to alleviate the power mismatching impact (e.g., aging of panels and risk of heat damage) and maintain the power supply in the case of PV string faults. In addition, the use of multiple power converters is due to the limited power ratings of the power semiconductor devices. Again, multilevel con- verters are gaining popularity in high-power PV systems,

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Blaabjerg et al.: Power Electronics Technology for Large-Scale REN Generation Fig. 12. Typical configurations of large-scale PV power generation systems connected to MVac grids: (a) double-stage via a boosting dc/dc converter, (b) single-stage using a boosting transformer, and (c) single-stage through a step-up inverter. Here, cascaded structures can also be used. The red dc/ac in Fig. 12(c) means that the inverter has boosting capability.

as discussed in Section III-A. The typical power rating of central PV inverters is several hundred kilowatts, while commercial central power inverters of several megawatts are also emerging and have become available in recent projects.

Furthermore, as seen in Fig. 12, many PV arrays are connected to combiner boxes using dc wires, which can incur a considerable ohmic loss in practice for large-scale PV power generation systems. Also, dc wires to carry large currents are difficult to bend in practical installations.

To address these issues, the maximum dc voltage for PV power converters has been shifted from 1000 to 1500 V,

Especially In Utility-Scale Systems , , . Beyond

the dc wire loss reduction, the 1500-V technology also brings several other benefits: reduced cable costs (as well as the overall system cost), increased reliability (fewer combiner boxes), and lowered installation investment.

In turn, it brings more energy production and lowers the cost of energy. Fig. 13 shows an example of an MV utility-scale PV power generation system based on the 1500-V technologies and SSTs, which can further increase the system efficiency, power density, and controllability (e.g., providing grid support). Notably, referring to the demands presented in Section II, the PV power convert- ers have to meet various requirements. Furthermore, the requirement of thermal management is one even in appli- cations of high efficiency, and modular multilevel technolo-

Gies , Enable The Generation Being Of High Power

quality, as well as higher reliability due to the increased redundancy and scalability (many modules in use). At the same time, the 1500-V technologies have several challenges. For example, due to the series connection of many PV panels needed to achieve the 1500-V dc voltage, potential-induced degradation (e.g., power gener- ation and efficiency may be lower) may become more severe, which also poses electric safety concerns. Moreover, the converters previously designed for 1000-V applications have to be redesigned to accommodate the higher dc voltage. To achieve this, advanced power devices can be adopted, or topological innovations should be considered.

In all, considering the significant cost reduction and energy increase, the 1500-V technologies are now standardized. As aforementioned, the use of multiple 1500-V converters for large-scale PV systems is common, and more will be seen. The accumulative power capacity of a single PV plant may challenge the entire grid operation and cause instabilities. In this regard, like the demand for large-scale WT power systems, advanced control is of importance to ensure the seamless integration of large-scale PV systems, which will be discussed in Section IV. In terms of power transmission, HVdc technology can be adopted, but it is not common as the location of large-scale PV power systems is relatively close to the load.

Systems

As it has been discussed in Section II, with the increased penetration of REN resources, the role of ES devices has become much more important in the energy transition.

Notably, ES technologies are not something completely new, and various applications of ES devices and systems have been seen in practice, ranging from small electronics to automotive applications to utility-scale grid systems.

It should be pointed out that, in all the listed applications, power electronics provides essential interfaces between the storage units and loads. Although there are many types of ES systems, this section gives an overview of the power Fig. 13.

Example of utility-scale PV power generation based on 1500-V technologies, where SSTs are adopted, increasing the controllability of the overall system.

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