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High-Level Penetration of Renewable Energy with Grid: Challenges and Opportunities
Md Shafiul Alam, Fahad Saleh Al-Ismail, M. A. Abido, Aboubakr Salem
TL;DR
High-level renewable integration offers environmental benefits but creates reduced inertia, uncertainty, fault ride-through, fault-current, reserve, and power-quality challenges. The paper reviews these challenges and associated control, storage, optimization, and protection approaches, documenting solution methods and research gaps. It concludes with future recommendations for improving renewable integration into the grid.
Problem
High-level renewable integration introduces technical challenges including low inertia, high uncertainty, limited fault ride through, high fault current, reduced reserves, and low power quality.
Method
The paper provides a comprehensive review of high-level renewable integration challenges, documented solution methodologies, research gaps, and future recommendations.
Results
The review clearly identifies and discusses major integration challenges and documents potential solutions supported by recent research articles.
Takeaways & Limitations
The identified gaps and future works define research areas for applying cutting-edge technologies and novel research to renewable integration problems.
Takeaways & Limitations
A reviewed primary-frequency-control scheme depends on a fast communication link that may pose a cyber-attack threat.
Abstract
from arXiv · showhide
The utilization of renewable energy sources (RESs) has become significant throughout the world especially over the last two decades. Although high-level RESs penetration reduces negative environmental impact compared to conventional fossil fuel based energy generation, control issues become more complex as well as total inertia to the system is significantly decreased due to removal of conventional synchronous generators. Some other technical issues, high uncertainties, low fault ride through capability, high fault current, low generation reserve, and low power quality, arise due to RESs integration. Renewable energy like solar and wind are highly uncertain due to intermittent nature of wind and sunlight. Cutting edge technologies including different control strategies, optimization techniques, energy storage devices, and fault current limiters are employed to handle those issues. This paper summarizes several challenges in the integration process of high-level RESs to the existing grid. The respective solutions to each challenge are also discussed. A comprehensive list of challenges and opportunities, for both wind and solar energy integration cases, are well documented. Also, the future recommendations are provided to solve the several problems of renewable integration which could be key research areas for the industry personnel and researchers.
1. Introduction
High-level renewable-energy integration can support cleaner generation but introduces stability, protection, uncertainty, frequency, and power-quality challenges. This review surveys these challenges, discusses solution methodologies, identifies research gaps, and recommends future work.
- Renewable energy generation is cleaner and cheaper than traditional synchronous-generator-based power generation, motivating expanded deployment.
- The world is expected to meet 36% of its energy demand from renewable sources by 2030, with solar and wind among the most promising technologies.
- High-level renewable integration complicates stable and reliable operation because wind speed and sunlight irradiation vary intermittently and unpredictably.
- Integration requires converter control and improved fault ride through because renewable sources must remain connected during system faults.
- Renewable integration can increase fault current, reduce reserves and inertia, destabilize frequency, and degrade power quality through harmonic injection.
- The review documents these challenges, discusses methodologies including storage and auxiliary devices, identifies gaps, and recommends future research.
2. Low inertia and frequency issues
Replacing synchronous generators with renewable resources reduces system inertia and reserve power, worsening frequency response. The section describes controller designs intended to emulate synchronous-generator behavior and improve frequency stability.
- High-level renewable penetration reduces total system inertia as classical synchronous generators are replaced.
- Variable-speed wind-turbine inertia is effectively decoupled by power-electronic converters, while solar PV plants provide no power-system inertia.
- Reduced inertia increases rate of change of frequency, potentially activating load shedding even after small load-generation mismatches.
- Replacing reserve generating units reduces reserve power and causes frequency deviation.
- New controllers emulate synchronous-generator behavior, including virtual synchronous-generator control, to enhance virtual inertia and stabilize frequency.
2.1. Wind based system
Wind-based systems lose effective frequency support when power-electronic interfaces decouple turbine inertia from the grid. The paper reviews de-loading, inertia emulation, synchroconverter, fast-reserve, and droop techniques to restore frequency-support capability.
- Frequency stability: The swing equation relates mechanical–electrical power imbalance to rotor acceleration, while ROCOF is inversely proportional to total system inertia.The paper uses this relationship to characterize frequency stability and motivate additional inertia-support methods.
- De-loading technique: De-loading shifts wind-turbine operation away from maximum power tracking to maintain reserve power for frequency regulation.Pitch control increases pitch angle, while speed control operates above or below the maximum-power-point speed.
- De-loading technique: Over-speed control releases additional reserve power by returning rotor speed from ωover toward ωmax during frequency instability.The paper describes over-speed control as preferable to under-speed control, whose recovery requires turbine-extracted power.
- De-loading technique: De-loading supports system frequency, but its reserve amount is not specific and long-term de-loaded operation can cause economic loss for turbine owners.Coordinated strategies can instead allow DFIG operation in MPPT mode when frequency support is unnecessary.
- Frequency-support techniques: Wind-system frequency-support methods include inertia emulation, virtual synchronous-generator behavior, fast power reserve, synchroconverter control, and droop control.These approaches aim to emulate classical synchronous-generator behavior through renewable-energy controls.
- Inertial response technique: Inertia emulation releases kinetic energy through added control loops, whereas synchroconverter control uses a synchronous-generator model to support frequency in weak grids.One-loop and two-loop strategies differ in loop structure, and synchroconverter control may require an additional damping controller.
- Inertial response technique: The additional inertia-control torque can slow the generator and release rotor energy, but constant torque may rapidly reduce rotor speed and delay controller operation.Adaptive inertia strategies dynamically adjust the inertia constant during frequency-response support.
2.2. Solar based system
PV systems operating at maximum power point lack reserve for negative frequency excursions, so frequency support requires deloading, inertial response, storage, or other control techniques. These approaches provide reserve and improve frequency response, but conventional deloading can distribute regulation unevenly across PV units.
- Frequency regulation limits: PV systems can reduce output during positive frequency excursions but cannot increase output during negative excursions while operating at maximum power point.Reserve must be created through deloading or other techniques for bidirectional frequency regulation.
- Support techniques: Three documented approaches for PV frequency and inertial support are energy storage, reduced-power operation through deloading, and inertial response.These techniques are summarized in Figure 11.
- Inertial response technique: Inertial emulation uses inner and outer control loops to regulate PV voltage and power references through the DC/DC converter.The frequency controller uses droop and virtual-inertia terms based on frequency deviation and its rate of change.
- De-loading technique: Deloading operates the PV system below maximum power to retain reserve power, with the reserve equal to Pmax − Pdelaoded.The retained reserve supports frequency regulation when additional output is required.
- Limitations and alternatives: Conventional deloading may produce non-uniform regulation because units with less reserve reach maximum power point sooner than units with more reserve.Energy storage, fuzzy logic, and particle-swarm-assisted control are also reported for reducing PV power variation and improving frequency response.
- De-loading technique: Adaptive deloading combines droop, active power-voltage matching, and vector-control loops to adjust PV output quickly for grid frequency regulation.A modified controller allocates output according to each unit’s reserve rather than delivering equal power from every unit.
3. Fault ride through (FRT) capability issues
Renewable plants must remain connected during grid faults, but PV and wind integration creates fault ride through challenges across different system types and grid-code settings. Reported solutions include improved controls, fault current limiters, FACTS devices, and energy storage, with cost, complexity, and scalability remaining constraints.
- Requirements and challenges: PV and wind plants are expected to remain connected during disturbances to maintain power-flow continuity and system reliability.This requirement is imposed by modern grid codes, which vary between countries.
- Control-based approaches: FRT techniques for PV, wind, and hybrid systems use improved control strategies, soft computing, and auxiliary devices.The literature categorizes techniques for different renewable system configurations in Figure 16.
- Control-based approaches: Model predictive control can provide fast and robust PV FRT, but switching between LVRT and normal modes requires an additional controller and increases cost.A Sugeno fuzzy controller is also reported with less overshoot and steady-state error than a classical controller.
- Auxiliary devices: Fault current limiters, energy storage, and FACTS devices can augment FRT, while FCLs are studied because of low cost, low standby loss, and high voltage withstand capability.The cited FCL types include bridge, braking-resistor, superconducting, and variable-resistive designs.
- Auxiliary devices: FCL-based FRT improvement is reported for PV, multiple wind-generator types, HVDC-connected wind, and combined wind-PV systems.DFIG systems have been studied with almost all listed FCL types, whereas fewer types have been studied for other renewable systems.
- Open challenges: External FRT devices can mitigate fault problems but increase control complexity and cost, motivating alternative methods such as dynamic current limitation.Methods developed for small-scale solar systems require modification for large-scale PV or hybrid PV/wind systems.
- Energy storage: Energy storage absorbs energy during disturbances and can support FRT, but its cost remains a limitation requiring optimal sizing or combined FCL-ESS designs.Supercapacitors are also proposed for reducing short-term PV fluctuations during normal operation.
4. Power quality issues
High-level renewable integration creates power-quality problems involving harmonics, voltage disturbances, reactive power, and filter scaling. The review documents filtering, FACTS devices, converter controls, and energy storage as mitigation approaches, while identifying practical limitations.
- Passive filters become less attractive as converter power ratings increase because their cost, size, and weight also increase.
- Shunt active and hybrid filters improve power quality by compensating current harmonics, with hybrid designs assigning lower-order harmonics to SAPF and higher-order harmonics to passive filters.
- Advanced converter controls can improve power quality by rejecting interharmonics, subharmonics, and disturbances or reducing unnecessary converter switching.
- FACTS devices, including TCSC, SVC, and STATCOM, are documented for addressing harmonics and other voltage-quality problems.
- Energy storage supports power smoothing and power-quality improvement, but batteries are poorly suited to frequent cycling because of low power density and limited life cycle.
5. Uncertainty Issues
Renewable uncertainty arises from weather, wind speed, solar irradiation, and demand variability, complicating reliable operation and planning. The review surveys probabilistic modeling, robust optimization, scheduling, storage, and advanced control approaches.
- Intermittent wind and solar generation produce uncertainty that affects system operation and requires improved long-term planning models.
- Probabilistic wind-turbine power-curve models use distributions and statistical parameters, but these assumptions may not match real operating conditions.
- Robust optimization represents uncertainty through uncertainty sets and supports generation-and-transmission expansion planning.
- Unit commitment and economic dispatch address renewable uncertainty through stochastic, deterministic, integrated electricity-gas, and multiobjective formulations.
- Model predictive, robust, coordinated, and virtual-inertia controls are used to mitigate uncertainty from wind-speed and solar-irradiation variation.
- Energy storage and soft-computing methods are employed to reduce uncertainty impacts and improve renewable-system reliability.
6. Current Challenges and Future Recommendations
Future work targets control, modeling, fault protection, inertia support, and storage-aware operation to address the technical challenges of highly renewable grids. The recommendations emphasize practical models, optimized virtual inertia, and broader system studies.
- Advanced control methods should account for high converter ratings and improve power sharing, rather than relying on ideal input-voltage assumptions.
- Fault-current limiters, including non-superconducting designs, are recommended for feasibility analysis because advanced control does not eliminate fault vulnerability.
- Renewable-system models should represent stochastic source behavior while reducing model complexity to support practical implementation.
- High renewable penetration raises concerns about low inertia, frequency instability, fault current, uncertainty, and degraded power quality.
- Improved inertia controllers and algorithms could optimize the virtual inertia required for stable high-renewable operation.
- Storage supports frequency and voltage, but future studies should consider storage lifetime in frequency support, voltage support, power-quality improvement, and uncertainty management.
7. Conclusion
The conclusion identifies major technical concerns in high-level renewable integration and reviews corresponding solution technologies. It also highlights research gaps and future work for improving renewable-system control and operation.
- The review identifies low inertia, high fault current, low power quality, and high uncertainty as major challenges of high-level renewable integration.
- Potential solutions for each challenge are documented using recent research articles and graphical representations.
- Research gaps are presented as challenges that may be addressed through cutting-edge technologies and novel research.
- The review is positioned as a reference for researchers studying major challenges and opportunities in renewable integration.
- The paper highlights future work for further improvement in renewable-energy control and operation.