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Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time
Ana Fernández-Guillamón, Emilio Gómez-Lázaro, Eduard Muljadi, Angel Molina-García
TL;DR
The transition to converter-connected renewable generation reduces effective grid inertia and affects frequency stability, motivating updated inertia analysis and control strategies. The paper reviews inertia and damping-factor evolution, compares generation-mix scenarios, estimates regional and national inertia changes, and discusses wind and PV frequency-control contributions. Europe’s averaged inertia decreased by around 20% over the last two decades as renewable integration rose from 14% in 1996 to 31% in 2016, while virtual-inertia methods and PV de-loading provide control approaches.
Problem
Converter-connected renewable generation reduces effective grid inertia, affecting system stability, reliability, frequency deviations, and ROCOF.
Method
The paper conducts a literature review, estimates country-level averaged inertia over two decades, analyzes damping-factor evolution, and surveys wind and PV frequency-control strategies.
Results
Europe’s averaged inertia decreased by around 20% over the last two decades, while renewable integration increased from 14% in 1996 to 31% in 2016.
Takeaways & Limitations
Renewable frequency support can use virtual or emulated inertia, and PV plants can reserve active-power headroom through de-loading.
Takeaways & Limitations
A single-loop wind control strategy does not restore frequency to its nominal value; an additional loop proportional to frequency deviation is required.
Abstract
from arXiv · showhide
Traditionally, inertia in power systems has been determined by considering all the rotating masses directly connected to the grid. During the last decade, the integration of renewable energy sources, mainly photovoltaic installations and wind power plants, has led to a significant dynamic characteristic change in power systems. This change is mainly due to the fact that most renewables have power electronics at the grid interface. The overall impact on stability and reliability analysis of power systems is very significant. The power systems become more dynamic and require a new set of strategies modifying traditional generation control algorithms. Indeed, renewable generation units are decoupled from the grid by electronic converters, decreasing the overall inertia of the grid. 'Hidden inertia', 'synthetic inertia' or 'virtual inertia' are terms currently used to represent artificial inertia created by converter control of the renewable sources. Alternative spinning reserves are then needed in the new power system with high penetration renewables, where the lack of rotating masses directly connected to the grid must be emulated to maintain an acceptable power system reliability. This paper reviews the inertia concept in terms of values and their evolution in the last decades, as well as the damping factor values. A comparison of the rotational grid inertia for traditional and current averaged generation mix scenarios is also carried out. In addition, an extensive discussion on wind and photovoltaic power plants and their contributions to inertia in terms of frequency control strategies is included in the paper.
Nomenclature
This section defines abbreviations used for renewable-generation technologies, generator types, frequency metrics, and wind power plants.
- DFIG denotes Double Fed Induction Generator, while PMSG and SCIG denote Permanent Magnet Synchronous Generator and Squirrel Cage Induction Generator.
- PV and RES denote Photovoltaic and Renewable energy sources, respectively.
- ROCOF denotes Rate Of Change Of Frequency, and WPP denotes Wind Power Plant.
- FSWT, VSWT, and HAWT denote Fixed Speed Wind Turbine, Variable Speed Wind Turbine, and Horizontal Axis Wind Turbine.
- EU denotes European Union.
1. Introduction
Power systems are transitioning from synchronous-machine generation to renewable sources, whose converter interfaces reduce effective grid inertia. This reduction affects frequency stability and motivates estimating inertia and damping while developing renewable frequency-control strategies.
- Synchronous machines provide stored kinetic energy that is automatically extracted after sudden power imbalances, affecting grid frequency.
- Wind, solar, and biomass generation overtook coal power in the EU for the first time in 2017.
- Power converters decouple renewable sources from the grid, reducing effective electrical-grid inertia when conventional generators are replaced.
- Low system inertia is associated with faster ROCOF and compromised frequency stability and transient response.
- The paper reviews inertia values, estimates country-level averages over two decades, analyzes damping-factor evolution, and surveys renewable frequency-response strategies.
2. Inertia analysis in power systems
The paper models generator inertia and damping through the swing equation, then aggregates generation and load behavior into equivalent system parameters. Historical estimates show that renewable integration changes equivalent inertia over time, while converter controls can partially restore wind-plant contributions.
- Swing equation: The swing equation describes synchronous-generator motion from mechanical and electromagnetic torque differences, with damping representing load response to frequency deviations.The model is expressed in the time or Laplace domain for small deviations around steady state.
- Inertia constant: H measures the interval a generator can supply rated power using kinetic energy stored in its rotating masses.H depends on the moment of inertia, base frequency, and base power.
- Equivalent system parameters: Equivalent inertia Heq aggregates synchronous generators coupled to the system, but varies with renewable generation across time, seasons, and operating conditions.For Germany in 2012, Heq was below 5.7 s during 50% of the year, below 5 s during 10%, and below 4 s during 1%.
- Historical evolution: EU equivalent inertia decreased nearly 20% between 1996 and 2016, compared with reductions of 2.5–3% in Asia, the USA, and South America.The estimates use annual averaged electricity and conventional-generation inertia values.
- Historical evolution: EU average inertia reduction was 0.6 s, while renewable supply increased nearly 20% and wind, biomass, biofuels, and PV surpassed hydro-power development.The paper also identifies EU countries with Heq reductions greater than 15%.
- Renewable contributions: Converter-interfaced PV and wind are decoupled from the grid, but wind plants can provide hidden inertia through stored rotational energy and converter control.Most wind-turbine inertia constants are 2−6 s, whereas PV has H ≈ 0 because it has no rotating masses.
- Renewable contributions: Including hidden wind-plant inertia reduced the estimated EU inertia change around 0.3 s, approximately 50% of the reduction calculated without it.The modified equivalent inertia combines synchronous and renewable-related contributions.
3. RES frequency control strategies
Renewable integration changes frequency dynamics by reducing synchronous inertia, motivating layered control and reserve strategies for PV and wind plants.
- Frequency-control context: Primary frequency control uses governors to adjust mechanical power from local frequency deviations over timescales up to the low tens of seconds.It responds automatically according to generator speed-droop settings but does not restore nominal frequency.
- Frequency-control context: Secondary control removes steady-state frequency deviations and maintains scheduled exchanges, while tertiary control performs economically efficient dispatch.In Europe, secondary control typically acts from seconds to 15 minutes after an incident.
- Frequency-control context: Low inertia increases frequency deviations and strongly affects ROCOF after generation-load mismatches.Higher renewable penetration reduces synchronous generators and reserves for primary and secondary control.
- PV strategies: PV plants can provide frequency support using energy storage or de-loading below the maximum power point to retain active-power headroom.For de-loading, the available reserve is ΔP = PMP P − Pdel, with the higher de-loaded voltage selected for stability concerns.
- Wind strategies: Wind plants use pitch-angle or over-speed de-loading, supplementary inertial-response loops, and stored rotor kinetic energy to provide additional active power.These approaches reduce supplied power below available aerodynamic power or release kinetic energy during frequency deviations.
RW T
Wind-turbine frequency controls emulate conventional-generator behavior or release stored kinetic energy, with some strategies combining responses to improve frequency performance.
- RW T: Hidden-inertia control adds power based on ROCOF and can use a second frequency-deviation loop until frequency returns to f0.The first loop releases kinetic energy by reducing generator speed; the second addresses frequency restoration.
- RW T: Fast power reserve supplies wind-turbine kinetic energy as additional active power, followed by an under-production period to recover the extracted energy.The boost is triggered when frequency deviation exceeds a predefined threshold and decreases rotor speed.
- RW T: Table 4 reviews wind-power-plant frequency-control proposals across plant integration levels and power-imbalance percentages.Some strategies are combined to improve frequency deviation after generation-load mismatches.
4. Conclusion
The review finds that renewable integration has reduced averaged European inertia, while wind turbines can provide inertia values comparable to conventional units through emulated hidden inertia.
- 4. Conclusion: Europe’s averaged inertia decreased by around 20% over the last two decades as renewable integration rose from 14% in 1996 to 31% in 2016.The comparison considers renewable generation decoupled from the grid.
- 4. Conclusion: Wind turbines present inertia values between 2 and 6 s depending on technology, similar to conventional generation units.The paper characterizes this contribution as emulated hidden inertia.
- 4. Conclusion: The paper reviews inertia estimation, damping-factor evolution, and frequency-control strategies for wind and photovoltaic plants in high-renewable systems.Averaged inertia values are estimated for different countries over the last two decades.