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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "A3: Wind, Wave and Tidal Energy".

Deadline for manuscript submissions: closed (9 September 2021) | Viewed by 25792
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Special Issue Editor

Prof. Dr. Clemens Jauch

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Wind Energy Technology Institute, Flensburg University of Applied Sciences, Kanzleistraße 91-93, 24943 Flensburg, Germany
Interests: control power; frequency stability; grid integration of wind power; inertial response of wind turbines; modelling and simulation; power systems; power system inertia; variable inertia flywheel; wind turbine; wind turbine control

Special Issue Information

Dear Colleagues,

Wind power penetration is steadily increasing in power systems around the globe. Therefore, wind turbines are no longer passive power sources; instead, they have to provide grid services. Most of these services revolve around grid stability, i.e., frequency stability and voltage stability. Wind turbines have to contribute to small signal stability as well as transient stability.

In addition, other services like mitigating harmonic distortion or flicker, as well as preventing the thermal overloading of grid equipment, or even black start capabilities, might be demanded from wind turbines.

All of these services require that wind turbines have to adapt their power infeed, both active and reactive, to the needs of the grid. Hence, the power is no longer passively driven by the prevailing wind at the rotor. Some of these grid services can be delivered by the wind turbine alone; others involve energy storages or interaction with other power generators.

Traditionally, the design of wind turbine controllers is aimed at reducing mechanical loads in the wind turbine and, at the same time, at increasing the energy yield. Now, the required grid services also have to be taken into account by the wind turbine controller. In many cases, grid services cause an increase of mechanical loads, or a decrease in the energy yield, sometimes even both. Increased mechanical loads cause a reduced service life of the wind turbine, or require a stronger design of the mechanical structure. Both options have a direct impact on the levelized cost of energy of the wind turbine.

This Special Issue covers research on the following topics:

Grid services with wind turbines
Grid services with wind turbines in combination with energy storages, or in combination with other generators
Impact of grid services on the mechanical loads in wind turbines
Impact of grid services on the levelized cost of wind energy
Impact of the grid services provided by wind turbines on the stability and operation of the affected grid

Prof. Dr. Clemens Jauch
Guest Editor

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Keywords

grid service
grid stability
grid operation
mechanical load
fatigue load
ultimate load
service life
wind turbine
wind turbine control
wind turbine design
wind energy yield

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Published Papers (11 papers)

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Research

23 pages, 6104 KiB

Open AccessArticle

Influence of Continuous Provision of Synthetic Inertia on the Mechanical Loads of a Wind Turbine

by Arne Gloe, Clemens Jauch, Bogdan Craciun, Arvid Zanter and Jörg Winkelmann

Energies 2021, 14(16), 5185; https://doi.org/10.3390/en14165185 - 22 Aug 2021

Cited by 4 | Viewed by 2043

Abstract

In many electrical grids, the share of renewable energy generation increases. As these generators are typically connected to the grid via inverters, the level of grid inertia decreases. Such grids may therefore suffer from high rates of change of frequency during power imbalances. Modern wind turbines can help in controlling the frequency in such grids by providing synthetic inertia. A controller to provide synthetic inertia with wind turbines was developed at the Wind Energy Technology Institute in collaboration with Suzlon Energy. For this controller the influence of providing synthetic inertia on the mechanical loads of the wind turbine is assessed for different grid frequency scenarios. Such a scenario-based load analysis has not been published before, especially as the scenarios are derived from real measurements. The effect of the loads strongly depends on the analyzed grid frequency behavior. Ten months of high quality grid frequency measurements of the Indian grid are analyzed in order to derive inputs for the load calculation. Different types of grid frequency abnormities are identified and categorized with respect to their severity. Based on the observed occurrences of the grid frequency abnormities, realistic scenarios for the load calculations are chosen. The load calculations are performed for a state-of-the-art Suzlon wind turbine generator. The load increases caused by the supply of synthetic inertia are calculated for individual components assuming an otherwise undisturbed power production of the wind turbine in turbulent wind. Furthermore, a hardware-in-the-loop test bench is used to show how the measured grid frequencies are actually perceived by the control system of a typical wind turbine. The original frequency data were recorded with high quality measurement equipment, which is faster and more accurate than a multi-function relay, often used in wind turbines. For exemplary time traces, the effect of the reduced measurement accuracy on the reaction of the wind turbine is shown. This aspect has not been investigated in the literature yet. The results show that wind turbines can provide synthetic inertia without a considerable effect on the lifetime of the wind turbine. However, there are still problems with providing synthetic inertia reliably at high power operating points, which have to be solved. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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15 pages, 1935 KiB

Open AccessArticle

Potential of Slip Synchronous Wind Turbine Systems: Grid Support and Mechanical Load Mitigation

by Dillan Kyle Ockhuis and Maarten Kamper

Energies 2021, 14(16), 4995; https://doi.org/10.3390/en14164995 - 14 Aug 2021

Cited by 7 | Viewed by 1902

Abstract

Wind power penetration into existing electrical power systems continues to experience year-on-year growth. Consequently, modern wind turbine systems (WTS) are required to comply with relevant grid codes and provide ancillary grid services to assist with overall grid stability. Adhering to these grid codes and services can cause additional mechanical loading on WTS, which can result in a reduction in service life of some of the drivetrain components, and instability if a sufficient means of damping is not present in the drivetrain. In this paper, a dynamic simulation model of a Type 1, direct grid-connected, fixed-speed (FS) slip-synchronous wind turbine system (SS-WTS) is developed to investigate its dynamic stability in response to the additional mechanical loads imparted onto it during transient events on the grid. The SS-WTS is not equipped with a power converter and, consequently, an understanding of its dynamic stability is critical to evaluate its ability to assist with grid services and maintain stability during transient grid conditions such as low-voltage ride-through (LVRT) events. An analytical transfer function model of a 1.5 MW geared direct grid-connected SS-WTS was derived and implemented in MATLAB/Simulink. It was found that the SS technology provides significant damping to the WTS drivetrain while maintaining dynamic stability during a severe LVRT event. Moreover, it was found that the degree of damping is directly proportional to the value of rated slip, and that high-speed drivetrains provide a greater degree of damping for a given value of rated slip. Furthermore, it is shown that the SS-WTS has the ability to assist with grid services such as primary frequency response, short-circuit strength, and reactive power compensation. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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13 pages, 3949 KiB

Open AccessArticle

LVRT Impact on Tower Loads, Drivetrain Torque and Rotational Speed—Measurement Results of a 2-MW Class DFIG Wind Turbine

by Mathias Arbeiter, Martin Hopp and Martin Huhn

Energies 2021, 14(12), 3539; https://doi.org/10.3390/en14123539 - 14 Jun 2021

Cited by 3 | Viewed by 1937

Abstract

With an increasing share in energy production, wind turbines have to fulfill strict grid requirements to support the grid in case of discontinuities. Here LVRT (Low Voltage Ride Through) compliance is one essential part, where the turbine has to stay connected to the grid in case of voltage drops and is not allowed to stop. This paper comprises measured low-voltage events from an LVRT campaign with a focus on the mechanical loads for the drivetrain and tower during these temporary and instant drops of power. Moreover, we analyze the turbine operation itself, like the rotational speed, which is essential to keep the turbine within its operational parameters and design limits. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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21 pages, 1913 KiB

Open AccessArticle

Fast Frequency Support from Hybrid Wind Power Plants Using Supercapacitors

by Qian Long, Aivaras Celna, Kaushik Das and Poul Sørensen

Energies 2021, 14(12), 3495; https://doi.org/10.3390/en14123495 - 12 Jun 2021

Cited by 6 | Viewed by 2088

Abstract

The concept of hybrid wind power plants (HWPPs) that consist of wind, solar and batteries has received a lot of attention, since HWPPs provide a number of advantages thanks to the complementary nature of wind and solar energy and the flexibility of batteries. Nevertheless, converter-based technologies, as interfaces of HWPPs to the utility grid, contribute to the reduction of total system inertia, making the system more volatile and creating additional threats to frequency stability. To address these operational challenges, the capability of supercapacitors (SCs) to provide fast frequency reserve (FFR) is explored in this paper to enhance the frequency response of the HWPP. Two topologies for integrating SCs into the HWPP are proposed: (1) connecting SC to the DC link of wind turbine (WT) via a DC-DC converter interface, (2) directly connecting SC to the DC link of WT without converter interface. Frequency controllers at the asset level are proposed for these two topologies accordingly. The idea of the proposed frequency controller is to provide frequency response by varying SC voltage in proportion to frequency deviation, namely droop-based FFR. A practical SC sizing method for FFR provision is also discussed. The simulation results have shown, that in the case of frequency event, the proposed frequency controllers for SCs in both topologies positively contribute to the frequency of the system by reducing the rate of change of frequency by at least 5% and improving frequency nadir by at least 10%, compared to the case where the SC has no contribution to FFR. However, the capacitor size requirement for directly connected SC is more demanding in order to achieve the same level of improvement. The performance of frequency support has been highly related to total system inertia and control parameters. Therefore, any change to the severity of frequency events or control parameters calls for the reevaluation of the capacitance. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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20 pages, 5440 KiB

Open AccessArticle

Software-in-the-Loop Simulation of a Gas-Engine for the Design and Testing of a Wind Turbine Emulator

by Alexander Rohr and Clemens Jauch

Energies 2021, 14(10), 2898; https://doi.org/10.3390/en14102898 - 17 May 2021

Cited by 4 | Viewed by 1826

Abstract

In order to investigate the grid integration of wind turbines (WT) of various scales and designs, a wind turbine emulator (WTE) is being built in Flensburg within the state-funded project GrinSH. The special feature of this WTE is the use of a large gas engine instead of an electric motor to emulate the behavior of a WT. In order to develop the controls of this innovative WTE and to design the upcoming test runs under safe conditions, a software in the loop model (SILM) was applied. This SILM contained a mathematical model of the wind turbine, mathematical models of the gas engine with an integrated controller, and a model of the generator and frequency converter unit, as well as a preventive modulator of the reference signal (PMRS). The PMRS module converts the reference signal of the emulated WT in such a way that the dynamics of the engine components can be calculated and balanced in advance to enable the required behavior of the entire SILM despite the dynamics of the gas engine. It was found that the PMRS module, developed and tested in this work, increased the ability of the WTE, based on a gas engine, to reproduce the dynamics of a WT. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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15 pages, 4622 KiB

Open AccessArticle

Implementation of Variable Blade Inertia in OpenFAST to Integrate a Flywheel System in the Rotor of a Wind Turbine

by Laurence Alhrshy

Energies 2021, 14(10), 2783; https://doi.org/10.3390/en14102783 - 12 May 2021

Cited by 2 | Viewed by 3032

Abstract

In this paper, the integration of the dynamic behavior of the flywheel system into the load simulation tool OpenFAST is presented. The flywheel system enables a wind turbine to vary the inertia of its rotor blades to control the power production and, most importantly, to affect the vibratory behavior of wind turbine components. Consequently, in order to simulate the behavior of a wind turbine with a flywheel system in its rotor, the variable blade characteristics need to be considered in the load simulation tool. Currently, computer-aided engineering tools for simulating the mechanical loads of wind turbines are not designed to simulate variable blade inertia. Hence, the goal of this paper is to explain how variable inertias of rotor blades are implanted in such load simulation tools as OpenFAST. OpenFAST is used because of it is free, publicly available, and well documentation. Moreover, OpenFAST is open source, which allows modifications in its source code. This add-on in the load simulation is applied to correct rotor mass imbalance. It can also be applied in many cases related to the change in the inertia of wind turbine rotor blades during its operation as, for example, atmospheric ice accretion on the blades, smart blades, etc. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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25 pages, 6223 KiB

Open AccessArticle

Grid Services and Stress Reduction with a Flywheel in the Rotor of a Wind Turbine

by Clemens Jauch

Energies 2021, 14(9), 2556; https://doi.org/10.3390/en14092556 - 29 Apr 2021

Cited by 5 | Viewed by 2148

Abstract

Wind power penetration increases in most grids and the sizes of wind turbines increase. This leads to increasingly tough requirements, which are imposed on wind turbines, both from the grid as well as from economics. Some of these partially contradictory requirements can only be satisfied with additional control mechanisms in the wind turbines. In this paper, such a mechanism, i.e., a hydraulic–pneumatic flywheel system in the rotor of a wind turbine, is discussed. This flywheel system supports a wind turbine in providing grid services such as steadying the power infeed, fast frequency response, continuous inertia provision, power system stabilization, and low voltage ride-through. In addition, it can help mitigate the stress on the mechanical structure of a wind turbine, which results from varying operating points, imbalances in the rotor, gravitation that acts on the blades, in-plane vibrations, and emergency braking. The study presented in this paper is based on simulations of a publicly available reference wind turbine. Both the rotor blade design as well as the design of the flywheel system are as previously published. It is discussed how the aforementioned grid services and the stress reduction mechanisms can be combined. Finally, it is concluded that such a flywheel system broadens the range of control mechanisms of a wind turbine substantially, which is beneficial for the grid as well as for the wind turbine itself. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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19 pages, 3555 KiB

Open AccessArticle

A Spectral Model of Grid Frequency for Assessing the Impact of Inertia Response on Wind Turbine Dynamics

by Feng Guo and David Schlipf

Energies 2021, 14(9), 2492; https://doi.org/10.3390/en14092492 - 27 Apr 2021

Cited by 7 | Viewed by 2627

Abstract

The recent developments in renewable energy have led to a higher proportion of converter-connected power generation sources in the grid. Operating a high renewable energy penetration power system and ensuring the frequency stability could be challenging due to the reduced system inertia, which is usually provided by the conventional synchronous generators. Previous studies have shown the potential of wind turbines to provide an inertia response to the grid based on the measured rate of change of the grid frequency. This is achieved by controlling the kinetic energy extraction from the rotating parts by its converters. In this paper, we derive a spectral-based model of the grid frequency by analyzing historical measurements. The spectral model is then used to generate realistic, generic, and stochastic signals of the grid frequency for typical aero-elastic simulations of wind turbines. The spectral model enables the direct assessment of the additional impact of the inertia response control on wind turbines: the spectra of wind turbine output signals such as generator speed, tower base bending moment, and shaft torsional moment are calculated directly from the developed spectral model of the grid frequency and a commonly used spectral model of the turbulent wind. The calculation of output spectra is verified with non-linear time-domain simulations and spectral estimation. Based on this analysis, a notch filter is designed to significantly alleviate the negative impact on wind turbine’s structural loads due to the inertia response with only a small reduction on the grid support. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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20 pages, 4768 KiB

Open AccessArticle

Grid Support with Wind Turbines: The Case of the 2019 Blackout in Flensburg

by Arne Gloe, Clemens Jauch and Thomas Räther

Energies 2021, 14(6), 1697; https://doi.org/10.3390/en14061697 - 18 Mar 2021

Cited by 11 | Viewed by 2552

Abstract

The work presented in this paper aims to show how modern wind turbines can help to control the frequency in a small grid which suffers from large power imbalances. It is shown for an exemplary situation, which occurred in Flensburg’s distribution grid in 2019: a major blackout, which occurred after almost two hours in islanding operation, affecting almost the entire distribution grid, which supplies approximately 55,000 households and businesses. For the analysis, a wind turbine model and a grid support controller developed at the Wind Energy Technology Institute are combined with real measurements from the day of the blackout to generate a fictional yet realistic case study for such an islanding situation. For this case study, it is assumed that wind turbines with grid support functionalities are connected to the medium voltage distribution grid of the city. It is shown to what extent wind turbines can help to operate the grid by providing grid frequency support in two ways: By supplying synthetic inertia only, where the wind turbines can help to limit the rate of change of frequency in the islanded grid directly after losing the connection to the central European grid. In combination with the primary frequency control capabilities of the wind turbines (WTs), the disconnection of one gen set in the local power station might have been avoided. Furthermore, wind turbines with primary frequency control capabilities could have restored the grid frequency to 50 Hz shortly after the islanding situation even if the aforementioned gen-set was lost. This would have allowed connecting a backup medium voltage line to the central European grid and thereby avoiding the blackout. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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19 pages, 1015 KiB

Open AccessArticle

Application of a New Dispatch Methodology to Identify the Influence of Inertia Supplying Wind Turbines on Day-Ahead Market Sales Volumes

by Henning Thiesen and Clemens Jauch

Energies 2021, 14(5), 1255; https://doi.org/10.3390/en14051255 - 25 Feb 2021

Cited by 2 | Viewed by 1665

Abstract

Power system inertia is an essential part of grid frequency control. The number of synchronously connected machines, which inherently provide inertia, is decreasing due to the transition to renewable energies. Conventional generation units are being replaced by renewable generation units which are connected to the grid via frequency converters. Some power systems already suffer from too little power system inertia. Hence, inertia is a valuable yet non-traded commodity. A day-ahead dispatch methodology to secure power system inertia was developed and is applied and assessed in this work. Day-ahead market data of the combined market of the Republic of Ireland and Northern Ireland is used. If the superimposition of sell and buy bids results in insufficient inertia, the dispatch algorithm is applied. In decreasing price order, non-inertia-providing sell bids get replaced by the following sell bids in the merit order. The iterative process is repeated until sufficient inertia is in the system. The provision of synthetic inertia by wind turbines is considered in the process. The costs for additional stored kinetic energy for the assessed time periods and scenarios result in costs ranging from 1.02 to 4.49 EUR/kgm². Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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16 pages, 2061 KiB

Open AccessArticle

A Controller for Optimum Electrical Power Extraction from a Small Grid-Interconnected Wind Turbine

by Tania García-Sánchez, Arbinda Kumar Mishra, Elías Hurtado-Pérez, Rubén Puché-Panadero and Ana Fernández-Guillamón

Energies 2020, 13(21), 5809; https://doi.org/10.3390/en13215809 - 6 Nov 2020

Cited by 13 | Viewed by 2427

Abstract

Currently, wind power is the fastest-growing means of electricity generation in the world. To obtain the maximum efficiency from the wind energy conversion system, it is important that the control strategy design is carried out in the best possible way. In fact, besides regulating the frequency and output voltage of the electrical signal, these strategies should also extract energy from wind power at the maximum level of efficiency. With advances in micro-controllers and electronic components, the design and implementation of efficient controllers are steadily improving. This paper presents a maximum power point tracking controller scheme for a small wind energy conversion system with a variable speed permanent magnet synchronous generator. With the controller, the system extracts optimum possible power from the wind speed reaching the wind turbine and feeds it to the grid at constant voltage and frequency based on the AC–DC–AC conversion system. A MATLAB/SimPowerSystems environment was used to carry out the simulations of the system. Simulation results were analyzed under variable wind speed and load conditions, exhibiting the performance of the proposed controller. It was observed that the controllers can extract maximum power and regulate the voltage and frequency under such variable conditions. Extensive results are included in the paper. Full article

(This article belongs to the Special Issue Grid Services with Wind Turbines and the Resulting Mechanical Loads)

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