Doubly fed induction generator‐based variable‐speed wind turbine: Proposal of a simplified model under a faulty grid with short‐duration faults

The aim of this study is to provide a simplified model of a variable‐speed wind turbine (VSWT) with the technology of a doubly fed induction generator (DFIG), which operates under faulty grid conditions. A simplified model is proposed, which consists of a set of electrical and mechanical equations that can be easily modeled as simplistic electrical circuits. It makes it an excellent tool to achieve fault ride‐through capability of grid‐connected VSWT with DFIGs. Both symmetrical and unsymmetrical grid faults, which cause symmetrical and unsymmetrical voltage sags, have been applied to the system in order to validate the model. The proposed simplified model has been compared with the traditional full‐order model under multiple sags (different durations and depths), and the results reveal that both models present similar accuracy. As the idea is to reduce the computational time required to simulate the machine behavior under faulty grid conditions, the proposed model becomes suitable for that purpose. The analytical study has been validated by simulations carried out with MATLAB .     

A review of wind energy technologies

Energy is an essential ingredient of socio-economic development and economic growth. Renewable energy sources like wind energy is indigenous and can help in reducing the dependency on fossil fuels. Wind is the indirect form of solar energy and is always being replenished by the sun. Wind is caused by differential heating of the earth's surface by the sun. It has been estimated that roughly 10 million MW of energy are continuously available in the earth's wind. Wind energy provides a variable and environmental friendly option and national energy security at a time when decreasing global reserves of fossil fuels threatens the long-term sustainability of global economy. This paper reviews the wind resources assessment models, site selection models and aerodynamic models including wake effect. The different existing performance and reliability evaluation models, various problems related to wind turbine components (blade, gearbox, generator and transformer) and grid for wind energy system have been discussed. This paper also reviews different techniques and loads for design, control systems and economics of wind energy conversion system.     

A robust power control strategy to enhance LVRT capability of grid‐connected DFIG‐based wind energy systems

This paper presents a new robust and effective control strategy to mitigate symmetrical voltage dips in a grid‐connected doubly fed induction generator (DFIG) wind energy conversion system without any additional hardware in the system. The aim is to control the power transmitted to the grid so as to keep the electrical and mechanical quantities above their threshold protection values during a voltage dip transient. To achieve this, the references of the powers are readjusted to adapt the wind energy conversion system to the fault conditions. Robust control strategies, combining the merits of sliding mode theory and fuzzy logic, are then proposed in this paper. These controllers are derived from the dynamic model of the DFIG considering the variations in the stator flux generated by the voltage drop. This approach is found to yield better performance than other control design methods which assume the flux in the stator to remain constant in amplitude. This control scheme is compliant with the fault‐ride‐through grid codes which require the wind turbine generator to remain connected during voltage dips. A series of simulation scenarios are carried out on a 3‐MW wind turbine system to demonstrate the effectiveness of the proposed control schemes under voltage dips and parameter uncertainty conditions.     

Fault ride-through capability of DFIG wind turbines

This paper concentrates on the fault ride-through capability of doubly fed induction generator (DFIG) wind turbines. The main attention in the paper is, therefore, drawn to the control of the DFIG wind turbine and of its power converter and to the ability to protect itself without disconnection during grid faults. The paper provides also an overview on the interaction between variable-speed DFIG wind turbines and the power system subjected to disturbances, such as short circuit faults. The dynamic model of DFIG wind turbine includes models for both mechanical components as well as for all electrical components, controllers and for the protection device of DFIG necessary during grid faults. The viewpoint of the paper is to carry out different simulations to provide insight and understanding of the grid fault impact on both DFIG wind turbines and on the power system itself. The dynamic behaviour of DFIG wind turbines during grid faults is simulated and assessed by using a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk in the power system simulation toolbox PowerFactory DIgSILENT. The data for the wind turbines are not linked to a specific manufacturer, but are representative for the turbine and generator type used in variable-speed DFIG wind turbines with pitch control.     

Co‐ordinated voltage control of DFIG wind turbines in uninterrupted operation during grid faults

Emphasis in this article is on the design of a co‐ordinated voltage control strategy for doubly fed induction generator (DFIG) wind turbines that enhances their capability to provide grid support during grid faults. In contrast to its very good performance in normal operation, the DFIG wind turbine concept is quite sensitive to grid faults and requires special power converter protection. The fault ride‐through and grid support capabilities of the DFIG address therefore primarily the design of DFIG wind turbine control with special focus on power converter protection and voltage control issues. A voltage control strategy is designed and implemented in this article, based on the idea that both converters of the DFIG (i.e. rotor‐side converter and grid‐side converter) participate in the grid voltage control in a co‐ordinated manner. By default the grid voltage is controlled by the rotor‐side converter as long as it is not blocked by the protection system, otherwise the grid‐side converter takes over the voltage control. Moreover, the article presents a DFIG wind farm model equipped with a grid fault protection system and the described co‐ordinated voltage control. The whole DFIG wind farm model is implemented in the power system simulation toolbox PowerFactory DIgSILENT. The DFIG wind farm ride‐through capability and contribution to voltage control in the power system are assessed and discussed by means of simulations with the use of a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk. The simulation results show how a DFIG wind farm equipped with voltage control can help a nearby active stall wind farm to ride through a grid fault, without implementation of any additional ride‐through control strategy in the active stall wind farm. Copyright © 2006 John Wiley &Sons, Ltd.     

Dynamic contribution of variable-speed wind energy conversion system in system frequency regulation

Frequency regulation in a generation mix having large wind power penetration is a critical issue, as wind units isolate from the grid during disturbances with advanced power electronics controllers and reduce equivalent system inertia. Thus, it is important that wind turbines also contribute to system frequency control. This paper examines the dynamic contribution of doubly fed induction generator (DFIG)-based wind turbine in system frequency regulation. The modified inertial support scheme is proposed which helps the DFIG to provide the short term transient active power support to the grid during transients and arrests the fall in frequency. The frequency deviation is considered by the controller to provide the inertial control. An additional reference power output is used which helps the DFIG to release kinetic energy stored in rotating masses of the turbine. The optimal speed control parameters have been used for the DFIG to increases its participation in frequency control. The simulations carried out in a two-area interconnected power system demonstrate the contribution of the DFIG in load frequency control.     

Dynamic evaluation of two configurations for a hybrid DFIG‐based wind turbine integrating battery energy storage system

Hybrid systems comprising battery energy storage systems (BESSs) and wind power generation entail considerable advances on the grid integration of renewable energy. Doubly fed induction generators (DFIGs) stand out among different wind turbine (WT) technologies. On the other hand, electrochemical batteries have proved to be valid for these purposes. In this paper, a comparative analysis is carried out between two alternative configurations for hybrid WT‐BESS systems, where the BESS is connected either outside or inside the DFIG. The modeling of these two configurations and the control systems applied for achieving the coordinate operation of the energy sources (DFIG and batteries) are illustrated. The hybrid systems under study are evaluated by simulation under normal operation (wind speed fluctuations and grid demand changes) and grid faults. Simulation results show that both configurations improve the grid integration capability of the WT, although the configuration with external BESS presents better results since it can provide additional active/reactive power injection. Copyright © 2014 John Wiley & Sons, Ltd.     

风特性对风力发电机组输出功率的影响

为了准确研究不同类型的风速对并网风电机组输出功率的影响,建立了并网风力发电机模型和不同类型风速的模型。在不同类型风速的扰动下,对异步风电机输出功率的波动进行了仿真分析;针对引起功率振荡最严重的阵风,进行了不同频率的阵风扰动下风电机组功率振荡的比较,对阵风扰动下风电机组间的相互影响进行了仿真分析。结果表明,在相同幅值的扰动下,阵风引起的风电机组功率振荡最严重,机组功率振荡情况与阵风扰动的频率有关,单台机组在阵风扰动下产生的振荡对其他机组也会产生一定的影响。     

A review of fault ride through of PV and wind renewable energies in grid codes

This paper presents a comprehensive review of fault ride through (FRT) in the grid code of 38 selected countries with an emphasis on renewable energy (REN) sources–related rules. Grid codes are the rules legislated usually by the transmission system operators (TSOs) to determine the grid integration requirements of electrical power generators. Each country establishes its grid code for satisfying the minimum required technical criteria and revises it frequently to cope with new modifications of the utility. Growing the penetration of REN sources have influenced many operational aspects of the power system such as protection, power quality, reliability, and stability. Thereupon, regulations must ensure the power system's secure and controllable operation of REN sources. FRT is one of the main parts of the grid code, and its characteristics affect the performance and rating of power system apparatus. FRT defines the performance of electric power generators during and in postfault conditions. FRT of solar photovoltaic (PV) and wind turbines (WTs) as the main REN sources of energy has great importance in the grid codes. In this paper, a comparison of FRTs in the grid code of 38 countries, including low‐voltage ride through (LVRT), zero‐voltage ride through (ZVRT), and high‐voltage ride through (HVRT) are provided and surveyed.     

Performance of PI controller for control of active and reactive power in DFIG operating in a grid-connected variable speed wind energy conversion system

Due to several factors, wind energy becomes an essential type of electricity generation. The share of this type of energy in the network is becoming increasingly important. The objective of this work is to present the modeling and control strategy of a grid connected wind power generation scheme using a doubly fed induction generator (DFIG) driven by the rotor. This paper is to present the complete modeling and simulation of a wind turbine driven DFIG in the second mode of operating (the wind turbine pitch control is deactivated). It will introduce the vector control, which makes it possible to control independently the active and reactive power exchanged between the stator of the generator and the grid, based on vector control concept (with stator flux or voltage orientation) with classical PI controllers. Various simulation tests are conducted to observe the system behavior and evaluate the performance of the control for some optimization criteria (energy efficiency and the robustness of the control). It is also interesting to play on the quality of electric power by controlling the reactive power exchanged with the grid, which will facilitate making a local correction of power factor.     

Economic evaluation of the dual mode CAES solution for increased wind energy contribution in autonomous island networks

Wind parks operating in autonomous island grids, such as those encountered in the Aegean Archipelago, face considerable wind energy curtailments, owed to the inability of local electricity networks to absorb the entire wind energy production. On the other hand, plans promoting the natural gas-based electricity generation in big islands (such as Crete) question the future of wind energy. To recover wind energy curtailments and benefit from the introduction of natural gas, the adoption of compressed air energy storage (CAES) systems suggests an appreciable energy solution. Furthermore, to improve the economic performance of the proposed system, it is decided that guaranteed energy amounts should be delivered to the local grid during peak demand periods. In an effort to obtain favourable negotiation conditions – for the selling price of energy delivered – and also improve the economic performance of the system, a dual mode CAES operation is currently examined. Proceeding to the economic evaluation of dual mode CAES configurations that ensure maximum wind energy recovery, the feasibility of the proposed system may be validated. Lower electricity production costs and considerable reduction of fuel consumption achieved – in comparison with the requirements of conventional peak demand power units – illustrate the system's advantages.     

Reactive power compensation and active filtering capability of WECS with DFIG without any over‐rating

This paper presents a novel approach for reactive power compensation and active filtering capability of a variable speed wind energy conversion system (WECS) with doubly fed induction generator (DFIG), without any over‐rating. First, the WECS is capable of capturing maximum wind power under fluctuating wind speed. Second, depending on the available wind power value versus nominal WECS power, power quality can be improved by compensating the reactive power and the grid harmonic currents, without any system over‐rating. The proposed rotor side converter (RSC) control manages the WECS function's priorities, between main active power generation and power quality management. To ensure high filtering performances, we used an improved harmonic isolator in the time domain, based on a selective pass band filter (SPBF) developed in our laboratory. Moreover, we took advantage of the high amplification effect of the rotor side‐controlled DFIG to compensate harmonic currents. Consequently, no over‐rating is necessary for the proposed additional active filtering capability. Simulation results for a 2 MW WECS with DFIG confirm the effectiveness and the performances of the proposed approach. Copyright © 2009 John Wiley & Sons, Ltd.     

Reliability models for DFIGs considering topology change under different control strategies and components data change under adverse operation environments

The doubly-fed induction generators (DFIGs) are widely applied for wind energy conversion. Their reliability models are important to risk analysis of wind power systems, which has not been fully investigated due to changing topology configuration and sensitive reliability data. In this paper, three reliability models are newly proposed for the DFIGs. Under the doubly-fed mode, the DFIG is logically in series configuration with low reliability. With outage of rotor-side converter, or operation of crowbar against short current, the grid-side converter or the power grid provides var support. The DFIG operates as an induction machine, avoids tripping and yields high reliability. The DFIGs' reliability is also dependent on vulnerability of the mechanical components to the strong wind, vulnerability of the electrical components to the short current, effectiveness of the fault-ride-through capability, and possibility of strong wind and severe fault. Logistic curve is applied to modify unavailabilities of the mechanical and electrical components under the adverse operation conditions. The proposed models help to quantify the lifetime and determine the key component of the DFIGs, thus improves reliability of the wind power systems.     

A novel method for obtaining virtual inertial response of DFIG‐based wind turbines

This paper investigates virtual inertia control of doubly fed induction generator (DFIG)‐based wind turbines to provide dynamic frequency support in the event of sudden power change. The relationships among DFIGs' virtual inertia, rotor speed and network frequency variation are analysed, and a novel virtual inertia control strategy is proposed. The proposed control strategy shifts the maximum power point tracking (MPPT) curve to the virtual inertia control curves according to the frequency deviation so as to release the ‘hidden’ kinetic energy and provide dynamic frequency support to the grid. The calculation of the virtual inertia and its control curves are also presented. Compared with a PD regulator‐based inertial controller, the proposed virtual inertia control scheme not only provides fast inertial response in the event of sudden power change but also achieves a smoother recovery to the MPPT operation. A four‐machine system with 30% of wind penetration is simulated to validate the proposed control strategy. Simulation results show that DFIG‐based wind farms can provide rapid response to the frequency deviation using the proposed control strategy. Therefore, the dynamic frequency response of the power grid with high wind power penetration can be significantly improved. Copyright © 2015 John Wiley & Sons, Ltd.     

Blade pitch control malfunction simulation in a wind energy conversion system with MPC five-level converter

This paper is on a wind energy conversion system simulation of a transient analysis due to a blade pitch control malfunction. The aim of the transient analysis is the study of the behavior of a back-to-back multiple point clamped five-level full-power converter implemented in a wind energy conversion system equipped with a permanent magnet synchronous generator. An alternate current link connects the system to the grid. The drive train is modeled by a three-mass model in order to simulate the dynamic effect of the wind on the tower. The control strategy is based on fractional-order control. Unbalance voltages in the DC-link capacitors are lessen due to the control strategy, balancing the capacitor banks voltages by a selection of the output voltage vectors. Simulation studies are carried out to evaluate not only the system behavior, but also the quality of the energy injected into the electric grid.     

Dynamic response of a stand-alone wind energy conversion systemwith battery energy storage to a wind gust

A mathematical model of each element of a stand-alone wind energy conversion system is developed. The model variables are expressed in the d-q rotor reference frame. The wind turbine was considered as the only source of power in this study. Using this model, the system response to a recorded wind gust is investigated by calculating the generator current, the rectifier current, the load current, the battery charging current and the battery voltage. The calculated results are then verified by comparing them with actual values obtained from a data acquisition system. Good agreement was achieved between the experimental and analytical results     

Control of wind generator associated to a flywheel energy storage system

In this paper, a doubly fed variable speed wind induction generator connected to the grid associated to a flywheel energy storage system (FESS) is investigated. The dynamic behaviour of a wind generator, including the models of the wind turbine (aerodynamic), the doubly fed induction generator (DFIG), a ac/ac direct converter, the converter control (algorithm of VENTURINI) and the power control of this system, is studied. Also investigated is a control method of the FESS system, which consists of the classical squirrel-cage induction machine supplied off the variable speed wind generator (VSWG) through a rectifier–inverter cascade arrangement. Simulation results obtained on the basis of the dynamic models of the wind generator are presented, for different operating points, to demonstrate the performance of the proposed system.     

电网不同故障下DFIG特性分析及保护措施建议

目前,国内外对DFIG的研究主要侧重于风力发电机组控制策略方面,而对于不同电网故障情况下DFIG的运行特性分析较少。鉴于此,在DIgSILENT／PowerFactory下建立TDFIG模型,利用含风电场的WSCC三机九节点仿真系统,进行了电网不同故障情况下的一系列仿真,重点分析了电网不同故障情况下DFIG的运行特性,研究了风电场与电网之间的交互影响及相应的保护措施,为大规模风电接入电网的运行控制提供依据。     

Design and real time implementation of type-2 fuzzy vector control for DFIG based wind generators

Doubly fed induction generator is very sensitive to voltage variations in the grid, which pose limitation for wind power plants during the grid integrated operation. Handling the uncertainity in wind speed and grid faults is a major challenge to fulfill the modern grid code requirements. This paper proposes a new control strategy for Rotor side converter using Interval type-2 fuzzy sets which can model and handle uncertainties in the system parameters. The presence of third dimension in the membership function, offers an additional degree of freedom in the design of the controller to counter the effects of fluctuations in wind speed and low voltage during severe grid fault conditions. A 2 MW DFIG connected to the grid is modelled in simulation software RSCAD and interfaced with Real time digital simulator (RTDS) to perform the simulations in real-time. The RTDS platform is considered by many research laboratories as real-time testing module for controller prototyping and also for hardware in the loop (HIL) applications. The controller performance is evaluated in HIL configuration, by performing the real-time simulations under various parameter uncertainties. The proposed controller can improve the low voltage ride through capability of DFIG compared to that of PI and type-1 fuzzy controller.     

A conceptual chemical looping combustion power system design in a power-to-gas energy storage scenario

Increasing global energy demand and the continued reliance on non-renewable energy sources, especially in developing countries, will cause continued increases in greenhouse gas emissions unless alternative electricity generation methods are employed. Although renewable energy sources can provide a clean way to produce electricity, the intermittent nature of many existing renewable energy sources, such as energy from the wind or sun, can cause instability in the energy balance. Energy storage systems such as power-to-gas may provide a clean and efficient way to store the overproduced electricity. In this work, a power-to-gas energy storage system coupled with a chemical looping combustion combined-cycle power generation system is proposed to provide base and intermediate load power from the unused electricity from the grid. Enhanced process integration was employed to achieve optimal heat and exergy recovery. The simulation results using ASPEN Plus V8.8 suggest that electric power generation with an overall energy efficiency of 56% can be achieved by using a methane chemical looping combustion power generation process with additional hydrogen produced from a solid oxide electrolysis cell. The proposed system was also evaluated to further improve the system's total energy efficiency by changing the key operating parameters.