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Wind energy systems are expected to supply the majority of future generation from renewables due to the maturity of technology and the availability of energy resources. In this paper, in order to improve the ability of tracking a desired power reference at high wind speed region, a pitch control strategy based on error-based active disturbance rejection control (ADRC) approach is proposed for variable speed wind turbine system, providing a novel practically appealing control structure. The stability analysis of the proposed pitch controller based on singular perturbation theory is provided. Next, an improved coyote optimization algorithm (COA) fused with L $\rm {\acute{e}}$ vy flight and Sinusoidal map is proposed to address the controller parameters tuning issue. Then, the effectiveness of the proposed solution is verified on a 1.5 MW FAST simulator taking into account both multi-step and IEC turbulence wind conditions. Finally, an analysis in frequency domain of the pitch control system with the proposed control strategy is presented. [ABSTRACT FROM AUTHOR]

As the wind power penetration increases, wind farms are required by the grid codes to provide frequency regulation services. This article develops a fully data-driven model predictive control (DMPC) scheme for the wind farm to provide temporal frequency support. The main technical challenge is the complexity and the nonlinearity of wind turbine dynamics that make the DMPC intractable. Based on Koopman operator (KO) theory, a specialized dynamic mode decomposition (SDMD) algorithm is proposed, which fits a global linear dynamic model of the wind turbines. The performance of learning dynamics is powered through integrating the physical knowledge of the wind turbine into the specialized observables of KO. To stabilize the rotor speeds in frequency regulation, the active power contribution is optimally dispatched in a moving horizon fashion. Simulation results show that the DMPC can efficiently learn and predict the wind turbine dynamics. During the frequency response process, the proposed method can effectively track the frequency support order specified by the utility grid operator while significantly stabilizing the rotor speeds. [ABSTRACT FROM AUTHOR]

This article presents a novel study of stator electrical faults detection using the wide-band spectral signatures in the controller signals of doubly fed induction generators (DFIGs). The possible wide-band spectral content of the controller signals are first defined with a set of analytical equations in order to characterize the wide-band spectral signatures and their correlation to operating conditions including stator electrical faults. A DFIG harmonic model and laboratory test-rig are used to validate the derived equations. The calculated, simulated and experimental results are in good agreement. Furthermore, it is shown that the wide-band stator electrical fault related spectral signatures carry considerable diagnostic potential for recognition of stator electrical faults, and therefore, can potentially be used to advance controller embedded fault detection techniques. [ABSTRACT FROM AUTHOR]

High temperature superconducting (HTS) generators could enable a lightweight and cost-effective direct drive (DD) wind turbines with large power ratings. The EU-funded EcoSwing project successfully demonstrated the world's first full-scale MW-class HTS generator on a commercial DD wind turbine. This paper focuses on the commissioning of the EcoSwing HTS generator on the wind turbine. The commissioning campaigns, including the rotor cool-down, excitation of the HTS field winding, and the power production of the generator, are presented in the paper. In the testing period, the generator was grid-connected for more than 650 hours and accumulatively produced more than 600 MWh to the grid. The target output power of the 3 MW class was reached. Throughout the real-life testing on the wind turbine, the generator performed well from the electromagnetic, thermal, and mechanical perspectives. Moreover, the generator even sustained three sudden short circuits in the converter system. The work reported has shown that HTS generators are technologically feasible for wind turbine applications, and the technology readiness level of HTS wind turbine generators has been improved to 6 $\sim$ 7 for the first time. [ABSTRACT FROM AUTHOR]

This paper investigates an offshore wind farm based on modular dual-active-bridge (DAB) isolated DC-DC converter. The DAB converter is adopted due to its compact design and the galvanic isolation between its input and output terminals. The synchronous generator in each wind turbine is operated individually with the generator-side rectifier to maximize the power production. The output terminals of multiple DAB converters are connected in series so that the high voltage for DC transmission is achieved and the expensive offshore step-up substation is eliminated. The DC transmission voltage is excluded from the primary side of the DAB transformer by its galvanic isolation. To balance the power sharing among the DAB converters, their input terminals are connected in parallel. In addition, the droop voltage balancing control is presented for the series offshore wind farm. As a result, the input and output voltages of each DAB converter are governed by the same droop characteristic, and the DAB converters’ output voltages are kept balanced. The time-domain simulation results show that the DC voltages of each DAB converter and the high voltage direct current (HVDC) transmission system remain stable during the temporal and spatial variations in the wind power generation. [ABSTRACT FROM AUTHOR]

This paper introduces a method which can accurately transfer active and reactive power between sets of three-phase windings of a multiphase induction machine. The machine has to have a multiple of three phases (3, 6, 9, etc.), with each three phases representing one set. The paper demonstrates first that the sets do not share the power in the same way as they share a torque. Therefore, the paper proposes a method that is capable of achieving accurate power sharing. This is ensured by considering the whole power that is transferred through the air-gap to/from a set, instead of considering only a power that is transferred between that set and a rotor shaft. A complete control algorithm of the proposed method is given. Experiments and simulations verify theoretical considerations and the proposed control. [ABSTRACT FROM AUTHOR]

This paper proposes a novel dual-stator brushless doubly-fed machine (DSBDFM) with cage-barrier rotor structure for torque density improvement in application of wind power generation. The torque density of electrical machine mainly relates to machine main dimensions and rotor coupling capability (CC). Therefore, optimizing the DSBDFM, especially the machine power distribution and rotor, affects machine torque density and rotor CC. At first, several rotor topologies are proposed and analyzed to strengthen rotor CC. Then, design principles and methods in deciding the main dimensions and power distribution for the developed DSBDFM are analyzed and illustrated in detail. Based on the machine structure and design methods, a 50-kW DSBDFM is designed, with outer and inner unit electrical machine power distribution, rotor CC, slot-pole combinations, and non-magnetic ring considered. The design example with detailed design procedure in this study provides designers a practical method in initial estimation of the dimensions and parameters for the designed DSBDFM. Meanwhile, a finite-element method is used to analyze performances of DSBDFM with winding series connection, which are verified by the experimental test. Through the results analysis, it is found that the designed DSBDFM satisfies the requirements with desirable performances, indicating the validity and rationality of design methods. [ABSTRACT FROM AUTHOR]

This paper presents a novel simulation approach to evaluate new ancillary service controls in the context of large-scale wind power integration. We adopt and compare different types of turbine inertial control methods with the proposed modifications to cope with realistic wind conditions in the field. The simulation procedure is started with the software-based simulation, in which we employ the high-fidelity wind turbine simulator FAST that models a real three-bladed controls advanced research turbine (CART3). The advantages of using FAST are that it can provide convincing simulation results and address the interactions between turbine electrical and mechanical systems. The developed controller model is then rapidly prototyped for the real-time simulation with the hardware-in-the-loop scheme. CART3 will respond to a virtual frequency event triggered in the emulated electric grid modeled in a digital real-time simulator. The introduced simulation platform streamlines the procedure of designing turbine auxiliary controls, and these simulations results give insights on the turbine controls and their impacts on the interconnected power system, as well as the effects on turbine mechanical components. For example, the results indicate that the inertial controls tend to reduce the out-of-plane mechanical loadings in region 2, while such loadings are dominated by the pitch actions in region 3. [ABSTRACT FROM AUTHOR]

This paper proposes an enhanced control scheme for a direct drive variable speed wind turbine with an interior permanent magnet (IPM) synchronous generator. The proposed control scheme incorporates maximum torque per ampere (MTPA) trajectory and maximum power extraction, which ensures the generation of required torque for maximum power extraction with minimum stator current. This in turn minimizes stator copper loss and excessive heat generated in the IPM synchronous generator. The performance of the proposed control scheme has been validated through rigours simulation and experimental studies under varying wind speed conditions. The simulation and experimental results demonstrate the efficacy of the proposed control method. [ABSTRACT FROM AUTHOR]

Under maximum power point tracking control, wind power is not dispatchable and can be highly disturbing to the supply–demand balance control in the context of microgrid particularly. To effectively dispatch wind power according to, e.g., operator command or market schedule, a variable utilization level (UL) scheme is proposed for a wind power plant (WPP) to fulfill the dispatch order while reducing the loss of total energy production. Considering different wind conditions, the proposed scheme directs the power output for each wind turbine (WT) according to a specific UL, which is adaptively adjusted according to WT rotor speed so that less reduction of energy production can be ensured. Meanwhile, more rotational kinetic energy stored can be stored in WPP, which can be later released for system support when needed. The proposed variable UL scheme is fully investigated in a doubly fed induction generator-based WPP and the results clearly indicate the proposed scheme can harvest more energy than the conventional same UL one while fulfilling the dispatch demand. [ABSTRACT FROM AUTHOR]

The definition of standard dynamic wind turbine models for power system stability analysis has been under development in recent years. These standard models, also known as generic or simplified models, have been developed under the framework of a new international standard, IEC 61400-27-1, the first edition of which was published in February 2015. These models are simplified representations of complex systems, such as wind turbines. Hence, the wind power industry urgently needs validation in comparison with real measurements to verify the accuracy and corresponding usability of the models. This paper presents the validation of a generic doubly-fed induction machine wind turbine model, commonly known as Type 3, based on a measurement campaign carried out in a Spanish wind farm. The wind turbine has been subjected to six voltage dips with different characteristics, magnitude, and duration, as well as several wind turbine operating conditions. The real behavior of the wind turbine is compared with the simulated response provided by the corresponding generic dynamic wind turbine model produced by the manufacturer Gamesa . Furthermore, the validation guidelines recently issued by IEC 61400-27-1 have been implemented, and the most important validation errors have been analyzed. [ABSTRACT FROM AUTHOR]

In recent years, with the ever increasing penetration of wind generation, great concerns have been raised since the constantly decline of system's inertia may considerably deteriorate the frequency stability of power system. In this regard, an inertial response control technique is proposed to enable wind generators to provide the power system with controlled inertia in need via releasing energy to or absorbing energy from the system. In this paper, an extended state observer (ESO)-based inertia emulation controller (InEC) is proposed for doubly-fed induction generators (DFIGs), which is shown to be robust to measurement noise and changing working conditions. In addition, to mitigate potential adverse impacts of rotor speed recovery control (RSRC) of DFIGs on system frequency, an ESO-based RSRC is derived, allowing a schedulable dynamic of speed recovery. Experiments carried out on a power hardware-in-loop testbed manifest that the proposed controllers outperform the traditional proportional–differential type InEC and proportional–integral type RSRC in performance and robustness. [ABSTRACT FROM PUBLISHER]

This study presents two methods to improve the maximum power point tracking strategy for a new wind power system that consists of a planetary gear, two permanent magnet synchronous machines, and loads. The generator, the electric motor, and the wind turbine are connected to the sun gear, ring gear, and carrier of planetary gear, respectively. With the aid of an electric motor, the system can track the maximum power without the full-power converter for generator because of its special operating principle. Conventional optimum torque (COT) control method with a torque error feed forward branch is proposed to obtain faster response compared with COT control method. Moreover, considering that the actual power cannot reach the theoretical optimum point due to the loss torque of the system, the COT control with torque loss compensation is described. A method is also proposed to estimate loss torque. Finally, the theoretical analysis is verified by simulation and experiment results. [ABSTRACT FROM PUBLISHER]

The emerging subsynchronous resonance (SSR) caused by the interaction of wind turbine generators (WTGs) with series compensation has aroused great concerns. For this particular issue, this paper is aimed to fill the gap between theoretical studies and actual observations. By analyzing the field data of 58 SSR events captured in a practical wind power system and examining the observed dynamics with previous theoretical results, the mechanism and characteristics of SSR are revealed in a more explicit and substantial way. The necessary conditions and dominant influential factors are identified and the underlying reasons are discovered. Theoretically derived as well as practically measured impedance models have demonstrated that the converter control of doubly fed induction generator (DFIG) produces negative resistance at the slip frequency and thus causes unstable SSR; while permanent magnet synchronous generators and self-excited induction generators are just passively engaged in those SSR incidents. The distribution of the oscillation frequency has also been examined with field measurements. It is discovered that WTGs at different locations participate into the same SSR mode and their frequencies are not fixed but keep changing with the time, the variation of grid topology, and the number of online generators. [ABSTRACT FROM PUBLISHER]

For wind-powered dc microgrids working auto-nomously, the available wind generation may be greater than the load demand, and hence, may cause supply–demand imbalance. To address this issue, this paper presents a new distributed load sharing control scheme, which coordinates the operation of multiple permanent magnet synchronous generators (PMSGs) in a dc microgrid. This scheme has a two-layer structure. Based on the distributed model predictive control approach, the upper layer controllers cooperatively optimize the operation of parallel-connected buck converters, which serve as the interfaces between PMSGs and microgrid bus. The optimized power references are sent to the lower layer wind generator controllers for execution. This way, the optimal load sharing among multiple PMSGs is achieved. The salient feature of the proposed scheme is that it optimizes the control actions in a distributed manner, which ensures that multiple PMSGs cooperate, rather than compete, with each other in achieving system-wide objectives. Simulation results verify the effectiveness of the proposed scheme. [ABSTRACT FROM PUBLISHER]

As the wind power generation develops, the doubly fed induction generator (DFIG) based wind power system is more and more likely to operate in the emerging weak network rather than in the conventional stiff network. Due to the comparatively large impedance of the weak network than the stiff grid, the DFIG system may be subject to the resonances due to the impedance interaction between the DFIG system and the weak network. Especially, when connected to a series π sections weak network, the multiple high-frequency resonances (MHFR) may occur and require careful studies. The impedance modeling of the DFIG system and the series π sections weak network is first demonstrated in this paper. Then, due to the multiple magnitude peaks of the series π sections of the weak network, the MHFR will be produced and can be theoretically explained based on the impedance modeling results. For the purpose of mitigating the MHFR, an active damping strategy which introduces a virtual impedance, including a phase leading compensation unit and a virtual positive resistance, is proposed and demonstrated. Simulations are conducted to validate the DFIG system MHFR as well as the proposed active damping strategy. [ABSTRACT FROM AUTHOR]

This paper presents a new hybrid wind turbine system that is formed by a continuously variable transmission connection of the turbine drive shaft with an air expander/compressor. A mechanical power split device is designed to synthesize the power delivered by the wind turbine and the air expander/compressor. A small-scale hybrid wind turbine system is mathematically modeled, analyzed, and validated using a laboratory-scale experimental test rig. By utilizing compressed air energy storage, it is shown that the hybrid wind turbine system can provide smooth power output under fluctuating wind speed conditions. Such a direct connection structure reduces the overall system cost by using one generator instead of two compared with the conventional CAES system structure. The study demonstrates the benefit of improved efficiency and flexibility brought to the turbine operation by the hybridization of wind energy and stored energy. [ABSTRACT FROM AUTHOR]

The intermittency of wind generation creates nonlinear uncertainties in wind power forecasting (WPF). Thus, additional operating costs can be incurred for balancing these forecasting deviations. Normally, large wind power penetration requires accurate quantification of the uncertainty-induced costs. This paper defines this type of costs as wind power uncertainty incremental cost (WPUIC) and wind power uncertainty dispatch cost (WPUDC), and it then formulates a general methodology for deriving them based on probabilistic forecasting of wind power. WPUIC quantifies the incremental cost induced from balancing the uncertainties of wind power generation. WPUDC is a balancing cost function with a quadratic form considering diverse external conditions. Besides, the risk probability (RP) of not meeting the scheduled obligation is also modelled. Above models are established based on a newly developed probabilistic forecasting model, varying variance relevance vector machine (VVRVM). Demonstration results show that the VVRVM and RP provide accurate representation of WPF uncertainties and corresponding risk, and thus they can better support and validate the modelling of WPUDC and WPUIC. The proposed cost models have the potential to easily extend traditional dispatches to a new low-carbon system with a high penetration of renewables. [ABSTRACT FROM AUTHOR]

Grid code specifications for voltage ride-through (VRT) of power generating units are based on time-domain envelopes of permissible terminal voltage magnitudes. Since an infinite number of voltage trajectories can be inscribed within a given enclosure, verifying that a given apparatus will withstand any such disturbance can be a daunting task. This paper tackles the VRT verification problem for doubly fed induction generator (DFIG)-based wind turbines. To this end, reachability analysis is employed to calculate sets that enclose all possible trajectories of the dynamic states of a DFIG-based system when perturbed by unknown-but-bounded grid voltage disturbances. [ABSTRACT FROM PUBLISHER]

Generic electrical simulation models of wind power generation have been developed as standards, such as the IEC 61400-27-1, to be used by wind industry, system operators, and academia for power system stability studies. In this paper, the IEC type 3 wind turbine model with wind turbine level voltage controller and with wind power plant level power factor controller is validated based on field measurements from a 52-MW wind power plant. In addition to the validation of the IEC type 3 wind turbine and wind power plant controller models, a comparison of the validation approaches, which are the full grid and play-back simulation, is provided together with a survey of the existing validation studies and recommendations for future modeling and validation tasks. The implemented IEC models are tuned to match the measurements accurately and the validated values for the control parameters of the reference wind power plant model are given. [ABSTRACT FROM PUBLISHER]

For the fixed-gain inertial control of wind power plants (WPPs), a large gain setting provides a large contribution to supporting system frequency control, but it may cause over-deceleration for a wind turbine generator that has a small amount of kinetic energy (KE). Further, if the wind speed decreases during inertial control, even a small gain may cause over-deceleration. This paper proposes a stable inertial control scheme using adaptive gains for a doubly fed induction generator (DFIG)-based WPP. The scheme aims to improve the frequency nadir (FN) while ensuring stable operation of all DFIGs, particularly when the wind speed decreases during inertial control. In this scheme, adaptive gains are set to be proportional to the KE stored in DFIGs, which is spatially and temporally dependent. To improve the FN, upon detecting an event, large gains are set to be proportional to the KE of DFIGs; to ensure stable operation, the gains decrease with the declining KE. The simulation results demonstrate that the scheme improves the FN while ensuring stable operation of all DFIGs in various wind and system conditions. Further, it prevents over-deceleration even when the wind speed decreases during inertial control. [ABSTRACT FROM PUBLISHER]

This paper presents the design of a dc grid-based wind power generation system in a poultry farm. The proposed system allows flexible operation of multiple parallel-connected wind generators by eliminating the need for voltage and frequency synchronization. A model predictive control algorithm that offers better transient response with respect to the changes in the operating conditions is proposed for the control of the inverters. The design concept is verified through various test scenarios to demonstrate the operational capability of the proposed microgrid when it operates connected to and islanded from the distribution grid, and the results obtained are discussed. [ABSTRACT FROM PUBLISHER]

A full-scale permanent-magnet synchronous generator (PMSG)-based wind turbine with dc-link voltage control via the machine-side converter has the potential to provide inherent low-voltage ride-through (LVRT) performance without additional hardware components. However, several important performance aspects related to this topology are not addressed in this literature. This paper investigates the impacts of the LVRT control on the stability and risk of resonance, successful operation, and fatigue in a full-scale PMSG-based wind power generation system. An analytical model, considering the double-mass nature of the turbine/generator and typical LVRT requirements, is developed, validated, and used to characterize the dynamic performance of the wind generation system under LVRT control and practical generator characteristics. To enhance the operation and reduce the fatigue under LVRT control, two solutions, based on active damping control and dc-link voltage bandwidth retuning, are proposed, analyzed, and compared. The detailed nonlinear time-domain simulation results validate the accuracy of the developed model and analytical results. [ABSTRACT FROM AUTHOR]

Wind power generation is increasing, rapidly forming a significant part of the power systems of the near future. Thus, incorporating wind power into frequency regulation seems necessary. Droop and virtual inertia are two key methods for involving the wind power in frequency regulation. However, when these methods are applied, undesirable effects can be induced. However, the mitigation of such impacts has not yet been analyzed. The rate of change of power (ROCOP) of a wind generator could be a major factor limiting the effective implementation of frequency regulation methods. Whereas high ROCOP leads to wear and tear and increases the maintenance cost, simply limiting the ROCOP of the generator by a ramp-limit control function will neutralize the desired impact of frequency regulation. In this paper, small-signal analysis is employed to study the impact of frequency regulation methods on the ROCOP of wind turbines considering the two-mass mechanical dynamics. Both doubly fed induction generator- and permanent-magnet synchronous generator-based turbines are considered. An effective solution, based on utilizing the converter dc-link capacitor, is proposed, analyzed, and compared to the conventional ramp-rate limit method in different aspects. Time-domain simulation is used to validate the analytical results. [ABSTRACT FROM PUBLISHER]

Brushless doubly fed induction generators (BDFIGs) are promising alternatives to doubly fed induction generators due to high reliability and low maintenance cost for the absence of brushes and slip rings. As a typical application in offshore wind energy generation, a feedforward method is applied to the grid-voltage oriented vector control to make the active and reactive power decoupled. Another challenge in practice is how to meet the demanding requirements of grid codes. The dynamic behavior of BDFIG is analyzed in detail under unbalanced grid conditions with the proposal of four different targets. To deal with these issues, an improved vector control strategy based on the proportional-integral resonant controller (PI + R) in a single synchronous reference frame is developed. The stability and robustness of the proposed control scheme under parameters uncertainties and variations are discussed as well. The advantage of this control strategy is that it is simple and fast in transient response. The effectiveness of the proposed control strategy is validated by the experiment results of a 3-kW prototype machine. [ABSTRACT FROM PUBLISHER]

The dynamic performance of wind power plants is difficult to model because of the large number of turbines that operate stochastically, with little information exchanged between the wind power plants and the serving utilities. In this study, we develop a linear dynamic equivalent model for large-scale wind power plants on the basis of data obtained at the point of interconnection in simulation. The modeling technique is based on adaptive parameter estimation of an equivalent model representing the dynamic performance of the plant. The developed model is tested against a large-scale plant model to confirm the effectiveness of the proposed technique. [ABSTRACT FROM PUBLISHER]

This paper proposes and implements a new single-phase magnetless flux switching machine (FSM) for rooftop wind power generation. The key is to integrate an outer-rotor FSM and an inner-rotor FSM with shared stator yoke and stator drum windings. In such a way, this proposed machine can not only realize flux switching function to regulate magnetic flux effectively, but also incorporate the field and armature windings within one stator to reduce the copper losses and improve the overall machine efficiency. Meanwhile, the structure is compact and rotors can be directly driven by blades to capture the wind power. The machine design guidelines from both magnetic and electric aspects are illustrated, the machine characteristics are analyzed by using time-stepping finite-element method, and the system performances are evaluated based on mathematical modeling. Both simulation and prototyping test are conducted to verify the validity of the proposed machine and system. [ABSTRACT FROM AUTHOR]

In this study, we developed a dynamic equivalent model for large-scale wind power plants based on an aggregation technique. The model is obtained in steps that include wind turbine clustering, development of slow dynamic models for the clusters, and aggregating of the models with the rest of wind power plant network. To confirm the effectiveness of the proposed method, the developed model is tested against the original large-scale wind power plant connected to the power system through computer simulations. [ABSTRACT FROM AUTHOR]

This paper proposes a new four-phase flux-reversal dc-field (FRDC) machine for the wind power generation applications. The key distinction of the proposed generator is to artfully design its stator pole and winding arrangement in a way imitating the permanent-magnet (PM) configurations, which the profound FRPM machine has. Hence, the proposed FRDC generator can provide the bipolar flux-linkage patterns and achieve performances similar to that of the high-quality FRPM generators. The proposed generator can regulate its controllable dc-field winding to vary its flux density effectively. Thus, it can achieve the constant-output-voltage characteristics at different situations in order to extend the battery life and to protect the power grid system. Without installation of any high-cost PM materials, the proposed magnetless FRDC generator can achieve better cost-effectiveness than its PM counterparts. The key performances of the proposed wind power generator are thoughtfully analyzed by the finite-element method, while the experimental prototype is also setup for verifications. [ABSTRACT FROM AUTHOR]

A fuzzy-logic sliding-mode control strategy is proposed to capture the maximum wind energy and reduce the generator-side current harmonics for a direct-driven wind power system. The control strategy mainly consists of a fuzzy logic controller for generating the optimum dc-side current and a double integral sliding-mode current controller capable of accurately tracking the optimum dc-side current. A resonant filter is also incorporated into the fuzzy logic controller to eliminate the generator-side current harmonics. Detailed design procedure, existence, and stability conditions of the proposed control strategy have been thoroughly investigated and presented. The capability and effectiveness of the proposed control strategy have been validated by comparative experimental results. [ABSTRACT FROM AUTHOR]

This paper investigates the spectral effects that stator supply harmonics can induce in wound rotor and doubly-fed induction generators. The purpose of this study is to clarify the nature and the origin of the reported significant inter harmonic effects in these machines. To this end, the authors derive analytical expressions that define the spectral signature frequencies of individual supply harmonics in the current and torque signals. The expressions are first validated through comparison with the spectra calculated using a harmonic time-stepped machine model and then by spectral measurements on a laboratory test rig comprising a 30 kW machine excited by a programmable power supply unit. It is shown that the presence of supply harmonics can result in multiple and variable wide-band interharmonic effects in the current and torque signals. Finally, the interharmonic current emissions for the wound rotor and the doubly-fed mode of operation are experimentally quantified using the relevant IEC standard indices. [ABSTRACT FROM AUTHOR]

Power fluctuation caused by wind speed variations may be harmful for the stability of the power system as well as the reliability of the wind power converter, since it may induce thermal excursions in the solder joints of the power modules. Using the wind turbine rotor inertia energy for power leveling has been studied before, but no quantified analysis or generic design method have been found. In this paper, the transfer functions from the wind speed to electrical power, electromagnetic torque, and rotating speed are built based on which the rotating speed controller is designed in the frequency domain for power leveling. Moreover, the impact of other parameters on power leveling, including the time constant of maximum power point tracking (MPPT) and the rotor inertia, are also studied. With the proposed optimal design, the power fluctuations are mitigated as much as possible, while the stability of the rotating speed is still guaranteed. Moreover, the oscillation of the electromagnetic torque is also reduced, and the performance of the MPPT is only weakened slightly. [ABSTRACT FROM AUTHOR]

This paper presents a surrogate-model-based optimization of a doubly-fed induction generator (DFIG) machine winding design for maximizing power yield. Based on site-specific wind profile data and the machine's previous operational performance, the DFIG's stator and rotor windings are optimized to match the maximum efficiency with operating conditions for rewinding purposes. The particle swarm optimization-based surrogate optimization techniques are used in conjunction with the finite element method to optimize the machine design utilizing the limited available information for the site-specific wind profile and generator operating conditions. A response surface method in the surrogate model is developed to formulate the design objectives and constraints. Besides, the machine tests and efficiency calculations follow IEEE standard 112-B. Numerical and experimental results validate the effectiveness of the proposed technologies. [ABSTRACT FROM AUTHOR]

To improve the wind energy dispatchability in the presence of non-Gaussian wind power uncertainties, this paper presents a stochastic coordinated control scheme for the doubly-fed-induction-generator–based wind-battery hybrid systems (WBHS). The proposed control scheme has a two-layer structure. Based on the non-Gaussian distributional wind power forecasts, an upper layer stochastic predictive controller coordinates the operation of wind and battery subsystems. The computed power references are passed to the lower layer wind and battery controllers for execution. This way, the combined power output of WBHS is brought to the desired dispatch levels. The salient feature of the proposed scheme is that it optimizes the control actions over the non-Gaussian wind power predictive distributions, thus handling the non-Gaussian uncertainties in wind power. The simulation results on actual wind data demonstrate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

The basic idea of the fractional frequency transmission system (FFTS) is to use lower frequency to reduce the effective electrical length of the ac transmission line, thus increasing its transmission capacity. Because the rotational speeds of the generators in hydro and wind power generation systems are very low, the FFTS is particularly suitable for transmitting electricity produced by renewable energy sources. The fractional frequency wind power system (FFWPS) provides an effective approach for integrating the large-scale and long-distance wind power into the grid. A novel method of the integration of the wind turbines in direct-driven FFWPS is introduced in this paper. A full-power ac–dc–ac converter is used as the grid-connection device in the experiment, and the experiment results verify the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents the development and experimental results of a supervisor strategy and a sliding mode control setup to improve the performance of hybrid generation systems. The topology in this study is conformed by a core comprising a fuel cell module and a supercapacitor module, in combination with an alternative energy source module and an electrolyzer. In particular, a wind power turbine is considered as alternative power source to attain a hybrid generation system fully relying on renewable energy. First, a supervisor strategy is proposed to manage the power flows of the subsystems and coordinate the system as a whole. Subsequently, a sliding mode control setup for combined operation of the dc/dc power converters of the fuel cell/supercapacitor core is presented to track the power references synthesized by the supervisor control. Both control levels, supervisor strategy and sliding mode controllers, are implemented and assessed through extensive experimental tests under different wind conditions and heavy-load changes. [ABSTRACT FROM AUTHOR]

This paper addresses comparative investigation on the permanent magnet (PM) wind power generators’ performance considering PM flux directions while generated electrical energy is being stored through a mechanical energy storage system (MESS). Radial- and axial-flux PM generators are dealt with, and since their equivalent circuit parameters are almost identical, their comparison can be reasonable. During the energy conversion through the MESS, since a power converter is required, the phase current of each generator contains harmonic components due to diode rectifier with high influence on the generators’ performance. In this study, the electromagnetic characteristics of each generator are analyzed based on the measured current to consider the influence of those harmonics, and their performance evaluation is also performed by experiment. [ABSTRACT FROM AUTHOR]

The papers in this special section brings together papers focused on the recent advancements and breakthroughs in the technology of modeling and enhanced active/reactive power control of wind power conversion systems, ranging from components of wind turbines to wind farms. [ABSTRACT FROM PUBLISHER]

Cascaded brushless doubly fed induction machines have attracted much attention because of the elimination of slip rings and brushes. However, only limited works have been reported on the control strategies due to the complex machine modeling. Further developments of these kinds of machines are still under investigation, especially for wind energy applications. This paper proposes a new control strategy for cascaded brushless doubly fed induction generators (CBDFIGs) based on direct power control. The effects of voltage vectors on the output active and reactive powers are first investigated. A vector selection strategy is then proposed to achieve flexible power regulation. In addition, the behaviors of the CBDFIG under unbalanced grid voltage conditions are studied and a power compensation scheme is developed to improve the power quality. The effectiveness of the proposed control strategy is validated experimentally on a laboratory prototype. [ABSTRACT FROM PUBLISHER]

Wind turbines are required to meet grid interconnection codes for voltage ride-through, which mandate that they should not disconnect during a specified range of grid disturbances. One of the tasks of a turbine’s design verification process is to ensure that it will be able to withstand any such event. This paper sets forth a powerful computational technique for design verification of Type-4 permanent-magnet synchronous generator-based turbines with fully-rated grid-side converters. The proposed method can provide assurance as to whether the currents and voltages will remain within acceptable bounds. To this end, reachability theory is employed to find sets that enclose all possible trajectories for the system states caused by a given set of unknown-but-bounded disturbances. The computation of the reachable sets is carried out by using on-the-fly linearization of the nonlinear system dynamics, zonotopes, and interval analysis. [ABSTRACT FROM PUBLISHER]

This paper deals with the design and the experimental validation in scale-lab test benches of an energy management algorithm based on feedback control techniques for a flywheel energy storage device. The aim of the flywheel is to smooth the net power injected to the grid by a wind turbine or by a wind power plant. In particular, the objective is to compensate the power disturbances produced by the cycling torque disturbances of the wind turbines due to the airflow deviation through its tower section. This paper describes the control design, its tuning, as well as the description of the experimental setup, and the methods for the experimental validation of the proposed concepts. Results show that the fast wind power fluctuations can be mostly compensated through the flywheel support. [ABSTRACT FROM AUTHOR]

The inexact power output from a pitch angle-controlled (PAC) horizontal-axis turbine, as influenced by short duration wind variations like turbulence and gusts, has been the subject of well-documented experimental and empirical studies. In this paper, an analytical interpretation of the phenomena is presented under assumption of two-parameter Weibull statistics for short duration wind variations. The popular concept of turbulence intensity is used as a parametric measure for randomness of wind speed. The formulations culminate in analytical expressions for two distinct metrics of power distortion, namely: (a) the short duration output power from the turbine as a mean distinct from corresponding ideal zero-turbulence value; and (b) the output power variability as a quantification of randomness around the mean. Estimates of both metrics are presented for operating conditions in accordance with the well-known IEC 61400-1 standards. Trends indicated by computed estimates are found to be consistent with empirically observed data from field studies that are reported in the literature. [ABSTRACT FROM PUBLISHER]

For the comparative investigation on direct-drive permanent magnet (PM) wind power generators, this paper presents the analytical approach based on transfer relations. Considering various generator types, this paper provides analytical solutions for those generators’ characteristic prediction, and the analyzed results are verified. In addition, since our analysis models are designed to be connected to a grid, the power loss characteristics according to grid output power variation are presented considering harmonics in generator current caused by the grid-connected power converter. [ABSTRACT FROM PUBLISHER]

This study proposes a power control strategy for a grid-connected variable-speed wind turbine, based on a doubly-fed induction generator (DFIG) with slip power recovery. The control objectives vary with the zones of operation (dependant on the wind speed), aiming to maximize the active power in the partial load zone and to limit it when operating within the full load zone, while regulating the stator reactive power following grid requirements. The control design, based on the second-order sliding modes (SOSM) and Lyapunov, uses a modified version of the super-twisting algorithm with variable gains which can be applied to nonlinear multiple inputs–multiple outputs (MIMO) systems. The well-known robustness of the sliding techniques, the simplicity of the algorithm, and the adaptive characteristic of its gains are used together in this study to obtain a controller able to deal robustly with the exacting challenges presented by these systems. An additional benefit of the proposal lies in the smoothness of the control action, an important issue regarding applied mechanical efforts. Representative results obtained by simulation of the controlled system are shown and discussed. [ABSTRACT FROM AUTHOR]

The performance of the doubly fed induction generator (DFIG) during grid faults is attracting much interest due to the proliferation of wind turbines that employ this technology. International grid codes specify that the generator must exhibit a fault-ride-through (FRT) capability by remaining connected and contributing to network stability during a fault. Many DFIG systems employ a rotor circuit crowbar to protect the rotor converter during a fault. Although this works well to protect the generator, it does not provide favorable grid support behavior. This paper describes an experimental investigation of an alternative FRT approach using a brake chopper circuit across the converter dc link to ensure that the dc-link voltage remains under control during a fault. Two different approaches to chopper control are examined and the resulting FRT performance is compared with that of a conventional crowbar approach. The new chopper-based control methods are experimentally evaluated using a 7.5-kW DFIG test rig facility. [ABSTRACT FROM AUTHOR]

Due to the variable characteristics of renewable generation, batteries used in renewable-power systems can undergo many irregular, partial charge/discharge cycles. In turn, this can also have a detrimental effect on battery lifetime and can increase project costs. This study presents a method of improving battery lifetime in a small-scale remote-area wind-power system by the use of a battery/supercapacitor hybrid energy storage system. The supervisory control algorithm and hardware implementation are described and projected long-term benefits of the proposed system are assessed by simulation. A representative dynamic model of the overall system, incorporating realistic wind-speed and load-power variations has been developed. An analysis is presented of the potential improvement in battery lifetime that is achievable by diverting short-term charge/discharge cycles to a supercapacitor energy-storage system. This study introduces a method by which supercapacitor energy storage systems and control algorithms can be evaluated and implemented in the application area considered. The composition of a prototype test system is described and experimental results are presented to demonstrate system feasibility. [ABSTRACT FROM AUTHOR]

In this paper, a two-time-scale coordination control method to mitigate wind power fluctuations using a battery energy storage system (BESS) is proposed. Two-time-scale maximal power fluctuation restrictions (MPFRs) are set for the combined output of the wind farm and the BESS: the maximal fluctuation of the combined power in any 1- and 30-min time window must be kept within \gamma 1\min\\% and \gamma 30\min \\% , respectively, of the rated power of the wind farm. A flexible first-order low-pass filter (FLF) with an optimization of time constant T_f is developed to limit the power fluctuation under restriction with smaller BESS capacity. Then, a coordination control method is developed on base of the FLF, which contains: 1) PSO-based time constant real-time optimizing algorithm; 2) remaining energy level feedback control; 3) two-time-scale coordination control strategy. Finally, an estimation of BESS capacity and power rating for a wind farm to achieve the MPFRs is presented, and the whole method is tested in a time-domain simulation system. [ABSTRACT FROM AUTHOR]

Protective relay settings for wind turbine generators often depend on short-circuit calculations performed using sequence network circuits in rms-based protection software. Traditional assumptions made in deriving the sequence network representation of induction machines result in some error in short-circuit calculations, which could potentially cause incorrect relay settings. A more accurate sequence network model of induction machines is derived in this paper to aid in short-circuit calculations in induction-generator-based wind plants. A rigorous mathematical approach to defining the stator short-circuit current equations under general faults is described and validated with transient simulations. The sequence network model of the induction machine is derived from the closed-form mathematical solutions. An example short-circuit calculation is performed on a simple network in this paper, and found to have a marked improvement in accuracy from traditional sequence network calculation methods for unbalanced faults. Resonant effects of power factor correction capacitors are also described. [ABSTRACT FROM AUTHOR]

In this paper, an improved efficiency control scheme for wind energy conversion systems (WECSs) with squirrel cage induction generators is proposed. Thus, the power harvesting from the WECS is increased and additionally expansion of the exploitable wind speed region toward the lower speed range is accomplished. The generator is connected to the power grid by means of two space-vector-controlled back-to-back converters. A minimum ohmic loss (MOL) controller is introduced in order to minimize the generator resistive loss that is accomplished by adjusting the d-axis stator current according to torque conditions. The implementation of the proposed controller is easy and cost effective because neither additional control signals nor the knowledge of the generator loss model is required. The effectiveness of MOL controller and its successful cooperation with two types of maximum power point tracking (MPPT) controllers, which are employed to maximize the wind turbine output power, are experimentally verified. The MPPT controller is implemented by using an adaptive search control and a fuzzy-logic-based control technique, since both are independent of wind turbine characteristics and widely used. Selective experimental results are presented to demonstrate the resulting improvements of the suggested control scheme. [ABSTRACT FROM AUTHOR]

Interaction between doubly fed induction generator (DFIG) Type 3 wind generators and series-compensated networks can lead to subsynchronous resonance (SSR) oscillations—a phenomenon observed in the real world. In this paper, impedance-based Nyquist stability criterion is applied to analyze the SSR phenomena. Impedance models of a DFIG along with its rotor-side converter (RSC) and grid-side converter, and a series-compensated network are derived in terms of space vectors. The DFIG impedance and the network impedance are analyzed to show the impact of wind speed, compensation level, and RSC current controller gain on SSR stability. Nyquist maps are also used to demonstrate the impact on SSR stability. Simulation studies are carried out to show SSR controller interaction. This paper successfully demonstrates that the interaction between the electric network and the converter controller is a leading cause of the SSR phenomena recently observed in wind generation grid integration. [ABSTRACT FROM AUTHOR]