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Investigation of LVRT capability of wind driven dual excited synchronous generator
Abstract
This paper proposes an effective control technique for low voltage ride through (LVRT) capability in dual excited synchronous generator (DESG) wind turbines. The proposed control technique is dependent on controlling the field circuit parameters. Where the active power is controlled by the field-current space phasor magnitude and the reactive power is controlled by the field-voltage space phasor phase. With the proposed control strategy, the DESG can generate additional reactive power to support grid voltage recovery under grid faults. The DC-link voltage is kept within an acceptable limit since the excess power, due to the power mismatch between the mechanical and armature power is stored in the generator inertia. Using the proposed control strategy, the DESG can enhance the LVRT capability efficiently without using extra protection circuits or any additional control techniques during fault conditions. To test the proposed control method, simulation, and experimental results for a 1.1 kW DESG wind turbine system were obtained.
... This method is based on managing the field-current space phasor magnitude and the field-voltage phase angle in order to control the active and reactive power provided by the DESG. Furthermore, in Ref. [12], the same authors developed a method for controlling the low voltage ride through (LVRT) capability in DESGs, acting on the field-voltage space phase, and exploiting the added reactive power to promote grid voltage recuperation under grid faults. The proposed strategy was validated through experimental tests and simulations for a 1.1 kW DESG wind turbine. ...
Due to an increasing demand for electric power and changes in the typology of loads, stability has become a major concern in power systems. As the system stability is directly related to the response of the connected generator, recent research has focused on enhancing generators’ stability and improving their response to load variations. This study focuses on adding another excitation winding on to the q-axis, perpendicular to the conventional excitation winding on the d-axis, to control both active and reactive power. This paper studies and compares the performance of the dual excitation synchronous generator (DESG) to conventional synchronous generators. The mathematical equations are derived, and a mathematical model is then developed. The experimental tests have been conducted using a laboratory model consisting of a two-phase synchronous generator driven by a DC motor with different loads. The obtained results and radial diagrams for the different loading types are presented and evaluated. Therefore, a new approach has been designed to connect the DESG directly to the power grid without any electronic components using a special coupling that works in one direction. Two perpendicular excitation coils, d and q, were formed from the existing coils, and the tests were carried out on all loads, ensuring that the revolving angle (i.e., the stability angle φ) was fixed. The results show that the proposed method offers significant cost savings, potentially amounting to 15–20% of the unit price. The experimental results confirm that the DESG significantly improves the generator stability by maintaining a constant rotor angle δ, which requires using an automatic angle regulator (AAR) in addition to the conventional automatic voltage regulator (AVR).
The dual‐excited synchronous condenser (DESC) has two field windings whose axes are located at direct and quadrature axes respectively. The DESC has more capacity of absorbing reactive power than that of the traditional synchronous condenser (TSC). However, the q‐axis field winding distributed on the d‐axis magnetic pole surface can decrease the magnetic circuit area of the rotor core of DESC, thus it may result in the increase of the total harmonic distortion (THD) of the magnetic field and the oversaturation of the rotor magnetic circuit. In order to obtain better electromagnetic characteristics of DESC, a q‐axis field winding structure is proposed by optimising the number of slots, slot pitch and slot dimensions. The influences of the number and pitch of slot in q‐axis on the air‐gap radial flux density and THD of DESC are studied under the d‐, q‐ and dual‐axis excitation modes. The dimensions of the q‐axis slots are optimised by combining the time‐stepping finite element method and Taguchi method, and the optimal structure of q‐axis field winding is obtained. A 10‐kVar DESC is developed to verify the simulation results. This study provides theoretical basis for the design and manufacture of the DESC.
Non-overlap winding technology continues to remain relevant in the design of wound-field machines as an alternative for high torque density permanent magnet machines. In this paper, the finite element analyses-optimisation of two variants of the non-overlap wound-field machines viz., wound-field flux switching machine (WF-FSM) and phase shifting wound rotor synchronous machine (WRSM), are compared, first in terms of their performance for large-scale converter-fed wind generator drives, then experimentally as sub-scaled converter-fed versus direct grid-connected wind generators, respectively. This study is unique because there are no prior attempts to design, optimize and analyse on these machines for medium-speed wind power generation, as well as experiment on sub-scale prototypes for direct-grid and converter-fed operation. All investigations are contemplated in the medium-speed wind generator drivetrain which provides a tradeoff for generator efficiency and size. From the global optimisation of both machines at large-scale power levels, the torque per mass of the WF-FSM is found to be 50 % lesser compared to the WRSM. This is due to approximate volume, with closely matched optimal split and aspect ratios. In terms of the sub-scaled experimentation, both generators can easily vary their generated output power to match with varying wind resource, but direct grid-connected WRSM generator yields better efficiency performance compared to the WF-FSM converter-fed operating mode, given that the generator terminal voltage of the former is highly regulated.
This paper presents a dual excited synchronous generator (DESG) as a novel suitable alternative generation system for wind energy conversion systems. A new control strategy was proposed for the DESG wind turbine system. With this control technique, the DESG could be operated as a constant-speed constant-frequency (CSCF) generation system with the benefit of adjusting the reactive power or as a variable-speed constant-frequency (VSCF) generation system with the benefits of simple control methods and lower copper losses. Using the space phasor model of the DESG, a direct relationship between the electromechanical torque and the armature reactive power in terms of the field current space phasor magnitude and the field voltage space phasor angle is provided. The proposed control strategy is based on controlling the field-current space phasor magnitude and the field-voltage space phasor phase angle to regulate the active and reactive powers of the DESG. Simulation results, based on MATLAB/SIMULINK, for a 1.1 kW DESG wind turbine system have been executed to verify the introduced control technique under various operating conditions. Also, experimental studies have been carried out to validate the simulation results using the proposed control algorithm.
With high access of wind energy conversion system (WECS) into the grid, it is essential that the wind energy producing companies have to conform the grid code standards laid down by the country. The low voltage-ride through (LVRT) capability is one among the obligations of all grid code standards. The LVRT capability can be enhanced by controlling the power electronic converters without any additional cost. This work proposes a novel adaptive hysteresis controller (AHC) for the LVRT capability enhancement of grid connected double-feed induction generator (DFIG)-driven WEC system. The active and reactive power flow at the DFIG is managed by modified PQ controller, and the results are compared with PQ controller. The DC link voltage (Vdc) under normal and fault conditions is controlled by fuzzy logic control (FLC). The proposed AHC controller is tested with 9 MW grid connected WEC system on Matlab/Simulink platform, and a detailed simulation was carried out under normal and fault conditions. The test results disclose the efficiency of the proposed approach.
Abstract The Dual‐Excited Synchronous Generator (DESG) has two sets of field windings. The magnitudes and directions of the two field currents can be regulated, so that the excitation magneto‐motive force can be in any direction. The electromagnetic characteristics of the DESG may be different with those of the Traditional Synchronous Generator (TSG) for the different rotor structures of the two generators. To obtain better electromagnetic characteristics, a time‐stepping finite element model (T‐S FEM) of the DESG is established. The air‐gap radial flux density, saturation, active and reactive power of the DESG in single‐ and dual‐axis excitation modes are calculated and compared with those of the TSG. The electromagnetic characteristics affected by the different ratio of q‐axis field current to d‐axis field current are studied. On this basis, an excitation control strategy that can enhance the steady‐state stability is proposed, and the steady‐state stability limits with and without excitation control are calculated by the T‐S FEM. A 10‐kW DESG is developed to verify the correctness of the simulation results. The results show that the DESG with dual‐axis excitation has better electromagnetic characteristics and stronger reactive power ability, and the steady‐state stability limit of the DESG is increased obviously through the excitation control.
Coordinated control methods involving a wind turbine (WT) and an energy storage system (ESS) have been proposed to meet several objectives, such as smoothing wind power (WP) fluctuations, shaving peaks, enabling power scheduling, and allowing low-voltage ride-through (LVRT). LVRT requirement is defined by grid operators, and it should be satisfied whenever grid faults occur. Several methodologies have been proposed for the LVRT both with or without the use of an ESS. Furthermore, using an ESS is more advantageous for several WP applications. By using an ESS, WTs can be operated in a more economic and reliable way. However, the installation cost of an ESS is high and it has operation range constraints for charging and discharging. Moreover, the WT operation condition and ESS state-of-charge (SoC) can be different when a grid fault occurs. Therefore, it is necessary to coordinate both units, WT and ESS, for reliable and economic operation during a grid fault. Thus, we propose a coordinated fuzzy-based LVRT method that considers the different operation conditions of a WT and an ESS. From the proposed method, the effective reference powers of a WT and an ESS are evaluated by considering the rotor speed and SoC in the fuzzy control algorithm. The effectiveness of the proposed method is validated by considering two case studies on ESS SoC and WT rotor speed violations.
Nowadays, most of the doubly fed induction generators (DFIGs) are equipped with rotor crowbar for the requirement of low voltage ride through (LVRT). The crowbar resistance is an important parameter, and it is selected taking rotor overcurrent and dc link overvoltage limits into consideration. However, the impact of grid impedance on the LVRT performance of DFIG and crowbar resistance is not adequately researched. This paper proposed an improved method to analyze the LVRT performance of DFIG system with rotor crowbar taking the influence of the grid impedance to fill this gap. The impedance of the present grid would be decreased in the future due to the installation of more wind farms to the grid. As a consequence, the performance of DFIG under LVRT with rotor crowbar is degraded. So the design rules of the crowbar resistance need to be reconstructed. Additionally, the analytical expression of the crowbar resistance is derived considering the grid impedance. The effectiveness of the proposed method is validated through theoretical analysis and MATLAB simulations.
An active crowbar protective circuit is an effective and common approach for low voltage ride through (LVRT) of a doubly-fed induction generator (DFIG). The crowbar resistance value and its switching scheme both have crucial effects on safety recovery. The effects encountered are correlative dependence and interplay so that analyzing them from a single factor, as most existing LVRT control methods would do, obtains a partial optimal solution. This paper connects these two factors to analyze their coordination effects on the LVRT control, and to also investigate whether the global optimal performance of these factors can be achieved. The principles for resistance selection and the schemes for crowbar switching are discussed first. Next the coupling relationship is analyzed based on statistical sampling simulation data with different resistance values and various switching schemes. The results demonstrate that their coordination has critical influence on rotor peak current, DC-link voltage and reactive power. The optimal coordination will be different according to specific requirements. Hence the global optimal combination could be achieved when all requirements are taken into consideration.
With the increasing wind power penetration, the dynamic behavior of modern power systems changes. Wind turbine generators (WTGs) should provide the ancillary services to enhance the transient stability of the power system. Currently, the installed WTGs are required to equip with the low-voltage ride-through (LVRT) capability based on the grid codes; that is, the WTGs should stay connected to the grid during short-term voltage drops. This paper reviews control stategies of the WTGs for providing the LVRT capability. First, the extra devices for limitating the transient current and voltage in WTGs are introduced. Then, the control techniques for the reactive power support without any additional components are reviewed. This investigation provides useful information to wind turbine manufacturers and grid operators.
Low voltage ride-through (LVRT) is one of the main requirements of the new grid codes for integration of wind farms (WFs) to power system. In this paper, to enhance the LVRT capability of double-fed induction generator (DFIG)-based WFs, a capacitive bridge-type fault current limiter (CBFCL) is proposed. The CBFCL is based on the conventional inductive bridge-type fault current limiter (IBFCL), in which the limiting impedance (LI) of the IBFCL is adapted according to WFs requirement to provide the reactive power needed of WFs after fault clearance. The WF has been modeled based on equivalent aggregated DFIG. To check the effectiveness of the CBFCL, its performance is compared with the IBFCL. The PSCAD/EMTDC simulation results show that the CBFCL is an effective and novel solution for LVRT augmentation and short circuit current limitation of WFs. Also, it was found that the CBFCL is more competent to satisfy the LVRT requirements than the IBFCL.
Nowadays, most double fed induction generators (DFIGs) based wind turbines are equipped with rotor crowbar connected in parallel with the rotor side converter (RSC). The parallel rotor side crowbar (PRSC) is used to protect the RSC and DC link capacitor by dissipating the rotor energy during grid fault condition. In this paper, two types of crowbar protections are used: one in the rotor winding and the second in the DC link. During the fault condition, the rotor winding crowbar connects in series with the rotor winding and RSC to decrease the RSC current and dissipate the rotor energy. The general PRSC does not have the ability to significantly decrease the over-current. To protect the semiconductor switches of RSC, DFIG should not be kept connected with the utility grids under severe faults. The DC link capacitor crowbar (DCCC) operates only if the DC capacitor voltage exceeds a threshold level. Both the series rotor side crowbar (SRSC) and the DCCC operate in coordination with each other to protect RSC and DC link during fault condition and improve the fault ride through (FRT) of the DFIG. Using the proposed SRSC, RSC continues its operation to control the DFIG during fault condition. Thereby, the reactive power can be injected to support the voltage at the point of common coupling. The behavior of the DFIG is investigated when the combined crowbars are operating with the proposed coordinated control approach and results are presented.
Low-voltage problem emerges in cases of symmetrical and asymmetrical fault in power systems. This problem can be solved out by ensuring low-voltage ride-through capability of wind power plants, through a static synchronous compensator (STATCOM). The purpose of this study is to reveal that the system can be recovered well by inserting a STATCOM with energy storage to a bus where a double-fed induction generator (DFIG)-based wind farm has been connected, so the bus voltage is maintained within desired limits during a fault. Moreover, a supercapacitor is used as an energy storage element. Modeling of the DFIG and STATCOM along with the nonlinear supercapacitor was carried out in a MATLAB/Simulink environment. The behaviors of the system under three- and two-phase faults have been compared with and without STATCOM-supercapacitor, by observing the parameters of DFIG output voltage, active power, speed, electrical torque variations, and d–q axis stator current variations. It was found that the system became stable in a short time when the STATCOM-supercapacitor was incorporated into the full-order modeled DFIG.
The electromagnetic stability issues of the grid-connected doubly fed induction generator (DFIG) system are usually overlooked. This paper presents a reduced order small-signal model that can be used to analyze the stability of DFIG’s dc-link voltage control system, especially under weak ac grid conditions. This model neglects DFIG flux and fast current control dynamics. However, the effects of operating points, grid strengths and control loops’ interactions on system dynamic performance are taken into account. An eigenvalue comparison shows the proposed model holds dominant oscillation mode featured by the detailed model and is suitable for stability analysis of dc-link voltage control system of DFIG. Influence coefficients reflecting control loops’ interactions are also presented. Application studies of the proposed model show it is suitable for illustrating the effect of grid strength on dynamic performance of the DFIG’s dc-link voltage control system. Meanwhile, phase-locked loop (PLL) and rotor-side converter (RSC) active power control (APC)/reactive power control’s (RPC) effect on system stability are also explored.
Wind energy installed capacity increased exponentially over the past three decades, and has become a real alternative to increase renewable energy penetration into the energy mix.
ABSTRACT | This paper presents a comprehensive study on the
state-of-the-art and emerging wind energy technologies from
the electrical engineering perspective. In an attempt to decrease
cost of energy, increase the wind energy conversion
efficiency, reliability, power density, and comply with the stringent
grid codes, the electric generators and power electronic
converters have emerged in a rigorous manner. From the market
based survey, the most successful generator-converter
configurations are addressed along with few promising topologies
available in the literature. The back-to-back connected
converters, passive generator-side converters, converters for
multiphase generators, and converters without intermediate
dc-link are investigated for high-power wind energy conversion
systems (WECS), and presented in low and medium voltage
category. The onshore and offshore wind farm configurations
are analyzed with respect to the series/parallel connection of
wind turbine ac/dc output terminals, and high voltage ac/dc
transmission. The fault-ride through compliance methods used
in the induction and synchronous generator based WECS are
also discussed. The past, present and future trends in megawatt
WECS are reviewed in terms of mechanical and electrical technologies,
integration to power systems, and control theory. The
important survey results, and technical merits and demerits of
various WECS electrical systems are summarized by tables. The
list of current and future wind turbines are also provided along
with technical details.
Due to the rapid increase of penetration level of wind generation connected directly to the bulk power system grid, a new grid codes have been issued that require Low-Voltage Ride-Through (LVRT) capability for wind turbines so they can remain online and support the electric grid post fault events instead of instantaneous tripping. This capability will increase the stability of the network and reduce generation shortage after the fault clearance. Each utility has its own grid codes for this LVRT. There are many types of wind generators, and currently the Doubly Fed Induction Generator (DFIG) is the most popular type among the leading wind turbine (WT) manufacturers. In this paper five LVRT methods for protection of DFIG during LV events are implemented and compared. The five methods are Crowbar, DC Chopper, series dynamic resistances, and two hybrid methods that combine DC chopper with Crowbar and DC chopper with series dynamic resistances respectively. These methods were tested under different types of fault including symmetrical and unsymmetrical faults and their performances were compared.
The main challenge for wind turbines is the grid voltage sag. Hence, the grid codes (GCs) define the tasks of both wind turbine and grid in different conditions e.g. grid voltage sag. The GCs force the wind turbine to remain connected to the grid and injects the reactive current in voltage sag that this action is well-known as a low-voltage ride-through (LVRT). The grid voltage sag may lead to dc-link overvoltage in permanent magnet synchronous generator (PMSG)-based wind turbine, which connects by back-to-back (BTB) converter to the grid. The BTB converter consists of a machine side converter (MSC) and a grid-side converter (GSC). This paper proposes the improved MSC controller, which uses de-loading droop to adapt the generated active power of PMSG in the grid voltage sags. In fact, the reference current of MSC is determined by grid voltage according to the grid voltage sag to implement the reactive current injection according to GC by GSC. To verify the performance of the proposed controller, it is simulated in MATLAB/Simulink environment and compared with conventional de-loading droop that uses the dc-link voltage as the input signal. The simulation results confirm the effectiveness of the proposed controller in enhancing LVRT.
The Brushless Doubly Fed Generator (BDFG) has a good application prospect in the fields of ship shaft power generation and wind power generation due to its simple structure, safety and reliability. In previous studies, the Brushless Doubly Fed Machine (BDFM) are all based on additive modulation and whose natural synchronization speed is
${(60f_{1})/((p_1+p_{2}))}$
. The natural synchronous speed and power density of BDFM are lower than those of traditional asynchronous machine. In order to increase the natural synchronous speed and expand the machine's operating range, this paper proposes a BDFG based on differential modulation and whose natural synchronization speed is
${(60f_{1})/|p_{1}-p_{2}|}$
. First, this paper analyzes the basic working principle of differential modulation BDFG and the key factors that prevent it from developing. Then, a design scheme of stator winding and rotor winding for differential modulation BDFG is presented. Finally, the finite element model of the BDFG is established on the basis of the winding design example and the experimental prototype is designed and manufactured. The steady-state performance of the differential modulation BDFG is analyzed according to the simulation and experiment, and the feasibility of the design scheme is verified.
Wind Energy Conversion systems (WECSs) based on synchronous generators devoid of rotor windings, like permanent magnet synchronous generators (PMSGs) and synchronous reluctance generators (SyRGs), have become popular due to their maintenance–free operation. As per grid-codes, WECSs ought to stay connected, at least for a short-while, to ensure reliability and stability, even if there is a fault in the grid. There are various techniques proposed in the literature, for implementing low voltage ride-through (LVRT). This paper explores a few methods for achieving LVRT for a synchronous machine-based WECS connected to the grid through a back-to-back connected full-rated converter. The machine side converter is controlled by field oriented control methodology to drive the generator at an optimum speed to harvest maximum power from the wind turbine. The grid side-converter makes use of grid voltage-oriented control algorithm to achieve decoupled control of real and reactive powers. LVRT capability of the system is attained using four different control techniques namely, modulation index control, de-loading, crow-bar protection and interchanging the roles of the two converters to arrest the rise in DC link voltage. These methods have been simulated in Simulink/MATLAB environment and the results obtained show that these techniques work very effectively for realizing LVRT in a synchronous machine-based WECS. Finally, experimental results are presented to demonstrate the successful working of the Maximum Power Point Tracking algorithm and LVRT capability on a laboratory prototype; a comparison of LVRT techniques is presented to enable the users to make an informed choice.
Fast transient and smooth steady state performance of wind energy conversion systems (WECSs) is essential for sustained power conversion and grid code fulfilment including
low voltage ride through capability. In this paper, an analysis of permanent magnet synchronous generator (PMSG) based WECSs under vector control and direct torque control provides the basis for combining salient features of the two control methods into a reliable and effective control system to provide enhanced machine performance. The system includes two hysteresis current controllers in connection with an inverters witching table. The proposed control provides fast transient and smooth steady state performance for machine side converter of PMSGs, which controls DC-link voltage under grid fault conditions. Extensive simulation results on a 2 MW PMSG based WECS demonstrate the desired performance of the proposed control system under both grid-normal and -fault conditions.
Power electronic-based wind turbine generators (WTGs) are capable of providing inertial response to the grid by releasing kinetic energy from the turbine blade; thus, as conventional power plants are retired, the reduction of online inertia can be compensated by designing frequency controls for the WTGs. Deployment of energy storage technology for renewable generations has been increased to have the renewables centralized with a system operator as an independent power supply by making up for their nature of generation. In addition, the cost of energy storage has dropped over time and global research activities on energy storage have been funded by private industries and governments. This paper investigates the opportunity of deploying an energy storage on a doubly-fed induction generator (DFIG)-based WTG to respond to the system frequency, and then explores dynamic capabilities of the energy storage-embedded DFIG to boost its contribution while the frequency response is being provided by the power system's online inertia; thus, enabling an effective delivery of ancillary services to the system. To do this, the dynamic models of a DFIG and energy storage are combined and simulated under the different operating conditions of a DFIG, such as subsynchronous, synchronous, and supersynchronous operations, using the PSCAD.
A review spanning across a 40-year period (1978–2018) and including a total of 332 applications has been addressed on theoretical and empirical wind resource extrapolation models applied in wind energy, which can be grouped into three main families: (i) the logarithmic models; (ii) the Deaves and Harris (DH) model; (iii) the power law (PL). Applied over 96 very heterogeneous locations worldwide, models have been tested against observations at upper extrapolation height and assessed by location characteristics, extrapolation range skills, and application economical advantages.
The logarithmic models can nowadays be considered unsuitable for extrapolating wind resource to hub height of current multi-MW WTs, mainly because exhibiting a limited extrapolation range capability (about 10–50 m median bin). Finer scores in extrapolating wind resource (mean absolute bias of 3.3%) and in predicting energy output (10.1%) were achieved by the DH model, also showing remarkable extrapolation range skills (10–80 m median bin). However, although among the most economical and forward-looking solutions, its need for accurate z0 assessment and u* observations resulted so far in great limitations to its large-scale application for wind energy purposes (less than 1%). Eventually, the PL confirmed the most reliable – and largely most commonly used (73.5%) – approach for wind energy applications. Out of the plethora of PL models developed in the literature, the PL(α)-αlower and the PL(α)-αI were the finest in predicting both extrapolated wind resource (mean absolute error of 4% and 4.4%, respectively) and energy output (8.9% and 5.5%), also exhibiting extrapolation range skills meeting modern WTs requirements. By contrast, the PL using α = 1/7 returned among the worst scores, yet resulting – since the simplest – the solution most frequently applied (19.6%). This study also demonstrated that extrapolation tools requiring the most expensive instrumentation equipment do not necessarily return the finest scores.
According to most grid codes, wind farms are required to inject reactive current into the connected power grid during fault. However, this requirement may lead to the system instability and the failure of low voltage ride-through (LVRT), especially for the wind farms connected to a weak grid. In this paper, we suppose that the instability of wind farm during LVRT is mainly due to the instability of the phase-locked loop (PLL), thus a state-space model of the wind power system including the PLL under the low voltage condition is established for stability analysis. The mechanism of the system instability phenomenon during fault is investigated and revealed. To address this problem, an energy storage system (ESS)-based stability control strategy is proposed to maintain the stability of the wind power system during fault. The detailed electromagnetic model of the test system including three wind farms is installed in PSCAD/EMTDC environment for simulation studies. Simulation results verify the correctness of the obtained stability analysis results and the effectiveness of the ESS-based stability control strategy.
Grid codes require wind farm to remain on-grid and inject specific reactive current when grid fault occurs. To satisfy the requirements, reactive power devices such as the static synchronous compensator (STATCOM) are usually used in wind farms. To produce reactive currents, the wind energy generation system (WECS) and the STATCOM are normally controlled with the phase locked loop (PLL)-oriented vector control methods. Due to the active power imbalance between the generation and consumption, the wind farm has the risk of losing synchronization with the grid under severe fault conditions. This paper analyzes the synchronization mechanism and of the wind farm and proposes a coordinated control scheme for the WECS and the STATCOM during severe grid fault period. The synchronization stability of both the WECS and the STATCOM is remained by the active power balancing control of the wind farm. The control objectives of the generator- and grid-side converters for the WECS are swapped to avoid the interaction between the dc-link voltage control loop and the synchronization loop. The synchronized STATCOM produces additional reactive currents to help the wind farm meet the requirements of the grid code. Effectiveness of the theoretical analyses and the proposed control method are verified by simulations.
Recently, the variable-speed wind turbine with doubly fed induction generator (DFIG) has drawn significant attention because of its many advantages such as small power converter rating, the ability to control the output power, etc. Nevertheless, the large penetration of DFIG wind turbines may affect the power system dynamics. This paper provides a general review of the DFIG wind turbine's impact on power system dynamic performances such as frequency stability, transient stability, small-signal stability, and voltage stability. Besides, ancillary services from the DFIG wind turbine for the power grid, such as frequency support, reactive power, and voltage support, are explained. Especially, the survey emphasizes power oscillation damping methods by the DFIG wind turbine.
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.
This paper proposes an artificial neural network (ANN)-based reinforcement learning (RL) maximum power point tracking (MPPT) algorithm for permanent magnet synchronous generator (PMSG)-based variable-speed wind energy conversion systems (WECSs). The proposed MPPT algorithm firstly learns the optimal relationship between the rotor speed and electrical power of the PMSG through a combination of the ANNs and the Q-learning method. The MPPT algorithm is switched from the online RL to the optimal relation-based online MPPT when the maximum power point (MPP) is learned. The proposed online learning algorithm enables the WECS to behave like an intelligent agent with memory to learn from its own experience, thus improving the learning efficiency. The online RL process can be reactivated any time when the actual optimal relationship deviates from the learned one due to the aging of the system or a change in the environment. Simulation and experimental results are provided to validate the proposed ANN-based RL MPPT control algorithm for a 5-MW PMSG-based WECS and a small emulated PMSG-based WECS, respectively. Index Terms—Artificial neural network (ANN), maximum power point tracking (MPPT), permanent magnet synchronous generator (PMSG), Q-learning, reinforcement learning (RL), wind energy conversion system (WECS).
Wind power has become an important source of renewable energy in a number of countries around the world, including Denmark, Germany and Spain. Thereupon, connection of wind farms to the grid and their dynamic behavior under different grid conditions has become an important issue in recent years and new grid codes have been introduced. One of the most important issues related to grid codes is the low voltage ride through (LVRT) or fault ride through (FRT) capability of wind farms. Based on such code requirements, wind turbine generators must remain connected to the grid and actively contribute to the system stability during various grid fault scenarios that result in a generator terminal voltage dip. Moreover, wind turbine generators should have the ability to supply reactive power during the faults. In addition, they should supply active and reactive power immediately after fault clearance to support the network frequency and voltage, respectively. In this paper, a comprehensive review of researches published about analysis, modeling and improvement of LVRT of wind turbines with doubly fed induction generator (DFIG) is presented. The review also concludes that more investigations should be carried out to completely fulfill the grid codes' requirements. In particular, reactive and active power requirements of grid codes should be taken into account in more depth in the future LVRT solutions.
A power coordinated control method is proposed in this paper. It is used to improve the low voltage ride through (LVRT) capability of wind turbines with permanent magnet synchronous generator (PMSG). Firstly, the factors that influence the LVRT of PMSG are analyzed in details. A coordinated application strategy of these factors is presented. Secondly, an improved power coordinated control method is proposed based on the theory analysis. The active power on the grid side is controlled to track the maximum wind energy. The active power output of the generator maintains the DC link voltage constant. Current limitation links are added to the converters. And the mode of pitch angle control is also modified. The LVRT burden of PMSG can be reduced by the pitch system in the whole wind speed range. Thirdly, the practicability and effectiveness of the proposed method is verified by the simulation in DIgSILENT/Power Factory.
This paper presents a novel modulated series dynamic braking resistor (MSDBR) control strategy for enhancing the fault ride-through (FRT) of doubly fed induction generator-based wind turbines. The proposed cost-effective protection scheme introduces a voltage booster that offers series voltage compensation capability and provides a means of power evacuation to mitigate the power imbalance during grid faults. To attain flexible and robust control solution for handling both balanced and unbalanced grid faults, the proposed scheme employs a modulated pulse width modulation (PWM) switching technique to control the stator phase voltage individually. The proposed transient management scheme allows the MSDBR to mitigate the impact from different types of grid faults and to fulfill with the recent grid code requirement. Also, reactive current injection capability during faults is also investigated with the proposed voltage reference algorithm. For the controller design, small-signal modeling is utilized with consideration of measurement dynamics for the tuning of controller parameters in order to ensure the system robustness and stability. Finally, the simulation results demonstrate the satisfactory performance of the MSDBR with its preferred allocation for enhancing the FRT performance against both balanced and unbalanced faults.
This paper proposes a virtual damping flux-based low-voltage ride through (LVRT) control strategy for a doubly fed induction generator (DFIG)-based wind turbine. During the transient states of grid voltage drop and recovery, the proposed virtual damping flux-based strategy can suppress rotor current with a smooth electromagnetic torque. During steady-state faults, a negative sequence current compensation strategy is adopted to smooth the electromagnetic torque and reactive power for asymmetrical grid faults, while the conventional vector control is used to inject reactive power into the grid to support grid voltage for symmetrical grid faults. The effectiveness of the proposed strategies is examined by the simulation with a 2-MW DFIG in MATLAB/Simulink and verified by the experimental results from a scaled-down 7.5-kW DFIG controlled by a DSPACE1006. In addition, the impacts of the magnetic nonlinearity characteristics of a practical DFIG are investigated under asymmetrical grid faults. Although the magnetic nonlinearity characteristics degrade the control effects, the proposed strategies can still improve the DFIG performances during asymmetrical grid faults. The results clearly demonstrate that the proposed strategies can effectively improve the DFIG transient behavior and achieve LVRT performances.
With the rapid growth of wind energy development and increasing wind power penetration level, it will be a big challenge to operate the power system with high wind power penetration securely and reliably due to the inherent variability and uncertainty of wind power. With the flexible charging-discharging characteristics, Energy Storage System (ESS) is considered as an effective tool to enhance the flexibility and controllability not only of a specific wind farm, but also of the entire grid. This paper reviews the state of the art of the ESS technologies for wind power integration support from different aspects. Firstly, the modern ESS technologies and their potential applications for wind power integration support are introduced. Secondly, the planning problem in relation to the ESS application for wind power integration is reviewed, including the selection of the ESS type, and the optimal sizing and siting of the ESS. Finally, the proposed operation and control strategies of the ESS for different application purposes in relation to the wind power integration support are summarized. The conclusion is drawn in the end.
Distributed generation inverters have become a key element to improve grid efficiency and reliability, particularly during grid faults. Under these severe perturbations, inverter-based power sources should accomplish low-voltage ride-through requirements in order to keep feeding the grid and support the grid voltage. Also, rated current can be required to better utilize reactive power provisions. This paper presents a reference generator capable to accomplish these two objectives: to keep the injected currents within safety values and to compute the power references for a better utilization of the inverter power capacity. The reference generator is fully flexible since positive and negative active and reactive powers can be simultaneously injected to improve ride-through services. Selected experimental results are reported to evaluate the performance of the proposed reference generator under different control strategies.
This paper reports a first-order sliding-mode control (1-SMC) design for controlling the doubly-fed induction generator (DFIG)-based wind turbine's rotor-side power converter. The design is particularly focused on keeping the generator successfully in operation under unbalanced grid voltage conditions, as today's grid codes require. Aside from controlling the stator-side active and reactive powers' average value, the rotor-side converter is commanded so as to remove the fluctuations affecting the electromagnetic torque and the reactive power during unbalanced voltage. The paper aims to put forward the bases of the proposed design together with the described algorithm's stability proof. Finally, the appropriateness of the sliding-mode control to deal with the aforementioned disturbed scenarios is supported by means of simulation results.
This paper gives a comprehensive review of the state of the art of wind energy conversion systems (WECS) and technologies, with an emphasis on wind power generator and control. First, different types of common WECSs are classified according to their features and drive train types. The WECSs are compared on the basis of the volume, weight, cost, efficiency, system reliability and fault ride through capability. The maximum power point tracking (MPPT) control, which aims to make the generator speed meet an optimum value to ensure the maximum energy yield, plays a key role in the variable speed WECSs. A comprehensive review and comparison of the four most popular MPPT control methods are carried out and improvements for each method are presented. Furthermore, the latest development of wind energy conversion technologies is introduced, such as the brushless doubly fed induction generator (BDFIG), the stator permanent magnet synchronous generators, the magnetic-geared generators, dual power flow WECS with the electrical variable transmission (EVT) machine, and direct grid-connected WECS. Finally, the future trends of the technologies are discussed.
This paper deals with a ride-through technique for permanent-magnet synchronous generator (PMSG) wind turbine systems using energy storage systems (ESS). A control strategy which consists of current and power control loops for the energy storage systems is proposed. By increasing the generator speed, some portion of the turbine power can be stored in the system inertia. Therefore, the required energy capacity of the ESS can be decreased, which results in a low-cost system. In addition, the power fluctuations due to wind speed variations can be smoothened by controlling the ESS appropriately. The effectiveness of the proposed method is verified not only by the simulation results for a 2[MW] PMSG wind turbine system, but also by the experiment results for a reduced-scale turbine simulator.
Wind turbines are increasingly being expected to provide oscillation damping to the power system to which they are connected. In this study, power oscillation damping control of variable speed wind turbines is studied. An energy storage device with a bidirectional DC/DC converter connected to the DC link of a fully rated converter-based wind turbine is proposed. As system oscillation is often induced by an AC fault, it is desirable for wind turbines to ride through the fault first and then provide a damping effect. During the fault period, the energy storage system (ESS) is controlled to assist the fault ride through process, and the line side converter (LSC) is controlled to provide AC voltage support in accordance with the grid code. Methods based on regulating the active power output of the ESS and modulation of reactive power output of the LSC are proposed so as to damp the oscillations of the power system. Matlab/Simulink simulations based on a simplified Irish power system demonstrate the performance of the ESS and LSC during fault periods and validate the damping effect of the proposed system.
In this paper, a hybrid control scheme for energy storage systems (ESS) and braking choppers for fault ride-through capability and a suppression of the output power fluctuation is proposed for permanent-magnet synchronous generator (PMSG) wind turbine systems. During grid faults, the dc-link voltage is controlled by the ESS instead of the line-side converter (LSC), whereas the LSC is exploited as a STATCOM to inject reactive current into the grid for assisting in the grid voltage recovery. The validity of the proposed system is verified by experimental results for a reduced-scale wind turbine simulator as well as simulation results for a 2-MW PMSG wind turbine system.
Wind energy is inexhaustible renewable. Unlike conventional fossil fuels, wind energy is clean, abundant energy that will be available for future generations. However, wind speed is a highly stochastic component which can deviate very quickly. Output power of the wind energy conversion system (WECS) is proportional to the cube of wind speed, which causes the output power fluctuation of the wind turbine. The power fluctuation causes frequency fluctuation and voltage flicker inside the power grid. In order to reduce the power fluctuation, various approaches have been proposed in the last decades. This article deals with the review of several power smoothing strategies for the WECS. Power smoothing methods of the WECS are primarily separated into two categories such as energy storage based power smoothing method and without energy storage based power smoothing method. The main objectives of this paper are to introduce operating principles for different power smoothing methods. The energy storage based power smoothing method is effective but installation and maintenance costs of a storage device are very high. According to the literatures review, without energy storage based power smoothing method can reduce the cost of the WECS extensively. Various methods have been proposed to generate a smooth output power of the WECS without energy storage devices. Simulation results are compared among the available methods. From the review of simulation results, the kinetic energy of the inertia control method is the highly efficient power smoothing approach.
This paper presents a comprehensive study on the latest grid code regulations enforced by transmission system operators on large wind power plants (WPPs). First, the most common requirements included in the majority of international grid codes are compared; namely, low and high voltage ride-through capabilities, active and reactive power responses during and after faults, extended range of voltage–frequency variations, active power (frequency) control facility, and reactive power (voltage) regulation support. The paper also presents a discussion on the global harmonization of international grid codes as well as future trends expected in the regulations. Finally, the evolution of different wind generator technologies to fulfill various grid code requirements is investigated. The presented study will assist system operators to establish their connection requirements for the first time or to compare their existing regulations with other operators. It also enables wind turbine manufacturers and wind farm developers to obtain a more precise understanding from the latest international requirements imposed on modern wind farms.
The increasing installed wind power capacity has caused wind power generation to become a significant percentage of the entire electric power generation. As a consequence, the power system operators have included wind power plants regulation to improve the control of the overall power system, both in steady-state and transient operation. Therefore, wind power systems are required to verify the grid connection requirements stated by the power system operators. In presence of grid voltage dips, the low voltage ride-through (LVRT) requirement compliance produces a mismatch between the generated active power and the active power delivered to the grid. The conventional solution assumes that the active power surplus is dissipated in a dc-link resistor. In this paper, a control scheme for the back-to-back neutral-point-clamped converter is proposed. Under grid voltage dip, the controllers for generator-side and grid-side converters work concurrently to meet the LVRT requirement by storing the active power surplus in the turbine-generator mechanical system inertia while keeping constant the dc-link voltage. Simulation and experimental results verify the proposed control scheme.
With the increasing percentage of wind power in the utility grid, grid codes require that wind turbines must have low voltage ride through (LVRT) capability. Considering that doubly fed induction generation (DFIG) systems have the tolerance capability, to overcome the drawback of LVRT operation of DFIG system using crowbar, a dynamic voltage restorer (DVR) is connected to the stator side to prevent the stator voltage fast changing, and then the rotor side converter (RSC) can be in normal operation. A mathematical model of the rotor voltage of DFIG during a symmetrical voltage dip is presented to describe the allowable changing rate of the stator voltage. A new connection structure of DVR is proposed where DVR and GSC share a DC capacitor. Based on the coordinated control of the DFIG system and DVR, the active power regulation and reactive power compensation can be implemented under the grid fault. The simulation results for a 1.5MW DFIG system is given to show the effectiveness of the proposed protection compared to the rotor crowbar during grid fault.
Compliancy to grid codes has been a driving force behind the development of variable-speed technologies for wind generation. This resulted in the doubly-fed induction generator (DFIG) to become a common technology. The operational benefits of the DFIG derive from the possibility of controlling the operating speed of the generator from the rotor windings. Even if constructively different, the Dual-Excited Synchronous Generator (DESG) is functionally identical to the DFIG and offers an equivalent controllability. However, the mechanical design of the DESG can be exploited to be comparatively better than a DFIG for low speed operations. Thus, DESG can be a valid candidate as non conventional variable-speed constant- frequency generators for wind power generation. This paper presents a novel control technique for wind turbines that allows the power flowing into the rotor to be actively regulated and even zeroed. The algorithm is exemplified on a DESG but can be equally implemented also on a DFIG. The control of the rotor power can be translated into a simplified power electronics configuration, since the grid-side inverter in the back-to-back converter can be replaced by an unregulated rectifier, leading perhaps to a cost reduction and to an increase of reliability.
Wind power seem to be one of the most interesting technology for electrical energy generation by renewable sources. It is necessary to set up carefully the wind turbine taking into account that the wind speed is variable with the time and there is a fluctuation of power output of wind energy conversion systems. There are many technical solutions adopted to increase the system efficiency by utilizing proper generators. In the paper the case of variable speed dual-excited synchronous machine is investigated. For its control possibilities, this machine is particularly suitable for variable-speed constant frequency applications like wind power generation systems. In the paper, after presenting the mathematical model of the wind turbine and the generator, a control technique for output power maximization is proposed. Results obtained by numerical simulations are discussed in the paper to demonstrate that using the proposed control algorithm it is possible to achieve a significant increment on the power generated in comparison with that obtained using an unregulated induction machine.