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This paper deals with the optimal state reference calculation for synchronous motors with a magnetically salient rotor. A look-up table computation method for the maximum torque-per-ampere (MTPA), maximum torque-per-volt (MTPV), and field-weakening operation is presented. The proposed method can be used during the drive start-up, after the magnetic model identification. It is computationally efficient enough to be implemented directly in the embedded processor of the drive. For experimental validation, a 6.7-kW synchronous reluctance motor drive is used. more...

In this paper, a predictive direct power control (DPC) method without sector information and voltage vector selection is proposed for three-phase grid-connected converters. Different from conventional predictive direct power controllers, the proposed strategy always adopts fixed voltage vectors, instead of selecting voltage vectors according to the angular information of the grid-voltage vector or the virtual-flux vector. The converter switching time is obtained by minimizing the squared errors of the instantaneous active and reactive power. Switch duty cycles are then calculated and equivalently reconstructed. The proposed strategy not only presents rapid dynamic response due to the use of the predictive controller, but also possesses excellent steady-state performance as a result of duty cycle reconstruction. The main advantage of the proposed control scheme, compared with the classical DPC strategies, is that it is unnecessary to use sector information of the grid-voltage vector, and selection of active voltage vectors is also not required here. Therefore, incorrect selection of voltage vectors and the resulting performance deterioration are surely avoided, without the need of any additional compensation measures. An in-depth comparison and experimental assessment are given to validate the effectiveness of the proposed strategy. more...

In the wind energy conversion system (WECS) based on the permanent magnet synchronous generator, the topology that a wind generator is connected to a voltage source inverter through a diode rectifier and a boost chopper is widely adopted for its low cost and high reliability. However, with the increasing power level of the WECS, the power switch device of a boost chopper in the topology has to endure high instantaneous voltage and current stress, thus reducing the system reliability. Although the traditional multiple boost converter reduces the transmission power level of single circuit units in terms of structure, it is still limited by narrow speed regulation range. Thus, a novel switchable-cascaded-mode boost chopper is proposed in this paper. The operation principle of the new converter and the generator speed control strategy are analyzed. At last, the effectiveness of the new converter is verified with an experimental prototype. The experimental results demonstrate that speed range is extended by using the new topology as generator-side converter in the permanent magnet wind generation system. more...

A predictive current controller with an extended-state observer (ESO) is proposed for grid integration of wind energy systems. In each sampling period, the proposed strategy calculates the converter switching time that minimizes a cost function defined as a sum of squared current errors, leading to constant switching frequency. To achieve excellent dynamic performance, the impact of sampling delay is analyzed, and detailed compensation methods are proposed. In addition, an ESO is constructed to suppress parameter variations and modeling errors, which affect the performance of the controller. The parameter tuning and the stability of the observer are analyzed. The proposed strategy not only presents rapid dynamic response due to the use of the predictive current controller but also possesses robust control performance as a result of the observation algorithm. Simulation and experimental results are given to validate the effectiveness of the proposed solution. more...

Due to the excellent dynamic performance, the Finite Control Set Model Predictive Control has been widely used in various types of converters. However, when Finite Control Set Model Predictive Control is adopted, the switching frequency of converters varies significantly with system operating conditions. Consequently, constant-frequency predictive control strategy has been proposed. Two active voltage vectors and a zero voltage vector are selected within each sampling period. The action time sequence is then calculated. Due to the unsymmetrical distribution of current variation rates around zero, the calculated value of the voltage-vector action time will turn up negative. According to common sense, the voltage-vector action time is greater than or equal to zero. The action time is normally forced to zero whenever a negative value is predicted, resulting in the control failure and performance deterioration. To solve this problem, this paper proposes modified strategy. The modified strategy examines the action time calculated out. When negative action time comes out, the modified strategy reselects the active voltage vector accordingly, instead of forcing the action time to be zero. Optimized action time sequence is further determined by minimizing the cost function. The effectiveness of the modified strategy is clearly verified by experimental tests, and analytical remarks are all founded in practical results. more...

For applications demanding high torque and high reliability transmission, coaxial magnetic gear (CMG) may be a promising substitute of the mechanical gearbox. However, with the increasing of unit capacity, CMG tends to have a big size with large pole number, which would lead to heavy computation burden if finite element method (FEM) is employed. Analytical methods are therefore expected. To date, only the exact subdomain model is capable of precisely predicting the magnetic field behaviors in an analytical manner through solving a matrix equation. However, as pole number of the CMG increases, the number of subdomain increases correspondingly, resulting in the growth of dimension of the matrix equation and thus the growth of computation cost. Therefore, the motivation of this paper is to develop a time-efficient solution for the subdomain model of the scaled-up CMG, which could efficiently reduce the size of the matrix to be inverted and computational burden. Finally, FEM is employed to confirm the validity of the developed analytical calculation. more...

This paper proposed a direct power control strategy for three-phase two-level voltage-source rectifiers based on extended-state observation. Active and reactive powers are directly regulated in the stationary reference frame. Similar to the family of predictive controllers whose inherent characteristic is highly dynamic behaviors, the proposed control strategy can present comparably enhanced transient performance. Furthermore, excellent steady-state power performance can also be observed when the proposed control scheme is adopted. In addition, another attractive feature lies in its robust performance, facilitating the successful suppression of performance deterioration caused by parameter variations and model inaccuracy. Detailed frequency-domain analysis is provided to verify its effectiveness. Experimental comparisons are also carried out among the finite control set model direct power control strategy, the vector-oriented control scheme and the proposed strategy, which demonstrate the superiority of the proposed scheme. more...

A diode bridge rectifier followed by a boost-chopper circuit is a common topology of the generator-side converter for the direct-drive surface-permanent-magnet-synchronous-generator-based wind energy conversion system. Owing to its nonlinearity, it is difficult for the system to maintain good performance within normal operating range under the ordinary proportional-integral control. In this paper, a piecewise nonlinear mathematical model for the whole system, including both generator and converter, is proposed based on the commutation points of the diode bridge rectifier for more accurate controller design. The input-output feedback-linearization-based nonlinear transform for the mathematical model of the system is piecewise made. Then, a speed controller is designed according to the converted linear model, considering the integral of time multiplied by the absolute error. The proposed strategy has the advantages of relatively simple transform of state variables for linearization and developed parameter tuning method. The parameters of the linearized controller for different model intervals are the same. Finally, simulation results indicate that the proposed nonlinear controller is able to reject parameter perturbation to some extent, and experimental results are presented with a 3-kVA prototype, demonstrating that the dynamic performance of the system is improved effectively. more...

This paper presents a robust model predictive current controller with a disturbance observer (DO-MPC) for three-phase voltage source PWM rectifier. The new algorithm is operated with constant switching frequency (CF-MPC). In order to minimize instantaneous d- and q-axes current errors in every sampling period, CF-MPC is implemented by selecting appropriate voltage vector sequence and calculating duty cycles. The fundamental of this algorithm is discussed and the instantaneous variation rates of d- and q-axes currents are deduced when each converter voltage vector is applied in six different sectors. A Luenberger observer is constructed for parameter mismatch and model uncertainty which affect the performance of the MPC. The gains of the disturbance observer are determined by root-locus analysis. Moreover, the stability of the disturbance observer is analyzed when there are errors in the inductor filter parameter. The proposed method has an inherent rapid dynamic response as a result of the MPC controller, as well as robust control performance with respect to the disturbance due to use of the combined observation algorithm. Simulation and experimental results on a 1.1 kW VSR are conducted to validate the effectiveness of the proposed solution. more...

A novel adaptive current controller for DFIG-based wind turbines is introduced in this paper. The attractiveness of the proposed strategy results from its ability to actively estimate and actively compensate for the plant dynamics and external disturbances in real time. Thus, the control strategy can successfully drive the rotor current to track the reference value, ensuring that the performance degradation caused by grid disturbances, cross-coupling terms and parameter uncertainties can be successfully suppressed. Besides, the two-parameter tuning feature makes the control strategy practical and easy to implement in commercial wind turbines. To quantify the controller performances, the transfer function description of the controller is derived. General disturbance rejection, robustness against parameter uncertainties, bandwidth and stability are also addressed. Simulation results, together with the time-domain responses, proved the stability and the strong robustness of the control system against parameter uncertainties and grid disturbances. Significant tracking and disturbance rejection performances are achieved. more...

With increasing wind power penetration, transient responses of doubly-fed-induction-generator (DFIG) based wind turbines gain attentive focus. Accurate prediction of transient performance of DFIG under grid faults is required with increasing wind power penetration. Taking into account the main flux saturation and deep-bar effect, this paper concentrates on transient responses and stability of the DFIG system under symmetrical grid faults. Their roles played in the enhancement of system transient stability are clarified. The analyses proposed contribute greatly to proper selection, design and coordination of protection devices and control strategies as well as stability studies. more...

This paper proposed an on-line optimizing duty cycle control approach for three-phase active-front-end rectifiers, aiming to obtain the optimal control actions under different operating conditions. Similar to finite control set model predictive control strategy, a cost function previously constructed based on the desired control performance is adopted here, which is essential for the solving process of the optimizing problem. On the other hand, differently, with respect to the proposed strategy, duty cycle signals are optimized, instead of possible switching states. The determination of optimal duty cycles is made by predicting the effect of duty cycles on instantaneous current variations and minimizing the cost function. Due to the adoption of behavior prediction, the proposed controller inherits the excellent dynamic characteristics of predictive controllers. Moreover, the application of optimal duty cycles determined by cost function minimization automatically ensures optimum operations of converters within each sampling period. Improved transient and steady-state features of the proposed strategy are confirmed by experimental validations and in-depth comparisons with linear controllers, deadbeat predictive controllers, and finite control set model predictive control strategies. more...

A novel control strategy is proposed in this paper for the rotor side converter (RSC) of doubly-fed induction generator (DFIG)-based wind power generation systems. It is supposed to enhance the low-voltage ride-through (LVRT) capability of DFIGs during great-level grid voltage dips. The strategy consists of a proportional-resonant (PR) controller and auxiliary PR controllers. The auxiliary controllers compensate the output voltage of the RSC in case of grid faults, thus limiting the rotor inrush current of DFIG and meeting the requirements of LVRT. Sequential-component decompositions of current are not required in the control system to improve the response of system. Since the resonant compensator is a double-side integrator, the auxiliary controllers can be simplified through coordinate transformation. The feasibility of the control strategy is validated by simulation on a 1.5 MW wind-turbine driven DFIG system. The impact of the RSC converter voltage rating on the LVRT capability of DFIG is investigated. Meanwhile, the influence of angular frequency detection and control parameters are also discussed. Compared with traditional vector control schemes based on PI current controllers, the presented control strategy effectively suppress rotor current and reduce oscillations of DFIG power and torque under grid faults. more...

In variable pitch control, the wind speed in the axis of the blade is used as the reference wind speed for wind blade synchronous control. In fact, the wind speed is changing with the height. With the increasing of the wind turbine capacity, the difference of the blade wind speed in different spin high degree is gradually increasing, making the loads of the blades tend to gradually increase, which has seriously affected the wind turbine life. This paper implemented the independent variable pitch control based on the traditional variable pitch control and obtained the amended pitch angle on the use of the state feedback control, which plus the collective pitch is the independent pitch. Through the simulation of the wind turbine at the step and the random wind, it follows that the independent variable pitch control of the wind power generation system can reduce the loads of blades and extend the service life of wind turbines, on condition that the output power is kept stable. more...

The wind turbine is a highly nonlinear system and its operating environment is extremely hostile. With the development of large-scale wind turbines and the introduction of flexible structures, vibration problems appear more and more serious. Random wind fluctuation, wind shear and tower shadow effect can all cause large ripples on drive-train torque. In this paper, two controllers are proposed for speed vibration suppression and torque ripple suppression. One is the observer-based state feedback generator torque controller, which can add damping to the first drive-train torsion mode. The other is the rotor collective pitch controller based on Active Disturbance Rejection Control (ADRC), which is designed to regulate generator speed. Both of the controllers are built on the basis of the model linearization method. The simulation results show that the control strategy can significantly reduce drive-train torque ripples, which demonstrates the excellent dynamic performance and robustness of the system, hence proves that the new method will have an important value in engineering application. more...

Wind power is a clean renewable energy source. Most countries are placing considerable faith in wind energy as an important and effective power technology for sustainable energy supply, thus providing particularly strong support to promote its development. China has abundant wind resources with a total theoretical wind power potential of 2, 200 GW. Developing wind power is of great importance to alleviate the energy crisis and environmental pollution resulting from the rapid economic growth of China in the 21st century. Recently, wind power in China registered a record level of expansion, and has doubled its total capacity every year since 2004. Many experts believe that China will be central to the future of the global wind energy market. This paper presented an overview of wind energy potential in China and reviewed the national wind power development in detail. The future prospect of wind energy development in China is analyzed. Its near future looks bright and the market will continue to develop at an ever-expanding rate foreseen for the next several years. more...

This paper deals with a practical sensorless control method for doubly-fed-induction-generator based wind power generation systems. The speed estimator is based on a novel control scheme called active disturbance rejection control. Encouraging simulation results have been obtained. more...

A predictive current control strategy can realize flexible regulation of three-phase grid-tied converters based on system behaviour prediction and cost function minimization. However, when the predictive current control strategy with conventional switching patterns is adopted, the predicted duration time for voltage vectors turns out to be negative in some cases, especially under the conditions of bidirectional power flows and transient situations, leading to system performance deteriorations. This paper aims to clarify the real reason for this phenomenon under bidirectional power flows, i.e.; rectifier mode and inverter mode, and, furthermore, seeks to propose effective solutions. A detailed analysis of instantaneous current variations under different conditions was conducted. An enhanced predictive current control strategy with improved switching patterns was then proposed. An experimental platform was built based on a commercial converter produced by Danfoss, and moreover, relative experiments were carried out, confirming the superiority of the proposed scheme. more...

Based on the grid voltage orientated vector control technique, a novel integral variable structure controller for current control of variable speed doubly fed induction generator (DFIG) wind turbines near cut-in wind speed is proposed. The proposed current controller can not only ensure generators' safe cut-in switches to the grid, but also maximum energy capture after switch motion without any regulations. By these, simulations of a 1.5 MW DFIG-based wind turbine near cut-in wind speed are separately conducted under integral variable structure control (IVSC) and PI control. The results show that IVSC strategy, which gives better dynamic response, less static error, smaller controller output dithering, stronger global robustness against generator parameters uncertainty and the grid voltage fluctuation, as well as needless controller regulation after cutting-in the grid, obviously preponderates over traditional PI control for DFIG-based wind turbines near cut-in speed. more...