Static and dynamic wind turbine simulator using a converter controlled dc motor

This paper describes a new wind turbine simulator for dynamic conditions. The authors have developed an experimental platform to simulate the static and dynamic characteristics of real wind energy conversion system. This system consists of a 3 kW dc motor, which drives a synchronous generator. The converter is a 3 kW single-phase half-controlled converter. MATLAB/Simulink real time control software interfaced to I/O board and a converter controlled dc motor are used instead of a real wind turbine. A MATLAB/Simulink model is developed that obtains wind profiles and, by applying real wind turbine characteristics in dynamics and rotational speed of dc motor, calculates the command shaft torque of a real wind turbine. Based on the comparison between calculated torques with command one, the shaft torque of dc motor is regulated accordingly by controlling armature current demand of a single-phase half-controlled ac–dc converter. Simulation and experimental results confirm the effectiveness of proposed wind turbine simulator in emulating and therefore evaluating various turbines under a wide variety of wind conditions.     

风力发电实验用模拟风力机

在风力发电机控制、风力机最大功率点追踪控制(MPPT，Maximum Power Point Tracking)等相关的研究中，风力机是必备的实验设备，但是，在没有风的情况下，或在实验室，就无法进行这些实验和研究。作者开发了一种模拟风力机，有了它，就可以在实验室随心所欲地进行风力发电的初期实验研究工作，从而缩短研发的周期和减小实验研究的费用。首先用6次多项式来拟合风力机的转矩特性曲线。然后根据当前的风速和风力机转速来计算风力机的转矩，将此转矩作为转矩指令控制感应电机来模拟风力机，感应电机通过控制逆变器来驱动。最后，给出了用此模拟风力机所做的MPPT实验研究结果，验证了该模拟风力机的良好特性。     

Control of a switched reluctance generator for variable-speed wind energy applications

This paper presents a novel control system for the operation of a switched reluctance generator (SRG) driven by a variable speed wind turbine. The SRG is controlled to drive a wind energy conversion system (WECS) to the point of maximum aerodynamic efficiency using closed loop control of the power output. In the medium and low speed range, the SRG phase current is regulated using pulsewidth-modulation (PWM) control of the magnetizing voltage. For high speeds the generator is controlled using a single pulse mode. In order to interface the SRG to the grid (or ac load) a voltage-source PWM inverter is used. A 2.5-kW experimental prototype has been constructed. Wind turbine characteristics are emulated using a cage induction machine drive. The performance of the system has been tested over the whole speed range using wind profiles and power impacts. Experimental results are presented confirming the system performance.     

Development of a 20 kW wind turbine simulator with similarities to a 3 MW wind turbine

As the use of wind power has steadily increased, the importance of a condition monitoring and fault diagnosis system is being emphasized to maximize the availability and reliability of wind turbines. To develop novel algorithms for fault detection and lifespan estimation, a wind turbine simulator is indispensible for verification of the proposed algorithms before introducing them into a health monitoring and integrity diagnosis system. In this paper, a new type of simulator is proposed to develop and verify advanced diagnosis algorithms. The simulator adopts a torque control method for a motor and inverter to realize variable speed-variable pitch control strategies. Unlike conventional motor–generator configurations, the simulator includes several kinds of components and a variety of sensors. Specifically, it has similarity to a 3 MW wind turbine, thereby being able to acquire a state of operation that closely resembles that of the actual 3 MW wind turbine operated at various wind conditions. This paper presents the design method for the simulator and its control logic. The experimental comparison between the behavior of the simulator and that of a wind turbine shows that the proposed control logic performs successfully and the dynamic behaviors of the simulator have similar trends as those of the wind turbine.     

Modeling of line side harmonic currents produced by variable speedinduction motor drives

An analysis method to model and calculate the line side harmonic currents produced by variable speed induction motor drives is proposed. An accurate analytical investigation of the line currents in the presence of a power conversion system with a load represented by a PWM inverter feeding an induction motor is carried out. With the obtained mathematical results it is possible to reproduce the distorted current waveforms injected by the whole drive system in the grid. Finally the simulation results are compared with the real harmonic distortion generated by a variable speed drive     

Regulation of WTG dynamic response to parameter variations of analytic wind stochasticity

Although variable‐speed operation can reduce the impact of transient wind gusts and subsequent component fatigue, this is still an unknown factor that must now be quantified. Reduction in drive‐train stresses caused by fatigue loads in high wind turbulence is fundamental to realizing both output power leveling and long service life for a wind turbine generator (WTG). This paper presents an evolutionary controller comprising a linear quadratic Gaussian (LQG) and neurocontroller acting in tandem to effect optimal performance under high turbulence intensities, for a variable‐speed, fixed‐pitch WTG. The control objectives are maximum energy conversion and reduction in mechanical stresses on the system components. The proposed paradigm utilizes generator torque in controlling the rotor speed in relation to the highly turbulent wind speed, thereby ensuring the extracted aerodynamic power is maintained at a constant value, while shaft moments are regulated. The performance of the proposed controller is compared with that of the LQG and it is found that the former is more efficient in maintaining rated power, minimizing shaft torque variations, and shows robustness to parameter variations. Copyright © 2007 John Wiley & Sons, Ltd.     

Drive train dynamics assessment and speed controller design in variable speed wind turbines

This paper deals with the speed controller design in DFIG based wind turbines, and investigates stability and performance of the drive train dynamics against different control strategies. It is shown that speed controller design based on the single mass drive train model may result in unstable mechanical modes, because it ignores the dynamics of the flexible shaft. Then, another control approach, known as feedforward compensation of the shaft torsional torque, is examined. It is shown that this control method results in poorly damped oscillations of torsional torque and turbine speed during the transient conditions. The open loop transfer function from the electromagnetic torque to the generator speed contains a dual quadratic function representing the dynamics of flexible shaft. The dual quadratic function comprises resonant and anti-resonant frequencies that greatly affect the stability of the drive train dynamics. Next, a step-by-step procedure for designing the speed controller based on the two-mass drive train model is proposed. The proposed speed controller provides stable closed loop system, zero tracking error, low-frequency disturbance rejection, and open-loop gain attenuation at the resonant frequency. At the end, performance of the proposed controller is investigated by the time domain simulations.     

Frequency support from doubly fed induction generator wind turbines

An assessment on the capability of a doubly fed induction generator (DFIG) wind turbine for frequency regulation is presented. Detailed aerodynamic, structural and electrical dynamic models were used in this study. A control loop acting on the frequency deviation was added to the inertia contributing loop in order to enhance the inertia support from the DFIG wind turbine. The possibility of de-loading a wind turbine to provide primary and secondary frequency response was discussed. A frequency droop controller was examined where the droop is operating on the electronic torque set point below its maximum speed and is operating on the pitch demand at maximum speed. It is also shown that by reducing the generator torque set point the DFIG wind turbine can provide high frequency response     

A new approach to direct torque control of induction motor drivesfor constant inverter switching frequency and torque ripple reduction

In this paper, a new approach to the direct torque control (DTC) of induction motor drives is presented. In comparison with the conventional DTC methods, the inverter switching frequency is constant and is dramatically increased, requiring neither any increase of the sampling frequency, nor any high frequency dither signal. The well-developed space vector modulation technique is applied to inverter control in the proposed DTC-based induction motor drive system, thereby dramatically reducing the torque ripple and speed ripple. As compared to the existing DTC approach with constant inverter switching frequency, the presented new approach does not invoke any concept of deadbeat control, thereby dramatically reducing the computations. Experimental results are illustrated in this paper confirming that the proposed DTC method has the above-mentioned features even at the low speed range down to ±1 r/min     

Multivariable control strategy for variable speed, variable pitch wind turbines

Reliable and powerful control strategies are needed for wind energy conversion systems to achieve maximum performance. A new control strategy for a variable speed, variable pitch wind turbine is proposed in this paper for the above-rated power operating condition. This multivariable control strategy is realized by combining a nonlinear dynamic state feedback torque control strategy with a linear control strategy for blade pitch angle. A comparison with existing strategies, PID and LQG controllers, is performed. The proposed approach results in better power regulation. The new control strategy has been validated using an aeroelastic wind turbine simulator developed by NREL for a high turbulence wind condition.     

A high-performance induction motor drive with on-line rotortime-constant estimation

A high-performance induction motor (IM) speed drive with online adaptive rotor time-constant estimation and a proposed recursive least square (RLS) estimator is introduced in this paper. The estimation of the rotor time-constant is on the basis of the model reference adaptive system (MRAS) theory; and the rotor inertia constant, the damping constant and the disturbed load torque of the IM are estimated by the proposed RLS estimator, which is composed of an RLS estimator and a torque observer. Moreover, an integral proportional (IP) speed controller is designed online according to the estimated rotor parameters; and the observed disturbance torque is fed forward to increase the robustness of the induction motor speed drive     

A novel power splitting drive train for variable speed wind power generators

In this paper a novel electrically controlled power splitting drive train for variable speed wind turbines is presented. A variable speed wind turbine has many advantages, mainly it can increase the power yield from the wind, alleviate the load peak in the electrical-mechanical drive train, and posses a long life time, also, it can offer the possibility to store the briefly timely wind-conditioned power fluctuations in the wind rotor, in which the rotary masses are used as storages of kinetic energy, consequently, the variable speed wind turbines are utilized in the wind power industry widely. In this work, on the basis of a planetary transmission a new kind of drive train for the variable speed wind turbines is proposed. The new drive train consists of wind rotor, three-shafted planetary gear set, generator and servo motor. The wind rotor is coupled with the planet carrier of the planetary transmission, the generator is connected with the ring gear through an adjustment gear pair, and the servo motor is fixed to the sun gear. By controlling the electromagnetic torque or speed of the servo motor, the variable speed operation of the wind rotor and the constant speed operation of the generator are realized, therefore, the generator can be coupled with the grid directly. At the nominal operation point, about 80% of the rotor power flow through the generator directly and 20% through the servo motor and a small power electronics system into the grid. As a result, the disadvantages in the traditional wind turbines, e.g. high price of power electronics system, much power loss, strong reaction from the grid and large crash load in the drive train will be avoided.     

Development of a comprehensive MPPT for grid‐connected wind turbine driven PMSG

This paper proposes a comprehensive MPPT method by which extraction of maximum power from wind turbine and its subsequent transfer through various power stages and final delivery to the connected grid are realized. In the proposed system, the operation of the wind turbine at its maximum efficiency point is maintained by control of grid‐tied inverter such that the shaft speed of the generator is set to result the desired optimum tip speed ratio of the turbine. The proposed comprehensive MPPT estimates the required DC link voltage for each wind speed using a unified system model, uses a loss factor to account for the system losses, and then controls the inverter to push the WT extracted maximum power into the grid. The comprehensive MPPT is developed and is validated in MATLAB/Simulink platform in a wide range of operating wind speed. The results ascertain that the wind turbine is made to operate at its maximum efficiency point for all wind speeds below the rated one.     

A computer program to predict the performance of slip energyrecovery induction motor drives

Details are provided, in the form of a flowchart, to permit the reconstruction of a computer program to predict the transient and steady-state performance of slip energy recovery induction motor (IM) drives. Slip energy recovery IM drives are different from most other drives in that the inverter is generally connected only after the machine has reached a predetermined speed. The initial conditions of the inverter are therefore nonzero and difficult to obtain. Three techniques that can be used to calculate the initial conditions are discussed. Theoretical predictions are supported by practical results     

A method of tracking the peak power points for a variable speed wind energy conversion system

In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-/spl omega/ curve correspond to dP/d/spl omega/=0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a DC motor fed from a commercial DC drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.     

Efficiency of three wind energy generator systems

This paper presents a method to calculate the average efficiency from the turbine shaft to the grid in wind energy converters. The average efficiency of three 500 kW systems are compared. The systems are: a conventional grid-connected four-pole induction generator equipped with a gear, a variable-speed synchronous generator equipped with a gear and a frequency converter, and a directly driven variable-speed generator equipped with a frequency converter. In this paper it is shown that a variable-speed generator system can be almost as efficient as one for constant speed, although it has much higher losses at rated load. The increased turbine efficiency that variable speed leads to has not been included in this paper. It is also found that a directly driven generator can be more efficient than a conventional four-pole generator equipped with a gear     

Modeling turbine wakes and power losses within a wind farm using LES: An application to the Horns Rev offshore wind farm

A modeling framework is proposed and validated to simulate turbine wakes and associated power losses in wind farms. It combines the large-eddy simulation (LES) technique with blade element theory and a turbine-model-specific relationship between shaft torque and rotational speed. In the LES, the turbulent subgrid-scale stresses are parameterized with a tuning-free Lagrangian scale-dependent dynamic model. The turbine-induced forces and turbine-generated power are modeled using a recently developed actuator-disk model with rotation (ADM-R), which adopts blade element theory to calculate the lift and drag forces (that produce thrust, rotor shaft torque and power) based on the local simulated flow and the blade characteristics. In order to predict simultaneously the turbine angular velocity and the turbine-induced forces (and thus the power output), a new iterative dynamic procedure is developed to couple the ADM-R turbine model with a relationship between shaft torque and rotational speed. This relationship, which is unique for a given turbine model and independent of the inflow condition, is derived from simulations of a stand-alone wind turbine in conditions for which the thrust coefficient can be validated. Comparison with observed power data from the Horns Rev wind farm shows that better power predictions are obtained with the dynamic ADM-R than with the standard ADM, which assumes a uniform thrust distribution and ignores the torque effect on the turbine wakes and rotor power. The results are also compared with the power predictions obtained using two commercial wind-farm design tools (WindSim and WAsP). These models are found to underestimate the power output compared with the results from the proposed LES framework.     

Efficiency testing of motors powered from pulse-width modulatedadjustable speed drives

This is a report of efficiency testing of an induction motor powered from three different pulse width modulated adjustable speed drives. The motor was operated at a range of speeds and torques, and each drive was operated at the highest, lowest, and an in-between carrier frequency. The motor was a typical NEMA design B motor, and the drives were typical, industrial scalar drives using the default volts/Hz setting. The testing showed that the drive efficiencies remained above 90% until torque was lowered to below 20% of rated torque. Combined motor and drive efficiencies remained above 80% until speeds or horsepower loads were lowered to below 20% of rated torque. The test was performed using a unique data acquisition scheme that permitted acquisition of a large number of torque settings for each speed selection     

A microcomputer-based induction motor drive system using currentand torque control

This paper proposes an induction motor drive with current and torque control. The current control based on the current error with the current controller yields h_l signal. The torque control based on the torque error with the torque controller yields a h_l signal. According to the h_l signal, the h_l signal and the appropriate voltage vector is selected by using a look-up table to control the induction motor drive to obtain a rapid speed response. The torque controller, current controller, and d-q frame transform are constructed by the hardware which reduce the running time of the microcomputer to obtain a high performance drive. Computer simulations and experimental results demonstrate that the proposed method can obtain a high performance induction motor drive. Meanwhile, employing the advantages of the added zero voltage vector to reduce the inverter switching frequency greatly increasing the efficiency of the inverter