With rapid development of wind power technologies and significant growth of wind power capacity installed worldwide, various wind turbine concepts have been developed. The wind energy conversion system is demanded to be more cost-competitive, so that comparisons of different wind generator systems are necessary. An overview of different wind generator systems and their comparisons are presented. First, the contemporary wind turbines are classified with respect to both their control features and drive train types, and their strengths and weaknesses are described. The promising permanent magnet generator types are also investigated. Then, the quantitative comparison and market penetration of different wind generator systems are presented. Finally, the developing trends of wind generator systems and appropriate comparison criteria are discussed. It is shown that variable speed concepts with power electronics will continue to dominate and be very promising technologies for large wind farms. The future success of different wind turbine concepts may strongly depend on their ability of complying with both market expectations and the requirements of grid utility companies.

Overview of different wind generator systems and their comparisons

Permanent magnet AC (PMAC) motor drives are finding expanded use in high-performance applications where torque smoothness is essential. This paper reviews a wide range of motor- and controller-based design techniques that have been described in the literature for minimizing the generation of cogging and ripple torques in both sinusoidal and trapezoidal PMAC motor drives. Sinusoidal PMAC drives generally show the greatest potential for pulsating torque minimization using well-known motor design techniques such as skewing and fractional slot pitch windings. In contrast, trapezoidal PMAC drives pose more difficult trade-offs in both the motor and controller design which may require compromises in drive simplicity: and cost to improve torque smoothness. Controller-based techniques for minimizing pulsating torque typically involve the use of active cancellation algorithms which depend on either accurate tuning or adaptive control schemes for effectiveness. In the end, successful suppression of pulsating torque ultimately relies on an orchestrated systems approach to all aspects of the PMAC machine and controller design which often requires a carefully selected combination of minimization techniques.

Pulsating torque minimization techniques for permanent magnet AC motor drives-a review

This paper reviews the trends in wind turbine generator systems. After discussing some important requirements and basic relations, it describes the currently used systems: the constant speed system with squirrel-cage induction generator, and the three variable speed systems with doubly fed induction generator (DFIG), with gearbox and fully rated converter, and direct drive (DD). Then, possible future generator systems are reviewed. Hydraulic transmissions are significantly lighter than gearboxes and enable continuously variable transmission, but their efficiency is lower. A brushless DFIG is a medium speed generator without brushes and with improved low-voltage ride-through characteristics compared with the DFIG. Magnetic pseudo DDs are smaller and lighter than DD generators, but need a sufficiently low and stable magnet price to be successful. In addition, superconducting generators can be smaller and lighter than normal DD generators, but both cost and reliability need experimental demonstration. In power electronics, there is a trend toward reliable modular multilevel topologies.

Trends in Wind Turbine Generator Systems

An original analytical study is developed concerning the torque undulation due to phase commutation on brushless DC motors. The results indicate that the relative ripple is independent of current, varies with speed, and may reach 50% of the average torque. The amplitude of the torque undulation and the duration of the commutation are analyzed, showing how this affects the torque-speed curve of the drive. Simulation and experimental results are presented that validate the theoretical analysis. >

Analysis of torque ripple due to phase commutation in brushless DC machines

In this paper, the authors investigate the iron loss of interior permanent magnet motors driven by pulsewidth modulation (PWM) inverters from both results of the experiments and the finite-element analysis. In the analysis, the iron loss of the motor is decomposed into several components due to their origins, for instance, the fundamental field, carrier of the PWM inverter, slot ripples, and harmonic magnetomotive forces of the permanent magnet in order to clarify the main loss factors. The Fourier transformation and the finite-element method considering the carrier harmonics are applied to this calculation. The calculated iron loss is compared with the measurement at each driving condition. The measured and the calculated results agree well. It is clarified that the iron loss caused by the carrier of the PWM inverter is the largest component at low-speed condition under the maximum torque control, whereas the loss caused by the harmonic magnetomotive forces of the permanent magnet remarkably increase at high-speed condition under the flux-weakening control

Iron loss analysis of interior permanent-magnet synchronous motors-variation of main loss factors due to driving condition

The authors study the torque ripple attenuation of permanent magnet synchronous motors caused by EMF harmonics and cogging torque. First, the magnet arc influence upon these variables is analyzed. Next, the impact on the harmonic contents caused by attenuation methods such as stator slot skewing, permanent magnet shifting and combinations of the two are presented. The last method makes it possible, using results of the simplest case (nonskewed slots and aligned magnets), to obtain the waveform of the EMF and cogging torque in more complex cases. The best combinations of the magnet arc and harmonic attenuation methods for minimizing electric torque harmonics and cogging torque are suggested. An analysis of cogging torque sensitivity to errors in the slot skewing angle is discussed. >

Torque ripple attenuation in permanent magnet synchronous motors

This paper presents the multiphysics design optimization of a permanent magnet synchronous generator (PMSG). A medium-power wind turbine (55 kW) is taken as the basis for the design optimization, whereas reducing its cost is the optimization goal. The design comprises the multiphysics generator model and the cost and losses models of the grid connection static power converters (SPC). The power converter command strategy is analyzed as it influences the system cost. Papers that dealt with the optimal design of the PMSGs usually did not consider the SPC cost and losses in the total system cost. The analysis of three power converter command strategies, implemented in the multidisciplinary design optimization, aims to define the phase angle of the generator current that leads to the lowest cost of the system composed by the generator and power converter.

Multiphysics Design Optimization of a Permanent Magnet Synchronous Generator

A methodology allowing one to include iron losses in voltage fed time stepping finite elements simulations is presented in this paper. Hysteresis, eddy currents and anomalous losses models are introduced in the 2D field formulation based on the magnetic vector potential. The influence of the losses on the behavior of an Epstein frame is given. Comparison between simulated and measured results shows the validity of the proposed methodology.

A new approach for iron losses calculation in voltage fed time stepping finite elements

A method allowing coupling of static converters with electromagnetic devices represented by finite elements analysis is presented Special attention is paid to the fact that the state-variable equations of the static converter are calculated automatically These equations are then solved simultaneously with those of the electromagnetic structure, step by step with respect to time

A general method for coupling static converters with electromagnetic structures

This work presents a new methodology for determination of hysteresis curves based on the Jiles–Atherton method. The magnetic induction is adopted as an independent variable which is available in the vector potential magnetic field formulation. The presented method can be directly incorporated in a finite element software.

Renato Carlson

Papers

Torque ripple attenuation in permanent magnet synchronous motors

Multiphysics Design Optimization of a Permanent Magnet Synchronous Generator

A new approach for iron losses calculation in voltage fed time stepping finite elements

A general method for coupling static converters with electromagnetic structures

A modified Jiles method for hysteresis computation including minor loops