Micro-wind energy conversion systems

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Brushless DC Permanent Magnet Generator for Micro-Wind Turbine Applications

Introduction

Worldwide focus has been directed for the past decades towards the advancement in the clean energy technologies as a counteraction to the fossil fuels dependency and nuclear technologies. Among the alternative energy production solutions, the wind energy has been the fastest-growing source of renewable energy contributing to the modern distributed electric power generation. Therefore, researchers have been prompted to develop optimized wind energy conversion systems in terms of design and components, such as wind turbines, electric generators, power electronic converters and their controls.
Since mechanical power of wind turbines strongly depends on the rotor area and on the cube of wind speed and since only 59.3% of the total wind power can be harvested, according to Bet’z law [1], it was ascertained that high-power wind turbines can produce more electric energy. Due to this fact, an exponential increase over the last two decades has been noted in the size of commercial wind turbines, as well as in their installed capacity across the world, as shown in fig. 1.1. The wind power capacity installed worldwide is expected to reach 500 GW by the end of 2016 [2].
This development of the wind markets and nevertheless of all renewable energy markets is motivated by the environmental issues emerged over the last years, like air pollution and climate changes due to CO2 emissions, as well as for the financial incentives policy support of various national governments for promoting the use of local manufacturing and by impelling utilities to purchase renewable energy. Nowadays, the wind energy systems can strongly compete to the conventional sources of electricity (coal, nuclear, gas, etc.) as they are one of the least cost renewable energy resource for new power generation [3].
As against the large-scale high-power wind turbine manufacturing, the small wind turbine market was formerly neglected. However, for the past years, the small wind-turbine capacity installed worldwide has shown an annual growth rate of 19 – 35 %. Subsequently, the small wind turbine market is estimated with a steady growth rate of 20 % from 2015 to 2020, so that its industry is forecasted to reach a cumulative installed capacity of close to 3 GW by 2020 [4].
The International Electrotechnical Commission (IEC) standard IEC 61400-2 defines the small-scale wind turbine as having a rotor swept area of less than 200 m2, leading to a rated power up to 50 kW delivered at a voltage below 1 kV AC or 1,5 kV DC. As a sub-category of small wind turbines, the micro-wind turbine has a rated power from 1 kW to 7 kW, and rotor swept area of less than 40 m2.

Micro-wind energy conversion systems

The micro-wind energy conversion system (μWECS) generates electricity by converting the kinetic energy from the wind into electrical energy. The micro-wind turbine, which is the main mechanical component of the μWECS, captures the energy from the moving wind by means of rotor blades and transfers it to the generator.
The generator shaft is driven by the micro-wind turbine to generate electric power and the mechanical coupling between them can be achieved directly or by means of a gear device. Finally, the generated electrical energy can be stored into batteries or used to power isolated loads distributed by an electrical grid (fig. 1.2). Several additional components are needed depending on the connection method, i.e. a DC/AC converter and a step up/down transformer unit for the utility /household grid connection and/or a DC/DC power converter for the battery bank charging [5].
The components of a μWECS can be categorized according to the power that they produce or convert in two major classes, i.e. (i) mechanical, represented by the turbine assembly and (ii) electrical, defined by the electric generator and power electronic converter(s). For an efficient and reliable conversion of the wind energy into electric energy a third component category is emerged related to the control systems of both mechanical and electrical components. For improved efficiency of the μWECS different combinations and designs of the above components can be encountered, resulting into several μWECS configurations that have been developed and which are further described below.
A simple classification of the micro-wind turbines can be made in terms of their rotor-axis orientation, leading to two main structures, i.e. of horizontal axis and of vertical axis (fig. 1.3). Various comparative studies can be found in the technical literature [1], [6] revealing the merits and drawbacks of each structure, which are synthetized in Table 1.1.
However, as specified in [1], the main classification criterion of μWECS topologies is represented by the drive train configuration. A brief schematic representation based on this criterion is given in fig. 1.4, followed by a description of several μWECS topologies that have been developed over the recent years.

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Fixed speed drive train topology

The fixed-speed topology was the first one developed for the micro-wind turbine systems (fig. 1.5). It consists of a multiple-stage gearbox coupled to a cage induction generator (CIG) without any power converter unit, but equipped with a capacitor bank for reactive power compensation and connected to the grid by means of a transformer. Even if it presents easy implementation and relatively low manufacturing cost, the inability to control even the narrow speed range at which it was designed for operation, resulting into lower wind energy conversion efficiency and high mechanical stresses on the system, has turned the attention to the semi-variable and variable speed micro-wind turbine systems. [8].

Variable speed drive train topology

Under variable-speed operation the energy conversion efficiency increases and the mechanical stress and maintenance requirements reduce, leading to a higher life cycle of the micro-wind turbine. Controlling and maximizing the wind power extraction becomes possible under variable-speed operation by employing pitch regulation and maximum power point tracking (MPPT) control. The micro-wind turbine systems that fall into this category can use partial-scale or full scale power electronic converters when connected to the utility grid. Although the additional power converter losses and the higher manufacturing costs that characterize these topologies, they still remain dominant on the micro-wind turbine market.

Table of contents :

Chapter 1 Brushless DC Permanent Magnet Generator for Micro-Wind Turbine Applications
1.1 Introduction
1.2 Micro-wind energy conversion systems
1.3 Description of the micro-wind energy conversion system under study
1.4 Conclusions
Selected references
Chapter 2 Modeling and Simulation of the BLDCPM Generator-Based Micro-Wind Energy Conversion System in View of Its Optimization
2.1 Introduction
2.2 Modeling description of the μWECS components
2.2.1 Micro-wind turbine model
2.2.2 Modeling of the generator-rectifier assembly
2.2.2.1 Dimensional features of the BLDCPM generator
2.2.2.2 Analytical simulation model
2.2.2.3 Semi-analytical simulation model
2.2.2.4 Finite element-based numerical model
2.3 Simulations of the micro-wind turbine system over base point operation
2.4 Conclusions
Selected references
Appendix 2.A Sizing Model of the BLDCPM Generator
Appendix 2.B VBScript for the Pre- and Post-processing in JMAG Designer of the
Chapter 3 Long-term Wind-Speed Profile Modeling and Simulation for the Micro-Wind Energy Conversion System
3.1 Wind-speed profile
3.1.1 Methods for simplifying the long-term wind-speed profile
3.1.2 Barycenter method applied to the computation of μWECS power losses over the long-term wind-speed cycle
3.2 Simulations of μWECS operating long-term wind-speed cycle operation
3.2.1 Simulations of analytical and semi-analytical models under long-term and reduced wind-speed cycle operation
3.3 Conclusions
Selected references
Appendix 3.A Calculation of μWECS power losses based on barycenter method
Chapter 4 Design Optimization of the BLDCPM Generator over Wind-Speed Cycle Operation
4.1 Introduction
4.1.1 Sources of complexity in the optimal design of a system
4.1.2 Design optimization
4.1.3 Difficulties in design optimization
4.2 Formulation of the μWECS design optimization problem
4.2.1 Objective function
4.2.2 Design variables and constraints
4.2.3 Proposed method for design optimization problem resolution
4.3 Optimization methodology and results of the BLDCPM generator design under wind-speed cycle operation
4.3.1 Single level optimization approach
4.3.2 Two-level optimization approach
4.4 Conclusions
Selected references
Chapter 5 Experimental Study of BLDCPM Generator Prototype
5.1 Experimental characterization of the BLDCPM generator prototype
5.1.1 Experimental study of BLDCPM generator no-load operation
5.1.2 Description of the laboratory experimental set-up
5.1.3 Experimental results
5.2 Conclusions
Selected references
Appendix 5.A Datasheet Semikron Semiteach –IGBT
Appendix 5.B Datasheet position sensor BaumerIVO GI321
General Conclusions and Perspectives

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