CHAPTER 1
INTRODUCTION

In this chapter, an overview of wind power generation and the evolution of wind power systems are briefly introduced, and the challenges and trends in wind power generation are discussed.

1.1 GLOBAL WIND POWER DEVELOPMENT

1.1.1 Global Environment Challenge and Energy Crisis

Nowadays, the human society consumes a huge amount of electricity every year. It is reported by the U.S. Energy Information Administration (EIA) that the global net electricity consumption has grown from 10,395 TWh in 1990 to 20,567 TWh in 2015 [1]. Since most of the electricity is generated from fossil fuels, the increase of the electricity net consumption will lead to large greenhouse gas emissions, and this may cause global warming. The Earth's average surface temperature has risen about 0.74°C for the period 1906-2005, which may cause the sea level rise, widespread melting of snow and ice, or some extreme weather challenges. Furthermore, burning of fossil fuels will produce dust and other chemical materials harmful to humans.

On the other hand, the fossil fuel reserves are limited and unsustainable. Oil will be exhausted in a few decades, followed by natural gas, and coal will also be used up in 200-300 years. The energy crisis brought by the exhaustion of fossil fuels is a long-range challenge for human beings. Many efforts have been made worldwide to try to find an alternative energy.

1.1.2 Renewable Energy Development

Renewable energy is defined as the energy that comes from resources that are naturally replenished on a human timescale such as sunlight, wind, rain, tides, waves and geothermal heat. Typically, the renewable energy includes wind power, photovoltaic (PV) power, hydropower, biomass power, and ocean power. As renewable energy is reproducible and has a low footprint of CO2, it is regarded as a favorable solution to both the global environment challenge and energy crisis. Rapid deployment of renewable energy has been reported in recent years. Global renewable energy policy multistakeholder network REN21 estimated that by the end of 2016, 30% power generation capacity will come from renewable energy and renewable energy will account for about 24.5% of global electricity generation [2]. Nowadays, the biggest renewable energy generation is from hydropower. However, since the location requirement of the hydropower is limited to lakes or rivers, the worldwide growth of hydropower has become slower in the recent years, which indicates that hydropower is very close to its capacity limit.

The non-hydropower renewable generation, including wind, PV, and biomass, has been growing very fast in the last 10 years. The non-hydropower renewable generation capacity reached 921 GW by the end of 2016, compared to 85 GW in 2004 [2]. The worldwide capacity share of different non-hydro renewable powers by the end of 2016 can be found in Figure 1.1. It is found that wind power has the largest capacity share among the non-hydropower renewable generations. Wind power has reached 56.8% of the non-hydro renewable power capacity.

Figure 1.1 Worldwide capacity share of different non-hydro renewable powers by the end of 2016 [2].

1.1.3 Wind Energy Development

The wind power generation is regarded as the most widely used non-hydro renewable energy generation. It has a high reserve and is renewable and clean. Besides it produces almost no greenhouse gas emissions. Now at least 83 countries around the world are using wind power to supply their electricity grids [3]. The capacity of wind power installation has grown rapidly for the past 15 years. The statistics show the worldwide total wind power capacity has grown from 24 GW in 2001, to about 487 GW in 2016 [3], as shown in Figure 1.2. China leads the accumulated wind power installation, followed by the United States, Germany, Spain, Indian, etc., as shown in Figure 1.3.

Figure 1.2 Worldwide total wind power capacity from 2001 to 2016 [3].

Figure 1.3 Accumulated wind power installation versus countries, end of 2015 [3].

At the same time, the wind power share in the mix of the power supply also increased in the world, especially in some European countries. In 2014, Denmark set a new world record by reaching a wind power share of 39% in the domestic power supply [4]. Spain has wind power share of more than 15% [5]. Worldwide, the wind energy production has reached around 4% of total worldwide electricity usage in 2014 [6].

1.2 EVOLUTION OF WIND POWER SYSTEM

With the increasing penetration of wind power into the grid, the technology of the wind power generation has undergone a rapid development. One of the typical features is the changing of the wind power system structures. Modern wind power systems are more efficient, more reliable and more intelligent than before.

1.2.1 Basic Structure of a Wind Turbine

The mostly used wind turbine is the horizontal wind turbine as shown in Figure 1.4. The blade, the shaft and the nacelle of the wind turbine are installed on a high tower. The blade rotates under wind flow and the wind energy is captured and converted into the mechanical energy in the shaft. The rotating angular speed of the shaft is increased using the gearbox so that it is compatible with the generator. The mechanical energy originated from the wind is converted into electric energy by the generator. Then the electricity is transmitted to the power electronic converter on the ground via the power cable, which is connected to the transformer in the grid. The nacelle provides space for components such as the shaft, the gearbox, and the brake on the tower, and can also target the turbine toward the wind flow direction by the action of the yaw.

Figure 1.4 Structure of a wind turbine system.

1.2.2 Power Flow in the Wind Turbine System

The function of the wind power generation system is to harvest the kinetic energy of the wind flow, convert it into the electrical energy and finally feed into the grid. The configuration of the wind turbine system (WTS), which is composed of the wind turbine, the gearbox, the generator, the power converter, as well as the transformer, can be simplified as shown in Figure 1.5.

Figure 1.5 Basic configuration of wind power generation system.

Wind Turbine: The kinetic energy in the wind is collected by the wind turbine, and it is converted into mechanical energy on the shaft of the wind turbine. The early wind turbines normally rotate at an almost fixed speed, while the modern wind turbines can adjust the rotation speed with the variations in wind speed in order to increase the wind energy harvesting efficiency [34].

Gearbox: The gearbox is used to adjust the rotating speed of the shaft and make it compatible with the generator. In some cases, for example, in directly driven wind power system with multiple-pole synchronous generators, the gearbox may not be used.

Generator: The generator converts the mechanical energy on the shaft into electrical energy. In different types of WTS, the generator can be caged generator (CG), doubly fed induction generator (DFIG), or permanent magnet synchronous generator (PMSG).

Power Converter: The power converter works as an interface between the generator and the power grid. It converts the original electrical energy from the generator, which may be unstable with respect to amplitude or frequency, into the relatively stable electrical energy, which is more accepted by the power grid. On the other hand, the power converter also controls the generator to cooperate with the wind turbine to achieve better energy harvesting efficiency.

Transformer: The transformer is used to step up the output of the power converter (normally around 690 V) to a higher voltage, and transfers the wind power to the distribution or transmission power lines.

1.2.3 Fixed-Speed Wind Turbine System

The fixed-speed WTSs emerged in the 1970s and were widely used during the 1980s and 1990s. The shaft of the wind turbine is operated at a fixed angular speed, independent of the wind speed. The scheme of the fixed-speed WTS is shown in Figure 1.6. The generator operates with a fixed rotor speed corresponding to the grid frequency. It is directly connected to the grid by a transformer.

Figure 1.6 Scheme of a fixed-speed wind turbine system.

The advantage of the fixed-speed WTS is its simplicity of structure. It has a drawback that it cannot realize maximum wind energy tracking according to the variations in the wind speed. Reactive power consumed by the generator needs to be compensated by the capacitor bank. Further, it has no grid fault support capability, which is now needed by the grid operator. It also has higher mechanical stress for the wind turbine.

1.2.4 Variable-Speed Wind Turbine System

The variable-speed WTS is widely used nowadays. Different from the fixed-speed WTS, the variable-speed WTS is able to adjust the rotor speed when the wind speed changes to realize the maximum wind energy harvesting.

The scheme of a variable-speed wind turbine is shown in Figure 1.7. The wind power is captured by a pitch-controlled wind turbine and sent to the generator. The generator is connected to...