Abstract: New energy sources are increasingly being used as a supplement and replacement for traditional large-scale central power plants. This paper describes the current status and future development trends of wind power generation, and also discusses the various applications of power electronics technology in wind power generation and its future prospects.
Keywords: power electronics, wind power generation, turbine generator, energy storage system
I. Introduction
Current societal needs have moved beyond simply solving problems with power electronics; they now demand the system integration of electrical energy processing. A more comprehensive, multidisciplinary solution is urgently needed. We will see a surge in energy storage systems. Furthermore, the increasing number of renewable energy sources and distributed generators requires new grid operation and management control strategies to ensure and even improve the quality and reliability of power supply. The widespread penetration of power electronics into energy systems will occur within the next 25 to 30 years, but will not have a significant impact on current major transmission networks. Advances in power electronics are primarily focused on applications in distributed generation and various loads. Power electronics plays a crucial role in distributed generation and the integration of renewable energy into the grid, and as these applications become increasingly integrated with grid-based systems, power electronics is being widely adopted and rapidly expanding.
II. Current Status of Wind Power Generation Technology
Wind power generation is becoming a widely accepted power generation technology. With the continuous development of wind power technology worldwide, wind power generation exhibits the following main characteristics: Installed capacity is continuously expanding, and the proportion of wind power generation in the world's total electricity generation has increased year by year, from less than 1‰ of the total to 5‰ in 2004; Generator unit capacity is continuously expanding. As an effective way to improve wind energy utilization and power generation efficiency, the mainstream generator unit capacity has developed from 500-750 kW before 1997 to the current batch installation of 3.6 MW units; Offshore wind farms are gradually becoming commercialized. Offshore wind farms have the characteristics of high wind speed, stable wind force, less interference, and large power generation, which can effectively utilize the generating capacity of wind turbine units; Wind power generation costs are continuously decreasing. While the construction investment cost of wind power is relatively high, the operating cost is very low.
III. Application of Power Electronics in Wind Power Generation Technology
The global wind turbine market has grown at an average annual rate of over 30% in the past five years, and wind energy is playing an increasingly important role in power generation. The design technologies used in existing wind turbines vary significantly, primarily in the integration of the wind turbine and the power generation system. A wind power generation technology where the rotational speed depends on wind speed has been adopted to maximize the capture of wind-generated energy. Variable-speed wind turbine technology can capture 5% more energy annually than fixed-speed technology, and both reactive and active power are easier to control, allowing for grid voltage control as the reactive power output is variable. The disadvantage of variable-speed turbines is the need for power conversion devices, increasing both the number of components and the complexity of control. The total cost of applying power electronics technology accounts for 7% of the entire wind turbine's cost. Due to the introduction of high-power semiconductor switching devices and advanced computer real-time control technologies with sophisticated algorithms, power electronics technology has undergone significant development and changes. These factors combined have led to the emergence of low-loss converters with good grid compatibility. This has also resulted in significant development of variable-speed wind turbines in recent years.
1) Speed-changing technology using a dual feedback induction generator (DFIG): The schematic diagram of this forced-switching power converter is shown in Figure 1. The converter consists of two three-phase AC-DC power converters connected by a DC capacitor battery. This structure ensures vector control of the machine's active and reactive power while reducing the harmonics injected into the grid by the power converter.
2) Variable speed technology using power converters throughout: The generator is completely decoupled from the grid. The generator's energy is rectified to a DC link and then converted into AC energy acceptable to the grid. Most of these wind turbine generators use multi-pole synchronous generators, although they may (but less often) use induction generators and gearboxes. Eliminating gearboxes has many advantages: reduced losses, lower costs due to the elimination of such expensive and heavy components, and increased reliability due to fewer rotating mechanical parts.
Figure 2 shows the schematic diagram of this full-power converter suitable for wind turbine generators. On the turbine side, a three-phase converter using a vector control strategy acts as the driver to control the torque generator. Two side-mounted three-phase converters allow the electrical energy converted from wind power to enter the grid and can control the reactive and active power entering the grid. It also aims to keep the total harmonic distortion (THD) as low as possible to improve the power quality delivered to the public grid. The purpose of the DC link is for energy storage; the energy captured by the wind is stored as charge in a capacitor and can then be injected into the grid immediately. The control signal maintains a fixed reference value for the DC link voltage Vdc.
3) Semiconductor device technology:
To improve the performance and reliability of power electronic variable frequency drives in wind turbine generators, power semiconductor devices with better electrical characteristics and lower costs are needed, as device performance determines the size, weight, and cost of the entire power electronics section used as the interface for wind turbine generators.
IV. Trends in Wind Power Generation Technology
1) Offshore Wind Power: The main future development trend of wind turbine technology is offshore installation. There are abundant wind energy resources at sea, allowing for the installation of wind turbines in many relatively shallow areas. Offshore turbines typically generate 50% more energy than onshore turbines installed nearby. This is because air resistance is lower at sea level. On the other hand, the platform structure and installation of offshore systems require more than 50% more energy than onshore systems. However, it should be noted that offshore turbines have a lifespan approximately 25-30 years longer than onshore turbines. This is because the lower disturbance levels at sea result in less fatigue load on the wind turbines.
Traditional heat flow air conditioning (HVAC) transmission systems are a simple and inexpensive solution for connecting wind farms to the grid. High-voltage direct current (HVAC) grid connection technology connects wind farm turbines to the grid and safely and efficiently delivers electricity to load centers. For offshore wind farms, DHVC transmission systems offer many advantages over HAC transmission systems.
1) The frequencies of the transmitting and receiving ends are independent.
2) The distance of DC power transmission is not affected by the cable load current.
3) Offshore installation is isolated from mainland disturbances.
4) The power flow is completely deterministic and controllable.
5) Low cable power loss.
6) Each cable has a high power transmission capacity.
HVAC transmission systems based on voltage source converters (VSCs) are attracting increasing attention, not only for large offshore wind farms connected to the grid. ABB has now marketed VSC-based solutions under the name "HVAC light," while Simens has named them "HVAC Plus." Figure 3 shows a schematic diagram of a VSC-based HVDC transmission system. This relatively new technology (commercially installed and operated in 1999) was only possible with the development of IGBT devices capable of self-turn-off current. This means that an active commutation voltage is no longer required. Therefore, VSC-based HVDC transmission systems no longer require strong offshore and onshore AC grids and can even start in completely paralyzed grids (dark start capability). However, this system also has other advantages: reactive and active power can be controlled independently, reducing the need for reactive power compensation and improving the stability of the AC grid at their connection points.
2) High-power medium-voltage converter topology: In order to reduce the cost per watt and improve the conversion efficiency of wind energy, the nominal power of wind turbine generators has been increasing in recent years.
The proposed different multilevel converter topologies can be divided into the following five categories:
1) Multilevel structure with diode clamping.
2) Multilevel structure with bidirectional switch interface.
3) Utilize the multi-level structure of the flying capacitor.
4) Multilevel structure with multi-element three-phase inverter.
5) Multilevel structure with cascaded single-phase H-bridge inverter.
As the rated power of devices increases and switching and conduction performance improves, the advantages of using multilevel converters become increasingly apparent. Recent papers have focused particularly on reducing harmonic content and electromagnetic interference (EMI) in the output and input voltages. More importantly, multilevel circuits require minimal input filters, or in other words, reduce the number of switching operations. Compared to two-level converters with the same harmonic levels, multilevel converters can reduce the switching frequency by 25%, leading to lower switching losses. Although conduction losses are higher in multilevel converters, the overall system efficiency depends on the ratio of switching losses to conduction losses.
The trend in the wind turbine generator market is to increase its nominal power (several megawatts) based on voltage and current ratings. This makes multilevel converters a perfect fit for the applications of these modern, high-power wind turbine generators. The increased voltage ratings allow the wind turbine generator's converter to be directly connected to the wind farm's distribution network, avoiding the use of bulky transformers (see Figure 4).
3) Energy storage technology for future wind farms: Energy storage technology has the potential to improve the technical and economic attractiveness of wind power, especially when it exceeds 10% of the total system energy (approximately 20%-25% of the system capacity). An energy storage system in a wind farm will be used for massive energy storage and to absorb or inject energy in shorter periods of time to maintain grid frequency stability during the average 15-minute wind island effect time.
Several energy storage technologies are applied in wind farms. Utilizing batteries as an energy storage system to exchange energy with the grid is well-known. Batteries used in renewable energy systems include lead-acid batteries, lithium-ion batteries, and nickel-cadmium batteries. Batteries have fast charge and discharge response speeds, but their discharge rate is limited by the chemical reaction and the type of battery. In power systems, batteries function as a voltage source. A new trend in the application of batteries in renewable energy systems is their integration with multiple energy sources (such as wind power, photovoltaic power generation systems, etc.) and with other energy storage systems that complement them. Simultaneously, many researchers are working to optimize battery cells to reduce maintenance costs and increase lifespan. For wind power applications, liquid (zinc-bromine) battery systems offer the lowest unit energy storage and transmission costs. Zinc-bromine batteries are completely different from traditional batteries, such as lead-acid batteries, in terms of concept and design. This type of battery is based on the chemical reaction of two common chemical materials: zinc and bromine. Compared to current lead-acid batteries, zinc-bromine batteries offer two to three times the energy density (75–85 watts per kilogram-hour) while saving space and weight. The power characteristics of this battery can be improved for different applications. In particular, zinc-bromine batteries do not deteriorate after repeated charge-discharge cycles. They have an excellent future in renewable energy applications.
With the development of energy storage technology, new energy storage methods such as flywheel energy storage, superconducting energy storage, supercapacitors and compressed air have been applied and developed accordingly. However, due to the characteristics of wind power generation and various energy storage technologies, the integration of these energy storage methods with wind power is still in the expansion stage.
V. Summary
In renewable energy grid integration, new power electronics technologies play a crucial role. Developing power electronic interface devices for planned turbine generators with the highest rated power should be feasible, thereby optimizing energy conversion, transmission, and reactive power control, reducing harmonic distortion, achieving low cost and high efficiency over a wide power range, and possessing high reliability and fault tolerance to subsystem component failures.
With the worsening global energy shortage, wind power has received increasing attention, and more large-scale wind farms are being connected to the power grid. Power electronics technology plays a crucial role in the grid connection and normal operation of wind power. The grid connection of large-scale wind farms will gradually reduce the cost of wind power generation, making it more widespread and enabling it to play a greater role in economic and social development.
For details, please click: Application and Prospect of Power Electronic Systems in Wind Power Grids
