1 Introduction
In recent years, with the increasing prominence of energy crises and environmental problems, countries around the world have been vigorously developing renewable energy industries such as wind power. Related technologies have developed rapidly, from stall-type to variable-speed constant-frequency wind power systems, and from geared to direct-drive wind power systems. my country's installed wind power capacity has also grown rapidly in recent years. To improve wind energy utilization efficiency and reduce wind power costs, the trend towards larger single-unit capacity wind turbines is a major trend in wind power technology development. The adoption of variable-speed, variable-pitch adjustment technology has become an important characteristic of large wind turbines of MW and above. In current variable-speed constant-frequency wind power systems, doubly-fed induction generators (DFIG) have the largest market share, while direct-drive systems using permanent-magnet synchronous generators (PMSG) are developing rapidly. With the continuous increase in installed wind power capacity, its impact on the power grid can no longer be ignored. Many countries have formulated new wind power grid connection rules, specifying functions such as low-voltage ride-through and reactive power support. my country will also introduce similar rules [1-3].
This paper starts with three typical wind power systems: stall-prone wind power systems, doubly-fed and permanent magnet direct-drive variable-speed constant-frequency wind power systems. Based on gearbox structure and generator type, the current wind power system structure is discussed, and the wind turbines used are discussed and analyzed. The wind power converter, which serves as the interface between wind power generation and the grid, is explained; with the increase in the single-unit capacity of wind turbines, high-power multilevel converters will be more widely used. Low-voltage ride-through and reactive power support in wind power systems are analyzed. Regarding the generators, converters, and low-voltage ride-through capabilities of wind power systems, relevant products and technologies from different wind power companies are introduced.
2. Several Typical Wind Power Generation Systems
Wind power generation systems can be classified into stall-type and variable-speed constant-frequency type based on generator speed. Variable-speed constant-frequency type can be further divided into doubly-fed and direct-drive type. Based on the transmission chain composition, they can be divided into geared type and direct-drive type. Geared type can be further divided into multi-stage gear + high-speed generator type and single-stage gear + low-speed generator type. Different classification methods can be used from different perspectives. This article discusses several typical wind power generation systems [4-5].
Figure 1 shows a typical stall-type wind power system, including a multi-stage geared system and a squirrel-cage induction generator (SCIG). The SCIG is directly connected to the grid via a transformer. Since the SCIG needs to absorb reactive power from the grid, capacitors are typically connected in parallel on the stator side for reactive power compensation to improve the power factor of the SCIG wind power system. Because the SCIG can only operate within a very narrow speed range above synchronous speed, this system is also called a constant-speed wind power system. Power regulation is achieved using constant-pitch stall, active stall, and pitch control. A soft starter composed of bidirectional thyristors enables smooth grid connection. Dual-speed generators can also be used to optimize its operation; a low-speed, small-capacity generator is used when wind speed is low, and a high-speed, large-capacity generator is used when wind speed is high. Companies such as Vestas, Bonus, Made, and Nordex offer stall-type wind power system products based on dual-speed generators. The SCIG stall-type wind power system has a simple structure, high reliability, and low cost, making it suitable for mass production. However, wind speed fluctuations directly translate into changes in electromagnetic torque, causing mechanical stress on the system; wind energy cannot be effectively utilized, resulting in low efficiency; and it cannot provide reactive power support to the power grid.
Figure 2 shows a typical doubly-fed variable-speed constant-frequency wind power generation system, including a multi-stage gear system, a doubly-fed induction generator, and back-to-back dual PWM converters. The DFIG stator side is directly connected to the grid, while the rotor side is connected to the grid through back-to-back dual PWM converters. The rotor-side converter adjusts the rotor frequency and speed to achieve variable-speed constant-frequency operation and controls the output power factor, while the grid-side converter maintains stable DC-side voltage. DFIG variable-speed constant-frequency wind power systems can operate over a wide speed range, typically within ±30% of synchronous speed. The capacity of the back-to-back converters accounts for only about 25-30% of the generator capacity. Companies such as Vestas, Gamesa, Repower, and Nordex offer DFIG wind power system products with maximum power ratings exceeding 5 MW. DFIG wind power systems can operate in both supersynchronous and subsynchronous modes, with a wide speed range. They can achieve maximum wind energy capture, optimize power output, improve wind energy utilization efficiency, reduce operating noise and mechanical stress on the drivetrain, absorb gust energy, reduce torque pulsation and output power fluctuations, and control both active and reactive power output to improve power factor and power quality. They can also quickly provide reactive power support during grid faults, helping to stabilize grid voltage. However, the multi-stage gearbox remains a major point of failure in DFIG variable-speed constant-frequency wind power systems, exhibiting issues such as friction loss, heat generation, and noise, requiring regular maintenance. The brushes and slip rings on the rotor reduce system reliability. Grid faults such as voltage dips have a significant impact on DFIG, generating large overcurrents on the rotor side, potentially damaging the converter. Therefore, the control strategies for achieving low-voltage ride-through and dynamic reactive power support are relatively complex.
Figure 3 shows a typical permanent magnet direct-drive variable speed constant frequency wind power generation system, including a permanent magnet synchronous generator and a full-power back-to-back dual PWM converter, without a gearbox. The PMSG is directly connected to the grid through the full-power converter, typically with a large number of pole pairs, low speed, high torque, large radial dimension, and small axial dimension, forming a ring shape. Eliminating the gearbox simplifies the transmission chain, improves system efficiency, reduces mechanical noise, decreases maintenance, and increases the unit's lifespan and operational reliability. The generator is isolated from the grid through the converter, thus exhibiting stronger ability to cope with grid faults and easier low-voltage ride-through capability compared to DFIG wind power systems. However, the cost of permanent magnet materials remains high; the converter has a large capacity and significant losses, further increasing its cost. Theoretically, permanent magnets are at risk of demagnetization at high temperatures, but in recent years, with the continuous improvement of permanent magnet material performance and the decrease in price, the PMSG + full-power converter has become a very attractive and promising solution. Currently, companies such as Zephyros, Mitsubishi, and Xinjiang Goldwind offer such products on the market.
The above discussion covers three typical wind power systems that currently hold a significant market share. Figure 4 shows the share of different wind turbine types in the world's annual installed capacity (1995-2004) [5]. The market share of SCIG wind power systems has been declining, from 75% in 1995 to 25% in 2004; the market share of DFIG wind power systems increased from almost zero in 1995 to 55% in 2004, replacing stall-type wind power systems as the dominant type in the wind power market; PMSG & EEG wind power systems (EESG will be discussed later) have grown slowly over the past 10 years, accounting for nearly 20% of the market share in 2004, and their growth momentum is currently faster. However, there are not only these three types of wind turbines on the market. Depending on the generator and gearbox, there are many other types of wind power systems that also occupy a certain market share; the following section will introduce other wind power systems starting from the three types of wind turbines introduced in this section.
With the rapid development of wind power technology, the single-unit capacity of wind turbines will continue to increase, thereby saving installation space and costs, especially for offshore wind power, due to higher wind speeds and larger spaces. Large-scale wind farms will become increasingly common, and the proportion of wind power in the power system will gradually increase. Power system operators will place higher demands on wind power grid connection. Technologies related to the coordinated control of large-scale wind farms, low-voltage ride-through and reactive power support capabilities, and the application of high-voltage direct current (HVDC) transmission in wind power generation will become topics of common concern for equipment manufacturers and system operators.
3. Types of generators in wind power systems
For the stall-type wind power generation system shown in Figure 1, Vestas proposed the concept of optimized slip (optislip). This involves using a wound rotor induction generator (Wrig). The Wrig stator is directly connected to the grid, and the rotor side has a variable resistor. The resistance value is adjusted by a power electronic converter, combined with pitch control, to optimize power output. Otherwise, it is similar to a typical stall-type wind power system. Currently, companies such as Vestas and Suzlon offer such products. Variable speed operation is achieved by controlling the rotor-side resistance dissipation. The wider the speed range, the greater the motor slip, the more power is consumed by the resistor, and the lower the motor efficiency. The energy in the resistor is lost as heat. A typical speed range is within 10% above synchronous speed.
For the DFIG wind power system shown in Figure 2, the presence of slip rings and brushes reduces operational reliability. Therefore, a brushless DFIG (BDFIG) can be used. BDFIG has dual stator windings: one stator winding is directly connected to the grid, similar to a conventional DFIG, while the other stator winding connects to a dual PWM converter. Compared to a conventional DFIG, it eliminates slip rings and brushes, but the motor manufacturing and control become more complex.
For the PMSG direct-drive wind power system shown in Figure 3, an electrically excited synchronous generator (EESC) can also be used, which is usually DC excited on the rotor side. The advantage of using EEC over PMSG is that the rotor excitation current is controllable, and the flux linkage can be controlled to achieve minimum loss at different power levels; moreover, it does not require the use of expensive permanent magnet materials, and also avoids the risk of permanent magnet demagnetization. Therefore, EEC is widely used in current direct-drive wind power systems, and Enercon mainly deals in this type of product. However, EEC requires space for the excitation winding, which will make the motor size larger, and the DC excitation of the rotor winding requires slip rings and brushes. Since PMSG is not a standard product, it has great flexibility in size and structure. According to the magnetic flux distribution, it can be divided into the following categories: radial flux PM machine (RFPM), axial flux PM machine (AFPM), and transversal flux PM machine (TFPM). Among them, RFPM has a simple and stable structure and higher power density, and has been widely used in high-power direct-drive wind power systems [6].
Based on the above discussion, other types of wind power generation systems exist, depending on the gearbox structure and generator. One example is the single-stage gear + full-power converter variable speed constant frequency wind power system. This system adds a single-stage gearbox between the wind turbine and the PMSG (Power Management System) in Figure 3, with a speed ratio of approximately 1:10. Compared to a direct-drive PMSG system, this structure allows for higher PMSG speeds; compared to a multi-stage gearbox system, it has fewer transmission components, making it a compromise. Companies like Multibrid and Winwind offer such products on the market.
Multi-stage geared constant-frequency wind power systems with full-power converters can use either PMSG or SCIG as generators. When using PMSG, the motor is smaller and more efficient. Compared to the conventional DFIG system shown in Figure 2, fault ride-through is relatively simpler to implement, but the converter capacity is larger; GE offers such products. When using SCIG, the motor is simpler to manufacture. Compared to the conventional stall-type wind power system in Figure 1, control is more flexible, and functions such as variable speed operation, flexible grid connection, and reactive power support are easier to implement. However, it also suffers from the issue of a larger converter capacity; Siemens offers such products on the market. With the decreasing cost of power electronic devices, multi-stage geared constant-frequency wind power systems will become more competitive.
4 Wind power converter
Power electronic converters play a very important role as the interface between wind power generation and the power grid. They are responsible for controlling wind turbines, transmitting high-quality power to the grid, and achieving low-voltage ride-through. With the rapid development of wind power generation and the continuous increase in the capacity of wind turbine units, the capacity of converters must also increase. Therefore, large-capacity multilevel converters have begun to be used. The following will discuss some typical converter topologies [7].
As shown in Figures 2 and 3, typical doubly-fed and permanent magnet direct-drive variable-speed constant-frequency wind power systems employ back-to-back dual PWM converters, including a generator-side converter (or rotor-side converter) and a grid-side converter, allowing for bidirectional energy flow. For PMSG direct-drive systems, the generator-side PWM converter adjusts the d-axis and q-axis currents on the stator side to achieve speed regulation and decoupled control of motor excitation and torque, enabling the generator to operate in a variable-speed constant-frequency state and achieving maximum wind energy capture below rated wind speed. For DFIG systems, the rotor-side converter adjusts the d-axis and q-axis currents on the rotor side to achieve speed regulation and decoupled control of active and reactive power. The grid-side PWM converters adjust the d-axis and q-axis currents on the grid side to maintain DC-side voltage stability, achieving decoupled control of active and reactive power, controlling reactive power flowing to the grid, typically operating at unity power factor, and also improving the power quality injected into the grid. Back-to-back dual PWM converters are a common topology in wind power systems, and there has been considerable research on them both domestically and internationally, mainly focusing on converter modeling, control algorithms, and how to improve their fault ride-through capability. International companies such as ABB and Aalstom, and domestic companies such as Hefei Sungrow Power Supply, all offer this type of converter product.
For direct-drive wind power systems, there are many converter topologies to choose from. Figure 5 shows an uncontrolled rectifier + boost converter + inverter topology. The boost converter achieves input-side power factor correction (PFC), improving generator operating efficiency, maintaining DC-side voltage stability, and controlling the electromagnetic torque and speed of the PMSG to achieve variable-speed constant-frequency operation. It also provides maximum wind energy capture below rated wind speed. Companies such as Enercon (international) and Hefei Sungrow Power Supply Co., Ltd. (domestic) use products with this topology.
With the continuous increase in the single-unit capacity of wind turbines, the voltage and current levels of wind power converters are also constantly improving. Therefore, multilevel converter topologies have received widespread attention. After adopting a multilevel approach, the converter can achieve higher voltage levels and obtain more levels (steps) of output voltage on the basis of the voltage withstand capability of conventional power devices, making the waveform closer to a sine wave, with less harmonic content, smaller voltage change rate, and larger output capacity. Figure 6 shows the topology of a three-level back-to-back dual PWM converter in a direct-drive wind power system. Compared with a two-level dual PWM converter, the number of power devices and capacitors is doubled, and clamping diodes are added. The DC-side capacitor consists of two identical capacitors connected in series. The midpoint of the capacitor is used as the clamping point of the converter, and the grid-side converter maintains the voltage balance of the two DC-side capacitors. The application of this structure in wind power is relatively mature, and there is a lot of research on it, mainly focusing on the optimization of control strategies [8]. Currently, several companies worldwide engaged in the research and development of high-power wind power converters and high-voltage frequency converters offer multi-level product solutions. ABB's converters for wind power, such as the ACS1000, use 12-pulse diode rectification in the rectifier and a three-level NPPC structure in the inverter, employing IGCT devices. Siemens also has similar applications, using high-voltage IGBTs as power devices. The French company Aalstom uses a flying capacitor-type four-level topology with IGBTs as power devices, and has also developed a flying capacitor-type five-level frequency converter based on IGCT.
5. Wind Power Low Voltage Ride-Through and Reactive Power Support
As the installed capacity of wind power generation continues to increase, its impact on the power grid can no longer be ignored. Many countries have formulated new rules and put forward new requirements for grid-connected wind power generation. These requirements include active and reactive power control, voltage and frequency control, power quality control, and fault ride-through capabilities. These requirements require wind power generation to gradually assume functions similar to those of traditional thermal power plants, maintaining grid connection during grid faults such as voltage drops, and quickly providing active and reactive power support to the grid to help restore and stabilize grid voltage and frequency [9-10].
5.1 Introduction to LVRT and Current Technological Status
lvrt (low voltage ride)
Low-voltage ride-through (LVRT) is an abbreviation for "Wind Power System Low-Voltage Ride-Through Capability". It refers to the ability of a wind power system to maintain grid connection and provide reactive power to the grid to support grid recovery when the voltage at the grid connection point drops, until the grid returns to normal operation.
Figure 7 shows the low-voltage ride-through curve specified in the German e.on standard. The grid voltage drops to 0 for approximately 150 ms. While the grid voltage is above the curve, the wind turbines must remain connected to the grid and provide reactive power to the grid during the voltage drop and recovery phases to help stabilize the grid voltage. The wind turbines are only allowed to disconnect from the grid once the grid voltage falls below the specified curve. This requires the wind power system to have strong low-voltage ride-through capabilities and the ability to quickly provide reactive power support to the grid.
Voltage dips are one of the most common faults in power grids, including single-phase, two-phase-to-ground, phase-to-phase, and three-phase faults. The types and proportions of these faults are as follows: single-phase-to-ground faults 70%, two-phase-to-ground faults 15%, phase-to-phase faults 10%, and three-phase faults 5%. They can also be divided into symmetrical and asymmetrical faults, with most voltage dips being asymmetrical. The depth of voltage dips varies, ranging from as low as zero, and the duration can range from 0.5 grid voltage cycles to several seconds.
Currently, there has been considerable research both domestically and internationally on countermeasures for doubly-fed and direct-drive variable-speed constant-frequency wind power systems during grid faults [11-12]. These measures include improving converter control strategies to enhance the system's dynamic characteristics and enable it to achieve a certain level of low-voltage ride-through (LVRT) capability. However, when the voltage drop depth is large, the effect of control strategies alone is limited. Therefore, it is necessary to add hardware protection circuits to enable low-voltage ride-through of the wind turbine and protect the converter. Typically, the duration of voltage drop is short, constituting a transient fault. When the duration of voltage drop is long, it is necessary to coordinate with pitch control and other measures to limit the wind energy captured by the wind turbine. Reference [13] provides a relatively comprehensive analysis and summary of protection circuits for doubly-fed and direct-drive variable-speed constant-frequency wind power systems both domestically and internationally. It classifies various types of protection circuits, explains their working principles and implementation methods in detail, and discusses their respective advantages and disadvantages. Adding protection circuits can effectively improve the LVRT capability of variable-speed constant-frequency wind power systems, enabling the wind power system to remain connected to the grid during grid faults and to quickly resume normal operation after the fault is cleared.
Most literature on LVRT (Leveled Voltage Rise) of variable-speed constant-frequency wind power systems focuses on three-phase symmetrical grid faults. However, asymmetrical faults account for the majority of grid faults. Therefore, research on asymmetrical fault ride-through has been increasing recently. Under conventional converter vector control, voltage and current only contain positive-sequence components, while asymmetrical faults also contain negative-sequence components. Using conventional control will cause power and DC-side voltage fluctuations [14]. Therefore, with the increasing requirements for LVRT functionality, strategies different from conventional control are needed for asymmetrical faults, such as positive-negative sequence decomposition or direct power control. To verify the LVRT capability of wind power systems, whether symmetrical or asymmetrical faults, a dedicated voltage drop simulation device is usually required to simulate different grid voltage drop faults. Reference [15] provides a detailed summary of existing research on voltage sag generators (VSGs) for wind power both domestically and internationally. It discusses three types of VSG implementation methods, analyzes the working principles and implementation methods of each method, and compares their respective advantages and disadvantages. Transformer-type VSGs have the advantages of simple structure, high reliability, and low cost, and are currently widely used. Power electronic conversion-type VSGs are popular due to their small size, portability, and powerful functions.
5.2 Application in Products
Currently, major foreign manufacturers of direct-drive wind power converters, such as Enercon, GE, ABB, and Siemens, have integrated these functions into their products to meet the new grid regulations' requirements for LVRT and reactive power support. Enercon wind turbines can maintain grid connection when grid problems occur. If necessary, they can also support grid voltage by transmitting reactive power during a fault. Once the fault is repaired and grid voltage is restored, the wind turbine immediately resumes power supply. Reference [16] provides simulation and test results for the Enercon direct-drive synchronous motor system – E66. The short-circuit current value transmitted to the grid depends only on the power rating of the converter system and can be controlled. It also provides the active power curve of the system output during a grid short circuit. During a fault, the wind turbine provides reduced power output. After the fault is cleared, it immediately transmits rated power. The remaining generator power cannot be transmitted to the grid side. Due to the fault and the resulting voltage drop, it is converted into heat energy on the additional resistor. Therefore, the impact of the fault on the generator's mechanical torque is minimized.
GE's Low Voltage Ride-Through (LVRT) technology for wind power enables wind turbines to remain connected to the grid and supply reactive power during major grid faults, allowing them to meet transmission reliability standards similar to those for thermal power generation. By improving generator and control design, LVRT technology enables wind turbines to operate uninterruptedly during severe grid disturbances. Wind Var technology maintains system stability and rapidly supplies reactive power to the grid when needed, reducing the risk of voltage collapse and mitigating the impact of grid disconnection. LVRT and Wind Var functions significantly improve the reliability of wind turbine operation, ensuring wind power is integrated into the grid as a "good neighbor." ABB's current converter for direct-drive wind power systems, with the highest power output, is the Multibrid M5000. This full-power converter uses a back-to-back structure and has a rated output power of 5.5 MVA. It features several key control characteristics: "fault ride-through" capability, reactive power control, grid-side "statcom" functionality, and continuous and dynamic voltage control. The generator-side converter uses direct torque control, offering advantages such as fast control, robustness, high availability, and high quality.
6. Conclusion
This article provides a broad overview of wind power generation technologies, including wind turbine models, generator types, wind power converters, low-voltage ride-through, and reactive power support. Wind power generation technology involves multiple disciplines such as mechanical manufacturing, automatic control, power electronics, and power systems. From a control perspective, it includes wind turbine control (e.g., pitch control), generator control and optimization, converter control, grid connection control, and coordinated control between various components. Currently, my country is investing heavily in wind power products, industry, and research, all of which will promote the rapid development of my country's wind power industry.
