Wind turbines are playing a major role in renewable power generation in the UK. The electrical generator system is one of the core parts in a wind turbine. Using information gathered from a minimum of 8 different sources/references (e.g. books, journal papers, company websites, etc…), write a report that identifies the operating principles, characteristics and application of the DFIG-WT. Use any appropriate/available software (e.g. MATLAB) wherever applicable within your report. Your report should discuss the following points.
- Introduction to the structure and operation of a generic wind turbine system.
- The major types of electrical generator used in wind turbines and their typical ratings.
- The operational principles of Permanent Magnet Synchronous Generator (PMSG).
- The operational principles of Double-Fed Induction Generator (DFIG).
- Provide comments and concise explanations on the strengths and weaknesses of PMSG and DFIG in terms of:
i) Industrial implementations.
ii) Offshore WT applications.
- Operation of the DFIG-WT in super-synchronous and sub-synchronous modes as well as the point of zero slip.
- Operation of the DFIG-WT in the pitch control range
- Fault ride through capability of DFIG-WTs and its operation under different AC faults (single-phase and three-phase).
The report should be limited to 2500-3000 words.
Renewable Power Integration
A brief history of wind power generation.
In Glasgow, Prof J. Blyth built the first electricity-generating windmill in July 1887. Then Prof. C. F. Brush buit a 12 kW wind turbine to power 408 batteries in his mansion. P. L Cour formed the Society of Wind Electricians in 1903 and held the 1st wind electricity subject in 1904. Joe and M. Jacobs founded the Minneapolis-based Wind Factory in 1927. George Darrieus designed the vertical-axis wind turbine (WT). 1941 saw Castleton, Vermont’s first MW WT connected to the grid .
Wind energy now generates 10.2% of U.S. electricity. China is the world’s largest wind energy producer. 16 countries generate 3.6 billion kWh of wind electricity in 1990. More than hundred countries—including Puerto Rico—produced 1,808 billion kWh of wind electricity in 2021 . Wind power capacity is detailed in the graph below .
Working and Construction
The rotor operates by rotating at the prescribed wind velocity, causing the connected rotor shaft to rotate within the nacelle. The nacelle, situated atop the tower, serves as a substantial enclosure housing the requisite mechanical and electronic components essential for electricity generation . The primary constituents of the system include the gearbox, generator, cooling system, yawing system, and brakes, as depicted in figure below.
Figure: Structure and components of wind turbine generator .
The initial procedure involves adjusting the rotational speed of the rotor shaft to a magnitude compatible with the operational requirements of the generator. The operational speeds of most generators are considerably higher than the practical operational speeds of wind turbine rotors. If a rotor were designed to accommodate a standard generator, its tip speed would exceed the speed of sound. Gears are highly advantageous in the context of altering the rotational velocity of a shaft. The gear that is coupled to the rotor shaft exhibits a higher number of teeth in comparison to the gear that is coupled to the generator. When transitioning from a gear with a larger diameter to a gear with a smaller diameter, the angular velocity of rotation increases while the torque decreases, while the overall power output remains constant. One primary issue associated with generators in wind turbines pertains to their ability to generate electricity that is compatible with the electrical grid specifications of the specific site. In the US, the electrical grid operates at a frequency of 60 Hz, whereas in the majority of European regions, the grid operates at a frequency of 50 Hz. To ensure the uninterrupted flow of current, it is imperative that the electricity possesses identical characteristics, meets the required quality standards, and is connected in a manner that avoids any disruption. WTs are equipped with either synchronous or asynchronous generators (induction), with the latter being the more prevalent option.
Generators used in wind turbines and their performance .
|Medium and Large WTs
|Residential, Low Power
|Small and Medium WTs
|Small and medium WTs
PM Synchronous generator
A Permanent Magnet Synchronous Generator (PMSG) employs permanent magnets in the rotor instead of conventional field windings. The implemented design facilitates the PMSG in generating electrical power with a consistent frequency, irrespective of the rotational speed of the rotor. The inclusion of this feature renders PMSG an indispensable element in a wide range of contemporary applications, including but not limited to wind turbines, electric vehicles, and other related domains. The functioning of a PMSG is predicated upon the principles outlined in Faraday’s law of electromagnetic induction. When the rotor, which is equipped with permanent magnets, undergoes rotation, it generates a dynamic magnetic field. The movement described causes the generation of an electromotive force (EMF) within the stator windings, resulting in the production of electrical energy. It is worth noting that the electromagnetic force (EMF) generated and, consequently, the resulting output frequency are directly influenced by the rotational speed of the rotor .
Permanent magnets offer a consistent magnetic field, thereby obviating the necessity of an external power source to stimulate the generator. The stator is comprised of windings arranged in three phases, which facilitate the induction of the output voltage. In contrast to traditional synchronous generators, the rotor of a PMSG lacks windings, slip rings, and brushes, resulting in decreased maintenance needs and improved reliability.
PMSG presents numerous advantages in comparison to conventional electric generators. The absence of losses related to rotor excitation contributes to its high efficiency. The lack of slip rings and brushes results in reduced maintenance requirements and increased reliability. Moreover, Permanent Magnet Synchronous Generators (PMSGs) possess the advantage of being more compact and lighter in weight, rendering them highly suitable for applications that prioritize space and weight constraints.
The utilization of Permanent Magnet Synchronous Generators (PMSGs) encompasses a wide range of applications. The multifaceted attributes of PMSG have rendered them a favored option for numerous applications. The following are several prominent domains in which PMSGs are frequently employed:
1. The utilization of Permanent Magnet Synchronous Generators (PMSGs) in wind turbines greatly enhances their operational efficiency and overall dependability.
Figure: PMSG in Wind Turbine System .
2. Electric vehicles (EVs) benefit from the inclusion of PMSGs due to their positive impact on efficiency, torque control, and size reduction.
3. Marine Propulsion: PMSGs are employed in marine propulsion systems owing to their notable attributes of high torque density, efficiency, and compact dimensions.
Doubly Fed Induction Generator (DFIG)
The DFIG principle is a commonly employed technique in the field of wind turbines. It operates on the basis of an induction generator that incorporates a rotor with multiple phases, as well as a slip ring assembly with brushes that enables reach the rotor windings. The elimination of the multiphase assembly is a viable option; however, there are inherent challenges pertaining to efficiency, cost, and size. An improved option entails the utilization of a brushless, wound-rotor, doubly-fed electric machine .
The fundamental concept underlying the DFIG is that the stator windings are linked to the electrical grid, while the rotor windings are connected to a converter through slip rings. This converter, in the form of a b2b VSC, effectively regulates both the grid and rotor currents. Therefore, the frequency of the rotor has the ability to deviate independently from the frequency of the power grid, which is typically 60 or 50 Hz. The utilization of the converter for regulating the rotor currents enables the independent adjustment of active and reactive power supplied from the stator to the grid, irrespective of the rotational velocity of the generator. The principle employed can be direct or vector control. The direct control method has demonstrated superior stability compared to the existing vector control technique, particularly in scenarios where the generator necessitates high reactive currents .
The rotors of doubly fed generators are commonly wound with a number of turns that is approximately two to three times greater than that of the stator. Consequently, the voltages across the rotor will exhibit an increase while the corresponding currents will demonstrate a decrease. Therefore, within the standard operational range of 30% near the nominal speed, the converter’s rated current is correspondingly reduced, resulting in a decrease in converter cost. One limitation of the system is the inability to perform controlled operations beyond the designated operational speed range due to the presence of rotor voltage that exceeds the rated value. Moreover, the amplification of voltage oscillations caused by grid fluctuations will occur. To mitigate the potential damage caused by elevated rotor voltages and the subsequent high currents on the IGBT of the converter, a protective mechanism known as a “crowbar” is employed.
When the crowbar mechanism detects excessive currents or voltages, it will cause a fault in the rotor by introducing a little resistance. To expedite the resumption of operations, it is imperative to employ an operational crowbar. The utilization of an active crowbar facilitates the controlled removal of the rotor short, allowing for the initiation of the rotor-side converter only within a desired time frame subsequent to the onset of the grid oscillation, provided that the residual voltage remains above 16% of the standard voltage. Hence, it is feasible to produce reactive current to support the grid’s restoration process following a voltage dip, thereby aiding in the grid’s recovery from the fault. In the context of zero ride-through voltage, it is a prevailing practice to defer the injection of reactive current until the conclusion of the voltage dip. This approach is adopted due to the inherent challenge of determining the precise phase angle at which the injection should occur in the absence of this temporal reference point .
In brief, a DFIM refers to a wound-rotor DFEM that exhibits numerous advantages when compared to a normal induction machine, particularly in the context of wind power applications. The induction generator possesses the capability to both import and export reactive power due to the presence of a power electronics converter that governs the rotor circuit. The aforementioned implications have significant ramifications on the stability of power systems, as well as the machine’s ability to provide grid support in the face of severe voltage disturbances, commonly referred to as LVRT. Furthermore, the regulation of rotor voltages and currents allows for the induction machine to maintain synchronization with the grid despite fluctuations in wind turbine speed. A variable-speed wind turbine demonstrates enhanced efficiency in harnessing the available wind resource compared to a fixed-speed wind turbine, particularly in situations characterized by low wind speeds. Moreover, in comparison to alternative variable speed solutions, the converter exhibits a relatively low cost. This is primarily due to the fact that only a portion of the mechanical power, typically ranging from 25% to 30%, is transmitted to the grid via the converter, while the remaining power is directly supplied to the grid from the stator. The efficiency of the DFIG is commendable due to the same underlying factor.
The control of pitch angle in wind turbines equipped with Doubly Fed Induction Generators (DFIG) significantly influences the dynamic performance and oscillatory behavior of the power system. Therefore, it is imperative for a Doubly Fed Induction Generator (DFIG) control system to integrate the pitch angle control alongside the Rotor-Side Converter (RSC) and Grid-Side Converter (GSC) controls. The effects of pitch angle can be observed in results shown at the end of the report.
Figure: DFIG-based wind turbine system .
Comparison between DFIG and PMSG
1. Offshore wind turbine applications:
The scholarly discourse often examines various generator systems by employing criteria centered on energy output and financial considerations. The DFIG exhibits certain advantages over the PMSG due to its characteristics of being lightweight and cost-effective. The converter for DFIG is specifically designed to accommodate only 25% of the rated power. This design choice has been instrumental in the widespread adoption and effectiveness of DFIG systems in wind energy applications. However, the specific circumstances surrounding offshore applications give rise to distinct limitations. The installation of offshore wind turbines will be conducted at locations characterized by robust currents that pose challenges in terms of accessibility. Hence, the reduction of maintenance requirements is a fundamental aspect. In comparison to a DFIG, a direct-drive PMSG necessitates less frequent maintenance. Specifically, the DFIG requires regular maintenance, particularly for its gearbox and slip rings .
2. Industrial applications:
|The speed range is restricted within a range of -30% to 30% around the synchronous speed. PWM inverters are characterized by their small capacity and low cost. Both reactive and active power can be fully controlled.
|The presence of slip rings is an inherent and inevitable aspect of certain systems.The necessary gears
|The maximum range of speeds.The utilization of gear can be circumvented. Both reactive and active power can be fully regulated.The absence of brushes results in reduced maintenance requirements.The field does not require a power converter.
|A power converter that can convert AC to DC and vice versa is necessary.The utilization of a multipole generator, characterized by its significant size and weight, necessitates the presence of permanent magnets.
DFIG operation in subsynchronous, supersynchronous, and zero-slip modes.
The doubly-fed induction machine has the capability to function in four unique modes, as depicted in figure above. The machine acts like a motor while the stator accepts power. In the subsynchronous mode, the rotor delivers power, whereas in the supersynchronous mode, the rotor takes power. The machine acts as a generator when the stator supplies power. In the subsynchronous phase, characterized by (ωstator > ωrotor), the rotor takes power. Conversely, in the supersynchronous mode, denoted by (ωstator < ωrotor), the rotor gives power. In order to maintain its magnetizing capability, the stator of the electrical machine draws reactive power from the utility, subsequently producing solely active power. Consequently, the utility supplies reactive power to the stator, while the stator delivers active power to the loads. The absorbed power is denoted by a positive algebraic sign, while the provided power is characterized by a negative sign .
When in the zero slip condition, DFIG operates as a synchronous generator with a main field, but leading stator currents augment the field. As a result, little or no relative motion occurs between the rotor and the air gap field, leading to zero slip. The rotor’s sole function is to provide a magnetic route for air gap flux. In theory, there will be no induced rotor currents. The permanent magnet’s mechanical input matches any real power component of the stator that the DFIG load causes.
Capability of DFIG in dynamic conditions.
The findings reported in the paper by reference  demonstrate the feasibility of maintaining stability in a DFIG under severe faults within the system network, without relying on help from any other equipments. The RSC is able to maintain its connection to the grid without sustaining damage in the event of a fault. This approach diverges from applications employing crowbar circuits by enabling the implementation of targeted strategies to enhance the voltage at the coupling point in the event of a fault, as mandated by the updated Grid codes.
In this report, the fact of the increasing popularity of wind power is evident, and thus measures have become mandatory to enhance the efficacy of wind power generation. Based on the requirement, the generation method can be chosen. The use of power electronics devices is also seen as inevitable as they provide high flexibility and control. Integration of wind power into the grid is highly important but not an easy task to achieve. Thus, proper integration solutions must be put forth. The use of DFIG has provided us with an opportunity to harness high amounts of wind power.
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