Due to mounting concerns about global warming and a depleting supply of fossil fuels, it is essential that renewable energy sources be developed. Wind power has expanded rapidly over the past three decades, making it the fastest-growing renewable energy source worldwide. Wind energy may be more widely adopted if advances in technology are made in wind turbine design and the generating system.

Since the 1990s, there has been a dramatic increase in the efficiency of wind power conversion systems. There are numerous distinct ideas and control techniques for wind turbines that have been developed to improve their dependability and efficiency and lower their overall cost.

Ⅰ. Techniques of Wind Turbine Generation

Traditional generating systems utilized by large wind turbines come in three kinds.

For the first, a conventional squirrel-cage induction generator (SCIG) and a multistage gearbox are used in conjunction with a direct gird connection to create a fixed-speed wind turbine system.

The second group consists of two subtypes:

a. Limited variable-speed designs 

b. Variable-speed designs that use a partial-scale converter.

To make the limited variable speed idea work, a wound rotor induction generator (WRIG) with a variable rotor resistance is used along with a power electronics converter and a pitch control mechanism.

A partially rated converter, a multistage gearbox, and a doubly-fed induction generator (DFIG) make up the variable-speed wind turbine system.

Finally, there is the gearless wind turbine system, sometimes known as a direct-drive or variable-speed wind turbine. Typically, a full-scale power converter is paired with a low-speed, high-torque synchronous generator to form the direct-drive generator system.

Ⅱ. Challenges in Direct Drive Generator

In a geared wind turbine setup, gearbox failure and the regular maintenance required are two major considerations. Energy output, drive train simplification, cheap maintenance, great efficiency, and reliability are just a few of the reasons why direct-drive generators are more interesting than geared generators.

Since these direct-drive generators run at low speeds and necessitate high torque to provide the required power, they typically employ a large air gap diameter, increasing the overall cost and bulk of the system.

To connect to the smart grid, a fully rated power converter is needed, which adds both expense and loss. Different types of generators that could be used in direct-drive wind turbines have been the subject of numerous studies.

Ⅲ. How do you overcome the challenges of direct-drive generators?

Permanent magnet synchronous generators (PMSGs) were found to be superior to electrically excited synchronous generators (EESGs) in terms of energy output, dependability, torque-to-cost ratio, efficiency, and lightweight design, making them the optimal choice for this application.

The PM machines are not your typical readily available machines; instead, their design allows for a considerable amount of flexibility, enabling the adoption of various topologies. Both the structure and the flux path's direction can be used to categorize the PM machine.

Ⅳ. Classification of Permanent Magnet Synchronous Generators

Depending on the orientation of the flux, the PMSG can be broken down into the

● Axial flux permanent magnet (AFPM) machine

● The radial flux permanent magnet (RFPM) machine

● The transverse flux permanent magnet (TFPM) machine

Ⅴ. Axial Flux Permanent Magnet Machine

The AFPM machine offers yet another option for direct-drive use. As can be seen in Fig. 1, this device produces magnetic flux along the axial axis rather than the radial axis. While the current travels radially, the flux from the PM moves axially. In comparison to a radial-flux PM machine, this one has a big diameter but a short axial length. Torus machine is another name for the common slotless AFPM machine with a single stator and dual rotor.

Fig. 1. Axial flux PM machine Source: IEEE Access

Merits

● Short axial length and easy winding

● Minimal cogging noise and torque

● Superior ratio of torque to size.

Demerits

● High usage of PMs when the diameter is high

● Structural instability

● Trouble keeping an air gap when the diameter is large

Ⅵ. Radial Flux Permanent Magnet Machine

As illustrated in Fig. 2, this is the most typical arrangement for PM machines, in which the flux lines lie in the radial plane and the current flows axially.

Fig. 2. Radial flux PM machine Source: IEEE Access

Applications

● Propulsion systems on ships

● Wind power generation

● Traction, etc

The RFPM machine has a low risk of structural failure and a high torque-to-weight ratio. This is why RFPM machines constitute the vast majority of low-speed megawatt generators on the market today.

The RFPM machine has greater performance across a broad variety of applications for direct-drive wind turbines. The machine's axial length and air gap diameter are also customizable. Since the axial length is greater and the diameter is lower, the copper loss will be concentrated at the terminal windings.

Types

The literature primarily discusses two types of RFPM machines:

● Surface-mounted

● Flux-concentrated.

Large-scale direct-drive wind turbines may benefit from the surface-mounted RFPM machine. The air gap flux density is less than the remanent flux density in the magnets on the rotor surface, which is different from the flux-focused topology.

Furthermore, because external rotor configurations are easier to install and maintain than internal rotor configurations, they are more practical for big direct-drive generators. Furthermore, a multi-pole construction with a large outer periphery can yield a larger torque density.

Transverse Flux Permanent Magnet Machine

W. M. Morday was the first to patent the idea for the TFPM machine in 1895. In the 1970s, the concept emerged again in a series of articles by E. R. Laithwaite et al. on the linear TFPM machine for railway applications. Last but not least, H. Weh's publication brought widespread notice to this apparatus.

The magnetic circuit that generates the driving force is located in a plane orthogonal to the direction of motion. One-sided surface-mounted TFPM machines, like the one depicted in Fig. 3, have the simplest topology of all TFPM machines.

Fig. 3. Transverse flux PM machine Source: IEEE Access

In contrast to traditional machines, TFPM machines provide additional space for the armature windings without sacrificing the area available for the main magnetic flux.

The pole pitch of a TFPM machine can be set extremely small in comparison to other machines. Compared to AFPM and RFPM machines, TFPM machines can have a larger torque density because of this. Complexity in three dimensions, a low power factor, and a large cogging torque are some of the issues with TFPM devices. This machine performs like a synchronous machine.

Merits

● Higher torque density

● Separate electrical and magnetic loadings

● Modular structure

● Fault tolerance

● Simple windings

Demerits

● Poor power factor

● High cogging torque

● High losses

● Complex manufacturing

Fairly designing and comparing a TFPM machine with a regular RFPM machine is necessary if it is to become a leading candidate in wind power applications. The designing procedure has three stages: conditions, investigation, and optimization.

Ⅶ. Summarizing the Key Points

● Wind power is the fastest-growing renewable energy source worldwide, and its development is essential to address concerns about global warming and depleting fossil fuels.

● Advances in technology are necessary to improve wind turbine design and the generating system to make wind energy more widely adopted.

●  Wind turbines have some demerits, including poor power factor, high cogging torque, high losses, and complex manufacturing.

● The latest innovations in wind turbine design and technology, such as the use of smart materials and advanced control systems, are revolutionizing renewable energy.