What type of generator is suitable for wind power generation?

The core task of a wind turbine is to efficiently and stably convert the mechanical energy that constantly changes with wind speed into electrical energy that can be connected to the grid. Therefore, generators suitable for wind power generation are not ordinary models, they must have the characteristics of adapting to wide wind speeds, withstanding complex loads, and being easy to grid control. The modern wind power industry mainly revolves around three technological routes, each with its own advantages and suitable for different scenarios.

1、 Double fed asynchronous generator: former industry mainstay

For a long time, doubly fed asynchronous generators have been the absolute mainstream in the wind power market. Its working principle is clever: the generator rotor is connected to a smaller frequency converter through slip rings and carbon brushes, while the stator is directly connected to the power grid.

The biggest advantage of this design is its economic efficiency. Because the frequency converter only needs to process about one-third of the power in the rotor circuit, the equipment cost is lower and the energy loss is also small. In addition, with extremely mature technology and a complete industrial chain, it has become a common choice for onshore wind power, especially megawatt level units, in the past two decades.

However, its drawbacks have become increasingly prominent with the development of the industry. Carbon brushes and slip rings are mechanical contact components that require regular maintenance in the harsh operating environment of wind farms and are potential failure points. In addition, due to the direct connection of the stator to the power grid, its ability to cope with voltage fluctuations in the grid (i.e. "low voltage ride through capability") is relatively weak.

2、 Permanent magnet direct drive synchronous generator: a model of reliability

Direct drive technology adopts a completely different approach. It completely eliminates the high-speed gearbox, allowing the wind turbine to directly drive a multi pole permanent magnet synchronous generator. Due to the low speed, the frequency of the emitted current varies greatly, so all electrical energy must be processed by a full power inverter before being sent to the grid.

This plan brings revolutionary advantages. Removing the gearbox with the highest failure rate greatly improves the reliability and maintainability of the unit, especially suitable for offshore wind farms that are not easily accessible. Permanent magnets provide a magnetic field without excitation losses, resulting in higher efficiency. More importantly, the full power frequency converter acts as a 'firewall', perfectly isolating the generator from the grid, making the wind turbine extremely insensitive to grid fluctuations and able to actively support the grid, resulting in excellent power quality.

The challenge lies in the need to create a generator with a huge diameter and numerous poles in order to generate sufficient frequency of electricity at low speeds, resulting in astonishing volume and weight, posing a challenge for transportation and lifting. Meanwhile, it relies on rare earth permanent magnet materials, which result in higher costs.

3、 Medium speed permanent magnet synchronous generator (semi direct drive): the current technological darling

In order to strike a balance between the reliability of direct drive and the compactness of traditional structures, compromise "semi direct drive" or "medium speed drive" solutions have emerged and quickly become the mainstream choice for large wind turbines, especially offshore models.

It retains a simple and sturdy medium speed gearbox (usually with only one planetary gear), which increases the wind turbine speed to a moderate level, drives a compact permanent magnet synchronous generator, and finally connects to the grid through a full power frequency converter.

This can be called a 'golden combination'. It significantly reduces the size and weight of the generator with a simple gearbox, solving the transportation problem of direct drive units, while retaining the high efficiency of permanent magnet motors and the excellent grid friendliness of full power frequency converters. Its reliability is much higher than traditional doubly fed systems. Although technology integration is difficult and the initial cost is not low, the advantages are significant in terms of the full lifecycle electricity cost.

Summary and Trends

Overall, the technological evolution path of wind turbines is clear:

Double fed asynchronous generators still have vitality in specific onshore markets due to their maturity and economy.

Permanent magnet direct drive synchronous generators firmly occupy a place at sea and low wind speed areas with their unparalleled reliability.

The medium speed permanent magnet synchronous generator successfully integrates the advantages of the first two, becoming the absolute technological leader in the current large-scale and offshore wind power development, representing the future direction of the industry.

The choice of generator is essentially a search for the optimal balance between initial investment, operation and maintenance costs, power generation efficiency, and grid connection requirements. As wind turbines become larger and offshore, the requirements for reliability and grid support capability have become overwhelming. This is also the fundamental reason why the technology route of permanent magnet generators combined with full power frequency converters has won.