Wind Power Generators – An Introduction

Electricity Generation: The Generator

The Generator is one of the most important components in a wind turbine and primordial to its system design. While the blades transfer the kinetic energy of the wind to rotational energy in the transmission system, the generator transforms that mechanical energy into electric power for the grid. The produced alternating current transmitted to the grid must match the grid frequency. Various power electronic devices ensure that the power produced is suitable to be fed into the grid.

We shall look at a few basic parameters on which Generator type depend on and discuss two of the most common turbine designs currently in the wind power industry.

1. Based on variability of speed:

There are Synchronous and Asynchronous generators.

Synchronous generators – operate at a constant speed, the synchronous speed, regardless of the magnitude of applied torque. The synchronous speed is the speed dictated by the grid frequency. It has one clear advantage compared with the induction (asynchronous) generator: it does not need a reactive magnetising current. However, it is much more expensive and mechanically complicated.

Asynchronous generators – allow deviations from the synchronous speed, called slip. This type of generator is more common in the wind energy industry. Slip is advantageous, especially at rated power, when power fluctuations due to changes in wind speed are undesirable. In the event of a wind gust, the slip enables the generator to speed up a little without causing a corresponding increase in the generated power output. Thus, a smooth power output is ensured while reducing loads on the blades, main shaft and gearbox. One of the main reasons for using an asynchronous generator as compared to a synchronous generator is that it implies less wear and tear on the gearbox. The major disadvantage is that the stator is dependent on a reactive magnetising current. As the asynchronous generator does not have any permanent magnets and is not separately excited, it is bound to obtain its exciting current from somewhere else – and thus to consume reactive power. The reactive power may be supplied by the grid or e.g. by the capacitor bank. Its magnetic field is only established when the generator is connected to the grid.

2. Based on materials used:

Wound Rotor Generator

Traditional designs use a wound rotor generator consisting of magnetically conducting iron and electrically conducting thread arranged in a coil. It requires excitation via slip rings or brushes.

Permanent Magnet Generator

The latest trend is toward permanent magnet generators. Since, the magnetic field is provided by the permanent magnets, there is no need for field windings or supply of current to the field. Also, since the power is taken from a stationary armature, there is no need for a commutator, slip rings, or brushes. Due to its simple construction, the PM generator is quite rugged. The operating principles of PM generators are similar to that of synchronous machines, except that these machines are run asynchronously. That is, they are not generally connected directly to the AC network. The power produced by the generator is initially of variable voltage and frequency AC. This AC is often rectified immediately to DC. The DC power is then inverted to AC with a fixed frequency and voltage or it is directed to DC loads or battery storage.

Modern PMGs require the use of rare earth materials as compared to earlier designs with Alnico or Ferrite. Rare earth magnets provide high energy, maximum efficiency and extreme stability when exposed to other electromagnetic fields. They are a logical choice when extra strength in a reduced size is important. For example, NeBFe magnets are 5 to 7 times stronger than ferrite magnets.

But the extraction of neodymium is reported to involve significant environmental damage. The separation of Neodymium from mined rocks is reported to result in toxic waste products.

3. Based on no. of poles:

Increasing the no. of poles of a generator allows it to run at a relatively lower speed. Thus cost savings can be made by using a lower stage gearbox. But multi-pole generators are larger and more expensive. For a given power output, you then have the choice between a slow-moving, large but expensive generator and a high-speed, smaller but cheaper generator. Based on the no. of poles, we thus have low-speed or direct drive, medium speed and high speed generators.

Table 1: Variation of Synchronous speed with no. of poles

Pole Number	50 Hz	60 Hz
2	3000	3600
4	1500	1800
6	1000	1200
8	750	900
10	600	720
12	500	600

4. The Doubly Fed Induction Generator (DFIG)

The DFIG concept corresponds to a variable speed wind turbine configuration with a WRIG and a partial-scale power converter on the rotor circuit. The stator is directly connected to the grid, whereas the rotor is connected through a power electronic converter. The power converter controls the rotor frequency and thus the rotor speed. This concept supports a wide speed range operation, depending on the size of the frequency converter. Typically, the variable speed range is +30% around the synchronous speed. The rating of the power electronic converter is only 25–30% of the generator capacity, which makes this concept attractive and popular from an economic point of view. The concept is used by many manufacturers such as Vestas, Gamesa, GE, Repower, Nordex, etc.

The power converter consists of two converters, the rotor-side converter and grid-side converter, which are controlled independently of each other. The DFIG has several advantages. It has the ability to control reactive power and to decouple active and reactive power control by independently controlling the rotor excitation current. The DFIG has not necessarily to be magnetized from the power grid, it can be magnetized from the rotor circuit, too. It is also capable of generating reactive power that can be delivered to the stator by the grid-side converter.

A DFIG being assembled on a GE 1.5 MW turbine

5. Direct drive generators

These are essentially synchronous generators of special design. The main difference from standard machines is that they are built with a sufficient number of poles that the generator rotor can turn at the same speed as the wind turbine rotor. This eliminates the need for a gearbox. Because of the large number of poles, the diameter of the generator is relatively large. Direct drive generators on wind turbines are frequently used in conjunction with larger power electronic converters. It is called Direct Drive technology because the wind turbine rotor is directly coupled to the generator without a gearbox. The gearbox is eliminated at the expense of an increase in size and weight of the generator.

Regen Powertech’s 1.5 MW Permanent Magnet Direct Drive

Enercon’s annular ring generator being manufactured

Geared vs. Gearless turbine system layout

Based on the size and design of the Generator, we have different types of turbine designs.

The standard industry model is a 1000-1500rpm generator used in conjunction with a 3 stage gearbox. The Direct-drive design eliminates the gearbox and uses a larger generator running at the much slower wind turbine rotor speed of 15-35 rpm. A more recent design concept is of the hybrid or semi-geared wind turbine which uses a smaller gearbox of 1 or 2 stages and a medium speed generator (100-150 rpm). Given below is a comparison of the three design concepts:

Parameter	Geared WPP	Direct-drive WPP	Semi geared WPP
Some sample manufactures	Suzlon, Vestas, GE, Siemens, etc	Enercon, Mitsubishi, Harakosan, Letwind	WinWinD, Multibrid, Clipper wind
Generator, speed range	1000-1500 rpm	15-35 rpm	100-150 rpm
Grid compatibility	Worst	Best	Better
Mechanical noise	More noise	Least noise	Less Noise
Maintenance	More rotating parts-hence more wear and tear	Least maintenance	Less Maintenance
Top head mass (THM)	Less than gearless	More weight	Less weight
Type of generator	Induction generator	Synchronous generator	Permanent magnet synchronous generator
Failure	More failures due to gearbox	Failure rate is less	May be less, time will tell, as newly introduced
Cost	Relatively less	More	More
Electric circuit complexity	Least complex	Most complex	Less complex

Bibliography

1. Overview of different wind generator systems and their comparisons_H. Li, Z. Chen
IET Renew. Power Gener., 2008, Vol. 2, No. 2, pp. 123–138
2. Comparison of various Wind Turbine Generators – Anagha R. Tiwari et. al.
Multidisciplinary Journal of Research in Engineering and Technology, Volume 1, Issue 2, Pg.129-135
3. Wind Power Technology by Joshua Earnest
4. Wind Energy - The Facts: A Guide to the Technology, Economics and Future of Wind Power, EWEA 2009
5. Wind Energy Explained – J. F. Manwell