Reducing the losses by one-third for this type of generator
Renewable energy sources already account for around 11.81% of today’s energy mix. The most successful of these is wind energy, occupying top place and delivering 5.7% of power consumption with an output of over 30 billion kilowatts in 20062. The technology has advanced in leaps and bounds since the first systems for exploiting wind energy – windmills – were introduced.
By Dr. Roland Zoller, Product Marketing DM-PM, VACUUMSCHMELZE GmbH & Co KG
The first wind energy systems of the early 80s notched up a mere 55 kilowatts in nominal output. By comparison, the largest of today’s modern systems have an output of 6 megawatts, although these “powerhouses“ are still the exception. The majority of wind energy systems today are based on a double feed asynchronous geared generator, and reach outputs between 500 kilowatts and one megawatt.
The system has a number of advantages which have contributed to its widespread popularity. Its modular structure of gear, generator and inverter is a widely accepted standard involving relatively low start-up investments. In addition, double-feed asynchronous generators are low in weight, and transformers are unnecessary since the stator windings can be connected directly to the power grid while the rotor windings are connected to an inverter. As a result, the inverter need only be dimensioned for one-third of the generator’s nominal output, a further factor that lowers costs.
The gears are an essential part of this system, which cannot be designed as a multi-pole system and requires high speeds to operate. However, the costs of the gear system and the complex maintenance they require – for example, oil must be regularly checked and changed – represent disadvantages of this type of generator. Slip rings are also high-maintenance components. The output of these systems is lower than, say, electrically excited synchronous generators.
Electrically excited synchronous generators – geared and gearless
Today, very few of the wind energy systems in operation use rotor induction generators with electrically excited synchronous generators.
These generators deliver higher efficiency than asynchronous generators, owing to the lack of remagnetization loss in the rotor. An exciter coil in the rotor produces the magnetic field. Although highspeed variations do not require sliding contacts since they include a rotating rectifier with pilot exciter. However this type of system also requires gears. The frequency generated depends on the generator’s rotation speed. Since this is not at a constant level in wind energy systems, the power generated cannot be fed directly into the grid, and an additional transformer is generally required.
A similar principle applies to direct-driven electrically excited synchronous generators. While they do not require gears, they require a transformer and inverter. Because this type of generator can support a high number of poles, a gear system is unnecessary or a singlestage planetary gear is sufficient. Direct drive involves a number of disadvantages. For example, the structure is not modular, since all components must be designed as integral parts of the wind energy system. The diameter, and thus the weight, of the synchronous generator increases with the pole number. In addition, electrically excited synchronous generators require slip rings, involving expensive maintenance procedures.
Permanently excited synchronous generators – the model of the future?
Supported by statutory innovation incentives, the technological development of wind energy systems will continue to advance. Given the annually falling remuneration scales in Germany’s Renewable Energy Act ( Erneuerbare-Energien-Gesetz, EEG ), an increasingly efficient method of converting wind energy into electric power is required. “Repowering“, the term used for the replacement of wind energy systems which may be considerably over 20 years old, is also a major concern for wind farm operators. In addition, these old, comparatively inefficient systems often occupy prime sites in coastal regions or mountainous areas.
Wind energy system prototypes using permanently excited synchronous generators are currently in operation at an array of test sites. In this design, the exciter output is replaced by permanent magnets, reducing the losses by one-third for this type of generator in comparison to electrically excited synchronous generators, and thus increasing output. The generator can be designed as a multi-pole system for smooth adjustment to lower rotation speeds. Here too, a gear system is unnecessary or a single-stage planetary gear is sufficient, although an inverter and transformer for grid feed-in are required.
Permanently excited direct-drive systems are lower in weight than electrically excited synchronous generators. Their permanent magnets are maintenance-free and do not require slip rings, so that it can be assumed that this generator type incurs significantly lower maintenance and operating costs throughout its lifespan.
Land sites favourable to wind generation are scarce, and are artificially rendered scarcer by the imposition of regional regulations restricting maximum height and distance. The solution to this problem is advanced development of offshore wind farms, for instance off Germany’s North Sea coast – a vision for which it is particularly important that the systems combine minimum maintenance with maximum efficiency. Here, the advantages of wind energy systems based on permanently excited synchronous generators come to the fore.
Permanent magnets from Vacuumschmelze GmbH & Co KG are designed for outstanding corrosion stability, an essential property in an environment of salt air and damp. The permanent magnets are also demagnetization-proof in specific constructions. Vacuumschmelze is primarily known as a leading supplier of magnetic sub-systems. These simplify assembly and operation for the customers since the magnetic poles are attached to metal pole pieces during manufacture by Vacuumschmelze, enabling customers to mount the magnetic poles directly onto the rotor. Vacuumschmelze GmbH & Co KG has extensive expertise in system assembly and magnetic design, and is thus a reliable partner in the development of permanently magnetically excited wind energy systems.
1) Information for 2006 from German Ministry for the Environment, Conservation and Reactor Safety.
2) Information from German association Bundesverband WindEnergie e.V.