There have been several phases of wind turbine generator technology and it is an area that continues to evolve. Some early wind turbines used synchronous generators, which were common in the power industry but were superseded in the wind industry by induction (asynchronous) generators during the 1980s. By the late 1990s, double-fed induction generators had become established in wind, in parallel with direct-drive synchronous generators.
While there remains interest in all types of wind turbine generators, direct-drive synchronous generators are now favoured by several major wind manufacturers. The German Federal Ministry of the Environment shows equal use of double-fed asynchronous generators and synchronous machines in wind turbines design from 2008. These figures may be skewed by the high proportion of synchronous direct-drive wind turbines from major German manufacturer Enercon, but the trend is clear.
The fundamental principle behind all electricity generators is that a conductor forced to move through a magnetic field will have a current induced in it. Until recently, this magnetic field — in large wind turbines — was invariably produced by electrical windings. This requires relatively little power, so it is the rotor that usually provides the magnetic field.
With a wind turbine using a synchronous generator, the magnetic field of the rotor rotates at the same speed as the magnetic field of the stator (stationary) part of the generator. This speed is governed by the frequency of the electricity supply. In Europe, electricity is generated at 50 cycles per second so a simple two-pole synchronous generator rotates at 3,000rpm. In the US, electricity is generated at 60 cycles per second, so the two-pole synchronous generator rotates at 3,600 rpm.
As the power from the turbine that is driving the rotor increases, the generator pulls against the electric field in the stator and there is a slight displacement. This tugging action generates electrical power and the greater the angle between the rotor field and the stator field, the greater the power generated. As the field generated by the rotor must correspond geometrically with the field in the stator windings, great care must be taken when switching on, or synchronising, to use the technical term. Synchronising when the fields are out of phase can cause damage.
Asynchronous, or induction, machines have a simple, rugged construction and are relatively inexpensive. The main difference is that the rotor usually has no windings but a number of linked conductors embedded in a laminated core and is often called a squirrel-cage induction generator. As with a synchronous generator, there is a rotating field in the stator windings produced by the electricity network to which it is connected. The rotor, however, rotates at a faster speed and it is the relative motion — the slip — that generates the electrical power. The slip is small, typically about 1% at full load, so a two-pole machine rotates at about 3,030rpm in Europe.
Despite their simplicity, induction machines do have disadvantages. The magnetic field in the rotor is effectively induced by the grid and requires what is termed reactive power. Although this can be reduced by using capacitors, this power usually has to be paid for. Some grid operators became concerned about these reactive power demands and this has led to increasingly stringent grid codes.
Both types of generator can be configured with more poles, which reduces their rotational speed. Four-pole machines rotate at 1,500rpm (1,800 rpm in the US), eight-pole machines rotate at 750rpm and 100-pole machines rotate at 15rpm. A direct-drive machine is simply a generator, usually synchronous, with a large number of poles. Few direct-drive machines are directly connected to the grid and power is generated at variable frequency, facilitating variable-speed operation. Power conditioning electronics make the output power compatible with the electricity network.
Double-fed induction generators
These are, in some respects, hybrid designs with features of both synchronous and asynchronous machines. The essential difference is that the rotors are fed with current through slip rings enabling varied speeds. The slip rings, plus the complex electrical controls add cost but can operate over a limited, although useful, speed range — and deliver, rather than absorb, reactive power.
Permanent-magnet generators are simply synchronous generators where the magnetisation of the rotor is due to its inherent magnetic properties.
Synchronous or Asynchronous. What will turbine designers choose?
Design trends have changed and the graph below shows the stark changes in Germany since 1996. European figures for 2007, however, gave equal prominence to asynchronous and double-fed induction generators, with direct drive, permanent-magnet generator and full power conversion behind.
Synchronous machines have many attractive features, but they are delicate at start-up and, without damping, may have power swings. So solutions have been explored. One is hydraulic transmission, which eliminates the gearbox and provides damping. UK firm Artemis Intelligent Power, supported by the Carbon Trust, is investigating a computer-controlled hydraulic transmission system, claimed to be as efficient as existing designs, but lighter and cheaper.
Through the various drive-train options — double-fed induction generator, direct drive with permanent magnet generator, squirrel cage induction generator — there is concern over efficiency losses, mostly associated with generator function over the full range of operational wind speeds. Tests have shown, however, that taking all the different component losses into account, the systems return a similar loss, around 9%.
Direct drives may have the edge
While efficiency influences the cost-effectiveness of a wind turbine, it is by no means the crucial factor. Direct-drive generators usually have added weight to consider, which means the cost of stronger bed plates and towers must be balanced against the savings from the higher energy yields and the likely lower maintenance costs from eliminating the gearbox. But advances in technology are reducing the extra weight of direct-drive systems, especially those with permanent-magnet generators. The tower head weight of the latest 101-metre-diameter direct-drive turbine by Germany’s Siemens is only about 30% heavier than the lightest of similar conventional machines with gearboxes. Enercon, another German direct-drive wind turbine manufacturer firm, claims that its 82-metre-diameter, 2MW turbine offers a 27% lower specific weight (nacelle weight divided by annual energy production) compared with its 66-metre turbine from 1995.
With several major manufacturers researching and launching direct-drive turbines, this may be where the future lies.
Other options to improve cost-effectiveness may also be on the horizon. One generator technology to watch for is the high-temperature superconductor. This is not a new form of generator, but one where the magnetising coils operate at very low temperatures with much lower electrical resistance. Losses due to heating are reduced, but at the cost of the equipment needed to deliver the low temperatures at which superconductivity starts to become worthwhile. Little is in commercial operation but there is considerable interest and research is taking place.