Revolution in the wind20 October 2000
ABB believes that applying its Powerformer technology to wind turbines could transform the economics of wind generation. A new installation in Sweden will put this theory into practice. Staff report
Vattenfall’s Näsudden III – representing the third generation of large wind power plants at this site on southern Gotland –will use ABB’s new Windformer technology.
Windformer is intended primarily for large offshore wind farms but Näsudden III is located on land to facilitate evaluation and testing.
The Windformer concept is a further application of advanced cable technology originally developed for Powerformer high voltage generators and subsequently also used in transformers, under the name Dryformer.
Windformer consists of a direct-coupled, variable-speed, high-voltage generator with a permanent-magnet rotor. Neither a gearbox nor a transformer is required with this design, which is expected to lead to reduced energy losses, lower operation and maintenance costs and a longer lifetime. This means a higher energy supply to the network and considerably lower generation costs.
Näsudden III will be financed by Vattenfall, the Swedish National Energy Administration and ABB. It is estimated that it will be operating in 2002, provided that all the necessary permits are granted.
The project reflects Vattenfall’s wind power strategy, which in part focuses on large offshore wind power stations with the aim of achieving competitive financial viability. Vattenfall is also currently preparing a permit application for three to seven wind power plants in Kalmarsund. Näsudden III continues Vattenfall’s pioneering track record in the construction and operation of large wind power stations. Näsudden I, with an output of 2 MW, was built in the early 1980s and was replaced ten years later by Näsudden II, one of the world’s largest wind power plants, with an output of 3 MW.
Wind turbines using Windformer are envisaged to have relatively high outputs, typically in the range 3 to 5 MW. This has a number of advantages. It reduces the required site area, since fewer machines are needed for a given station output. This in turn diminishes visual impact and noise. The voltage (over 20 kV) produced by the generator is converted to DC by means of diodes. The turbine generators are connected in groups, the power being transmitted by cable to a network station with inverter, linked directly to the utility grid.
A key aim of Windformer is to achieve maximum simplicity, with the following main components:
Rotor Iron - permanent magnets
Stator Iron - cables
Rectifier - Diodes
Transmission - Cables
Inverter - Transistors.
It is anticipated that Windformer turbine generators will be deployed in clusters, with a typical cluster generating 40 MW. A Windformer wind farm would comprise several clusters.
The cluster is connected to a high-voltage utility grid through an inverter located in a network station. Offshore wind farms use land-based network stations, which are easily accessible for maintenance and service work. The network station controls the real and reactive power output individually, making it possible to connect the wind farms to even a weak network. With Windformer technology, variations in wind speed or tower shadow due to other wind turbines will not cause fluctuations in grid voltage, which could affect nearby consumers. This is an important consideration, particularly for weak electrical networks.
By using an inverter to control the DC voltage, the generator speed is indirectly regulated, optimising energy production in the process. The turbine blades are pitch-regulated; however, regulation is mainly used to adjust the input power to prevent turbine overspeed.
A permanent-magnet rotor converts the kinetic energy to electrical power. The generator is directly connected to the turbine, and operates at a frequency in the range of 5 to 10 Hz. The choice of voltage, which is at least 20 kV, depends on the optimisation of the Windformer system. A diode rectifier converts the low-frequency AC voltage to DC.
The voltage configuration for the wind farm is chosen on the basis of, among other things, the existing network connection and the local regulations governing it, the output of the wind farm, the maximum energy output per surface unit, the distance to the network connection, and its environmental (especially visual) impact.
As an example, a conventional utility-scale wind farm currently consists of 10 to 100 turbines each delivering 500 kW to 2 MW with asynchronous generators rated at 0.7 kV. The generators are connected in series with a step-up transformer. In a large wind farm, a substation transformer steps the voltage level up to 130-230 kV.
The ABB Windformer power system has several distinct advantages: it exhibits low losses, the power electronics in each wind turbine generator are reduced to a minimum, thereby increasing availability, AC flicker in the network is reduced, and the real and reactive powers are controlled separately.
To sum up, the land-based regulating system optimises the energy output for an entire wind farm. This is in stark contrast to systems that regulate the power output of each individual wind turbine.
Using Powerformer technology
The Windformer concept includes a cable-wound generator (Powerformer), connected directly to the turbine. The variable speed of around 18 rev/min reduces stresses as well as noise emissions at low wind speeds.
Windformer does not have a gearbox, which reduces both losses and maintenance requirements. It is worth noting that a conventional gearbox-driven 1500 rev/min fixed-speed generator rotates the same number of times in one month as the direct-driven Windformer generator does in ten years.
Windformer’s smaller number of vulnerable components contributes to its extremely high reliability and availability. These characteristics and the reduced maintenance are vital for offshore applications, where access to the turbine generators is limited.
Special measures have been taken to ensure that the generator withstands severe climatic conditions, such as constant dampness and salt air.
The stator is based on Powerformer technology, which means that round cables are used instead of square, insulated copper conductors, as in a conventional generator.
Since the electrical field is totally confined within the cable, there are none of the usual problems with discharges at the end windings and connections, as occur in conventional generators. Losses in the windings are lower due to the higher voltage level. The use of cables also reduces the risk of phase-to-phase faults. The concept in itself reduces the short-circuit currents. And there are fewer safety issues involving the generator because of the smaller number of electrical components. Summing up all of these features, ABB argues that a generator based on Powerformer technology offers higher availability, improved efficiency, and, since fewer components are used in the installation, lower operating and maintenance costs.
Further improvements were made to Powerformer technology in the course of developing Windformer. For example, the entire Windformer generator winding is constructed without cable joints.
In all previous Powerformer applications the magnetic flux is induced in the rotor by coils on the pole cores. The magnetising current and the slip rings that are required are a direct cause of electrical losses.
In Windformer, the stator’s magnetic field is provided by permanent magnets, and auxiliary equipment for inducing the magnetic flux in the rotor circuit is not necessary. A rotor with permanent magnets does not need to be cooled since the losses are low. In addition to improved efficiency, the permanent-magnet rotor offers other advantages, such as better availability, higher reliability, and reduced maintenance.
During development of the Windformer generator special attention was given to optimisation of the magnetic circuit. This included dimensioning the circuit so as to avoid demagnetisation of the permanent magnets.
The rotor is multipolar with a large air-gap diameter of over 6 m, making it probably the largest permanently magnetised rotor ever to have been manufactured.
Windformer is adapted for offshore operation. For example, a corrosion-resistant material is used for the permanent magnets that will withstand the harsh environment and minimise maintenance. Also, the Powerformer stator cable windings are designed to resist the harsh marine climate, so there is no risk of electrical flashover.
The cables used for the stator windings are cross-linked polyethylene (XLPE) insulated and contain no oil. Distribution transformers, with their oil and epoxy insulation, are not needed, eliminating the risk of fire and high-risk oil handling. Also, virtually all parts of the generator can be recycled at the end of its useful life.
Less cost more output
Overall, it is estimated that Windformer can boost the production of electricity by 20 per cent relative to an equivalent sized machine using conventional technology. This is due to the variable speed, the absence of a gearbox, the reduced electric losses and the higher availability, which in turn stems from having a simpler system with fewer components. Simplicity also contributes to reduced operations and maintenance costs over the plant lifetime. The technology makes it possible to build wind farms with capacities ranging from 6 MWe up to 300 MWe.
Wind generated electricity is still relatively expensive, but successful deployment of Windformer promises to greatly increase its competitiveness
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