German wind turbine manufacturer Enercon has erected a 4.5 MW wind turbine near Magdeburg, Germany. The E-112 wind turbine, which will be the largest unit of its kind in the world, is now undergoing testing.


Wind is one of the fastest growing sources of energy for power generation
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Wind may not yet be a major player in the global power generation market but it is certainly one of the fastest growing. And with the increase in its use, manufacturers are developing larger units which will one day see this technology become a mainstream player in the power sector.

During the summer of 2002, German-based wind manufacturer, Enercon, installed the prototype of its 4.5 MW turbine. The unit, which is the largest of its kind in the world, is now undergoing testing near Magdeburg, Germany.

Construction and assembly

The development and construction of the new turbine, produced by Enercon, were supported by the German Federal Ministry of Economics and Technology in collaboration with the project sponsor Jülich.


Figure 2. Performance curve for the E-112
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The E-112 rotor blades each measure over 52 m in length and weigh 20 t. The full nacelle cover weighs about 19 t. To simplify transport and handling, the generator was made in nine individual parts, plus the cap. The nacelle hood consists of no less than 14 parts.

The generator of the wind turbine was almost completely assembled on the Sket MAB premises in Magdeburg. Enercon’s supplier of steel towers, SAM, constructed the steel parts for the rotor and stator; while the interleaving and winding was carried out in the generator production area at Magdeburg. The pole shoes were manufactured at Enercon’s induction works at Aurich.

Construction of the prototype at Egeln was planned to take three weeks but was actually achieved in less than two weeks. The prototype unit was set up in a wind farm near Magdeburg which already had E-66 and E-40 units to allow people to compare the size and sound level of the prototype against smaller units. This also demonstrated that the unit would fit into the surrounding landscape.

Construction of the E-112 was very different from previous units, which required just a few lifts. With the E-112, the nacelle components were raised to a height of

124 m, item by item, in pre-assembled sections of up to 110 t. The heavy components were lifted by two 800 t lattice tower cranes working in tandem. To ensure safe lifting, erection was carried out during periods of low wind speeds.

After a brief pre-assembly on the ground, the main carrier went up together with the nacelle cover. Next followed the stator in two halves. According to Enercon, a particular challenge was the “millimetre-exact” installation of the disc rotor into the stator. After this, the hub and axle were put up jointly, followed by the spinner.

Finally, the rotor blades were individually flanged on to the hub adapter.

Testing

Such a large piece of equipment called for the building of a test stand, weighing 260 t, on the banks of the Weser river. The stand was developed by Enercon engineers using principles derived from bridge construction. The blade test stand is neither fixed to the ground nor welded, but is entirely bolted. This means it can be easily dismantled and reassembled.

It is also possible to use a pitch motor to turn a rotor blade by up to 180°. This allows the blade to be brought into exactly the required position and turned on both sides for testing.

In one test, the root of one E-112 blade was firmly clamped, and the blade itself pulled towards the ground at six locations. The tension in the blade, which according to Enercon “looked like a loaded catapult”, was gradually increased. The stress was increased to 150 per cent of the maximum load expected in operation.

The stress test was one of four tests which the blades were subjected to before they could be shipped to the prototype wind turbine in Egeln. The blades withstood the stress tests without any damage.

In addition to the stress test, blades were subjected to deformation, resonant frequency, and elongation tests. The deformation of the blades was found to be within the usual tolerance of one or two per cent of the calculated values. Wire strain gauges were attached to the relevant points of the internal structure of the blade to analyze elongation.

The results of the investigation were utilized for the continuing development of the blade structure.

To investigate resonant frequency, the blade on the test stand was made to oscillate. Data received from the vibration sensors agreed with the calculated values.

Future Projects

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With the erection complete, the entire operating performance of the turbine will be under close scrutiny for at least a year. A key part of the work will be to gain experience with the newly developed electronic converter controller. Measurements will be compared to results predicted by computer. Attention will be paid to the forces and stresses acting on the turbine and its components.

Only after extensive testing and the installation of two or three more prototypes will Enercon offer this machine for offshore and onshore use. The E-112 is already designed for offshore applications.

The second prototype will be set up this year, also onshore in Wilmelmshaven. The third E-112 project is also likely to be undertaken this year by project company, Winkra. This will be located 500 m off the coast of Wilhelmshaven.


New rules

With the growing size of actual wind turbine units and the growth in the utilization of wind power in Europe, the rules regarding grid connection changed in January of this year.

The new technical requirement primarily affect two physical parameters: the frequency and the voltage. In the past, if the frequency varied slightly, wind turbines automatically disconnected themselves from the grid. Now, they stay connected, like large power stations, in order to support it. Currently there are wind farms with an installed capacity of more than 5000 MW – equivalent to large power stations. Voltage is closely associated with reactive power, and regulators have now specified a range corresponding to that of large power stations.

Further, there is now the question of the operating behaviour on the grid when faults occur. In the past, small units disconnected from the grid as quickly as possible when the voltage changed even slightly. Now they must remain on the grid if, for example, there is a short on the grid. The short-circuit capacity is needed for the sake of system stability and grid protection.

In northern Germany for example there is 3000 MW of wind capacity that could not afford to be disconnected. Although north Germany could draw power from elsewhere within the UCTE grid, 3000 MW would not be available immediately.

A loss of 3000 MW is about the permissible limit for the European UCTE grid. As soon as more than 3000 MW is taken off, the grid frequency goes outside its permissible range. If an even greater generation capacity fails, there would be a danger of grid collapse.

At this point, it would be essential to regain the balance betweengenerated and consumed energy in the remaining grid to avoid a grid collapse and blackout. This is a risk that would have become increasingly real as the amount of wind power grows and the new rules were not implemented.