Vestas V90-3MW wind turbine nacelle (Photo: Wikipedia/Davagh)
The nacelle is the wind turbine’s upper enclosure. This is where wind energy captured by the blades is converted to electricity for the grid. This energy is proportional to the swept rotor area and to the wind speed to the third power.
The conversion of wind to electricity takes place in the drive train, which is a series of mechanical power transmission components culminating in the electrical generator shaft. First in line is the main shaft, which bears the hub and the blades. The rear of the main shaft usually goes in the slow-rotating side of a gearbox. Not all wind turbines have gearboxes however.
The next component after the gearbox is the electrical generator. The generator’s construction is linked to whether or not the nacelle design includes a gearbox. In front of the generator is a large disc brake that has the ability to keep the turbine in a stopped position.
The generator produces three-phased electrical power, which must then be transformed to the higher voltage of the grid. There is one transformer for each phase, and these three transformers are sometimes placed in the back of the nacelle and sometimes at the bottom of the tower.
Electrical power leaves the nacelle through a power cable, which hangs freely from the centre of the nacelle all the way to the bottom of the tower.
This cable twists as the nacelle yaws to face the shifting wind direction. The wind turbine control system keeps track of the number of cable twists, ensuring that it is kept within safe limits.
Testing a complete nacelle at LORC
Rigorous testing of the fully assembled nacelle is an important step in reducing the cost of failures once the wind turbine is producing power at the sea. Each component and subassembly in the nacelle is, of course, produced in accordance with predetermined quality levels and thoroughly tested before it is introduced to the design. Nevertheless, this is no guarantee that the assembled nacelle will be tough enough for the job or fulfill the demands for grid connection.
It is not easy, however, to test a fully assembled nacelle, and less ambitious compromises are often chosen, leaving the final test to be undertaken with the nacelle mounted atop a tower and real wind blowing at the blades.
A powerful test bench - the LNT, Lindoe Nacelle Testing - is being built at LORC Test Center to allow wind turbine producers to test their fully assembled nacelles on the ground, under controllable conditions. This means that all functions of the nacelle can be tested simultaneously, including the pitch control system.
Test load parameters can be accelerated so that years of wear and tear from violent weather can be simulated over the course of just a few months. The justification for such abuse is that it is less expensive to quickly wear down a prototype wind turbine in the test bench in order to discover its weaknesses than it is to replace recurring faulty components at a wind farm at sea.
The LNT will include two test docks, both of which can be reached by a large 1000T gantry crane. It will be possible to control the nacelle’s environment including the electrical interface to the grid to simulate unfavourable conditions, such as grid disturbances.