SIEMENS

Virtually any improvement that enhances the efficiency of a power plant is good for business and the environment. That is particularly true when it comes to optimizing the performance of downstream generators, which are responsible for converting the rotational energy of a plant’s turbines into electrical power. To this end, in 2007 Siemens teamed up with the Universities of Bayreuth, Freiburg, and Dortmund as well as with industry partners Infineon Technologies AG, cable manufacturer Leoni AG, and Nanoresins AG, a supplier of nanoparticles. The joint project, which has the support of Germany’s Federal Ministry of Education and Research, is known as “Nanotechnology in Insulation Systems for Innovative Electrical Applications” - or NanoIso for short.

The basic idea behind the project is simple. When an existing power plant is being retrofitted with more powerful turbines, it would also make good technological sense to install a larger generator - were it not for the complexity and cost of this procedure. However, there is an alternative. By swapping the electrical conductors inside the generator for ones that can carry more current, the generator’s output can be increased without having to replace the entire installation. Even so, this solution is not without complications. A generator consists of a rotor and a stator. The rotor is a current-carrying bar magnet that is turned by the turbine; the stator consists of coils made of copper bars, which surround the rotor. The rotational movement of the rotor induces an electrical voltage in the stator, which causes an electric current to flow.

If the copper bars in the coils are to carry more power, they must be made thicker. However, as there is no additional space available within the generator housing, this means that the layer of insulation coating the copper bars must be made thinner. This, in turn, means that the insulation must provide much better protection against disruptive discharges - which is precisely the aim of NanoIso. By developing new insulation materials containing nanoparticles, it is possible to make the insulation thinner and thereby increase the efficiency of existing generators.

Greater Resistance to Erosion. The rotation of the rotor inside the generator results in potential differences of as much as 27,000 volts between the copper bars of the stator windings. This can cause the air to ionize, leading to partial discharges in the form of small lightning flashes that destroy the insulation. The result is so-called erosion channels, which eat through the material and can lead to shorting. The current method of preventing this is to incorporate mica in the plastic insulation material. Tiny scales of this mineral - some five micrometers thick and several millimeters in length - block the path of the erosion channels, so that it takes longer for them to reach the metal. But because of the mica, the layer of insulation has to be several centimeters thick - valuable space that could be occupied by thicker copper windings.

In addition to mica, researchers on the NanoIso project have also incorporated particles of sheet silicates just one nanometer thick into the insulation. These were developed in cooperation with the University of Freiburg. Because of their huge surface area in relation to their volume, these nanoparticles offer greater resistance to erosion channels. “Laboratory tests show that the nanoparticles improve resistance against partial discharges by as much as a factor of ten,” explains Dr. Peter Gröppel from Siemens Corporate Technology.

As good as all of this sounds, hurdles still remain. Scientists in Freiburg are investigating possible interactions between the nanoparticles and the plastic insulating material. Researchers from the University of Dortmund are testing the service life of the new insulation. And a team in Bayreuth, Germany is looking at how best to process the nanoparticles. Meanwhile, Siemens is responsible for collating all this new information. The ultimate aim is to develop an insulation material that meets the full range of industrial requirements, including that of being quick and easy to manufacture. The next step toward a more efficient generator will be to install copper conductors fitted with the new insulation.

The resulting generator will be provided by power company RWE. In the future, when one of RWE’s power plants needs to be upgraded, the generator will be fitted with the new technology instead of being replaced at great expense. “We don’t know exactly which power plant this will be,” Gröppel explains. But he’s confident that in a few years the knowledge gained from this joint research project should be helping to make power plants operate more energy-efficiently.