Image of the stator in a hydroelectric generator in one of the Sira-Kvina power company's power plants. Photo: Sira-Kvina power company
Fluctuations in energy prices and an increase in varying solar and wind power in the power system mean that hydropower producers have to start and stop production more often. This has consequences for the various parts of the hydropower generator, which are not designed for so much variation.
A lot of variation causes more wear and tear of the insulation
One of the parts that gets increased wear is the stator. Stator is a central part of the generator where electric current is generated as a result of a varying magnetic field. The stator consists of current-conducting copper windings that are covered with several layers of insulation. With an increase in start/stop in power production, the copper windings and insulation will be exposed to more frequent temperature changes and thus thermomechanical stress.
As the copper windings and the insulation react differently to temperature changes (they expand/contract differently) this can cause the insulation to detach from the copper windings and lead to an electrical discharge ("lightning" between the windings). In the worst case, this can cause the generator to break down.
Mapping the negative effects of variable operation
The researchers at HydroCen have therefore taken a closer look at how quickly the temperature in the stator windings drops after a stop in production. The aim is to be able to better map when and how quickly the negative effects of frequent starts and stops occur.
The researchers have analyzed operating data from a small selection of Norwegian hydropower plants.
The results indicate that the temperature in the stator windings drops rapidly in the first couple of hours after stopping, and that it therefore does not take long before they are exposed to thermomechanical stress. The cooling rate depends on both the load and the temperature before and after the reduction/stoppage in production.