The Power Qualities of Wind Turbines

Doctoral thesis, 2000

The power quality of wind turbines is dealt with in this dissertation. The thesis consists of four parts. The first part describes the electrical systems used in wind turbines. The second part presents the results of measurements of different types of wind turbines connected to different types of grids. The measurements include voltage and frequency variations, flicker, transients and harmonics. The third part deals with future standards for measuring and testing wind turbine power quality. In the last part, regulatory requirements concerning the power quality of wind turbines are discussed. Special emphasis has been given to flicker and flicker calculations according to new recommendations for the grid connection of wind turbines. The operation of wind turbines has an impact on the power quality of the connected grid. Depending on the grid configuration and the type of wind turbine used, different power quality problems may arise. All wind turbines have an uneven power production following the natural variations of the wind. If the wind turbine is operating at fixed-speed, the tower shadow and wind speed gradients will result in fluctuating power. The power fluctuations caused by the turbine may cause flicker disturbances. In order to evaluate the significance of flicker, measurements and subsequent flicker calculations must be performed. In the case of variable-speed wind turbines, one drawback is the injection of harmonic currents into the grid. Depending on the type of inverter used, different orders of harmonics are produced. The new recommendations provide tools for predicting the interaction between wind turbines and the grid. Wind turbines which, in combination with the grid, are likely to cause power quality problems can at an early stage of planning be rejected and replaced by a more proper type of wind turbine.