Wind-turbine harmonic emissions and propagation through a wind farm

Abstract: The increasing demand for sustainable energy is one of the driving forces behind the increasing use of wind power by means of wind-turbines in electric power systems. Modern wind turbines commonly employ variable-speed-generator technology associated with a power-electronic converter as part of the grid connection. A drawback of the use of power electronics is the presence of harmonic emissions. Consequently a systematic study on distortion from wind power installations is needed; this holds for individual wind turbines as well for complete installations. In the work, measurements and analysis of harmonic emissions were performed on a number of wind-turbines in several wind parks in Northern Sweden. The measurements on the individual wind turbines reveal that the harmonic emissions are different from each other, even for different turbines from the same manufacturer. However in general the characteristic harmonics dominate the harmonic emissions. Furthermore, a long-term measurement shows that the dominant frequencies in the emission change with time. The total emission from a wind park into the public grid is determined by the emission from individual turbines and by the properties of the wind park. To study the impact of the wind park on the propagation a ``transfer function'' method has been introduced, and applied by means of calculation and simulation. The method is based on a mathematical model that predicts the harmonic propagation from the wind turbines to the public grid in the frequency range up to 50 kHz. Applying the model to three example parks reveals that, the amplitudes at the resonance frequency are strongly dependent on the resistance of both underground cables and transformer, especially at high frequencies. In other words, the higher order harmonics are damped a lot. In conclusion, wind park harmonic emissions into the public grid are due to the combination of emission from individual wind turbines and the propagation through the collection grid.