This paper discusses a number of ways in which wind power installations can impact the harmonic levels in the power system. Wind turbines are an additional source of harmonic emission, especially when it concerns ‘‘non-characteristic harmonics.’’ Parallel resonances can amplify the emission from individual turbines. A mathematical model is developed to quantify this amplification. Series resonances can result in high currents, driven by the back ground voltage distortion in the transmission grid, flowing into the wind park.Weakening of the transmission grid will increase lower order harmonics but reduce higher order harmonics