Søren Bækhøj Kjær, Ph.D. - R&D Consultant, Danfoss Solar Inverters
Harmonic issues are to be known leading to wind power plant curtailment or even replacement of components. State of the art calculation principles given in standards and guidelines are leading to an imprecise estimation of harmonics at the PCC using the harmonic current emission of a single inverter. Overestimation of harmonics will cause extensive filter design while under estimation will cause potential curtailment risk. The background for this paper is the method applied for wind turbines with focus on the validity of the method. The method is investigated by measuring the harmonics content of multiple inverters (up to ten) and the sum at the point of common coupling on the low and high voltage side of the mains transformer. Moreover, field measurements from a plant consisting of 800 single inverters are considered. This paper shall find principles to estimate THD at PCC out of measurement/simulation data from one inverter under test The analysis will differentiate between a steady state harmonic analysis for stiff grid and a dynamic simulation of harmonic for ''weak'' grid both considering impedances between inverters. The obtained results get compared with analysis/simulations of the measured results from 10 inverters in the lab Finally the standard calculation method is compared with measurements on a power plant, consisting of approximately 800 inverters in parallel with the network. The presentation will consist of a brief introduction to the problem followed by a deep investigation using measurement results and wraps up with a final conclusion.