Field measurements of harmonic distortion and the role of the DC-link inductor

Field measurements are shown of the distortion in the line current for power-electronic converters used in industry in Sweden and Europe. The role of the smoothing DC-link inductor has been investigated, both its contribution to the current distortion and its influence on the converter's immunity to variations in the line voltage. A small inductance causes a large current distortion and a large inductance can cause commutation notches in the voltage if the load current is a large fraction of the short-circuit current of the supplying network. Also the size of the DC-link capacitor has influence on the line current. A small capacitor can cause high frequency ripple in the line current due to the switching of the valves in the voltage source inverter. A nonzero sequence third harmonic current can also be present when the line voltage is unbalanced. The most common waveforms of the current are presented, supplemented with some simulations.