Online grid-adaptive control and active-filter functionality of PWM-converters to mitigate voltage-unbalances and voltage-harmonics - a control concept based on grid-impedance measurement

This work presents a control concept for a PWM-converter switching with medium-low frequencies (2.5 kHz) based on grid-impedance measurement to adapt the converters control online to variable grid-conditions and to improve the voltage-quality at the converters connection point. Thus, this work contributes to the actual problem of connecting distributed renewable energy sources to the grid. To improve the voltage-quality, additional active-filter functionality is implemented to mitigate unbalances and harmonics (5^th and 7^th). To meet these control objectives with medium-low switching-frequencies, and thus with medium-low control-frequencies (5 kHz), the control-design is done using discrete time-domain analysis. The theoretical approaches are verified in experiments on a 22 kVA laboratory setup composed of a voltage-source back-to-back converter connected with an LCL-filter to the mains, an interior permanent-magnet synchronous-machine and a grid-impedance analyzer. The results reveal that stable converter operation is reached over a wide range of grid-impedances and that it is possible to improve the voltage-quality with the additional active-filter functionality adapting the measured grid-impedance for the control.