Symmetrical Component Model for a Transformerless, Grid-Tied Current Controlled Inverter under Unbalanced Conditions

Accurate analysis of fault currents in inverter-based systems has always been a challenge. Most methods used today to predict fault currents in inverter-based systems employ the classical symmetrical component method. Typically, the inverter is assumed to act as an ideal current source providing balanced currents during faults. This assumption will work in special circumstances, as for the case when wye-delta transformers are interfaced to the inverter, but is problematic in other system arrangements. Hence, this paper proposes a modified symmetrical component model for current-controlled inverters experiencing unbalanced faults. The new symmetrical component model accounts for unbalances, the inverter control, and the output filter characteristics. The model is derived and justified mathematically. Finally, simulation studies using the PLECS simulation environment have shown that the new model offers superior prediction over the classical assumptions.