Power Oscillation Characterization and Component Sizing For Asymmetrical Fault Ride Through of Grid Forming Converters

Recent fault ride-through codes require the grid-connected converters, that interface renewable energy resources, to maintain grid connection during fault. In this work, we have analyzed the oscillation in the injected real and reactive power that a grid forming converter introduces as it attempts to improve the voltage at the point of common coupling during an asymmetrical fault ride through. A new current reference generation method is proposed that facilitates closed-form quantification of both real and reactive power oscillation during fault ride through operation. Effect of the resulting voltage oscillation at twice the grid frequency on the input DC bus capacitors corresponding to real power oscillation is analyzed, and the subsequent oscillatory current requirement for the DC bus capacitor is derived for appropriate component sizing. Additionally, it gives control over the negative sequence current injection during fault to assist fault identification based on bus capacitor current capability and allowed bus voltage oscillation. The theoretical analysis presented in this work is validated through simulation in PLECS for different symmetrical and asymmetrical fault scenarios. Experimental result for a 3kV A system with a three-phase, grid forming converter during both symmetric and asymmetric fault ride through are provided as well.