Double Synchronous Unified Virtual Oscillator Control for Asymmetrical Fault Ride-Through in Grid-Forming Voltage Source Converters

In this work, a double synchronous unified virtual oscillator controller is proposed for grid-forming voltage source converters to achieve synchronization to the fundamental frequency positive- and negative-sequence components of unbalanced grid voltage without any phase-locked loop. The proposed controller leverages a positive-sequence virtual oscillator and a negative-sequence virtual oscillator, a double-sequence current reference generator, and a double-sequence vector limiter. Under fault conditions, the controller enables to limit the converter output current below/at the maximum value allowable by the converter hardware while retaining synchronization regardless of the nature of grid faults. Consequently, symmetrical and asymmetrical fault ride-through can be achieved without the need for switching to a backup controller. This article presents the implementation and detailed analysis of the double-synchronous structure, which enables simultaneous synchronization to both sequences during current-unconstrained and -constrained operations. Validation of the proposed controller is provided through laboratory hardware experiments.