Double Synchronous Unified Virtual Oscillator Control for Asymmetrical
Fault Ride-Through in Grid-Forming Voltage Source Converters
Abstract
A double synchronous unified virtual oscillator controller (dsUVOC) is
proposed for grid-forming voltage source converters to achieve
synchronization to the fundamental frequency positive- and
negative-sequence components of unbalanced or distorted grids. The
proposed controller leverages a positive- 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 clamp the converter output current below the maximum value
limited by the converter hardware while retaining synchronization
without a phase-locked-loop (PLL) regardless of the balanced or
unbalanced nature of grid faults. Consequently, balanced and unbalanced
fault ride-through can be achieved without the need for switching to a
back-up controller. The paper presents the systematic development of the
double-synchronous structure along with detail design and implementation
guidelines. Validation of the proposed controller is provided through
extensive control-hardware-in-the-loop (CHIL) experiments.