Adaptive Pre-Synchronization and Discrete-Time Implementation for Unified Virtual Oscillator Control

Unified virtual oscillator controller (uVOC) is a nonlinear time-domain controller which offers robust synchronization and enhanced fault ride-through for grid-following (GFL) and grid-forming (GFM) converters without the need for switching to a back-up controller. An adaptive pre-synchronization method is proposed for uVOC to enable smooth start-up and seamless connection to an existing grid/network with non-nominal frequency and/or voltage magnitude at the point of coupling (PoC). Furthermore, we evaluate the efficacy of different discretization methods for discrete-time (DT) implementation of the nonlinear dynamics of uVOC and demonstrate that zero-order-hold (ZOH) discretization fails at sampling frequencies up to tens of kHz. DT implementation of uVOC using second-order Runge-Kutta method is presented, which offers a reasonsable compromise between computational overhead and discretization accuracy. In addition, an inductor (L) or an inductor-capacitor-inductor (LCL) type input filter used in typical voltage source converter (VSC) applications leads to voltage deviation at the converter output terminal depending on the power flow. A terminal voltage compensator (TVC) for such voltage deviation is proposed. The efficacy of the proposed methods are demonstrated through laboratory hardware experiments.