Robust Grid-Forming Control Based on Virtual Flux Observer

TL;DR

The paper tackles stability robustness of grid-forming converters under varying grid strengths by introducing a virtual-flux observer-based synchronization integrated with load-angle control and direct terminal-voltage regulation to realize voltage-source behavior. It develops decoupled estimation dynamics through targeted gains in the virtual-flux observer and PI for grid frequency, and uses pole-placement to shape the voltage loop, with load-angle relationships connecting $p^*$ to $\delta^*$ via $p^* = \kappa U_g V^*/(\omega_g L) \sin\delta^*$ and $\delta^* \approx \alpha_p p^*$ where $\alpha_p = \omega_g L /(\kappa U_g V^*)$. Numerical simulations and experiments on a 20 kW platform demonstrate robust performance across a wide range of grid impedances, including improved PCC-voltage stiffening and comparable or faster transient responses than a reference RFPSC method. The results illustrate that sensorless, observer-based GFM control can achieve reliable grid integration of distributed generation with tunable bandwidth and damping, maintaining stability and performance in both strong and weak grids. The work therefore offers a practical pathway to enhance robustness of GCCs in modern power systems.

Abstract

This paper investigates the design and analysis of a novel grid-forming (GFM) control method for grid-connected converters (GCCs). The core novelty lies in a virtual flux observer-based synchronization and load angle control method. The terminal voltage of the converter is directly regulated to provide voltage-source behavior. The control parameters are designed for decoupling and pole placement. The proposed method exhibits strong robustness in stability and dynamical performance across varying and uncertain grid strengths. The robust control performance of the proposed method is first demonstrated by small-signal analysis, then validated by experiments on a 20 kVA power conversion system.

Figures (11)

• Figure 1: Circuit topology of GCC and block diagram of the proposed GFM controller. Superscript $s$ denotes variables in $\alpha\beta$-frame.
• Figure 2: Definition of controller coordinate frame and load angle $\delta$.
• Figure 3: Small-signal model of grid synchronization after decoupling.
• Figure 4: Small-signal model of the closed-loop system.
• Figure 5: Closed-loop pole trajectories of the proposed method under total impedance variation from [parse-numbers = false]L=0.1 to [parse-numbers = false]L=1.