Identification of Resonance and Instability From Algebraic Virtual Impedance of Grid-Forming Control in the Perspective of Negative Sequence

Virtual impedance (VI) is commonly implemented in a static manner, typically neglecting the derivative term. This letter investigates fundamental issues associated with algebraic virtual impedance (AVI), emphasizing its characteristics related to negative-sequence impedance. The analysis reveals that AVI exhibits capacitive behavior in the negative-sequence impedance, inducing significant harmonic currents near a particular negative resonant frequency. Furthermore, the influence of control delay is analyzed, illustrating that AVI can readily become non-passive within the negative-frequency region of the frequency domain, leading to harmonic instability. Experimental results are provided to validate the theoretical analysis.