Transient Stability Analysis of Grid-Forming Converters with Current Limiting Considering Asymmetrical Grid Faults

TL;DR

This work tackles transient stability of grid-forming converters under asymmetrical faults with current limiting. It develops a $P-\delta$ framework that explicitly accounts for positive- and negative-sequence components and the elliptical current limiter, showing that the inner-loop controllers act as equal resistances in both sequences. A key finding is that the negative-sequence active power, $P_o^{-}$, is negative across faults, degrading transient stability and reducing the critical clearing time; these curves agree with EMT simulations and experiments. The results extend traditional symmetrical-fault stability analysis to asymmetrical faults and inform design of current-limiting schemes for grid-forming converters.

Abstract

Under asymmetrical faults, analyzing the transient stability of grid-forming voltage-source converters (GFM-VSCs) becomes essential because their behavior fundamentally differs from that under symmetrical faults. When current limiting is activated under asymmetrical faults, the point-of-common-coupling voltage of a GFM-VSC contains both positive- and negative-sequence components, and the interaction between these components generates a non-negligible negative-sequence-driven active power. However, the transient stability of GFM-VSCs under asymmetrical faults has not been sufficiently investigated, and the influence of negative-sequence-driven active power remains unclear. Accordingly, this letter derives the P-δ curve of a GFM-VSC with an elliptical current limiter under asymmetrical faults by explicitly accounting for negative-sequence effects. This enables a more accurate transient stability assessment when extending conventional symmetrical-fault analyses to asymmetrical conditions. The theoretical analysis is validated by the agreement between the derived P-δ curve and both simulation and experimental results.

Figures (5)

• Figure 1: Block diagram of a GFM-VSC with an elliptical current limiter.
• Figure 2: Equivalent sequence networks of a grid-connected GFM-VSC with ECL: (a) positive-sequence. (b) negative-sequence. (c) zero-sequence.
• Figure 3: $P\text{--}\delta$ curves under pre-fault, SLG, DLG, and LL fault conditions.
• Figure 4: Transient stability analysis of the GFM-VSC using $P_o$, $P_o^{+}$, and $P_o^{-}$ curves under a SLG fault.
• Figure 5: Experimental results under an SLG fault: (a) relationships between $E_{\mathrm{ref},a}-V_{oa}^{+}$ and $i_{oa}^{+}$, and between $-V_{oa}^{-}$ and $i_{oa}^{-}$; (b) response for a 0.27 s fault duration (Case I). (c) response for a 0.28 s fault duration (Case II).