Abstract
Grid-forming converters can suffer from control interaction problems in grid connections that can result in small-signal instability. Their inner-loop voltage controller tends to interact with the outer-loop power controller, rendering the controller design more difficult. To conduct a design-oriented analysis, a control-loop decomposition approach for grid-forming converters is proposed. Combined with impedance-based stability analysis, the control-loop decomposition approach can reveal how different control loops affect the converter-grid interaction. This results in a robust controller design enabling grid-forming converters to operate within a wider range of grid short-circuit ratios. Finally, simulation and experimental results, which validate the approach, are presented.
Highlights
Grid-forming converters can suffer from control interaction problems in grid connections that can result in small-signal instability
Grid-following Voltage-source converters (VSCs) feed the power to the grid via the vector current control, which is synchronized with the voltage at the point of connection (PoC) [3]
The output impedance model of the VSC observed at the PoC in Fig. 1 can be used to analyze the VSC-grid interaction, which is derived according to Fig. 2 as where i2 j2Īø1)iā2
Summary
Voltage-source converters (VSCs) have become more predominant in modern power systems as the penetration of renewable energy sources increases [1]. The adverse effects of VSC control loops on the VSC-grid interaction explained in terms of the VSC terminal dynamics remain unknown To resolve this problem, the impedance-based stability analysis has been adopted to analyze the VSC-grid interaction. The impedance-based stability analysis has been adopted to analyze the VSC-grid interaction In this approach, the entire system is partitioned at the PoC of the VSC, where the small-signal VSC impedance and grid impedance are modeled separately to characterize the voltage-current relationships viewed from each side [18]. The detrimental impact of the voltage feedforward loop on the grid-forming VSC impedance has been identified in the low-frequency range This effect can destabilize the system in stiff grids [21]; this work ignores the dynamic impacts of the outer-loop power control.
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