Comparison of the impact of different current reference calculation methods on grid interface converter design under asymmetrical grid faults

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The proportion of power electronic Voltage Source Converter (VSC) interfaced resources in electrical transmission systems is expected to dramatically over the coming decades as the power system is decarbonised. Unlike synchronous generator interfaced resources the fault current characteristic of VSC interfaced resources is fully controllable, and so the current injected by such resources during asymmetric grid fault scenarios is determined primarily by the control strategy applied to them. Three of the most prominent Current Reference Calculation Methods (CRCMs) are analysed in this paper: Injecting only positive sequence current as the conventional standard method; Injecting unbalanced current to suppress the active power oscillations which can limit the required sizing of the dc-bus capacitor; Flexible positive- and negative-sequence control provides a flexible way of balancing regulation of phase voltage against negative sequence voltage suppression. An analytical comparison is made with each precise grid voltage dip variation and with varying active and reactive power set-points based on the GB grid code and maximum current limitation of semiconductors. The performances of the three methods are discussed and validated in Matlab.