Analytical Evaluation Approach of Complex Inter-Inverter Interactions in Emerging Hierarchical-Infeed LCC-UHVDC Systems

Compared to traditional multi-infeed HVdc (MIDC) systems with inverters coupling only in the ac-side, more complex inter-inverter interactions exist in emerging hierarchical-infeed
LCC
-UHVdc (HIdc) systems with inverters coupling in both the ac- and dc-sides. However, the effectiveness and accuracy of the earlier interinverter interactions evaluation approach in HIdc systems are limited since it is established by the empirical reasoning without rigorous theoretical basis. Thus, in this article, the hierarchical-infeed voltage interaction factor (HVIF) index is first used to quantify the complex inter-inverter interactions in HIdc systems. The salient feature of the HVIF is that both the voltage magnitude and angle couplings at various inverter buses are considered in its definition. Hence, the definition of the HVIF enables the proposed analytical evaluation approach to be more effective than the earlier approach. Moreover, with various dc control modes comprehensively considered, the inverter power-voltage dependence terms in the quasi-steady-state power-flow model of HIdc systems are derived based on the rigorous theoretical analysis. This lends the fundamental mathematical basis to subsequently develop the analytical calculation method of the HVIF, which makes the proposed approach more accurate than the earlier approach. Finally, both the simulated and experimental results sufficiently validate the proposed approach.