Selection of feedback signal for the inverter side of an HVDC system

This paper presents a small signal stability analysis of the most often used inverter control strategies as well as some of the alternative inverter control methods suggested in literature. The eigenvalue analysis and the root locus technique are used to study the dynamic influence of various inverter control methods on the overall power system stability. The positioning of corresponding transfer function zeros is used as the first screening tool in the analysis. The results show that reactive current feedback would be the most beneficial inverter feedback strategy. Direct current feedback can be used to increase the system robustness to the second harmonic instability. Except for the reactive current feedback, all considered feedback signals are vulnerable to instabilities at some frequencies, which is indicated by right-half plane zeros. These unstable zeros will limit the maximum possible feedback gain. The last section studies the location and movement of the eigenvalues affected by reduction in the AC system strength. The results show that reactive current and direct voltage feedback are the most beneficial inverter control methods for HVDC systems with weak receiving AC systems.