This research paper addresses the issue of enhancing the operational availability of NPC three-level converter-based high voltage direct current (HVDC) transport systems during alternating current (AC) grid fault conditions. During short-circuit faults in power transmission lines, voltage sags can occur, causing fluctuations in the DC link voltage of converter systems. These voltage sags have the potential to induce reversed power flow and lead to the tripping of VSC-HVDC transmission systems. The objective of this paper is to develop a nonlinear control technique that investigates the fault ride-through (FRT) capability of VSC-HVDC transmission system characteristics during voltage sag events. To achieve this, we conduct semi-experimental investigations using Processor-in-the-loop (PIL) simulations and analyze the results. Symmetrical and asymmetrical voltage sag events with different remaining voltages are applied to an AC grid, and their effects are observed for varying durations.

The proposed nonlinear control technique aims to mitigate the impact of voltage sags on the operational availability of HVDC transport systems. By analyzing the semi-experimental results, we aim to gain insights into the FRT capability of the VSC-HVDC transmission system. This research contributes to the field by providing a comprehensive understanding of the behavior and performance of HVDC systems under AC grid fault conditions. The findings of this study show the effectiveness and high efficiency of the proposed nonlinear control strategy and ensure uninterrupted power transmission even in the presence of voltage sags caused by AC grid faults.