Analysis and improvement of the reactive power cascade control strategy for MMC-HVDC receiving-end converter connected to weak grid

The weak AC grid in a modular multilevel converter modular multilevel converter based high voltage direct current (MMC-HVDC) receiving-end system requires faster reactive power support from the receiving-end MMC. However, under the MMC traditional constant AC voltage dual-loop control strategy, the increase in MMC's reactive power support speed will reduce the system stability. The reactive power cascade control strategy is a novel MMC control strategy. The analysis in this paper demonstrates that it possesses advanced performance under faster reactive power support, which is required by the MMC connected to a weak AC grid, but, some of its control links and operations can cause impacts exceeding that a weak AC grid can undertake. To solve this problem, based on the original control strategy, this paper proposes an improved reactive power cascade control strategy for weak AC grid conditions. The complete framework and the detailed design of primary control links of the improved cascade control strategy are displayed in this paper. Finally, a digital simulation has been conducted to validate the analysis and the proposed improved control strategy in this paper.