Improved droop control strategy for an MMC-MTDC connected to offshore wind farms with dynamic correction of the actual operating point

Abstract

To solve the problems of overload of converter stations and to reduce operating loss of multi-terminal High-Voltage DC (MTDC) grids based on modular multilevel converters (MMC), an improved droop control strategy is proposed in this paper. The novelty of the suggested strategy is that two droop coefficients aimed at reducing system loss and saving power margin are non-linear weighted combined to realize multi-objective optimized operation in the full operating voltage range. First, a power allocation mechanism model of an MTDC system is built to show the influence of the droop coefficient on the DC voltage under the droop control strategy. Then, the droop coefficient under minimum operating loss is calculated and the adaptive droop coefficient is introduced to save enough power margin when the system meets large unbalanced power. In addition, a self-coordinating weight factor with a low-voltage acceleration factor is employed so that MMC can correct the operating point dynamically. Finally, a derived five-terminal MMC-MTDC is built in PSCAD/EMTDC and the effectiveness and feasibility of the proposed control method is validated by simulation.