Research On Power Control Strategy Of MMC-HVDC Interconnected System

In recent years,modular multilevel converter based flexible HVDC transmission projects have achieved successful applications at home and abroad.The topology of multiple modules in series not only improves the quality of the output AC voltage waveform,but also increases the capacity and voltage level of the converter station,so that flexible DC transmission has great advantages in the fields of new energy grid-connected consumption and regional grid interconnection.MMC-HVDC can independently control active power and reactive power at the same time,and can realize flexible power control strategy.The power control strategy can adjust the instantaneous exchange power between the DC system and the AC system to ensure the safe and stable operation of the interconnected system.In this paper,the reason of power oscillation of the generator near the converter station in the MMC-HVDC interconnected system under transient disturbance is analyzed,and an AC control strategy for flexible HVDC transmission based on active power adaptive setting method is proposed.This control strategy can make MMC-HVDC present the power transmission characteristics similar to that of AC transmission lines,quickly suppress the power oscillation of generators,and maintain the safe and stable operation of the MMC-HVDC interconnected system,thus improving the transient stability of the system.The correctness of the analysis process and the effectiveness of the control strategy are verified in the application scenarios that the large power grids are interconnected by MMC-HVDC.Aiming at the application scenarios that the large power grid supplies power to the regional small grid through the mixed channel of MMC-HVDC and AC transmission line,a flexible HVDC transmission AC control strategy based on additional frequency control and active power adaptive setting method is proposed.This control strategy can adapt to the MMC-HVDC to switch between the island operation mode and the network operation mode,and ensure that the system can operate safely and stably after the transient disturbance of the regional small power grid in the island operation mode,so that the large power grid can play the role of power support and frequency support for the regional small grid.For the MMC-MTDC system,the flexible HVDC transmission AC control strategy based on the active power adaptive setting method is optimized for multi-terminal flexible HVDC grid.According to the flexible and controllable characteristics of MMC-MTDC,a coordinated AC control strategy is proposed,which can realize power support between multiple AC systems,thus improving the applicability of flexible HVDC transmission AC control strategy.