Reactive Power Coordinated Control Strategy for Hybrid Cascaded Systems With Large-scale Renewable Power Hierarchical Access

With the continuous increase of the proportion of renewable energy generation, the sending power grid strength of the UHVDC transmission system becomes weaker, and the voltage stability problems may exist. In order to improve the voltage stability of the weak AC system, based on the hybrid cascaded UHVDC system with large-scale renewable power hierarchical access, the paper proposes a steady-state reactive power coordinated control strategy for the MMC (Modular Multilevel Converter) station and the wind farm to participate in the voltage regulation of the sending-end grid. The operating characteristics of the cascaded system, the reactive power support capacity of weak AC system and the factors that affect the reactive power support capacity of MMC under different operating conditions are analyzed respectively. In order to solve the problem that MMC is weak in reactive power support for weak AC system at full or near full load, an improved control strategy to expand the reactive power limit of MMC is proposed, and the related parameters of the improved control strategy are designed. Simulation examples show that the proposed control strategy can effectively support the voltage of weak AC system and improve the voltage stability of AC system.