Control Strategy And Performance Analysis Of MMC-MTDC System

With the development of flexible DC power transmission technology,Modular Multilevel Converter based Multi-Terminal Direct Current(MMC-MTDC)technology has been able to achieve high-voltage,large-capacity power transmission and interconnection of multiple AC systems across multiple regions.MMC-MTDC technology has become the focus of research in the field of DC transmission.This paper mainly studies the coordinated control strategy of MMC-MTDC system and its performance under the improved control strategy.Firstly,this paper analyzes the topology and operating principle of MMC,deduces the modeling process of MMC's classic Thevenin model,studies the hierarchical control strategy of flexible HVDC transmission system,especially the detailed modeling of converter-level control strategy,and analyzes the current coordinated control strategy of MMC-MTDC system.A 21-level MMC-MTDC model was built on PSCAD / EMTDC.The MMC uses the classic Thevenin model,and the model is verified by analyzing th e operating characteristics of the system.Then,in view of the problem of insufficient DC voltage control ability of the master-slave control strategy,this paper presents a DC voltage control strategy based on power addition.By analyzing the internal mechanism that causes DC voltage fluctuations,this paper adds power reference signals to the input of the constant DC voltage outer loop controller,which can reduce the DC voltage fluctuation of the system and improve the power adjustment speed of the system.Simulation analysis under different working conditions validates the effectiveness of the power-added control strategy.Finally,this paper analyzes the performance of MMC-MTDC system under DC line failure.The characteristics of the DC line unipolar ground fault and the inter-pole short-circuit fault of the MMC-MTDC system are analyzed and verified by simulation first.Then the system adopts the original master-slave control.The strategy and the power-added control strategy are compared and analyzed under the above two faults.The results show that the power-added control strategy reduces the DC voltage fluctuation under the single-pole grounding fault and reduces the short-circuit current peak.In order to effectively reduce the short-circuit current,this paper uses a resistance current limiting method.Through simulation verification,this method can greatly reduce the short-circuit current.