Research On Reactive Power And Voltage Coordination Control In AC Side Of MMC-HVDC System

Since the Modular Multilevel Converter(MMC)can adjust the reactive power independently,the MMC can be used as a potential reactive power source to participate in the voltage regulation of AC system.When the voltage drop occurs in AC system due to fault,the MMC can provide reactive power support.Based on the actual flexible DC power grid pro ject,this paper focuses on the coordinated control strategy between the reactive power output of MMC and the voltage sag of AC power grid on the basis of the research on the mathematical model of MMC and the basic control strategy of MMC.Firstly,on the basis of studying the topology and basic working principle of MMC,the mathematical model of MMC is constructed,and the overall control strategy of MMC-HVDC system is analyzed in detail,which lays a good theoretical basis for the establishment of simulation model and control strategy of MMC.Secondly,the mathematical model of MMC is established in d-q coordinate,and a MMC double hopping control strategy based on the d-q coordinate system is proposed.It mainly includes an inner wheel current controller and an outer wheel power controller.The dual edge source MMC system model was established on the PSCAD simulation platform,and the control strategy was verified,and it was shown that this control strategy was effective.It has good dynamic response ability and reliable decoupling control ability of active and reactive power.Furthermore,aiming at the problems of the reactive power and voltage co ntrol strategy adopted by MMC in steady-state condition,the reactive power cascade control strategy of MMC in transient condition is proposed.An actual flexible direct power grid engineering model is built in RTDS simulation system to simulate and verify the reactive power cascade control strategy.The results show that compared with the steady-state reactive power control strategy of MMC voltage control strategy and reactive power cascade control strategy are more reliable and flexible in reactive power output.Finally,in order to improve the transient stability of the communication system,the MMC overcurrent phenomenon is suppressed,and the no pollution grade control strategy is improved,and based on the inefficient serial control strategy,the instantaneous integral reset link after fault and the bridge arm current auxiliary control link of MMC are added.As a result of the simulation,the improved no pollution series control strategy is superior to the original no series serialization control strategy,and as a result,the improved no pollution series control strategy can effectively reduce the overvoltage level after the transitory state of the AC system and effectively reduce the system’s temporality.The stability is enhanced and the MMC overflow phenomenon is effectively restricted.