Operation Scenario And Control Strategy Of DC Grid With DC-DC Converters

The MMC-HVDC based DC grid technology has significant advantages in solving the integration of new energy sources,the long-distance power transmission and the power supply in urban core areas.In order to improve the reliability,economy and flexibility of system operation,the DC-DC converters will be used to interconnect different DC networks to form a complex DC grid with multiple voltage levels in the future.Therefore,the study on the DC grid with DC-DC converters has theoretical guiding significance and engineering application value.In this paper,the F2F-MMC topology of DC-DC is adopted for its significant advantages in the high voltage and high power scenes.At present,the control mode of F2F-MMC converter is relatively simple,which is difficult to meet the requirements of different operating scenarios.Besides,the researches on the operation scenarios and the coordination control of DC grid with DC-DC need to be further studied.Therefore,this paper conducts researches on the DC grid with DC-DC converters,which include the converter control of DC-DC,the operation scenarios of system,the coordination control and etc.The MMC converter is an important part of F2F-MMC converter,thus the mathematical model,topology and modulation mode of MMC converter are introduced.The mathematical model of F2F-MMC converter is derived and analyzed.The controller design of F2F-MMC converter is presented and the usual control configuration is implemented.Based on the traditional control of F2F-MMC converter,an adaptive control of DC-DC is proposed to realize the flexible switching between the constant DC voltage control and the constant power control,and the power stability of the converter is improved with the control.In order to reduce the transient fluctuations generated in sub-module fault,a redundancy protection strategy for DC-DC with smooth clearance is proposed,by which the redundant sub-modules are further divided into hot-reserved modules and cold-reserved modules.When the sub-module faults occur,the hot-reserved modules can replace the fault sub-modules,and cold-reserved modules can be inserted smoothly with the proposed control.In this way,the smooth transition of the fault clearance can be realized and the operation reliability of the converter can be improved.Finally,the validity of the adaptive control and redundant protection control is verified on PSCAD/EMTDC.According to the operating conditions of DC grid,three operating scenarios are proposed:local self-balancing,supported by the upper network and supported by the lower networks.By selecting appropriate parameters,the flexible switching of different scenarios can be realized.With the operating scenarios,the frequent fluctuations of DC-DC can be avoided and the normal operation of system can be guaranteed under different conditions.In order to optimize the power transmission between the networks with different voltage levels,two coordination control strategies were proposed for the proposed operation scenarios.Under the scenario 2,a coordination control strategy was proposed to reduce the AC and DC disturbance,by which the power distribution is optimized when the station is out of operation.Under the scenario 3,a coordinated control based on OSCR is proposed,which is conducive to the safe and stable operation of the system.Finally,a simulation model is built on PSCAD/EMTDC and the simulation results verify the effectiveness of the proposed operation scenarios and coordination controls.