Research On MMC Flexible DC Power Distribution Center Based On DC Fault Current Blocking

For DC transmission and distribution systems,it is of great significance for its safe operation to quickly and effectively block the fault current when a bipolar short circuit fault occurs on the DC-side.The existing DC circuit breaker technology is not mature enough,AC circuit breaker response is slow,so the modular multilevel converter(MMC)sub-module topology with DC fault current blocking capability is an ideal choice.The sub-module hybrid MMC based on half bridge sub-module(HBSM)and full bridge sub-module(FBSM)is a new and practical MMC topology which has the capability of blocking the DC-side fault current and also saves hardware costs.Firstly,this article introduces the status quo at home and abroad,including the research status of DC distribution center and MMC,and the research status of DC-side fault current blocking methods.It is clear that the sub-module hybrid MMC based on HBSM and FBSM is used to block the DC fault current.Secondly,the sub-module hybrid MMC is studied in detail.The performance characteristics of several single-module topology and MMC system-level topology with DC-side fault current blocking capability are compared and analyzed.The working principle of the HBSM and FBSM is analyzed in detail,and the mathematical model of the sub-module hybrid MMC is established.According to the mathematical model,the inner-loop current controller,outer-loop controller and circulating current suppressing controller of the sub-module hybrid MMC are designed.According to different types of faults that may occur on the DC side,the HBSM and FBSM configuration principles for the sub-module hybrid MMC are deduced;According to the traditional MMC nearest level modulation strategy,a sub-module hybrid MMC with capacitor voltage balance is designed for the nearest level modulation strategy.Finally,a DC power real-time modulation technique for balancing the load strategy is proposed for a 10 kV circuit with multiple interconnects in a DC distribution center to achieve load balancing among multiple circuits.A four-terminal DC power distribution center simulation model is built on the PSCAD/EMTDC simulation platform.The system power fluctuations,converter out of operation,and DC-side bipolar short circuit fault are simulated separately.For power fluctuations and converter out of operation,whether or not to install a power modulation device for comparative simulation analysis verifies the effectiveness of the proposed DC power modulation strategy for DC power distribution center control.For the DC-side bipolar short circuit fault,the simulation analysis is performed,and the converter of it uses all the HBSM,all the FBSM,and the sub-module hybrid MMC with half of HBSM and FBSM respectively,which verifies the DC-side fault current blocking capability of the sub-module hybrid MMC.