Research On DC Side Fault Clear Scheme For Half-bridge Submodule Based MMC-HVDC

The Modular Multilevel Converter(MMC)has received extensive attention and application in the field of DC transmission by virtue of its flexible and expandable topology and the advantage of better output characteristics compared to the traditional Voltage Source Converter(VSC).With the rapid development of power electronics technology,flexible DC transmission technology is also changing with each passing day.From the previous low-level,low-capacity to high-level,high-capacity,the flexible DC transmission system are putting forward even higher requirement on the DC-side fault isolation speed.The half-bridge submodule(HB-SM)topology is widely used in modular multi-level converter based HVDCs(MMC-HVDC),but this submodule topology is vulnerable to DC-side faults.DC fault current will be present for a long time under the feed of the grid side current.This paper first analyzes the transient characteristics of the DC side faults of the MMC-HVDC transmission system and obtains the analytical expression of the fault current through the equivalent circuit at the time of fault.Based on the analysis of the fault mechanism,the fault criteria are summarized and the corresponding fault detection methods are provided.The corresponding simulation model is built on the PSCAD/EMTFDC electromagnetic transient simulation platform to verify the effectiveness of the fault detection method.In view of the existence of rectifier effect,slow isolation speed,and high overcurrent level,of the existing Single-Thyristor Switch Scheme(STSS)and Dual Thyristor Switch Scheme(DTSS),this paper improves the topology of redundant sub-modules and proposes a DC side fault removal method for MMC-HVDC systems equipped with topology-improved sub-modules.By applying the proposed method to the two-ends MMC-HVDC system,the problem that the arc of the short circuit point cannot be extinguished because of the rectifier effect of the half-bridge sub-module based MMC-HVDC is solved;the overcurrent level of the converter arm caused by the AC-side current-feed in the traditional “DTSS” is limited;the decay rate of the DC-side fault current is greatly accelerated.For non-permanent DC-side faults,it is not necessary to disconnect the AC circuit breaker,which facilitates quick restart of the entire system and improves system reliability.Based on the fault detection method and the fault removal method,the DC-side fault protection process and the system restart sequence of MMC-HVDC after fault are proposed.Once the fault is detected by the proposed fault detection method,the redundant sub-module method will be used to cut off the fault.After the DC fault is removed,the system turns to the restart mode.Finally,the simulation validates the effectiveness of the entire process.