Current Limiting DC Circuit Breaker Based On Double State Commutation And Its Reclosing Strategy

With advent of fully controllable devices: IGBT,the flexible DC transmission which relied on the modular multilevel converters(MMC)gradually has practicality,it has the advantages of large transmission capacity and conveniently net-connection of distributed resources.due to the much smaller impedance of DC transmission systems,the fault current development is fast,the amplitude is high,and without zero crossing point,so the AC circuit breaker is not suitable for the flexible system,it is necessary to study a reliable and fast DC system fault isolation scheme,in addition,the system has many transient faults,and the reclosing function of the system needs to be considered.The contents of this article are as follows.(1)Introduce the current research of fault isolation scheme of flexible straight system,which the fault isolation scheme include DC circuit breaker has the most advantages,which can not only avoid the shutdown of the whole system because of the converter station blocking,but also make it easier to achieve rapid reclosing,hybrid DC circuit breaker simultaneously has the advantages of mechanical and solid-state type,which is the hotspot of current research,and can further reduce on-state loss,improve performance and reduce costs after combining with mechanical DC circuit breaker.(2)Elaborated the key technologies involved in the study and application of currentlimiting DC circuit breakers,including the main components,basic commutation methods,DC arc characteristics,arc extinguishing technology,the cascade voltage and current sharing technology of power electronic switches,and systematically analyzed the working principle of current-limiting DC circuit breakers in DC systems.(3)Introduce the mathematical model of the flexible DC system based on MMC and its controller design principle,the basic control strategy,and the switching of MMC control strategy under different working conditions from the dynamic characteristics of the DC system.(4)On the basis of the existing scheme,a current-limiting DC circuit breaker topology with reclosing function(RCL-HDBC)is proposed,introduced the topology in detail,derived the principle in phased,and the selection basis of main component parameter is given.The scheme use the thyristor as the solid-state device of commutation branch,and uses the precharged capacitor to provide conduction and shutdown voltage-drop for the thyristor,and inputing the current limiting branch can quickly limit the current after the fault,consume the energy on the non-fault side through the self-discharge/absorption branch while charging the pre-charged capacitor,so it has the ability to quickly reclose;The circuit breaking part adopts IGBT with higher power full control device,which can adapt to higher breaking current after the current limiting treatment of the device;Add the landing branch,discharge the fault side inductance after completing fault isolation.An landing branch is added to discharge the fault inductance after breaking,and reduce the energy absorption pressure of the lightning protector.(5)Combined with the advantages of mechanical circuit breakers,a dual-stage commutation current-limiting topology(DS-DCCB)is proposed,and the structure introduction,work analysis and parameter selection are also carried out.The scheme inputs the currentlimiting branch in advance through the optimization of the structure,which further reduces the requirements for the electrical strength of the solid-state device,improves the economy,and can adapt to the larger fault current by increasing the number of IGBT modules in parallel with the current-limiting branch,and proposes an modified reclosing strategy by switching the MMC control mode based on this topology to improve the reclosing speed.(6)The 320 kV/1.6kA pseudo-double-ended DC transmission model is established in the simulation software to verify the proposed topology and reclosing strategy.The results confirmed DS-DCCB topology has good current limiting effect,fault current peak reduced by73% compared with the ABB scheme,compared with RCL-HDBC fault current peak reduced by 45.4%,which is conducive to make the circuit breaker configuration lightweight;The shortest fault breaking time is only 5.53 ms,which can quickly switch DC faults,and the modified reclosing strategy based on DS-DCCB can shorten the fault recovery time by 63.9%compared with the traditional strategy,and realize rapidly reclosing.In addition,based on the proposed scheme,the DS-DCCB scheme has improved reliability,speediness,economy and reduced control complexity.