| Using the traditional commutator converter(LCC)DC transmission technology,it has the advantages of large capacity transmission,convenient system power regulation,asynchronous networking and so on,but it can not change the power flow direction,easy to happen commutator failure.Flexible DC transmission technology can make up for the failure of LCC commutation,and the power can be reversed,but there are problems of low voltage level and large power consumption after operation.Hybrid cascade transmission system combines the advantages of both and complements each other’s disadvantages.Currently,it has been applied in ±800k V Baihetan—Jiangsu hybrid cascade transmission project,which is the first time in the world to adopt the cascade transmission technology of LCC and MMC on the inverter side.As the DC transmission line of the hybrid cascade transmission system is 2140.2 km long,the mountainous terrain it passes through is also very complex,which is prone to ground faults.Therefore,it is very important to study the DC line protection suitable for this project.Firstly,according to the topological structure and actual engineering situation of LCC and MMC,combined with the principle of their control operation,Baihetan-Jiangsu hybrid bipolar cascade transmission system with ±800k V is established.The rectifier station and inverter station LCC adopt double 12-pulsating thyristor commutator,while the low end MMC of inverter station adopts modular multilevel converter.After that,the two kinds of traveling wave main protection of DC line are simulated and analyzed on the simulation model to analyze their reliabilityThen,it is observed that the hybrid cascade transmission system is a true bipolar DC transmission system.When the ground fault occurs on the DC line,the ground pole line will flow through a larger value of current.At the same time,the current of the measuring points of the positive and negative lines and the ground terminals at both ends of the converter station meet Hough’s current law when the DC side of the line is on the positive ground fault.By studying the characteristics of the fault current loop closed when the DC side is on different ground faults,the positive and negative faults can be quickly identified.After that,the fault pole is determined by the positive and negative of the ground pole current and the setting of the bipolar DC line current.When the rectifier station and inverter station are identified as positive faults,it can be determined that the DC line has a ground fault.Based on this,the longitudinal protection method of DC line based on fault current loop is proposed,and through the simulation analysis of PSCAD software,it is concluded that the protection sample rate is low,the protection is sensitive and reliable,and has good engineering practicability.Finally,after analyzing the traveling wave propagation process when single-phase ground fault occurs inside and outside the ±800k V DC transmission line,considering the structural characteristics of Baihetan-Jiangsu hybrid cascade transmission system,the equivalent impedance of the measuring points at the head and end of the line is obtained.According to the difference of high frequency impedance measured at both ends of the DC line when the fault occurs inside and outside the DC line area,the longitudinal protection method of long-distance LCC-MMC hybrid cascaded DC transmission line based on high frequency impedance is proposed.Through the simulation analysis,the principle of the protection method is simple,withstand the transition resistance,can move quickly. |
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