Research On Line Protection Of MMC-HVDC Transmission System

Compared to traditional HVDC transmission,the flexible HVDC transmission system can be used as a static synchronous compensator to perform reactive power compensation to the AC system.It does not have the problem of commutation failure.It can provide power for passive systems and can also independently adjust active power and reactive power.Not only that,the HVDC system has a low level of harmonics,and it is suitable for forming a multiterminal DC transmission system.Therefore,the flexible DC transmission system is the future of DC transmission.At this stage,the fault clearing capability of flexible HVDC transmission is poor,and the percentage of overhead lines fault is high.How to quickly detect line faults is an urgent problem to be solved.In response to these problems,this article has carried out corresponding research,analyzed the fault characteristics of DC lines,and proposed corresponding protection principles.The fault characteristics of the DC line were studied.Theoretical analysis of the bipolar fault and single-pole fault current generation mechanism: bipolar fault current generated by the sub-module capacitor discharge,inductor freewheeling and AC side feed;the single-fault fault current is mainly distributed by the line When the capacitor is charged and discharged,the fault current exhibits a fluctuating characteristic,and the larger the transition resistance,the faster the attenuation of the fluctuation.The simulation results verified the analysis and showed that the bipolar fault current has a large rate of rise and it needs to speed up the protection operation time.Based on the transmission characteristics of fault traveling waves,the blocking effect of current-limiting reactors on high-frequency signals was analyzed to propose the boundary protection of the protection.Using the change of voltage after fault,a method of fault selection is proposed.The calculation method of wavelet packet decomposition is used to define the transient energy.A single protection method based on transient energy is proposed.A large number of simulation results show that this method can quickly and accurately determine faults in or out of the zone.According to the characteristics of the fault current,the voltages on the current-limiting reactors of different bipolar faults and unipolar faults at different fault positions are analyzed.After the fault-selection is performed by the aforementioned method,a cross-connection protection based on the voltage of the limiting reactor is proposed according to the voltage characteristics of the reactor.Simulation results show that this method can quickly and accurately determine faults in or out of the zone.