| The multi-terminal flexible DC grid based on modular multi-level converter has the advantages of flexible power flow control,convenient for clean energy access,easy to implement multiple power supply and multiple points of power receiving,and is an ideal power grid form for energy interconnection.However,the topology of multi-terminal flexible DC grid is lattic or ring.The capacitances of multiple converter stations discharge at the same time when a DC line fails.The fault current presents the characteristics of high rising rate,high peak value and fast spreading speed.The converter stations have limited ability to withstand overcurrent,so the DC side fault greatly threatens the safety of converter device.Without high-speed tripping,DC line fault will cause the converter stations on both sides of the faulted line to be blocked,and even lead to the blackout of the whole grid.Transmission reliability is greatly affected.Therefore,when a single line fails,the protection should quickly and accurately identify the faulty section and isolate it.In order to limit the fault current,the current-limiting reactors are generally installed at both ends of the DC line in the flexible DC grid,thus providing the boundary for DC line protection.Using this boundary,this paper deeply analyzes the fault characteristics of multi-terminal flexible DC grid DC line,and proposes two protection methods.The main work and achievements are as follows:(1)A non-unit protection method for DC line in flexible DC grid based on current differential difference between poles is proposed.The DC side fault characteristics of the multi-terminal flexible DC grid based on MMC is analyzed.Analysis shows that for different types of fault on the DC side,there are obvious feature differences between the positive and negative abrupt current of the same side line,as well as between the fault line and the normal line on the same DC bus.Thus,a fault identification criterion based on the difference between the ratio of poles current differential is constructed,and the faulted pole selection is completed according to the ratio of the positive and negative voltage of the line;In addition,according to the impact of lightning interference on protection,the identification criterion for lightning interference is proposed;And the fault isolation scheme based on DC circuit breaker is designed.(2)A DC line protection method in flexible DC grid based on the voltage variation characteristics of current-limiting reactors is proposed.The polarity characteristics of the reactor voltages between the faulted line and the normal line are different when a pole-to-ground fault occurs on the DC side of the multi-terminal flexible DC grid based on MMC.In case of a pole-to-pole fault,the reactor voltage characteristics between the faulted line and the normal line on the same DC bus are different.Therefore,based on the polarity relation of current-limiting reactors voltage integral,an identification criterion of pole-to-ground fault is constructed.For pole-to-pole fault,the identification criterion is proposed based on the voltage difference between the positive pole and negative pole current-limiting reactors.And then the whole protection scheme is designed.(3)The effectiveness and reliability of the proposed protection scheme are analyzed by simulation.Using the electromagnetic transient simulation software PSCAD/EMTDC,taking the 9-terminal MMC-HVDC model proposed by Cigre working group B4-58 as a reference,select some of the converter stations and their lines,and build a ±500kV three-terminal flexible DC power grid model based MMC.A large number of simulations have been performed on various situations that affect protection action.The simulation results show that the proposed protection can correctly identify internal and external faults.Lightning interference,fault location,transition resistance and other factors will hardly affect the protection criteria.In addition,the proposed protection scheme only uses single-ended data,which is fast and easy to implement. |
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