Research On Single-end Quantity Protection Of Ring-shaped Flexible DC Transmission Network

Compared with traditional point-to-point DC transmission systems,flexible DC transmission technology has more flexible control,stronger power supply reliability,and better economic benefits of investment,and has obvious advantages for asynchronous AC grid interconnection systems.Based on the topology of the voltage source converter,this paper studies the protection of the transmission line of the ring network flexible DC transmission network.Firstly,the type of topology of flexible DC transmission technology is understood.The two-level converter is used as the basic topology of the converter in this paper.In the simulation model of the ring network flexible DC transmission,the control mode selected by the main converter station is fixed DC voltage and fixed reactive power control,and the control mode selected by the slave control station is fixed active power and fixed reactive power control mode;followed by the SPWM modulation strategy And the simulation model of the ring network flexible DC transmission system was built in PSCAD/EMTDC software.Secondly,according to the unique characteristics of this type of ring network flexible DC transmission system,the protection area inside and outside the area is divided.Taking the VSC1 side protection as an example,the out-of-area faults are AC side faults,converter faults,and DC transmission line L₂₃ faults.In this area,the faults in the area are the fault of DC transmission line L₁₂ and the fault of DC transmission line L₁₃.The analysis and formula derivation of the catadioptric path of the traveling wave of the internal and external faults of the ring network flexible DC transmission system and the upward wave of the path are used to propose a fast DC based on the cosine similarity of the line mode current for the internal and external faults of the system Protection scheme.Then,use the line mode current of the DC line detected by the measuring terminal to obtain the rate of change by derivation to achieve protection start.According to the polarity of the initial traveling wave of the line mode current of the I and II loops detected by the measurement terminal on the DC side,the cosine similarity of the line mode current of the I and II loops is calculated to determine the inside and outside the fault.For the fault line selection of multi-terminal annular DC transmission lines,a protection method using the difference of low-frequency transient energy of the line mode current of I and II circuits is proposed.By analyzing the topological boundary of the voltage source converter,the current transmission line is obtained.In the event of a fault,the low-frequency transients of the line mode current of the faulty return line are significantly higher than the low-frequency transients of the line mode current of the non-faulty return line.This feature is used to achieve fault line selection protection.For the multi-terminal ring-shaped DC transmission line,a fault pole selection protection method based on the integral difference of the square of the pole current is proposed.The difference of the integral value of the current square is used as the criterion of fault pole selection.Finally,through the simulation model to verify its theoretical analysis,it is concluded that the protection scheme can act quickly when the area of the DC transmission system fails,and the protection does not act when the out-of-area fault occurs,verifying the effectiveness of the protection scheme And feasibility.