Research On Fault Analysis And Protection Principles Of DC Lines In Multi-terminal Hybrid HVDC System

Multi-terminal hybrid HVDC transmission system can provide a larger capacity,more economical and flexible transmission mode,while combining the technical advantages of LCC-HVDC and MMC-HVDC,it will occupy an important position in the future power system development.DC line protection is an important guarantee for safe and stable operation of DC system.However,compared with the traditional two-terminal LCC-HVDC system,the control strategies of multi-terminal HVDC converter station with different principles are different,and the topology of DC line is more complex.There are many T-junction buses.Therefore,the study of fault characteristics and protection principles of multi-terminal hybrid HVDC transmission line has important theoretical and engineering guiding significance.This paper takes multiterminal hybrid HVDC transmission line protection as the research object,and carries out the following research work:(1)The equivalent characteristics of LCC and MMC transient traveling waves are studied.On this basis,the traveling wave refraction and reflection law on line,line terminal and Tjunction bus are deduced.The correctness of the deduction is verified by comparing the simulation and theoretical calculation values.Then the refraction transfer function of T-junction bus is analyzed in frequency domain.The conclusion is that T-junction bus greatly reduces the low-frequency component of fault transient traveling wave,and has no obvious influence on the high-frequency component,showing band-stop characteristics.Finally,the influence of equivalent parameters of T-bus on the refraction transfer function at T-bus is analyzed.(2)Based on the traveling wave refraction characteristics of multi-terminal hybrid HVDC transmission lines at T-junction bus,the simulation model built by PSCAD/EMTDC is used to compare the differences between multi-terminal HVDC transmission lines and two-terminal systems when they are in fault.The effects of the action characteristics of traveling wave protection based on polar line electrical quantity and traveling wave protection based on polar wave modulus are studied respectively.The simulation results show that the T-junction bus connection will not have a significant impact on the characteristic value of protection action based on the change rate of electrical quantity.The single-terminal traveling wave protection can be applied to multi-terminal hybrid DC lines,but it will reduce the characteristic value of protection action based on the magnitude of electrical quantity,which may lead to the rejection of traveling wave protection.(3)The characteristics of transient current polarity and medium and low frequency energy of multi-terminal hybrid DC transmission lines under different rsection faults are analyzed respectively.Based on this,two different fault section identification methods,i.e.the comparison of maximum polarity of transient current wavelet transform modulus and the difference of transient current energy based on wavelet transform,are proposed.The multiterminal hybrid DC transmission line model is constructed by PSCAD/EMTDC.The correctness and validity of the proposed method are verified.Finally,the strategy of fault line isolation is given by combining fault area identification and protection logic after line fault.