Research On Protection Principle Of Flexible DC Transmission Lines Based On MMC

Flexible DC transmission system is a new technology emerging in the past two decades.Compared with the traditional HVDC transmission system,the characteristics of flexible DC transmission based on modular multilevel converter(MMC)are as follows: flexible module structure,light expansion,low device operation loss,no risk of commutation failure,small harmonic distortion rate,high output voltage quality.With the rapid development of DC transmission technology,the transmission technology based on MMC has a high degree of compatibility,and has broad application potential in the future.After the operation of the DC power grid,it can effectively solve many problems such as large-scale long-distance power transmission,mixed AC/DC transmission,and centralized access to renewable energy.However,at present,the protection principle of the DC grid is not perfect.For long-distance power transmission using overhead lines,improving line protection is of great significance to the ability to respond to faults and the stable operation of the power grid.This paper analyzes the fault characteristics of MMC lines,and proposes two protection schemes for MMC lines with different lengths.First of all,in order to ensure the uninterrupted operation of the DC lines in a fault,the speed of protection should ensure that the fault can be reliably removed before the converter is locked.By searching the literature,this paper analyzes the structure and working principle of three types of high-voltage DC circuit breakers.Based on the characteristics of high-voltage DC circuit breakers in actual engineering,the range of breaking time for high-voltage DCCB is summarized,and then give the operating time expressions that should be satisfied if the pilot main protection and local main protection can operate reliably.This also lays the foundation for the subsequent research on the main protection of DC lines based on MMC.Secondly,facing the short and medium lines of MMC-MTDC,in order to explore the better performance of the pilot protection,a sensitive enough and fast response to line fault pilot protection criterion is designed.The DC line is equivalent to the accurate Bergeron model which can compensate the capacitance current,effectively solving the influence of the line capacitor current.Considering the factor that the line parameters have a greater influence on the Bergeron model,a protection floating threshold that can eliminate the influence of line parameters is designed.Based on the difference of Hausdorff distance between the current original value and the deduced value in the case of internal/external faults,a pilot protection principle based on the waveform difference of MMC-MTDC lines was proposed.Theoretical analysis and simulation results show that the proposed pilot protection principle has high sensitivity,can reliably and accurately identify internal and external faults in the line,and can meet the requirements of MMC-MTDC systems in terms of quickness index.Finally,facing the short lines of MMC-HVDC,a type of main protection based on the local electrical quantity was proposed,which is sensitive enough and can identify the fault in a very short period of time window.The paper analyze the dispersion phenomenon of the line-mode traveling wave.By using the energy ratio difference between the forward and the backward traveling waves under the condition of forward/reverse fault,a start-up criterion,which can accurately represent the arrival time of the wave-front,was designed.On this basis,based on the characteristics of the same-mode different-frequency time difference,a single-ended location-type main protection principle for MMC-HVDC transmission lines was proposed.Theoretical analysis and simulation results show that the proposed protection principle can be applied to ultra-long-distance transmission lines of 900 km and above.The longer the line length is,the better the operating performance will be,which can adequately compensate for the sensitivity of the existing main protection applied to ultra-long transmission lines and the rapidness problem of the pilot protection.