| Flexible DC transmission system can adjust active power and reactive power independently at the same time and supply power to passive system.It has the advantages of low harmonic level,flexible and reliable transmission mode,and does not require reactive power compensation and no commutation failure.It can be applied to flexible transmission,asynchronous networking,large-scale new energy grid connection and other fields.It has a good promotion prospect.However,due to the particularity of voltage source converter topology,the current rises rapidly and has a large peak value when the DC line fault occurs.This is in contradiction with the weak overcurrent capability of power electronic equipment.At the same time,the common backup protection current differential protection is difficult to meet the requirements of flexible DC system.Moreover,the flexible HVDC system can not realize fault self clearing by controlling the trigger angle like the traditional HVDC system.Therefore,flexible DC transmission line protection and fault treatment methods are of great significance to improve the safe and reliable operation performance of the system.At the beginning of the study of this paper,the flexible DC technology is applied to the transmission system.Therefore,the research of this paper is based on the flexible DC project at that time,as well as the research on its fault characteristics and protection methods domestically and abroad.Based on this research,this paper starts from the steady-state operation characteristics and fault transient characteristics of modular multilevel converter flexible DC transmission system.The main protection and backup protection methods,fault treatment scheme and fault location method of flexible DC transmission line are studied.The main work of this paper includes the following aspects:(1)A main protection method of modular multilevel converter based high voltage direct current(mmc-hvdc)overhead line based on wavelet entropy is proposed.Based on Zhangbei four terminal HVDC project,the steady-state operation characteristics of the system are studied.The fault mechanism and process of different faults are analyzed in detail.The transient characteristics of different faults are analyzed.It lays a foundation for the research of the main protection method.The mechanism of time-frequency distribution difference of pole-to-pole faults,pole-to-ground faults,lightning disturbances,lightning strikes,sub-modules short circuit faults,and single-phase grounding AC faults are revealed.Combining the characteristics of wavelet entropy which has strong characterization ability for disordered signals,a main protection method based on wavelet entropy is proposed.It solves the problem that transient time-domain waveforms are similar and difficult to distinguish quickly in different scenarios.The proposed protection algorithm can meet the demand of flexible DC system for 3 ms operation of main protection.It has good operation performance,and can effectively distinguish the internal faults and the external faults.In addition,it can accurately identify different fault types of the internal faults.The proposed protection method has excellent ability to withstand the transition resistance and anti-interference.It is not affected by the fault distance and the transient signal generated by sub module short circuit fault.It can be applied to multi terminal flexible HVDC transmission system.(2)A backup protection method based on the amplitude ratio of the sixth harmonic component of the voltage at both ends of the current limiting reactor is proposed.The time-domain and frequency-domain characteristics of fault current in different stages of pole-to-pole fault are studied.The sixth harmonic generation mechanism of voltage and current with different number of bridge arms conducted is revealed.It lays a foundation for the research of protection principle.The mechanism of the variation of the equivalent impedance of the transmission line with the length,frequency and fault transition resistance is analyzed.Combined with the attenuation effect of current limiting reactor on harmonics,a backup protection method based on the sixth harmonic amplitude ratio of voltage at both ends of current limiting reactor is proposed.The influence of line length on the proposed protection method is studied.Based on the capacitive and inductive characteristics of measured impedance,the fault area can be distinguished effectively by setting reasonable setting value.The proposed protection method has good tolerance to transition resistance and anti-interference ability.(3)A new fault treatment strategy for full bridge modular multilevel converter(FBMMC)flexible HVDC transmission system is proposed.When a transient fault occurs,the fault is cleared and restored based on the DC fault ride through mechanism of FBMMC.When permanent fault occurs,fault location and maintenance are carried out based on the principle of energy conservation.Combined with the conduction mode and operation characteristics of full bridge sub module,the control strategy and sub module trigger strategy of full bridge modular multilevel converter are studied.The pole-to-pole fault mechanism of DC side before and after converter blocking,and the fault current blocking principle of FBMMC are analyzed.Combined with the DC fault ride through characteristics of FBMMC,a DC transient fault clearing control strategy is proposed.The mechanism that the energy stored in the fault line will be fed back to the capacitor of the converter sub module after the FBMMC locked is studied.The mathematical relationship between fault distance and feedback energy is analyzed.A fault location method based on energy conservation principle is proposed.The influence of line distribution parameters is considered from the mechanism.It is not affected by sampling frequency and has strong noise tolerance.It is suitable for cable lines and has good ranging accuracy. |
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