Research On The Overvoltage For Monopolar Earth Fault In UHVDC Transmission Line

Monopolar earth fault is a kind of fault type with high probability and serious overvoltage in UHVDC transmission line,therefore,the monopolar earth fault is also the focus of UHV DC transmission line overvoltage research.The overvoltage distribution along healthy line for different grounding fault locations of faulty line is obtained by simulating and calculating,and the process of the overvoltage for monopolar earth fault in DC line can be divided into two periods including the first voltage jump and the second voltage jump.The factors may influence the overvoltage are classified into general and critical factors according to the severity,the general factors include the structure and parameters of transmission line,DC control system,tower grounding resistance,the midpoint line tower arrester,transmission line length and transmission power,while the critical factors include grounding fault location,DC filter main capacity and DC filter type.The influence of the factors above on the overvoltage is analyzed,and DC filter main capacity is proved to be the most critical factor affecting the overvoltage of the DC line grounding fault.The mechanism of these two periods and the influence mechanism of DC filter main capacity on monopolar earth fault overvoltage are analyzed according to electromagnetic coupling and wave process of two poles.When the grounding fault occurs on the faulty line,2 current jumps will be produced along the line.Meanwhile,2 reverse current jumps will also be produce along the healthy line because of electromagnetic coupling between the poles.2 current pulses will charge the line-to-ground capacitor reversely,resulting in the 2 voltage jumps on the healthy line on the basis of rated voltage.What's more,the DC filter main capacity influences the second current jump and second voltage jump.The research shows that,the DC filter capacitance parameters have certain influence on single-pole grounding fault overvoltage,and the main capacitor of DC filter influences the overvoltage most significantly,which is the critical factor to limit the overvoltage on transmission line,while the DC filter inductance parameters have little effect on the overvoltage.The increasing trend of overvoltage tends to be slow with the increase of capacitance parameters.The overvoltage of transmission line can be basically limited within 1.7 times of rated voltage as the main capacitor of DC filter is limited within 2?F.