Research On Transient Protection For HVDC Transmission Line

Currently, the fault characteristics had not been well used by most of the traveling wave protections for HVDC transmission line, which caused unreliable protecting; at the same time, current longitudinal differential protection can not fulfill its role of backup protection result from the influence of the unbalance current. In this paper, the boundary protection criterion is proposed based on the characteristic differences between internal and external fault, meanwhile, traveling wave based differential protection criterion is raised as backup protection according to the transmission characteristic of traveling wave.The frequency characteristics of the line and the DC boundary are analyzed in this paper, including the influence caused by them to transient characteristics. Then we find that the transient component contains boundary frequency and position frequency. The position frequency is generated by superposition of the traveling wave and the boundary frequency is produced by the frequency characteristics of the line boundary. The spectrum differences of voltage are offset because of the zero points in the transfer function of voltage, however, the spectrum differences of current are relatively clear. Then the effective frequency band [500Hz,3000Hz] is found between the internal and external fault under the interference of the line and the transition resistance.Based on the former analysis, the boundary protection is carried out, which employs the Wavelet Transform to pick up the high frequency components of the fault current. Traveling wave based differential protection criterion is raised based on the whole electrical quantities.The simulation of PSCAD results show that:When the length of the line is 1000km, the boundary protection can identify the ground fault at the end of line with 500Ohm transition resistance and the operating time is less than 10ms. The principle of traveling wave based differential protection is provided with great transient and steady-state character; the protection can avoid the influence of the unbalanced current generated by line-to-earth capacitance and VDCOL control link; when the length of the line is 800km, the protection can identify the ground fault with 500Ohm transition resistance and the operating time is less than 20ms. Both of them can distinguish internal fault from external fault and judge the faulty line exactly.