DC/AC Cross Line Fault Analysis

With the increasing number of HVDC project, the dc transmission lines cross ac transmission lines or ac power distribution lines is common,due to the power transmission corridor is limited. When the dc transmission lines cross the ac lines, the electrical distance between ac and dc lines is short,so cross line fault is likely to occur.In this case,the fault current will present a different characteristics and impact the protection of the ac transmission lines,causing the protection miss-operation or rejected to operation.In this paper,we mainly analyzing the characteristic of the fault current on the ac system under cross line fault, to be the theory basis of the ac transmission protection study in cross line fault.There is no theoretical except simulation,so the difficulties of the study to cross line fault is the dc system equivalent impedance calculation in the fault point.As the converter which is a typical non-linear element connected the dc side to the ac side of the dc system, so it is the top priority to calculate the equivalent impedance of the ac side through the converter to the dc side.To analyze the equivalent impedance,we first overall calculate the relation between voltage and current at the dc port.From the analysis of it,we divided the equivalent impedance into two parts,they are respectively the influence of the transformer branch and the influence of the ac system impedance.using time domain method and switching function,respectively,we get the inverter equivalent impedance.Result show that the impedance is closely related to the commutation overlap angle.Cross line fault will cause the transformer dc bias,which can influence the equivalent impedance. We firstly analyze the mechanism of dc bias, and the relationship between the dc current in the transformer winding and the the second harmonic produced by excitation current is got.At last the influence to the equivalent impedance is calculated.Finally, the sequence network to calculate the ac fault current in the ac/dc cross line fault is exposed, simulation verify the validity of the analysis.