Distance Protection Scheme for DC Distribution Systems Based on the High-Frequency Characteristics of Faults

Due to the advantages of flexible and efficient power conversion, large power supply radius, and high-power quality, flexible dc distribution system has become an important research trend. However, when dc fault occurs, the short-circuit current provided by the converter is nonlinear due to different controls. In this case, it is difficult to identify the correct faulted area based on the converter's varying impedance. Aiming at the above-mentioned problems, based on the characteristics of fault transient components in the system, the circuits of fault high-frequency signal in the converters are analyzed. Then the high-frequency constant impedance equivalent models of converters are established, which are not affected by the control strategies. On this basis, a distance protection method based on the feature of high-frequency fault transient is proposed. This method utilizes the electrical transients of several milliseconds before and after the fault, extracts the high-frequency components of the voltage and current to map the faulted section, and realizes the fault region identification by combining the protection setting and coordination. The detailed model of a six-terminal flexible dc distribution system is built on power systems computer aided design (PSCAD)/electro magnetic transient in dc system (EMTDC) electromagnetic transient simulation platform. The simulation results show that the proposed method has good speed and selectivity. It can reliably identify faulted areas and can withstand transition resistance and system noise.