Transient measured impedance-based protection scheme for DC line faults in ultra high-voltage direct-current system

DC line fault is a major threat to the security of ultra high-voltage direct-current system due to complex terrain and terrible weather conditions over long distances. This study proposes a DC line backup protection method on the basis of transient measured impedance (TMI), which is defined as the ratio of transient voltage and transient current at both sides of DC line. The characteristics of TMIs are theoretically analysed for DC line faults and external faults, and these characteristics are used to construct a protection scheme. Since the TMIs only depend on the system structure, the proposed method is able to identify the faults correctly, unaffected by the fault resistance and DC line attenuation. A selection method for the faulted line is also proposed based on the ratio of transient component on the bipolar lines. Comparison with the transient-component-based protections and conventional protections demonstrates the promising performance of the proposed protection. Numerous simulations on PSCAD/EMTDC platform and the tests with field fault-recording data have validated the effectiveness of the proposed protection scheme.