Non-unit Protection for Asymmetrical DC Line Faults in Bipolar LCC-HVDC Transmission Systems

The faults in high voltage direct current (HVDC) transmission lines cause a sudden rise in DC, resulting in over-stressing of converter valves and transformer windings. DC line faults of longer duration, or permanent nature, will lead to a severe disturbance in the associated AC network as well. Thus, the HVDC line faults must be detected immediately to interrupt the fault current by initiating proper actions of control and protection. This paper implements a new method to detect the asymmetrical (pole-to-ground) faults in the conventional bipolar LCC-HVDC transmission system. The proposed method defines a Fault Indicating Parameter (FIP) based on the transient voltage and current elements of each pole. The transient elements are obtained based on single-end measurements. Single-end or non-unit measurements lead to the development of simple, cost-effective and fast protection criteria. Also, the method is applicable to faults close to the line boundary and of high resistivity. A two-terminal bipolar LCC-HVDC transmission system, based on +/- 500 kV, 1000 MW and 900 km length, is used to evaluate the performance of the proposed technique in offline mode, using MATLAB/Simulink software. Also, the results are validated using an OPAL-RT real-time simulator under divergent fault conditions. The simulation results prove the robustness, selectiveness and accuracy of the proposed scheme.