Abstract
The existing current differential protection has been designated as the backup protection for LCC-HVDC lines. However, its reliability is very limited under the dc line high-impedance faults. In this letter, the challenges of the existing current differential protection are firstly revealed to offer a basic background. These challenges are shown to be caused by the existing disturbance blocking criterion, which imposes the unnecessary long time-delay under the dc line high-impedance faults. Moreover, a novel disturbance blocking criterion is proposed to identify the concerned disturbances, which is realized by a combination detection of the ac faults and commutation failures in LCC inverters. Compared with the existing disturbance blocking criterion, the proposed criterion has no long time-delay even under the dc line high-impedance faults. This salient feature makes the concomitantly proposed current differential protection operate much more quickly under the dc line high-impedance faults, and thus much more reliable than the existing one. Finally, the simulation results based on an actual bipolar LCC-HVDC link using the PSCAD/EMTDC program validate the proposed criterion and protection.
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