A Differential Pilot Protection Scheme for MMC-Based DC Grid Resilient to Communication Failure

Primary single-end line protection strategies of modular multilevel converter (MMC)-HVdc systems are difficult to make a tradeoff between fast detection speed and high reliability. To improve reliability, the pilot protection schemes based on communication and data exchange can be adopted. However, the communication-based schemes suffer from potential communication failure problems, such as data error, data loss, and time synchronization error. To avoid blocking of protection devices during communication failures, a resilience-oriented differential pilot protection method is proposed in this article. To address the problem of synchronization error, a startup element based on the multiresolution morphological gradient (MMG) of traveling wave (TW) is proposed. For the problems of data error and data loss in communication, the sampled data are preprocessed by morphological filtering (MF). Also, the correlation of TWs is used to identify the internal and external faults; the ratio of the morphological gradient of pole voltages is adopted to discriminate the faulty poles. A four-terminal MMC-based dc grid model is built in PSCAD/EMTDC interfaced with the optical fiber-based communication system built in MATLAB/Simulink. The simulation results show that the protection scheme can effectively identify the faults against serious communication problems of 1‰ bit error rate and 5% data loss.