The University of Pittsburgh has been a partner with Mitsubishi Electric Corporation in a joint research collaboration concerning the development and the protection needs of modular multilevel converter (MMC) based high voltage direct current (HVDC) transmission for DC fault case scenarios. The related research activities have included a) HVDC system modeling, b) DC fault analysis, c) DC protection system design, and d) DC protective relaying schemes.
The detailed HVDC system modeling pursuits included MMC capacitor initialization and balancing procedures, controller implementation (DC and AC voltage, current, and power regulators), as well as exhaustive model validation. The fault analysis entailed multiple rounds of extensive fault case scenarios including 1) a parametric trends and their impact upon DC peak transient currents, 2) exploration of a characteristic signal trend that would provide helpful in achieving the protection needs, and 3) sensitivity analysis for the exploration of the effectiveness of a novel. In the end a DC fault protection method was designed with associated protective relaying schemes.
The primary result of the research work is a fault section identification protection algorithm for which a patent application has been submitted. This protection algorithm ensures that a system reclose will be attempted for potentially temporary DC faults on the overhead line section, but also eliminates a system reclose attempt on permanent cable section faults. An additional outcome of the work is a novel method of suppressing circulating currents that exist in the MMC half bridge converter topology during DC fault events. With the first chapter of this research collaboration coming to a close, the next stages of research thrusts are in discussion.
Modular Multilevel Converter for HVDC Applications