Parameter coordination of modular multilevel converter for robust design during DC pole to pole fault

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Abstract
Sub-module over-current caused by dc pole-to-pole fault in modular multilevel converter based HVDC (MMC based HVDC) system is one of the important research objects about its electrical characteristics. In this paper, the fault mechanism before and after the converter blocked was analyzed respectively and the circuit model for the analysis of sub-module over-current was explored. The analytic equation for over-current calculation was deduced and a detailed analysis was also performed. The results indicate that the sub-module over-current is the ac system three phase short circuit current superposed the discharging current before the converter blocked, and the sub-module over-current is the ac system three phase short circuit current superposed the valve reactor freewheeling current after the converter blocked. The appropriate valve reactance and sub-module capacitance slow down the rate of fault current rise to leave enough time for the protection. This paper provides references for sub-module components electrical design.