Loading Analysis of Modular Multi-level Converter for Offshore High-voltage DC Application under Various Grid Faults

The modular multi-level converter has become an interesting candidate in high-voltage DC systems due to its higher voltage levels and modular construction. Low-voltage ride-through is an important grid requirement for modular multi-level converter-high-voltage DC since not only causes control challenges but may also result in overstressed components for the modular multi-level converter. However, the thermal loading of the modular multi-level converter under various grid faults has not yet been clarified. In this article, the power loss and thermal performance of the modular multi-level converter during grid voltage dips are studied. The impacts of two typical grid faults to the modular multi-level converter in terms of operating and loading conditions are analytically researched and simulated. It has been found that the operating and loading conditions of the modular multi-level converter under low-voltage ride-through strongly depend on the types and severity values of grid voltage dips. The thermal distribution among the three phases of the modular multi-level converter may be quite uneven, and some devices are much more stressed than the normal operating condition, which may be of importance for the design of the cooling system.