Abstract
Two medium voltage direct current (MVDC) grids can be connected via an isolated DC-DC modular multilevel converter (DC-DC MMC) to enhance the power transmission and supporting abilities. DC power, DC terminal currents, arm energy balancing and active/reactive power through inner AC link need to be controlled in such a DC-DC MMC, and the submodule and grid fault situations should also be addressed. Therefore, a multi-object controller is inherently required. This study thus proposes a multi-object control frame that can realise not only DC power regulation but also inner dynamic control, reactive power optimisation and fault riding through. To facilitate the controller design, a detailed dynamic model of DC-DC MMC is also derived, and such a model is turned into different forms according to different control objects. The proposed control method and the main design considerations are given in a detailed mathematical way. A full-scale simulation system containing a 6 MW DC-DC MMC, a 20 kV MVDC grid, a 10 kV MVDC grid, and several sources and loads is built to verify the control compatibility, and a down-scale low-power prototype is also built to test the control performance.
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