Bipolar output direct-coupled DC–DC converter applied to DC grids

Abstract

The unipolar-input bipolar-output DC–DC converters with megawatt rating are the key equipment in the multi-terminal HVDC and DC grids, especially in the DC wind farm. This paper presents a novel direct-coupled transformerless DC–DC converter to interconnect unipolar DC system with bipolar DC transmission line based on cascaded sub-modules. The sub-modules can be achieved by active clamping (clamped diode), half bridge and full bridge. The proportional-integral control strategy is employed to balance the capacitors' voltages in several sub-modules and the energy between the upper and lower arms. Compared with other bipolar output DC–DC converters, the new direct-coupled DC–DC converter offers several merits: (1) The volume and the cost can be reduced due to the absence of the transformer and the large-scale sub-module capacitance; (2) There is no extra AC–DC conversion process, so it can achieve higher efficiency; (3) The IGBTs only turn on or off once in a period, which results in lower power loss; (4) High step-up ratio, redundancy, and fault tolerance can be achieved by changing the number of sub-modules. Finally, the simulation of the proposed DC–DC converter based on MATLAB/Simulink platform is given to verify the feasibility of the topology and the accuracy of the theoretical analysis.

Keywords

• HVDC power transmission
• insulated gate bipolar transistors
• switching convertors
• DC-DC power convertors
• wind power plants
• bipolar output direct-coupled DC–DC converter
• DC grids
• unipolar-input bipolar-output DC–DC converters
• DC wind farm
• direct-coupled transformerless DC–DC converter
• unipolar DC system
• bipolar DC transmission line
• bipolar output DC–DC converters
• extra AC–DC conversion process