A High Step-up Ratio Soft-Switching DC-DC Converter for Interconnection of MVDC and HVDC Grids

Abstract

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency dc-dc converters with high voltage step-up ratio to interconnect medium-voltage (MV) dc distribution grids and high-voltage (HV) dc transmission grids. This paper presents an isolated bidirectional soft-switching dc-dc converter combining two-level converters in parallel on the MV side and a modular multilevel converter (MMC) on the HV side. A dedicated control method of the proposed converter is presented. By the proposed method, a certain reactive current is injected into the MV side by the MMC to ensure soft-switching on the MV side. The proposed converter presents low power-semiconductor total device rating and low semiconductor losses over a wide power range at variable input and output voltages.

Existing System

Modulation Methodology.

Proposed System

A novel high voltage step-up ratio isolated bidirectional soft-switching dc-dc converter is proposed in this paper for interconnection of MVDC and HVDC grids. The proposed converter combines TLCs connected in parallel on the MV side and the MMC on the HV side. Thanks to the dedicated control strategy, the efforts of semiconductor-device series connection can be significantly reduced on the MV side. Consequently, the employment of a massive amount of high-voltage insulated transformers are avoided. Compared to the existing solutions, the proposed converter presents low semiconductor losses over a wide power range at variable input/output voltages.

Architecture

Configuration of the proposed MVDC-HVDC converter