Modular DC/DC converter topologies for off-shore DC collection point

Abstract

With the development of the economy, the demand for energy has been substantially growing. Wind power, particularly from offshore wind farms, is one of the best solutions. In Europe, installed capacity had exceeded 8GW by 2105 and will probably reach 40GW in 2020. These ambitious plans require the establishment of large-scale wind farms with more efficient transmission systems. The high voltage direct current (HVDC) transmission system is an effective way to deliver large-scale energy over long distances with lower power losses. However, due to the growth of large-scale offshore wind energy system, the connection between the farms and HVDC transmission lines is more challenging. Medium-voltage DC (MVDC) collection networks are a promising technology for such integration, aiming to eliminate voltage difference. High-voltage high-power DC/DC converters are the key enabler for MVDC grids. But the present lack of suitable high-voltage high-power DC/DC converter topologies is preventing the development of DC networks. Several high-voltage high-power topologies have proposed in previous studies, but most such topologies involve design compromises in terms of switching losses, limited conversion ratios, and lack of modular design, lack of electrical isolation features. This thesis presents three novel modular DC/DC topologies, which are developed based on the conventional modular multilevel converter (MMC), to enable the integration of off-shore wind farms with high voltage direct current (HVDC) transmission systems. The first topology is a unidirectional single-phase modular DC/DC converter, and the second is a unidirectional threephase modular DC/DC converter consisting of a windfarm-side three-phase modular multilevel (MMC) inverter and a series-connected diode rectifier module linked by a special decoupled medium frequency transformer. The third topology is a bidirectional three-phase modular DC/DC converter where a three-phase MMC inverter produces a controllable AC voltage connected at the primary side of a three-phase decoupled medium frequency transformer. The secondary output voltages decoupled into three identical but 120-degree phase-shifted voltages. Simulations using MATLAB/Simulink are reported to demonstrate the effectiveness of the proposed converters. Moreover, a low voltage scaled-down prototype of the unidirectional II single-phase modular DC/DC converter is developed to validate its feasibility experimentally. Keywords: Modular multilevel converter, offshore wind farm, unidirectional/bidirectional modular DC/DC converters, HVDC system.