Isolated Cascaded DAB DC-DC Converter to Boost Medium DC Voltage to HVDC

Abstract

The offshore wind farms typically use ac system to collect power from each generator, with the voltage increased by means of high, heavy step-up transformers. DC collection grids have also recently been taken into consideration as solution to simplify and minimize the offshore windfarm platforms. DC collection grids offer an additional method to reduce complexity of offshore windfarms. However, to increase the DC voltage for HVDC transmission requires the development of high-power and high voltage converters, which presents a technical challenge. This research makes use of an isolated cascaded dual active bridge (DAB) DC-DC converter to boost medium DC voltage to HVDC. Isolated Cascaded DAB DC-DC Converters are connected in series on the output side and in parallel on the input side to obtain a high transformation ratio and high power. The bidirectional DAB DC-DC converters cab be designed with power densities in the variability of tens to hundreds of kilowatts, depending on the components used and the switching frequency at which the converters function most effectively. The input parallel output series (IPOS) topology, 225 kV HVDC can be generated from a 5 kV MVDC input by cascading DAB DC-DC up to 30 stages. This converter family is useful due to its scalability and flexibility, since the power and voltage ratings can be increased while still using the same cells. MATLAB Simulink simulations are performed and verified the elementary operating characteristics of the system.