Takushi Jimichi

Design and loss analysis of a medium-voltage DC-DC converter intended for offshore wind farms

In the last decade, many projects of offshore wind farms have been planned and conducted to compliant each government policy. To increase the energy yield, attention has been paid to the replacement from AC collection grid to DC collection grid. At the moment, many applications such as electric vehicles and DC power supplies have adopted an isolated DC-DC converter with low-loss magnetic material. However, the low-loss magnetic material has the limitation of the size and shape. Therefore, such magnetic material cannot be used directly for high-power application. This paper discusses suitable designs for a 10 MW 1 kV/±25 kV DC-DC converter, focusing on the use of low-loss magnetic materials. The discussion deals with system structure, circuit topology, phase number, semiconductor devices, and transformer structure. The loss analysis conducted by computer simulation and theoretical analysis verifies that the medium-voltage DC-DC converter can achieve the efficiency of 98.5% with the submodule-based structure, the three-phase dual-active bridge topology, the hybrid pair of 5th- and 6th-generation IGBTs, the shell-type transformer, the nanocrystalline core, and the switching frequency of 1.5 kHz.

A modular multilevel cascade converter (MMCC) with a resonant high-frequency link

This paper presents a modular multilevel cascade converter (MMCC) topology based on a resonant high-frequency link, which is applicable for transformerless medium-voltage motor drives requiring a wide range of operation frequency. The todays most practical topology for medium-voltage motor drives proposed in 1990s, contains a bulky and costly phase-shifted multi-winding transformer. On the other hand, existing MMCC topologies based on double or triple star-configured cascade voltage-source converters can incarnate transformerless motor drives, however being subject to limitation of the operation frequency due to the interference between the inter DC-voltage balancing control and the output-voltage control. This paper proposes an MMCC topology based on a resonant high-frequency link, which is characterized by controlling high-frequency zero-sequence voltage and current via the resonant link for the inner DC-voltage balancing control between the supply- and motor-side converters. Simulation results obtained from a 6.6-kV 800-kVA model confirm the validity and effectiveness of the proposed MMCC topology.

Comparison of single-phase and three-phase dual-active bridge DC-DC converters with various semiconductor devices for offshore wind turbines

This paper topics the effect of the phase-number and the semiconductor-device selection on the volume and weight of a medium-voltage (MV) DC-DC converter intended for offshore wind turbines. In recent years, many papers have discussed the transposition from MVAC to MVDC collector grids in offshore wind, where the main motivation is to reduce losses of the system. The authors have identified that the MV DC-DC converter can achieve the efficiency of 98.5% with the three-phase dual-active bridge (DAB) topology and the hybrid pair of 5th- and 6th-generation IGBTs at the switching frequency of 1.5 kHz. Following the efficiency, the volume and weight of the MV DC-DC converter are also important factors in offshore wind turbines. However, from the intuitive point of view, it seems that the single-phase configuration with less number of components and the latest IGBTs with lower losses and better thermal conductivity take less volume and weight than the proposed configuration. In order to clarify the doubt, this paper carries out volume and weight analysis of the MV DC-DC converter with various arrangements of the phase number and semiconductor devices. In the analysis, the worst operation point of the input and output voltages and the required thermal resistance of the cooling system as well as the semiconductor and transformer losses are considered. Moreover, regarding the medium-frequency transformer, the high-frequency loss characteristics of the magnetic cores and windings are taken into account by dimensional analysis. Finally, the analysis elucidates that the MV DC-DC converter with the three-phase DAB topology and the hybrid pair of 5th- and 6th-generation IGBTs can obtain both high efficiency of 98.5% and less volume and weight compared to other configurations.