Novel 5 MW DC Converter Put Into Operation


As part of the Flexible Electrical Networks (FEN) research campus, the Institute for Power Generation and Storage Systems at the E.ON Energy Research Center (E.ON ERC) commissioned the world's most powerful medium-voltage (5 kV), medium-frequency bi-directional DC-DC converter. 


Design of the DC-DC Converter

Mittelfrequenztransformator Figure 1: 5 MVA medium-voltage, medium-frequency transformer, with a nominal frequency of 1000 Hz, weighing less than 700 kg

The transformer (see Fig. 1) of the DC converter, which provides galvanic isolation, was designed and built in a close cooperative effort between the FEN industrial partners Schaffner Group and thyssenkrupp electrical steel, the RWTH Institute for Electrical Machines (IEM) and the Institute for Power Generation and Storage Systems (PGS). The controllers of the PGS converters were adapted for the DC converter with support of General Electric Company.

As part of the FEN research campus projects, a medium-voltage, multiterminal 5 kV DC Campus Grid is being constructed at RWTH Campus Melaten. This DC grid connects the multi-megawatt converter test benches of PGS at E.ON ERC with the wind turbine test bench of the RWTH Center for Wind Drives. This infrastructure creates a unique test environment in which not only new DC-DC converters but also DC components, protection technology and operational management concepts can be tested.

The bi-directional DC-DC converter operates according to the three-phase dual-active bridge (DAB) principle and enables a bidirectional, flexibly controllable power flow while providing galvanic isolation to meet all requirements of future DC networks. The converter is part of several DC-DC converters, all in the same performance class, developed at E.ON ERC by the Institute for Power Generation and Storage Systems.

Parallel to the adaptation of the controllers of the megawatt class converters of the PGS medium-voltage DC- laboratory, the 5 MW medium-frequency transformer, which uses thyssenkrupp grain oriented silicon steel (GOES), was constructed by the industrial partners. Operating at a frequency of 1000 Hz, the gravimetric power density of the transformer increases with a factor of 15 up to 20 times compared to a conventional 50 Hz dry-type transformer.

The soft-switching DAB DC converter developed and tested here is unique in its power class and forms the heart of future intelligent DC substations for DC distribution grids. Furthermore, this DC converter can be used in large collector arrays for wind and PV farms, as well as in high-power, fast charging stations for electric vehicles or in industrial DC networks.