Bidirectional Medium-voltage Converter with High-voltage SiC Devices for Increased Integration of Renewable Energies and Inner-city Storage in Innovative Grid Structures

Contracting Authority/ Sponsors: Bundesministerium für Wirtschaft und Energie (BMWi)
Project Partners: STS Spezial-Transformatoren Stockach GmbH & Co. KG, Bombardier Transportation GmbH, VAG Freiburg
Project Focus:      
Single Phase 15 kV medium voltage inverter for rail applications. Frontview.
© Fraunhofer ISE
Single Phase 15 kV medium voltage inverter for rail applications. Frontview.
Single Phase 15 kV medium voltage inverter for rail applications. Rearview.
© Fraunhofer ISE
Single Phase 15 kV medium voltage inverter for rail applications. Rearview.

The aim of the "SiC-BiNet" project is the demonstrator development of a highly compact single-phase bidirectional AC/DC converter with high-voltage SiC devices for connection to the medium-voltage grid. Here, galvanic isolation is provided by an RF transformer, which uses new core structures and adapted insulating materials to withstand the high voltage gradients of the high-voltage SiC devices.

In the project, a single-phase medium-voltage converter for coupling the DC streetcar grid (750 V) with the AC railway grid (15 kV / 16.7 Hz) was developed and put into operation. In addition, two measurement campaigns were carried out in the substations of VAG Freiburg and, based on these results, a simulation model was created for sections of the VAG network. In addition, system concepts for the integration of storage facilities were developed, simulated and evaluated.

Based on an extensive topology research, where different topologies were collected, described in detail and evaluated, the most suitable topology for the demonstrator was selected. A novel multilevel dual active bridge (DAB) with an H4 full bridge at the low voltage input, a medium frequency transformer and an ANPC full bridge at the medium voltage output was selected for the DC-DC controller. The inverter is designed as an ANPC full bridge with LCL filter. DC-DC-converter and inverter were simulated in detail. Using the results from the semiconductor characterization of the 15-kV MOSFETs, a switching frequency of 30 kHz was specified for the DAB and 16 kHz for the inverter. Considering the very high voltages, these are very high switching frequencies.

The necessary inductors, such as the medium frequency transformer and the main and line chokes of the LCL filter, were developed by STS and optimized with the results of the commissioning tests.

For the control of the ANPC full bridges, special control methods were developed to avoid overvoltages at the semiconductors. For good isolation and noise immunity, all signals were transmitted via optical fibers. A low-voltage model was specially developed for the initial tests of the control system in order not to run the risk of destroying the expensive medium-voltage power electronics in the event of possible faults.

The complete system was set up in a control cabinet with dimensions of 2000 mm x 140 mm x 80 mm (H x W x D) and commissioned in the newly occupied medium-voltage laboratory of Fraunhofer ISE.

The work was carried out in close cooperation with the project partners Bombardier, VAG, STS and Fraunhofer ISE.

More Information on this Topic:

Research Topic

Power Electronics and Grid Integration

Business Area

Power Electronics, Grids and Smart Systems