SuperGrid – Components and Systems for HVDC Transmission from Generators and Storages to Suppliers in the European-North African Electrical Network

High-Efficiency Power Electronics for Medium-Voltage Applications

Duration: May 2011 - April 2014
Contracting Authority/ Sponsors: Fraunhofer-Gesellschaft in the course of the "Märkte von Übermorgen" (Markets of Tomorrow) Program
© Fraunhofer ISE
Test bench to determine the switching energy of medium-voltage transistors.
© Fraunhofer ISE
Laboratory prototype of the newly developed step-up converter with 10 kV SiC transistors.
© Fraunhofer ISE
First measurements show a maximum efficiency value of 98.5% for the rated power of 28 kW. Still higher efficiency is achieved for a lower output voltage and a lower duty cycle.

Power electronics is becoming increasingly important for the electricity distribution network. The number of power converters in the chain between the generator and the consumer is rising, particularly due to the expansion of renewable electricity generation, such as photovoltaics and wind energy. The application of high-voltage semiconductor components of silicon carbide (SiC) makes it feasible to connect efficient power electronic systems directly to the medium-voltage grid. Increasing the voltage level of the converters reduces the currents on the one hand and on the other hand, the connection via an additional 50 Hz transformer can be omitted.

The band gap of silicon carbide (SiC) is about three times greater than that of silicon. As a result, SiC components have a very high breakdown field strength, which means that power semiconductor components can be manufactured with significantly higher reverse voltage for the same chip thickness. This type of component is thus predestined for use in power converters in medium-voltage applications. New power electronic solutions for the systems technology of future power supply and transmission are being developed with the first available semiconductor prototypes.

Novel 10 kV SiC MOSFETs and internally antiparallel connected SiC JBS diodes, each with a rated current of 10 A, are used. A test bench to characterise the MOSFETs was developed, with which the switching energy of medium-voltage transistors can be determined.

A step-up converter was developed as a power converter which can boost an input DC voltage of 3.5 kV to an output voltage of 8.5 kV. The low switching energy of SiC MOSFETs enables a very high switching frequency of 8 kHz, which corresponds to about 10 times the value for conventional medium-voltage converters with silicon semiconductors. The higher the switching frequency, the smaller the passive components can be dimensioned, which reduces material consumption, volume and costs.


About »SuperGrid«

Further aspects of the SuperGrid project in different business areas focus for instance on "Storage Technology for Linear Fresnel Power Plants". The topic of computer modeling of potentially suitable energy systems is addressed in the article "Energy-economic Analysis of a Future Supergrid between North Africa and Europe".