Joint project "SwiSS solid-state SiC transformer"

Local electric grids can be destabilised in terms of voltage and overcurrent when a large number of renewable electricity sources are installed therein. To overcome these challenges, new and more flexible power-electronics-based technologies such as SST are required. To reach higher efficiencies in power electronic converters, the novel material SiC can be used for the active semiconductor devices. However, SiC is not yet as advanced as conventional Silicon-based technologies. In-depth research and development is required to establish reliable, energy- and cost-efficient systems.

1. Investigation of SiC material fundamentals and development of adequate device analysis and fabrication methods.
2. Development of a novel, entirely passive cooling system for SiC devices based on the ther-mosyphon concept.
3. Development and realisation of a highly-efficient SiC-based 25kW, 3.8kV single phase AC to 400V DC SST.
4. Application and sustainability assessment of SiC-based SST for the future Swiss electric grid.

Several new technologies which contribute to the realisation of efficient, compact and reliable SiC-based power electronic systems have been developed and demonstrated for the first time. The main results include a new analysis method for the SiC/SiO2 interface in SiC metal-oxide-semiconductor field-effect transistors (MOSFET) using free electron laser, an entirely passive and yet powerful cooling method for SiC devices, and the demonstration of an extremely com-pact AC/DC SST based on the latest 10 kV SiC MOSFETs, whose efficiency is unprecedented (AC/DC-stage: 99.1%; DC/DC-stage: 99%). The individual technologies were designed to be compatible with each other in order to facilitate converters that combine all advantages in one system.

Furthermore, potential future applications of SiC SST in Switzerland have been assessed, using the example of a real distribution grid containing 100 transformers. In the event of massive PV integration in accordance with the targets of the Energy Strategy 2050, the researchers recommend hybrid transformer technology to simultaneously stabilise medium and low voltage grids, while minimising the overall ecological footprint.

Implications for research

Because of the interdisciplinary nature of the project, several research fields are impacted: novel SiC material analysis methods are now available for SiC researchers; a non-energy drawing cooling concept was demonstrated for the first time for SiC devices; the feasibility of highly efficient and extremely compact SiC-based AC/DC SSTs was demonstrated, together with precise characterisation methods; and a method for judging the overall ecological sustainability of grid-based assets was established.

Implications for practice

The technological findings and developments can be used by companies in the field of power electronics to accelerate their own developments of SiC technology and passive cooling systems. Furthermore, distribution grid operators can use the results of this project to judge how many renewable energy sources their grids are capable of integrating, and to decide which technolo-gies are bestsuited to prevent grid destabilisation.

"SwiSS Transformer" – Solid State SiC Transformer

The joint project consists of four research projects

3.3kV SiC MOSFET and diodes for advanced power electronic systems

• Prof. Jens Gobrecht, Labor für Mikro- und Nanotechnologie, Paul Scherrer Institut, Villigen; Dr. Hans Sigg, Paul Scherrer Institut, Villigen

Integrated 3D Cooling SiC Power Module Packaging

• Prof. John R. Thome, Laboratoire de transfert de chaleur et de masse, EPF Lausanne

SwiSS Transformer - P3: 99% Efficient Solid State SiC Transformer Cell Demonstrator

• Dr. Dominik Bortis, Departement für Hochspannungstechnologie, ETH Zurich

Application and sustainability of SiC SSTs in the Swiss electrical grid

• Prof. Nicola Schulz, Institut für Aerosol- und Sensortechnik, Fachhochschule Nordwestschweiz, Hochschule für Technik, Windisch