High Power Electronics and System Technology

In order to bring forward the renewable energy expansion as set down by the German government, small decentral power plants and large multimegawatt-scale photovoltaic plants as well as large-scale storage systems must be installed. In addition to the classic grid feed-in, hybrid power plant solutions (e.g., PV/storage power plants) as well as industrial prosumer concepts become important.

Within the scope of numerous developments for the industry as well as research projects, we have built up an in-depth expertise around power converters from the low and medium voltage range up to the multi-megawatt range.

Our R&D objective has always been to optimize the overall system by applying new technological approaches to reduce the costs of power electronic systems during manufacture, installation, and operation. Our extensive laboratory infrastructure with test and development facilities for DC and AC systems in the voltage range up to 40 kV and power ratings up to 20 MVA forms the basis for our services on behalf of our industrial partners and contracting authorities.

In the low and medium voltage range, silicon carbide (SiC) transistors can be used to build compact and efficient power converters with and without galvanic isolation. The resulting higher switching frequencies open up new possibilities, e.g., for circuits with medium frequency transformers or for compensation of harmonics up to the kHz range. In the future, single and three-phase inverters will be able to operate directly on medium voltage without a transformer. This opens up opportunities for new application opportunities, especially for renewable energy and power grids. SiC transistors also play a significant role in mobile applications, such as railroads, when it comes to saving weight and installation space.

Our R&D-Services in the field of »High Power Electronics and System Technology« include:

• High-power Converters for PV and Battery Systems and for Hybrid Power Plants
• Development of High Power Electronics for Medium Voltage
• Test of Power Semiconductors and Modules up to 20 kV / 3.6 kA
• Comparative Investigations of Power Electronics Topologies
• Development of New System Concepts for Large-scale PV Plants
• Application of Semiconductor Prototypes, Especially High-blocking SiC Devices
• High-resolution Measurements on Systems in the Medium Voltage to 20 MVA Range
• Development of Control Algorithms and Embedded Software

Depending on the voltage and power range of your application, different topologies and control algorithms are suitable for building efficient power converters. In today's rapidly growing market environment for power converter systems, solutions which are well-tailored to the application area become a competitive advantage. In close cooperation with our partners and customers, we develop suitable concepts and control technologies or even optimize existing solutions. Before developing a hardware design, we carry out thermal and electrical simulations to ensure fault-free operation. thus carrying out all development steps up to the pre-series prototype.

In grids that are increasingly fed with decentralized generators and storage systems, inverters already are taking over important system services such as local voltage regulation or frequency stability. In the future, grid-forming inverters and systems with black start capabilities will be important for stabilizing the power grid in the case of low or completely absent feed-in from rotating generators. At Fraunhofer ISE, we can simulate, implement, and test these functions.

With the increase of the DC system voltage in large-scale PV systems from 1000 V to 1500 V, the connection power of new product generations of decentralized multistring inverters will increase to several hundred kilowatts. More and more, this device class will also be used in large power plants, which were previously reserved for central inverter solutions. Inverters of the megawatt class can play out their advantages in the area of hybrid power plants, e.g., together with battery storage or hydrogen generation. In exchange with our partners, we can find suitable and future-oriented solutions.

In the future, the energy transition will increase the demand for converters in the medium-voltage range. Not only in wind power, but also in large photovoltaic power plants with outputs of over 100 MVA and very large PV panel surfaces, it makes sense to collect the energy at medium voltage level by means of power electronic converters to keep cable losses low.

Silicon carbide (SiC) devices with reverse voltages greater than 10 kV are ideally suited for use in medium voltage power electronics. SiC transistors in this voltage range are now available and we use them regularly for the development of power converters. Compared to silicon (Si) transistors, considerably simpler topologies can be used with the same system voltage.

But the high reverse voltages as well as the extremely high switching speeds of these new components also pose new challenges, for which we are developing solutions. Other areas of application include the use of power converters for grid stabilization and for railroad technology.

Active semiconductor switches are the core element in every power converter. They directly determine the system behavior, efficiency, switching frequency and thermal management. The exact knowledge of the static and dynamic behavior is therefore absolutely necessary for the optimal design of a system. Especially with newly available components, the data sheet values are often not yet available in detail or are inaccurate.

In our laboratories we can characterize power transistors in the voltage range from 100 V to 20 kV. Our test benches can be adapted to all common package types for discrete transistors from flip-chip to SMD to through-hole packages. Special power semiconductor modules can also be measured. We use the latest broadband current and voltage sensors to record the characteristics.

To create the best possible design of a power electronic circuit, the optimal topology for the specific application must be considered. With state-of-the-art software such as MATLAB/Simulink®, PLECS®, PSpice® or FloTHERM®, a large part of this work can be done before the first hardware components are delivered.

The criteria for choosing an optimal power electronics topology are very diverse. The suitable technical and economic aspects (efficiency, power density, component costs, etc.) must be weighed accordingly and depends on the application.

The researchers at Fraunhofer ISE have long-standing experience in the detailed investigation of existing topologies as well as in the simulation and verification of the functionality of novel topologies. Our spectrum covers all types of current/voltage conversion: DC/DC, DC/AC, AC/DC and AC/AC.

In PV power plants, the technical requirements on the inverters and the grid connection strongly depend on the system technical setup of the overall system. Depending on geographical and environmental conditions, there may be further requirements to consider, e.g., derating characteristics or cooling and maintenance concepts.

Innovative circuitry and active elements in the DC distribution lead to more efficient and less expensive inverters with the advantage of adapted grounding and protection systems. Particularly in hybrid generation systems that combine renewable energy sources with storage technologies, the optimization of the design enables higher efficiency while reducing investment costs. In addition to benchmarking market-available power converter systems for known plant specifications, we develop and evaluate new plant concepts, including DC distribution and grid connection.