Fraunhofer: Electronic systems for tomorrow's power grids

Source:Fraunhofer Institute

To meet the Germany's goal of generating 80 percent of its energy requirements from renewable sources by 2050, power grids must also be completely overhauled.

Germany’s transition to a new energy economy requires more than simply switching to renewables. To meet the country’s goal of generating 80 percent of its energy requirements from renewable sources by 2050, power grids must also be completely overhauled. It’s not just a question of creating electricity highways to transmit power over long distances: the infrastructure also needs to be adapted. In the past, it consisted of a small number of large, centralized power plants, which generated electricity that was then distributed to all users. Now there are more and more small-scale generators connected to the grid, whose wind turbines, biogas plants, and solar panels feed in varying amounts of power at different times. To assure that power supply remains stable and reliable throughout the grid under these changed conditions, radical reengineering is called for to create a decentralized network. Two aspects are of particular importance. Firstly, the restructured grid must deliver power to users at all levels dependably, from private households to industry. And secondly, the efficiency of distribution grids must exceed its present level to enable optimal utilization of the available resources.

Ways of thinking are changing

“Changes will have to be made on numerous levels, from the major European power grids and the distribution networks to factories, homes, and electric vehicles,” points out Professor Lothar Frey, director of Fraunhofer IISB. The potential for change is particularly high in homes and offices. “When you think about it, the way we do things today is crazy,” says Frey. “Electricity is supplied by the grid at 230 volts and used to power electronic devices such as computers, printers, TVs, hi-fi systems and fluorescent lighting. Almost all of these devices have their own internal power supply unit (PSU) that converts 230-volt alternating current (AC) into the direct-current (DC) voltage required by the device. Because these PSUs are usually made of cheap components to minimize costs, their conversion efficiency is relatively low – in other words they transform part of the electricity into unwanted heat. This is a huge waste of energy.” Electrical losses from connecting electronic devices to the AC grid supply via a PSU are 40 to 80 percent higher than if they were connected directly to a DC supply. Moreover, the internal PSU makes the devices bigger, heavier, and more expensive.

It would make more sense to convert the 230-volt AC grid supply into direct current at a central point inside the building and then provide DC power to specific circuits and types of load at, say, 24 or 380 volts. Furthermore, an increasing number of buildings are now equipped with solar panels, which natively generate DC. Rather than converting their output into AC, as it is now, it could be input directly to a DC network. The same applies to the DC output of solar storage batteries.

The best place to start creating a DC network is in rooms that require little power, such as offices or living rooms, by installing an AC-to-DC converter in the wiring circuit serving those rooms. Researchers at Fraunhofer IISB have developed components to make this technically possible, including a converter the size of a pack of playing cards capable of serving all power outlets in a living room, and a DC network manager with a total capacity of 120 kilowatts, capable of serving an entire office building or several single-family homes. The converter has an efficiency of 98.5 percent and its efficiency is far superior to that of the PSUs in use today. “With conventional technology, the switchgear needed to control a domestic power supply of 20 kilowatts would fill an entire cabinet,” explains Professor Frey. “We can provide the same functions using efficient power electronics in a box no bigger than a telephone directory.” In addition to its use in office and residential buildings, the new technology is also of interest to industrial and commercial users, whose refrigerators and cooling systems, variable-speed motors, and lighting systems can also be operated more efficiently, reliably, and at lower cost using DC power.

As part of the SEEDs project funded by the Bavarian Ministry of Economic Affairs, Fraunhofer IISB is currently working on a holistic solution for its own institute building based on existing technologies and components. The aim is to reduce peak loads and energy losses and to integrate secondary sources of energy, such as waste heat, process cooling, and process gasses. In this way, the institute itself will serve as a research and demonstration platform for efficient energy management, and as a model that can be adapted to the more complex requirements of industrial plants.

Reconfiguring the grid for the energy transition

On a different scale, power electronics also plays an essential role in the German and European energy supply system. High-voltage DC (HVDC) transmission offers considerable advantages for long-distance power lines of the sort Germany needs to transport energy generated by offshore wind farms in the north of the country to electricity consumers in the south. An HVDC system basically consists of a transmission line with converter stations at each end. The first of these converts AC power from the conventional grid into DC, and the second converts the transmitted DC power back into AC.

The advantage of this method is that energy losses are 30 to 50 percent lower than in an AC transmission system. Modern DC transmission lines can be operated at voltages of up to several hundred thousand volts – and the higher the voltage, the lower the transmission losses. The cables can be installed as overhead power lines on overland routes, or buried underground, or laid as submarine cables.

Each end of the DC transmission line terminates in a substation containing up to several thousand inverter cells with semiconductor power switches. Each of these inverter cells weighs around 50 kilograms and stores a quantity of energy roughly equivalent to the explosive charge of a hand grenade. The Fraunhofer researchers have designed a fail-safe system that prevents the propagation of faults beyond a failed inverter cell, enabling the system as a whole to continue operating without interruption. Another advantage of this multi-level converter concept, according to Professor Martin März, deputy director of the IISB and head of the Energy Electronics department, is that: “The passive filters used to limit the impact of electrical disturbances in conventional systems require an area the size of an entire football pitch. The new technology makes it possible to construct systems that fit into a standard industrial building or mobile container.”

While Germany will certainly need its AC grids for a long time to come, in future they will co-exist with DC grids. Power electronics will be essential as a means of linking the two types of grid. “In my view,” says Professor Lothar Frey, “the way we send power through the grid will soon resemble the way we transmit data over the Internet. There’s electronics at every web interface, and in future the same will be true of power grids. The different elements, such as long-distance transmission lines, local substation networks, decentralized energy storage units, the many new generators, and of course the end consumers, will be linked via a multiplicity of power electronic nodes. Together with intelligent control systems, this will have a stabilizing effect on the grid as a whole,” affirms Frey.

New components for electromobility

Another power source that will be exploited by tomorrow’s power grid is the energy stored in the batteries of electric vehicles. But before that can happen, electromobility must address a number of technical issues – and these can be solved with the aid of power electronics. In a typical high-voltage vehicle power-net, the battery delivers an output of about 400 volts to drive the electric motor. At the same time, the battery has to supply the lower voltages to operate the lighting, air conditioning, servo-assisted steering, car radio, windscreen wipers, and other essential vehicle functions. In most cases, these electrical loads require different voltages and currents. The necessary interfaces are provided by power electronic converters, which need to be very compact and reliable. Moreover, they have to comply with electromagnetic compatibility (EMC) requirements to ensure that they do not interfere with other electronic components or vehicle systems.

To meet EMC requirements, and in order to save space and weight and reduce the need for costly wiring, the researchers chose to place individual power electronics components as close as possible to the functions they control, rather than grouping them in a central location. Martin März refers to this approach as “point-of-action-focused system integration.” As a result, he and his researchers were able to reduce the number of connectors by two thirds and eliminate many heavy, expensive, thick high-voltage cables. This means the electronic inverter, which converts DC power from the vehicle power-net into AC power to drive the electric motor, can be mounted directly on the motor or integrated into the drive system, as in electric wheel hub motors. The power converter that generates the necessary low-voltage supply from the vehicle network is installed in the battery compartment, together with the charging device that allows the vehicle’s battery to be recharged at any charging station. “We have developed an innovative fast charging solution based on a DC system that does not require an extra fast charger, which makes our solution particularly economical,” says März.

To prove that these ideas also work in practice, the IISB researchers have already demonstrated their concept creating a hybrid version of an Audi TT. To do so, they completely developed all power-electronics systems along the energy chain, from the charging point to the wheels, including the charging device, the battery system, all monitoring functions, voltage converters, and the powertrain. The converters built by the researchers are extremely compact and yet do not require any extra cooling cycle.

Redesigned from the materials through to the processes

To reduce the size and increase the reliability of power converters, it is frequently necessary to adopt an entirely new approach when choosing materials, designing electronic components, packaging and systems, and manufacturing. Fraunhofer IISB conducts research in each of these areas. Its scientists have a special preference for silicon carbide (SiC) materials, because they can be used to produce electron devices with particularly low loss characteristics and high temperature resistance. To separate chips from SiC wafers, the IISB has developed a high-output method that is based on the concept of thermal laser separation (TLS).

New technologies are also available to increase the reliability of bonded, soldered, and sintered joints, and for the polymer casting techniques frequently used to prevent flashover. Once they reach the prototype stage, the researchers integrate them in their demonstrators, where they are subjected to an exhaustive range of tests, including accelerated ageing, to determine their load and wear resistance. The most important parameters when testing power electronics are efficiency and power density. The IISB has set a number of benchmarks in this domain, for example in 2013 when its researchers set the record for the performance of an SiC power converter by reaching a power density of 100 kW per liter of volume. In 2014, they became the reference for the design of DC-DC converters by reaching a conversion efficiency of 99.3 % with gallium-nitride power switches.

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