Abstract. 1 Introduction. 2 Transition to Smart Grid

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1 Semiconductors as Key Components in Future Smart Grid Installations Martin Schulz, Infineon Technologies, Warstein, Germany, Abstract This paper deals with the electronic components needed to support a highly efficient, reliable and secure smart grid structure that is supposed to develop from the mere static grid installed today. It highlights the role semiconductors will play in this new grid structure and explains, why projects like the German Energiewende and the plan to eliminate the need of nuclear power cannot become reality without sophisticated semiconductor solutions. 1 Introduction The classical, centralized power supply grids today face a transition into a so-called smart grid. This smart grid can be understood as a system of systems or the sum of interlinked mini-grids with massively decentralized energy generation, storage and consumption. Though todays large scale power plants will remain the major backbone of these grids for the years to come, the change will allow the more efficient use of a growing portion of renewable energies, finally eliminating the usage of fossil fuels and nuclear power. Besides the power generation and transmission, storing of energy, communication and security issues will be part of this change, making semiconductors in all ranges the dominant key component of these new mega-structures. Projects like the Artemis Internet of Energy are starting points to evaluate possibilities and generate standards to enable manufacturers to master the challenges that will come with the change. 2 Transition to Smart Grid Centralized power generation as done today benefits from plants with huge power densities that predictably operate on demand. Regulation strategies are well established and throughout the last 50 years, an interconnected European power grid grew to become one of the most complex technical achievements. Generators, rotating at fixed frequencies and controlled using the external excitation provide stable frequency, constant voltage and the amount of reactive power needed. With the urge to make use of renewable energy to fight global warming and reduce CO 2 emissions, windmills and solar arrays started to become a growing fraction of power sources. However, both generate electricity stochastically, depending on the availability of their particular primary energy. As their output voltages are of fluctuation nature and in case of solar cells are of DC-character, power electronic became necessary to transfer the power delivered into a form that can be fed into the grid. Inverter Technology based on Insulated Gate Bipolar Transistors (IGBT) became the industrial standard for this particular task. Additionally, the transport of and use of electricity will change in a smart grid compared to today. Locally generated power will be used locally, thus eliminating the losses during transportation. Energy storage will at least partially compensate the lack of continuity in power generation. This will contribute to cutting peak power demand. At the same time, transport across long distances has to be achieved at maximum efficiency to interconnect off-shore wind parks to the continents or transfer energy on a global scale as envisioned in the Desertec Project. This is the domain of High Voltage Direct Current power transmission (HVDC), a typical application for thyristors and bipolar diodes.

2 3 Regenerative Energy Generation Sun, wind and biomass are three major sources of renewable energy to generate electricity. Especially photovoltaic solar applications and wind power plants benefit from the use of power electronics. 3.1 Photovoltaic PV-Collectors generate a DC-voltage and the magnitude of output power is a function of solar radiation. To feed energy into the grid, a minimum voltage level is required. Furthermore, the DC-voltage has to be transferred to an AC voltage compatible to the mains. This is a classical task for power electronic components. Schematically, Figure 1 hints out the blocks, a solar power plant may consist of. The dashed Figure 1: Schematic view to a solar power plant lines denote optional components. The DC-AC-converter is a mandatory component and essential to generate a grid-compliant AC output. Today, solar plants are installed from several hundreds of watts up to the megawatt scale. This requires a wide range of power semiconductor components. The driving force in improving existing solar inverters for the European market during the last years has been advancement in system efficiency. Modern solar converters thus have reached maximum efficiencies of more than 98%. Recently, a visible trend is the step away from 2-level converters towards multilevel topologies. Mainly the 3-level inverter is more and more in focus. The so-called Neutral Point Clamped (NPC)-topologies are preferred in higher power levels. This leads to systematic advantages regarding electrical losses and physical sizes of wound goods in filter components. Figure 2 depicts the often used NPC-1 topology which is well established in solar inverter designs. It is predestined to be used in a power range up to several hundreds of kilowatts. Figure 2: 3-Level NPC-1 topology and power semiconductors from 30 to 300A to support the design of 3-level converters

3 3.2 Wind Energy Generation In 1983, German energy provider RWE was involved in building the first 3MW windmill Grosse Windkraft Anlage (engl.: Large Windmill System) called Growian. It used a Leonard Converter to feed energy to the grid [1]. Today, windmills feature output powers of up to 6MW per device. Double fed induction generators coexist with synchronous machines. Both, permanently and separately excited machines are in use. Special requirements for the power electronics in use arise from the wide variety of boundary conditions as well as lifetime and availability of the installations. Depending on the location, the power plant may be subject to ambient temperatures from 30 o C in cold regions to +50 o C in warmer zones. Relative humidity can exceed 90%, sulfurous atmosphere, salty mists and dust in deserts are factors that have to be considered in power electronic design too. Especially components mounted in the nacelle or even the hub suffer from vibration, leading to further stress for the power semiconductors [2]. The electrical interface between generator and grid can be designed on module- or subsystem level. The power electronic subsystem, or Stack, can be considered an off-the-shelf component, available in power ranges up to megawatts. Figure 3 gives an impression of a MODStack HD, designed for a throughput of 2MW. Figure 3: Stack assembly for a wind power application with 2MW throughput. In this application, the most important thing to care for is robustness. The predicted lifetime is demanded to reach 20 to 25 years along with a warranted availability of 97%. 4 Energy Transport One of the major challenges coming with the extended use of regenerative energy, is the geographic distance between the point of generation and the area where the energy is finally needed. Transferring energy from an off-shore windfarm in the North Sea to the industrial centers in the middle or even the south Europe comes with two separated difficulties. Besides political aspects, the extension of the grid infrastructure is an obstacle to overcome.

4 Using AC-voltage to transfer energy over long distances is not a viable option. The losses that occur will make this an non ecological approach. Starting from some hundred kilometers, High Voltage Direct Current (HVDC) transmission is to be favored. DC-transmission is most efficient in both, electrical losses and material in use as it can be done on a single-wire setup. HVDC is well established and, among others, connects England to the European continent via cable. Core of these transfer systems are semiconductors in disc designs. Thyristors and diodes are installed to transfer energy in a GW-range using bipolar DC-voltages of up to ±800kV. Today, the converter needed to create an AC-voltage from this DC-line is based on thyristors as well. Figure 4 gives an overview on this kind of devices. Figure 4: Disc-shaped thyristor and diode devices Currently, research is ongoing to use IGBT-based multilevel converters to replace the thyristors in the DC-AC-converter. Here too, efficiency is the driving force. Expanding the interconnection beyond European borders would allow to integrate the regions with the highest energy yield, Noth Africa and Middle East, into a transcontinental grid. The vision of the Desertec Project pictured in figure 5 clearly shows, that thousands of kilometers would have to be crossed. In this vision, HVDC becomes the technology of choice for the necessary long-distance connections. Figure 5: A transcontinental energy grid, Source: Desertec Foundation

5 5 Energy Storage The European supply grid, grown during the 50 years past, has become one of most reliable and most stable grids in the world. It also became one of the most complex structures ever built. The main target now is, to integrate renewable energies harvested from wind farms and solar arrays in industrial scale into this grid and at the same time reduce the use of fossile fuel. This in turn may not lead to any loss in stability or availability, leading to the need of decentralized energy storage devices. Today, pumped hydro power plants are state of the art to store larger amounts of energy by pumping water uphill. Due to their structure, pumped hydro power plants are bound to geography, thus they cannot be build anywhere and cannot simply be scaled down to form decentralized energy storages. For energy storage, weight and volume are less important factors. Chemical storages like lead batteries or Lithium-Ion batteries can be considered as an alternative and even fuel cells may be an option. Flywheel may be used to support short-term demand or add bursts of power in case of short-term brownouts. Large-scale project using Vanadium Redox Batteries (VRB) with capacities of several MWh have been installed throughout the last decade as well. These large capacities are useful on grid scale, but not necessarily needed in lower levels of the grid. In decentralized storage devices, as for example in private homes, capacities of only a few kwh would be enough to provide the energy buffer needed locally. Excess energy from a solar array can be stored and enhance the possibiliy to locally stabilize the grid or cut peak demands. The application Energy Storage, no matter if pumped hydro power, batteries, fuel cells or flywheel can only achieve highest efficiency and availability if power electronic is used to control the flow of energy and the individual point of operation. 6 Smart Grid, Elektromobility and Energy Management A Smart Grid is an interconnection of power generating and power consuming entities, controlled by a superordinated, decentralized authority to align and optimize energy generation and consumption in a timely manner. One key component in such an installation is the communication among the participants to coordinate energy demands and generation. The evenly important component remains power electronic to efficiently control the flow of energy. The third important part is measurement equipment. It will be needed to provide data in order to adapt pricing for energy depending on the actual market situation. With the picture shown in figure 6, the setup of a Smart Grid and the power electronic subsystems included can be described. Figure 6: Schematic overview of a Smart Grid Conventional power plants (A) will remain a backbone of the supply grid for the years to come. Still power electronic is an essentail part of these plants and is mainly involved in auxilliary devices such as pumps, cooling or ventilation systems.

6 In power generation systems based on renewable energies (B), power electronic forms the interface between the generation setup and the supply network. The superordinated authority can decide, whether the energy flow is directed to a certain consumer or a storage device (C) to be used at a later time. Efficient long-distance energy transport (D) is achieved by using thyristor based HVDC-lines. In pumped hydro power plants, IGBT-based inverters take care of operating pumps at maximum efficiency. In chemical storage devices, highly efficient AC/DC-converters allow rapid storing of energy. Despite the higher power levels needed for grid-scale systems, knowledge gained from the automotive sector regarding battery charging topologies can synergetically be used for this field of application. Measurement technology (E) in Smart Grids has to be smart itself, thus develops into Smart Metering. It allows for dynamic changes of energy prices, making energy less expensive during times with excessive generation and more expensive in case of high demands. In addition to regulate the whole grid to achieve a high level of stability, even single houses can be considered a miniature version of a Smart Grid. This way, a Smart Grid can be considered a so called System of Systems intermeshing several levels of subsystems into one complex superstructure. This context can be explained for a house as a small cell in this structure by means of figure 7. Figure 7: An intelligent house as a subsystem of a Smart Grid Using photovoltaics, the house is supplied with regenerative energy on an irregular base. At the same time, the house is part of the grid and power can be taken from the grid at any time. The battery of an electric vehicle can act as storage device, which leads to further demands regarding the charging point. To charge the car and allow the house to take back energy from the battery, the charger has to be of bidirectional nature. Most important is, that this device will be controlled to adapt the charging mode. In case the user decides to charge very rapidly, high output power is demanded. To treat the battery with care, the most common charging method will use lower power levels for an extended periode of time. As a consequence, the charging system has to achieve contradicting goals. While high power is demanded on one side, best possible efficiency has to be achieved even under partial load conditions. Research is currently ongoing to incorporate newly developed semiconductors into this application featuring resonant DC/DC-converters [3]. Charging systems like this are expected to achieve a lifetime of 15 years at least while the predicted operating time exceeds the one of a car. Even with a single car, charging- and discharging cycles can take several ours every day, resulting in a demand of no less than operating hours. The charging spot itself is not necessarily installed in a protected location so here too, all worst-case scenarios regarding ambient temperatures and atmospheric conditions have to be considered. To support balancing of available power sources like solar cells and battery storage, the home appliances that consume higher power levels are build as smart consumers as well. The superordinated control consists of a building energy management (BEM) that gathers data from smart metering devices. This way, decisions can be made, what consumer or storage is supplied with energy at what time or if energy is taken from a battery or fed into the grid. The scenario hints out, that power electronic, communication and security technology will become the dominant building blocks in this sort of supply network.

7 7 Conclusion Power electronics both in module and disc design, is a mostly unrecognized part of the existing supply network already. New applications in Smart Grids demand innovative approaches to enhance efficiency and increase power density to build smaller power electronic devices. Especially mobile and electric vehicle applications demand low weight and volume and even in private houses, space is not necessarily available in excess. Furthermore, supply networks are expected to achieve a very long lifetime, especially if compared to classical consumer electronics. Robustness and longevity will be the most pressing needs to be fulfilled. Therefore, new developments in semiconductor material and the interconnection technology in use are to be expected [4, 5]. References [1] Internet source Wikipedia, see [2] Martin Schulz, How to Handle Electricity from µw to MW in Windmill Applications Power Electronic Europe, Issue [3] Artemis Joint Undertaking, Internet of Energy for Electric Mobility, [4] Karsten Guth et al. New assembly and interconnects beyond sintering methods PCIM 2010 Nuremberg, Germany in May 2010 [5] R. Ott et al. New superior assembly technologies for modules with highest power densities, PCIM, Nuremberg, May 2010