Semiconductors as key components in the smart grid

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 minigrids with massively decentralized energy generation, storage and consumption. Though today’s 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 megastructures.

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 CO2 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 offshore 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.

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.

Photovoltaics

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 lines denote optional components. The DC-AC-converter is a mandatory component and essential to generate a grid-compliant AC output.

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Fig. 1: Schematic view to a solar power plant

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 for a power range up to several hundreds of kilowatts.

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Fig. 2: 3-Level NPC-1 topology and power semiconductors from 30 to 300A to support the design of 3-level converters

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. 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°C in cold regions to +50°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.

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. 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%.

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Fig. 3: Stack assembly for a wind power application with 2MW throughput.

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 of Europe comes with two separated difficulties. Besides political aspects, the extension of the grid infrastructure is an obstacle to overcome.

Using AC-voltage to transfer energy over long distances is not a viable option. The losses that occur will make this a 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.

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Fig. 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 integrating the regions with the highest energy yield, North 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.

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Fig. 5: A transcontinental energy grid, Source: Desertec Foundation

Conclusion

Power electronics both in module and disc design, is a mostly unrecognized part of the already existing supply network. 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.