The next big thing is small
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The next big thing is small
The reliable low-loss transmission of large amounts of electrical power is key to the efficient use of renewable sources of energy, and high-voltage direct current (HVDC) transmission gains an increasingly important role in this field. This technology makes it possible to transmit power generated far away from the load centers to where it is needed with low loss. But the air-insulated DC switchgear and lines used for HVDC systems so far have a comparatively large footprint. With the development of gas-insulated DC systems, Siemens is now opening new perspectives for future power grids.
The next big thing is small
“The space requirements of the converter stations must be reduced immediately,” explains Dr. Denis Imamovic, who is heading the development of gas-insulated DC transmission systems at Siemens. “For example, you’ll need efficient transmission systems to get wind power generated in the north of Germany to the load centers in Bavaria, where the use of conventional power sources is reduced in favor of renewables. The advantages of DC transmission can be brought to full use in such applications, but the switchgear that belongs to the converter stations still requires plenty of space.” This is because air-insulated technology is used.
The platforms that are currently being built for the grid connection of several offshore wind farms in the North Sea are a striking example: BorWin 2, a platform that has recently been anchored in the North Sea bed and will transmit up to 800 megawatts of wind power to the mainland via HVDC at a transmission voltage of 300 kilovolts, boasts an impressive height of 73 meters and a width of 51 meters. The platform houses two complete air-insulated DC switching bays along with the actual converter technology.
Still, the transmission of the generated power from the wind farms on the high sea to a converter station on the mainland is only the first step. It is equally important to deliver the power to where it is finally needed, from the North Sea coast to the industrial centers in the south of Germany, for instance.
In view of the high demand for power and the long transmission distances, it is foreseeable that much higher transmission voltages will be used for DC transmission in Germany in the future, especially for onshore purposes, explains Prof. Claus Neumann of the TU Darmstadt, an acknowledged expert in high-voltage transmission technology. According to Prof. Neumann, this will result in the need for even more space for the systems, because air is a poor electrical insulator, as everybody knows. It is thus essential to leave enough space between the devices. Many times, there’s a gap of up to five meters or even more between the different tree-high components. Furthermore, gas-insulated lines are of interest as an alternative to cables in cases when underground transmission is required.
This is where Dr. Imamovic and his project team have identified the ideal starting point: “So far, there hasn’t been any mature technology for gas-insulated DC systems. However, DC switching technology is one of the crucial components for the future of HVDC: Power generation and grid infeed are becoming increasingly distributed, which means that we will see more infeed and nodal points in the future, so space requirements will be a matter of high priority. Gas-insulated switchgear is substantially more compact, and, hence, more space-saving than air-insulated switchgear. Both versions are readily available for AC transmission – just think of Siemens’ 8D type range of gas-insulated switchgear, a success story since 1968. But it’s a different story with HVDC. Controlling a direct current electrical field is an extremely tricky thing, so it hasn’t been possible to build compact gas-insulated systems for DC applications so far. But now we have managed to develop an insulator that can withstand the stress induced by high-voltage DC. Our all-new design, an innovation on the world market, enables us to build gas-insulated systems for DC. For the market-ready 320 kV systems, this translates into space savings of up to 95 percent.”
According to Dr. Imamovic, the new technology will be able to play to its strengths in offshore as well as in onshore applications: “Space is at a premium on platforms, and the new systems are so small that they’ll no longer require their own room,” he says. “This provides enormous cost benefits. And it’s the same onshore, where space can be a substantial cost factor as well, especially in metropolitan areas. In such cases, we are now also able to provide our customers with a solution thanks to considerably smaller systems. The smaller footprint facilitates the approval procedure for the stations. A building is not absolutely necessary for the new systems. All live parts are fully encapsulated, so the systems can be safely installed even under demanding environmental conditions such as in coastal regions. The systems can also be combined perfectly, because we have also developed modules that enable gas-insulated systems to be connected to cables as well as to overhead lines. The modularization of the technology makes the systems flexible and makes it possible to rely on cost-efficient ways of shipping.”
Siemens’ gas-insulated high-voltage DC switchgear for voltages up to 320 kilovolts has already reached market maturity, and the Siemens engineers are working hard to develop and test systems of even higher voltage levels – up to 500 kilovolts at present. A pilot installation that was assembled from the new components and simulates the grid connection of an offshore wind farm is currently being tested. The aim is to prove the long-term operational safety of the technology under dielectric and thermal stress, which is simulated by the temporary application of increased voltages and currents. The pilot installation is a great step forward for the developers of this project, which is of crucial importance not only for the German energy transition. Customers from several business areas have already expressed serious interest in the new switchgear.
Thanks to its modular design, Siemens’ proven HVDC (high-voltage direct current) PLUS converter technology reduces the complexity of the entire transmission system and thus the footprint of the converter stations of HVDC systems. That’s why it is ideally suited for the transmission of power from offshore wind farms.
Operating as a full bridge, Siemens’ HVDC PLUS is blazing the trail for multi-terminal systems. HVDC PLUS as a full bridge is based on proven multilevel converter technology. The crucial factor is the ability to reverse the voltage. Within a given DC corridor, the full bridge can handle any given voltage level. In the case of a line fault, which can be caused by bad weather or some other environmental impact, for instance, the full bridge ensures immediate and reliable fault current interception: Reverse polarity is applied to the line for a moment, which reliably interrupts the current and deionizes the flashover. Then the full bridge performs two or three restart attempts to reestablish the power flow in a quick and safe manner. HVDC PLUS arranged in full bridge topology is the ideal solution for overhead line applications and provides much more operational safety in DC transmission.
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