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The new MVDC PLUS solution (medium-voltage direct current) from Siemens closes the gap between HVDC and local distribution grids connects autonomous systems, increases the efficiency of power transmission, and simplifies decentralized power trading.
by Ingo Petz
The sun is shining on Erlangen this morning. From the conference room, housed on the top floor of Siemens Building 74, the view stretches out over office buildings, family houses, pastures and forests, to an overarching blue sky. It is a good place to think about and develop new ideas.
Mirko Düsel, CEO of the Transmission Solutions Business Unit, is presenting the future – the next development in the rapidly changing electricity market as the energy transition continues. To illustrate this dynamism, Düsel first turns to Southeast Asia. “Indonesia, for example, consists of more than 1,000 islands. The country wants to achieve a certain level of social prosperity and increase manufacturing output. How can this be achieved? By building microgrids on all the islands, autonomous systems that are linked together without creating a dependency. Decoupling and simultaneous linking would work brilliantly with DC links.”
DC has been around for a very long time and we have mastered it. Through the entire value chain to consumption, in new installations as well as in operation and service.
Mirko Düsel, CEO Transmission Solutions Business Unit, Siemens
Microgrids in remote areas – i.e., networks that supply themselves with wind or solar power or through their links with each other without being connected to the grid – are just one field of application opened up by the viable and economic MVDC solution from Siemens that will ensure power transmission and distribution is better managed, controlled, and optimized in future. This solution is not new, but based on proven HVDC PLUS technology.
This basic version of the system can be used over a distance of up to 200 kilometers, and in a power range of 30–150 megawatts. The core component, which is available in specified sizes, are the voltage-sourced converters (VSC) in a modular multilevel converter (MMC) design. MVDC solutions are cost-efficient – making them very attractive to regional distribution system operators (DSO) and those at the subtransmission medium-voltage level.
The Islands Commission, which represents twenty-three European regional island authorities from 11 countries, argues “for the potential of islands to become pioneers in climate action and energy transition by delivering innovative smart solutions.” These new DC applications in the medium-voltage range can help islands realize that potential. For instance, the MVDC PLUS system enables wind turbines on offshore islands to be connected to the AC grid on the mainland. Smaller wind farms, medium-sized solar plants and remote communities can be connected to the grid, and sensitive, weak distribution networks can be stabilized. In addition, regional medium-voltage networks can be connected to each other to share power, increase their security of supply and reduce their dependence on the high-voltage networks.
MVDC offers the opportunity to build
hybrid medium-voltage grids
How did these new applications for DC come about? “Until now energy was produced where it was needed,” says Düsel. “The energy transition allows us to generate power where it is actually available, where there is a lot of wind or sun. That means having to bridge longer distances.” The distance has roughly quadrupled. For economic reasons, DC was used because compared with AC it results in significantly lower transmission losses. For a few years now Siemens HVDC PLUS technology has set the standard when it comes to developing and constructing power transmission systems in the power range of up to 1,000 megawatts.
The next step: using medium-voltage direct current transmission (MVDC) as an efficient transmission link in AC medium-voltage networks of 30–150 kilovolts. Düsel explains: “Renewable energy is produced at a low voltage level, and we have an increasing number of DC consumers. I’m thinking of electric cars and e-mobility. Then there are data centers that operate with DC.”
The energy transition allows us to generate power where it is actually available, where there is a lot of wind or sun.
Mirko Düsel, CEO Siemens Transmission Solutions
“It is the next step towards network expansion to combine our knowledge of HVDC with local DC generation. MVDC allows to manage short-circuit currents and optimize networks through a hybrid approach, using a mix of AC and DC. This in turn reduces the investment costs. I then only have to change to AC when I really need AC networks.” In addition, MVDC is simply more economical than MVAC, because it offers a higher capacity at lower voltage.
So the expansion of wind power and solar energy systems has huge consequences for the transmission and distribution of electricity. It will become more complicated and decentralized, with risks for power quality and grid stability. Until now, power generation was not that complicated. Produced in power plants, it was carried over high-voltage lines to regional distribution companies, where it was either transported to major industries in the high-voltage sector or to cities in the medium-voltage range. The lowest level comprises households and smaller businesses that receive a low voltage of less than 1 kilovolt.
Power transmission was in one direction, and there was no need to bridge large distances to urban and industrial areas. So losses during transportation were manageable and the susceptibility of the network to disruption could be predicted. But now there is more and more decentralized feed-in of renewables into the grid when weather conditions are good, and more electricity is drawn from the grid when they are bad. So the grid needs to be more intelligent and serviceable.
PLUS technology by Siemens makes it possible: A high energy density can be transmitted in even the smallest area and in hybrid medium-voltage networks. In addition, the solution provides a feature that allows the capacity of networks to be increased through reactive current – or reactive power compensation. This has benefits not only in terms of cost-effectiveness and efficiency, but also for the environment. It will result in an MVDC grid with lower electricity masts. And diesel generators would no longer be required during power shortages.
One further important application for the MVDC PLUS technology is on the horizon, which could have a big impact, above all on the energy market of the future. Ohm’s law says that alternating current flows where the least resistance is. But in the case of DC the flow direction can be controlled. “It offers a great advantage to independent energy producers,” Düsel is convinced. “It will be easier for them to transmit electricity via DC links, achieve bundling, and so sell their energy.”
He is sure that Siemens’ modular MVDC system will conquer the market and fill the gap between HVDC and local distribution grids. “DC has been around for a very long time and we have mastered it,” Düsel says confidently. “Through the entire value chain to consumption, in new installations as well as in operation and service.” You could say DC has a home. And that the future has begun.
26, 09, 2017
Ingo Petz lives and works as a freelance journalist in Berlin.
Picture credits: Simeon Johnke
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