Siemens Rolls Out MVDC Transmission System to Bolster Distributed Generation

Siemens has launched a new direct-current (DC) transmission system for alternating current (AC) grids of between 30 kV and 150 kV. The medium-voltage system that can bridge distances of up to 125 miles is designed to help grid entities handle ever-growing volumes of power fed into the distribution system from distributed and renewable power.

The MVDC PLUS (Medium Voltage Direct Current Power Link Universal System) introduced on October 4 is based on Siemens' high-voltage direct-current transmission (HVDC) technology, but it is offered in three variants: for a transmission capacity of about 50, 100, and 150 MW at DC transmission voltages of 20 to 50 kV. Comparatively, the company's HVDC system transmits power at voltages of more than 220 kV.

Siemens has introduced a new medium-voltage power transmission solution that uses direct current applications to increase power transfer capability while also minimizing losses. Courtesy: Siemens

According to the company, the system meets growing needs to expand and upgrade grid infrastructures that are increasingly burdened with decentralized power generation, which is leading to congestion at distribution and transmission levels. The MVDC system, it said, is easy to integrate into existing networks-it needs no additional power electronics-and transmits power transmission over long distances.

The MVDC system connects grids via active elements that control load flow and provide reactive power. It can also decouple grids with different frequencies, voltage levels, and quality with a DC link. It operates with voltage-source converters in a modular multilevel converter design, which convert AC to DC and vice versa. And, as significantly, current on the transmission route can flow in both directions.

Pivotal to the Energy Transition

According to Mirko Di1/4sel, CEO of Siemens Transmission Solutions Business Unit, the development will be pivotal to the energy transition. Indonesia, for example, a country made up of 17,500 islands, can now consider installing microgrids on each island, and link it all together with these autonomous systems without creating a dependency. "Decoupling and simultaneous linking would work brilliantly with DC links," he said.

In the past (arrows shown in blue), power has typically flowed from conventional generation source to consumer in one direction via alternating current (AC). The medium-voltage direct current (MVDC) solution system (arrows in green) could allow for omnidirectional energy flows at larger distances. Courtesy: Siemens

Another possible application is to connect wind turbines on offshore islands to the AC grid on the mainland. And yet another is to connect smaller wind farms, medium-sized solar plants, or remote communities to the grid, and in the process, stabilize weak distribution networks.

Also noteworthy is the MVDC system's capability to connect separate regional medium-voltage networks to share power, which would reduce their dependence on high-voltage networks.

"Until now energy was produced where it was needed," said Di1/4sel. "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 Promise of Medium Voltage Direct Current

DC has typically been used because it offers significantly lower transmission losses. But HVDC systems, which are capable of transmitting power of up to 1,000 MW, aren't practical everywhere. That's why MVDC, integrated in AC medium-voltage networks of 30 kV to 150 kV, could offer a sound solution, Di1/4sel said.

MVDC allows grid operators to manage short-circuit currents and optimize networks through a hybrid approach, using a mix of AC and DC, he explained. "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, he said.

For now the system's applications are centered around low-voltage uses. "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."

But in the future, because the flow direction can be controlled in DC-as opposed to AC, which flows where the least resistance is-the MVDC system could offer "a great advantage to independent energy producers," Di1/4sel said, because "It will be easier for them to transmit electricity via DC links, achieve bundling, and so sell their energy."

-Sonal Patel is a POWER associate editor (@sonalcpatel, @POWERmagazine)

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