Edison used direct current, or DC, to distribute power. But when he tried to send electricity for distances longer than a mile, his network started losing voltage. Edison’s rivals George Westinghouse and Nikola Tesla pounced on the shortcoming, and their system transforming and transmitting alternating current, or AC, has since become the norm all over the world.
But DC has never gone away. In fact, it has been staging a quiet comeback as an efficient way to transmit power from remote locations like offshore wind farms or zip it long distance across large states like Texas, and possibly even the entire United States. IEEE Spectrum recently reported that the latest incarnation of DC transmission — high-voltage direct current, or HVDC — was “readily available and increasingly affordable, and could replace the old equipment to make long-distance electric power transfers between the eastern and western United States possible.” Some of the HVDC lines could be buried along existing railroad rights-of-way to quicken the installation process.
We talked to GE Power’s HVDC expert Rafael Bonchang about the technology’s latest applications. Here’s an edited version of our conversation.
GE Reports: Why did Edison lose his “current war” with Westinghouse more than a century ago?
Rafael Bonchang: The technology was not there to increase the DC voltage needed for the [efficient] transmission of bulk power from point A to point B. AC faced the same challenges, but electrical engineers backed by Westinghouse figured out how to build practical transformers. That totally flipped the situation and gave AC an advantage, in particular for power transmission and distribution. Transformers allowed them to step up voltage and transmit power longer distances.
GER: So Westinghouse and AC won. Why are we still talking about DC?
RB: Direct current has always had very attractive qualities. For example, alternating current oscillates at certain frequencies. These frequencies can be different from grid to grid, and you cannot connect them unless you synchronize the frequencies first. You don’t have this problem with DC, because it’s direct, it’s constant. This feature is quite useful since it makes the power system more stable. HVDC interconnectors can help prevent cascading power outages, for example.
GER: North America has several separate power transmission systems. Does this problem exist elsewhere in the world?
RB: Absolutely. Brazil and Uruguay come to mind. The grids in these two countries run at different frequencies. Another good example is Saudi Arabia, which runs at a totally different frequency from the rest of the Middle East.
GER: What are some other benefits of HVDC?
RB: When you send power from point A to point B, it’s never 100 percent efficient, and you always lose power. But your HVDC transmission losses are way lower than with AC. The longer the distance of the power interconnectors, the more attractive HVDC becomes. With HVDC, you can also send more power over the same transmission line corridor — as much as three times more, in fact. This is huge considering that the cable itself could represent up to 70 percent of the total cost of the project. Another big benefit is that you can run HVDC though underground and underwater cables. This is difficult with AC, for various technical reasons.
GER: Why don’t we have HVDC everywhere, then, especially given that these lines could efficiently bring power from, say, remote and offshore wind farms to cities that consume a lot of electricity?
RB: It is happening, especially with offshore windfarms, but historically there have been challenges. For example, wind farms generate alternating current, and the HVDC connectors at each end of the cable that transform AC to DC, and DC to AC at the other end, have been very expensive until recently. AC transformer stations are a lot cheaper. You have to make the calculation whether the HVDC interconnector makes economic sense. As I already mentioned, the farther the distance, the more losses you rack up with an AC system and the more attractive HVDC becomes.
GER: When did HVDC start coming back?
RB: HVDC has been coming back for a while. GE has been working on the technology for over 50 years. The first big breakthrough was the mercury-arc valve that allowed us to convert AC into HVDC and build some of the early HVDC interconnectors. One interconnector in Canada for Manitoba Hydro brought bulk power from hydropower stations in the north to the load centers and cities, like Winnipeg, in the south.
GER: Are you still making these valves?
RB: No. The mercury-arc valve paved the way and showed that high-voltage DC transmission had its place in the market. The next big innovation was the thyristor, a powerful semiconducting device that lent itself to converting AC to DC and DC to AC. It allowed us to build a 2,000-megawatt interconnector between France and the U.K., for example. That link still remains the largest subsea and most utilized HVDC interconnector in the world.
GER: Why did France and the U.K. need such a massive power link?
RB: Like I said, connecting different grids allows you to improve your grid stability, take advantage of differential energy prices and reduce your generation margin.
GER: What is a generation margin?
RB: Two countries would normally keep a certain amount of generation capacity in their back pockets for a bad day when something goes wrong. But it’s unlikely that your grid will have a problem at the same time there is a problem in the grid of the neighboring country. By connecting the grids with an HVDC link, you can reduce the amount of margin and have fewer power plants on standby, which is expensive.
GER: What is the state-of-the-art of the HVDC technology today?
RB: The latest technology today is something we call the voltage-source converter, or VSC, which is using semiconducting transistors. Compared with the thyristor, these converters have a much smaller footprint and greater flexibility.
GER: Why is size important?
RB: It allows us to build converter stations in densely populated areas and bring HVDC to centers of cities where real estate is very expensive, for example. This is definitely one of the big future markets.
GER: What are other business opportunities for HVDC?
RB: Definitely renewables. Our attitudes have changed a lot in terms of the sources of power generation we want. As a result, offshore wind is one of the main areas we are looking at. With the VSC technology, you can fit converter stations on platforms in the sea that support 1,000 megawatt interconnectors. We have already deployed one in the North Sea, to support Germany’s DolWin3 offshore wind project.