Here’s a bit of old news: The world wants more renewable power. The tricky work of feeding it into our homes, schools and offices doesn’t often make the headlines — but figuring it out is key to changing the energy mix.
Dense cities surrounded by expensive land are no place to build a wind farm; they work best in windswept seas and on vast, open plains. Likewise, a giant solar installation works best in the remote desert — not the most habitable place. That’s why energy experts are increasingly interested in building electricity superhighways that can ship renewable electrons efficiently across long distances to customers. It’s a difficult and expensive task that will take years and cost tens of billions of dollars in the U.S. alone.
And distance is just one challenge. Wind and solar power themselves are tough to pin down. Unlike electricity from gas-fired power plants, which can work around the clock, wind and solar aren’t generally available on demand. They depend on environmental factors beyond our control — the fickle breeze, cloud cover and the setting of the sun.
Further complicating matters is the issue of inertia. Although most people haven’t thought about inertia since their last physics class, it’s a key factor that helps keep our lights on. That’s because today electricity comes to sockets in our homes in the form of alternating current, or AC. As the name implies, AC moves along a wire in a sine wave, changing direction — or alternating — 120 times per second between plus and minus in the U.S. (In Europe, it switches 100 times per second.) If the frequency gets out of whack a little, lights can start to flicker, oven clocks may slow down and machines may not work properly. Larger frequency disruptions may cause conventional power plants to disconnect from the grid as a way to prevent damage to their expensive turbines and generators — and lead to blackouts.
The frequency is set by spinning generators inside power plants. When huge generators inside conventional power stations supplied all of our electricity, maintaining this constant frequency was relatively easy. That’s because even if you take your foot off the gas, the inertia of the heavy generator will keep it spinning. But maintaining a constant frequency can be problematic when the wind suddenly stops blowing. Although wind turbines also have inertia, they typically grind to a halt much faster. It’s even worse with solar farms, which have virtually no inertia and stop producing power as soon as the sun disappears.
That’s why grid operators, as well as governments, are increasingly looking at another way to transmit electricity: high-voltage direct current (HVDC) transmission. Direct current, or DC, is much simpler than AC: It can flow in either direction at a constant plus or minus voltage. DC has been around since the days of Thomas Edison, but it gave ground to AC because, back then, it was hard to transmit efficiently over long distances.
But today, those technological challenges have been largely solved, and modern HVDC links can transmit three times as much power over the same transmission line corridor as AC. A 2018 study commissioned by the U.S. Energy Information Administration found that HVDC lines “have a number of potential benefits including cost effectiveness, lower electricity losses, and the ability to handle overloads and prevent cascading failures. These attributes mean that HVDC lines could, if properly configured, help mitigate some operational issues associated with renewable generation.”
In May, GE Reports visited Stafford, a town in the British Midlands where GE Renewable Energy’s Grid Solutions unit designs, tests and builds some of the most advanced HVDC systems. We sat down with GE Grid Solutions power guru Colin Davidson to talk about the technology. Here’s an edited version of our electrifying conversation.
GE Reports: Thomas Edison was a big fan of DC, but he lost the war of the currents to AC proponents Nikola Tesla and George Westinghouse in the 1890s. I thought the case was settled. Why are we still talking about DC?
Colin Davidson: Well, the truth is that DC never completely went away. Most power plants, whether they burn coal to produce electricity or use wind, generate AC current. For DC transmission, you have to convert AC to DC and then back again. In the beginning, this conversion was very difficult, but people kept trying. GE was actually very early in the game. In the 1930s, they built an experimental HVDC line using mercury arc rectifiers to convert the current and ran it 23 miles from Mechanicville to Schenectady in New York, where GE had its headquarters. But back then, the converters were still very expensive and GE didn’t see the potential of the technology. The first true commercial HVDC line didn’t happen until the 1950s. We started slowly building from there, and even a decade ago, HVDC was still quite a niche industry. It wasn’t really big at all. But in the last 10 years, the explosion in the number of projects around the world has been phenomenal.
GER: What happened?
CD: It’s a number of things. People want more power all the time, but it’s hard to build overhead lines for environmental reasons. One way to build new links is to bury the cables underground, and it’s much cheaper to do that with DC than with AC. Plus, of course, we’ve got renewables, particularly here in Europe, where we have a lot of offshore wind. Wind farms make a huge difference in the way electricity’s generated, but they are often in the wrong place. Germany’s a case in point. They shut down their nuclear power plants, which are mainly in the south of the country, but they have a lot of wind generation up north. They need a way to ship power across the country, and they’re building HVDC corridors to transmit some of that power.
In North America, it’s picking up too. The U.S. has three AC grids: the eastern grid, the western grid and Texas, which wanted its grid to stay out of reach of federal regulators by not crossing state lines. You also have [the Canadian province of] Quebec doing its own thing, too. So there you have four large AC areas in North America potentially operating at different frequencies. But an HVDC link can tie them all together and allow them to exchange electricity, for example.
GER: How does HVDC do this?
CD: HVDC converters can essentially manufacture and match the frequency at which the destination grid operates. Without getting too much into the details, the converter at the end of the line can essentially behave like a traditional generator with a power source that can be turned on or off very quickly. So when something changes, it can quickly compensate.
GER: I think we have to slow down a little.
CD: OK. So the voltage and direction of the alternating current switches, or alternates, back and forth at a set frequency described by sine waves. The HVDC technology we are developing here in Stafford can chop up the sine waves of the alternating current into smooth DC lines in the converter station at the beginning of the HVDC link. From there, the DC travels over a cable to the converter at the end of the link, where another converter rebuilds it into the desired AC sine waves that precisely match the characteristics of the destination AC grid.
GER: How do you chop the AC up?
CD: You use a device called a rectifier, which straightens the AC sine waves. The first such device was the mercury arc rectifier. That was the grandfather technology they used for the Mechanicville line. These rectifiers were based on technology similar to the glowing tubes inside old televisions, but scaled up. As the industry evolved, it embraced semiconductor devices, like thyristors and special transistors. The latest version — power transmission’s equivalent of a 4K TV set, if you will — is a semiconductor device called insulating gate bipolar transistor (IGBT). We use IGBTs to build up a system called a voltage source converter (VSC). These power electronics are much smaller and much more efficient and powerful than anything in the past.
You can think of a converter using VSC technology as a black box that allows you to break down and build up any sine wave you want. Inside the black box there are lots and lots of little individual VSC converters, controlled by a computer, that step up or step down a wave by 2,000-volt increments. It’s kind of like a lot of square pixels on a computer screen forming a circle, one step at a time.
GER: Two thousand volts? That seems like a lot! Wouldn’t the circle be too choppy?
CD: Two thousand volts in our industry is quite small. We have hundreds of these converters — we call them submodules — lined up in a series. The submodules are connected in a series to make “valves,” and six valves — two for each phase of the AC system — go to make up the complete converter. We can quite easily produce a sine wave of about 400,000 volts with lots and lots of little steps in it, so it’s a very good approximation of the sine curve.