Primer on transmission: AC vs. DC

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The AC vs. DC question has been around since the earliest days of electric power, going back to that golden era when Edison and Westinghouse duked it out-verbally, of course-over the issue. (Edison liked DC; Westinghouse favored AC.)

EL&P asked Ben Damsky, EPRI project manager for transmission and substations, to weigh in on the debate for this month's Power Pointers.

What were the technical issues involved in the AC-DC debate?

It was really a question of system architecture and the distances involved. Long-distance transmission requires high voltages for efficiency. With AC, you can use step-up transformers to boost the voltage for transmission and then step it back down near the customer for low-voltage consumption. But transformers don't work for DC power, so there was no way to optimize the voltage. Since Edison's DC concept was restricted to low voltage, a central station could serve a radius of only a couple of miles. Considering the tremendous growth in electrification and long-distance transmission in the early 20th century, there's no doubt AC was the right choice.

Has the technical base for DC improved since then?

It certainly has for special applications. In the mid-30s, the mercury arc valve was developed, which could rectify AC-basically convert it to DC-after its voltage had been raised. This opened the possibility for high-voltage DC transmission (HVDC). However, the equipment was extremely large and expensive and required a lot of maintenance. The smaller, solid-state thyristor valve, developed in the 1970s, was a big improvement. But even with thyristors, a DC converter station is huge-typically covering a dozen acres. You have to have a compelling need for DC to justify the expense.

Can you give our readers an example?

The case is quite clear-cut for undersea cable. Cables have a lot of stray capacitance; so when you put voltage into one, you are essentially charging up a big distributed capacitor-the cable itself. Only after the cable is charged up will you get power out the other end. But with AC, the charging happens every half cycle because the current is constantly switching directions. This doesn't matter much if it's a short cable, but if it's 20 or 30 miles long, more current is being used to charge the cable than is coming out the other end. At some length, you get nothing on the far end. With DC, the cable only needs to be charged once, and you don't have this extreme loss. So with long cables, DC is essentially the only choice. There's a 50-mile DC cable connecting mainland Canada to Vancouver Island, for example, and a similar undersea cable is being installed right now between Connecticut and Long Island.

What about overhead HVDC transmission?

Well, you don't have the big stray capacitance problem that you do with cable. With overhead transmission, the idea is to offset the extra cost of DC converter stations at each end of the line by spending less on the line itself. This is feasible. DC requires fewer conductors-two instead of the three for three-phase AC transmission-and the towers can be smaller, allowing a narrower right-of-way. Also, for AC lines, the insulators that separate the conductors from the tower have to be specified for peak voltage rather than average voltage; with DC, the peak voltage and average voltage are the same. Of course, you have to allow for transient peaks with both types of current, but overall, DC lines require less insulation for a given line voltage. The lower material and construction costs add up over a long line, with the breakeven point believed by many to be around 300 miles.

But with a line like that, you're giving up a lot of flexibility.

Right. We're talking here about long-distance bulk transfer between two points. You generally don't take power off the line in the middle because you'd need another expensive converter station. It's sort of like the difference between local and express trains. With the locals, people get on and off all along the line-very flexible, but not that efficient if the end of the line is your destination. Express trains are very efficient at getting a lot of people from one big population center to another at considerable distance. The Pacific Intertie, which runs from Oregon to Southern California, has both AC and DC lines that run in parallel, so it can serve both needs.

So HVDC transmission makes the most sense when you've got a lot of generating capacity in one place and a lot of load to be served hundreds of miles away.

Yes, and it's especially attractive if you're connecting areas that have complementary power needs, as with the Pacific Intertie. In the summer, the Pacific Northwest has a lot of hydropower available because of the snow melt and runoff; this is just when Los Angeles can use extra power to cover its big summer air-conditioning peak. Then in the winter, when the Northwest's reservoirs are largely depleted, Southern California can help them with their winter heating peak. So the power flows north or south, depending on the different regions' peaking needs. Another advantage of DC systems are their extreme stability; when a DC line is tied into an AC grid like this, its controls can be programmed to automatically damp out problems caused by line faults or other disturbances. So the western grid is more stable because the Pacific Intertie has a DC segment.

It sounds as if DC is now a true complement to AC systems rather than a rival.

Very much so. The number one application for DC today is connecting two AC systems that are not synchronized, allowing transfer of power from one AC grid to another. The ultimate case is where one grid is operating at 50 Hz and the neighboring grid is at 60 Hz. By installing an AC-to-DC converter station and a DC-to-AC converter station back-to-back where the grids meet, you can change power from one frequency to another continuously. Because direct current is not in the form of a sine wave, it has no frequency or phase angle, so any mismatch between the AC systems is cleaned up as the power goes through the DC link.

What about links in the United States?

The problem here is mainly one of phase angle. Most of the country is either on the eastern or western grid, and these systems are always slightly out of phase with each other. There are now numerous DC links operating between the eastern and western grids, adding a great deal of control and flexibility to the U.S. power system.

What lies ahead?

Superconductivity and DC are natural partners. You'd have to use cable rather than overhead lines because of the liquid nitrogen cooling required. The superconducting cable will have the same stray capacitance issue I mentioned earlier, so for longer lines, you'll have to use DC. Also, when you run AC through a superconductor the losses are very low, but with DC they're zero. With no resistance, a grid based on DC superconducting cable could operate at something like 20 or 50 kV, not 500 kV as now. And that means you're going to deliver a lot of current. It's certainly decades off, but if we can make it happen it will be a tremendous advance. elp

Click here to enlarge image

Damsky received his B.S. in physics from Princeton and his M.S. from the University of Pennsylvania. He can be reached via phone at 650-855-2385 or e-mail at bdamsky@epri.com.

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POWERGRID International

March 2014
Volume 19, Issue 3
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ELECTRIC LIGHT & POWER

January 2014
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