Energy transportation: AC vs DC

Energy transportation: AC vs DC

Pedro Peres

The subject of high-voltage transmission lines continues (HVDC) is based on the areas of electric power transmission and eletrònica because of the necessary converters.
I’m currently finishing up a degree in Electrotechnical Engineering at FCT/NOVA in Portugal, where I specialize in power electronics and electrical drives.

Abstract—Since the beginning of the 20th century, alternating current (AC) rules our world in matters of energy generation and distribution. But in the recent past, the development of computers and micro and power electronics, in general, have made most of our loads to be direct current (DC). The development of power electronics is also slowly solving the main problem of DC transmission which is converters. These reasons are making countries and energy companies reevaluating the old dichotomy AC vs DC power transmission.

Index Terms—AC line, DC line, HVDC, HVAC, DC vs AC

hvdc
HVDC technology can help to lower the cost of energy and improve grid reliability

INTRODUCTION

The generation, transmission, and distribution of electricity started with DC in the 19th century, patented by Edison, but was quickly discontinued and substituted by AC. AC could be easily increased to very high levels with the use of a transformer, reducing the losses in power cables. On the opposite, DC voltage couldn’t be transformed as easily which meant low voltage in transmissions lines resulting in huge losses.

Another reason for the early AC hegemony was the cheaper AC machines. DC machines are expensive and require maintenance often while AC machines, especially the multi-phase induction machine, are cheaper to build and can go for long periods without much maintenance.

The total hegemony of AC over DC lasted till the 1950’s when the first commercial HVDC line, Gotland 1, was constructed in Sweden, connecting the Swedish mainland with the island of Gotland. [5]

  1. The line original capacity was 20MW at 100kV which was later expanded and added a second line (Gotland 2). The original line was already dismantled but the lines Gotland 2 and Gotland 3 still operate together, forming a bipolar line with a total capacity of 260 MW.

Nowadays, there are many advantages in using DC transmission over AC transmission, which is among the most important:

  • Less investment cost: A transmission line for DC will always cost less for AC. The problem is the converters at both ends of the line, which will almost always cost more than a transformer.
  • Cheaper maintenance: The maintenance of an HVDC line is cheaper than an AC line.
  • Lower losses: A DC line has fewer losses than an AC line rated for the same power, one of the reasons being the absence of skin effect in DC transmission. However, the losses in the converter stations should not be ignored, especially for low power and short lines.
  • Better connectivity: In terms of transmission, two AC lines sometimes are impossible to connect (one of the reasons may be different frequencies). DC lines can always be connected provided they have the same voltage. In terms of generation, a DC source is also easier to connect to the grid, since we only have to worry about the voltage level, while in AC we have to worry about voltage level, frequency, and phase to be able to connect it without making a dangerous short-circuit.
  • The reactive effects of the line can be ignored.

Every line can be considered an RLC circuit due to parasitic capacitances and inductances, which means in AC we will have reactive power circulating along with side active power, this means more current will be needed for the effective power usage, increasing losses. This effect greatly increases in underwater or close to earth cables. In DC this problem does not exist making it almost always the only solution for under-water power transmission.

  • Control: In HVDC the ability to control the active power in the line is very easy to do.
thyristor valves
Thyristor valves at Ygne converter station, Gotland

STATE OF THE ART

Nowadays DC transmission lines are more common, especially for underwater transmission for the reason explained earlier. Today, DC is also normally preferred for very long-distance transmission.[6] The current technologies for HVDC lines make it possible to transfer up to 10 GW of power for 2000km with voltages between 800 and 1100 kV.[3] In terms of generation, today solar power is be- coming more and more popular because it is clean and free energy that can be harvested with a solar panel. This type of generation is DC and because we use AC lines it has to be converted before the connection with the grid is made. In 2017, it is reported that Germany added more 1.75GW of installed solar power making it a total of 42.9 GW.[2]

In terms of conversion, nowadays, thanks to modern semi-conductor and converter technologies, both AC-DC and DC-DC converters can have very good efficiency ratings, which increase with the power rating (being the AC-DC the most pronounced one). [7] The converters in HVDC power stations can have losses as low as 0.7 %. [3]

mercury arc valve
Mercury arc valve at Ygne, Gotland

NEW IDEAS AND CONTRIBUTIONS

Nowadays the battle for AC vs DC distribution is not only in very long transmission lines.

best alternative. The new industrial DC systems use electronic speed drives that can be powered by DC lines, making AC distribution inside the installation almost not needed nor desired due to the necessity for AC-DC-AC conversion. Some of these industrial equipment and drives can be very sensitive to power disturbances, but this problem can also be solved with power converters, as it can easily filter the disturbances. [4]

CONCLUSION

This article was exposed to why at the beginning of the electrical revolution it was AC who ruled the world. It also shows why decades later DC started to come back as an alternative.

In conclusion, AC was better for distribution in the past due only to the transformer and cheaper AC machines. With the invention of semiconductor devices, this advantage started to fade and DC was given another opportunity. In the articles cited it seems to be a consensus that DC might rule the world someday as it is increasingly becoming more and more a part of modern distribution grids and power generation.

REFERENCES

  1. Abb: The Gotland HVDC link. https://new.abb.com/systems/hvdc/references/the-gotland-hvdc- link. Accessed: 2018-11-11.
  2. Germany installed 1.75 gw of PV in 2017. https://www.pv- magazine.com/2018/01/31/germany-installed-1-75-gw-of-pv-in- 2017/. Accessed: 2018-11-15.

Because of the increasing popularity of the concept

  1. Siemens: Hvdc classic. https://www.siemens.com/global/en/home/products/ene

of distribution generation, sustainable development with renewable energy and the ever-increasing domestic DC loads, in [7] is proposed that the power distribution line within a building be mainly DC with some AC lines for potential AC loads. This would be done by having a centralized AC- DC converter powering the whole building’s DC lines. The study concludes that this would improve the building and converters efficiency (since fewer converters were needed).

For the industry, DC distribution is also becoming very attractive. As they demand highly reliable supply of power DC has been proposed to be the

voltage/high-voltage-direct-current-transmission-solutions/hvdc- classic.html. Accessed: 2018-11-16.

  1. M. E. Baran and N. R. Mahajan. Dc distribution for industrial systems: opportunities and challenges. IEEE Transactions on Industry Applications, 39(6):1596–1601, Nov 2003.
  2. Marene Larruskain, I Zamora, A.J. Mazon, Oihane Abarrategui, and J Monasterio. Transmission and distribution networks: Ac versus dc. 10 2018.
  3. R. Liu. Long-distance dc electrical power transmission. IEEE Electrical Insulation Magazine, 29(5):37–46, September 2013.
  4. Huey Pang, Edward Lo, and Bryan Pong. Dc electrical distribution systems in buildings, 12 2006.

What are your thoughts?