The University of the Basque Country GISEL research group of the Department of Electrical Engineering is at work to improve the technology needed to convert alternating current (AC) lines to direct current (DC) lines.

Today most electricity transmission lines are carrying AC. In some cases DC lines are also being used. Grid operators and researchers are becoming aware that in some cases DC lines are more suitable than AC ones.

High Voltage Test Facility at ABB.  Click image for the largest view.  Image Courtesy ABB, Zurich, Switzerland.

High Voltage Test Facility at ABB. Click image for the largest view. Image Courtesy ABB, Zurich, Switzerland.

The GISEL research group is working to improve the technology needed for conversion. The aim is that direct current transmission should be done in a more straightforward, smoother and consequently less expensive way.

Electricity is normally transmitted by means of AC, but it is not the only way and not always the best one. In some cases, high voltage direct current (HVDC) is used. In Spain, for example, as the press release points out, there is only one DC line, the one that connects mainland Spain with the Balearic Islands; all the remaining ones transmit electricity by means of AC.

Marene Larruskain, one of the engineers in the UPV/EHU’s GISEL group fills in the background with, “Direct current continues to be highly suitable for underwater and underground lines. Less investment is needed to build direct current lines, and there are fewer losses in electricity transmission. However, as most of the lines in the power grid are alternating current ones, converters are needed to change the type of electricity transmission, and they are very costly. That is why direct current lines are appropriate beyond a certain length.”

Larruskain continues with the fact that use is made of HVDC lines to transmit electricity over very long distances; indeed, the longest lines that exist are direct current ones. “There are facilities worldwide that transmit direct current electricity. The typical examples are the transmission lines of large hydraulic power stations: the Itaipu power station in South America, the Three Gorges power station in China, etc.” said Larruskain.

Right now almost all of the transmission of electricity is done by means of alternating current lines. “Our aim is in no way to replace these lines by direct current ones. Our proposal is based on using direct current as a solution in cases where there are problems with the alternating current lines,” explained Larruskain.

Renewable energies would be an example of this. Renewable energies are produced in a very irregular way; the wind, for example, could blow very strongly at some moments and very lightly in others. And the output may not coincide with moments of peak energy consumption. “One way of solving the problem caused by this situation for the electricity supply could be to connect the farms or parks of various countries where renewables are produced. That way, if at one moment one region has a high consumption of energy but is not producing renewable energy, its demand could be met by using renewable energy which is being produced somewhere else,” she explained.

At very long distances HVDC could have a very beneficial effect. The variability in production of renewables could be much more easily balanced with DC. The UPV/EHU’s GISEL group is proposing that these international grids should be direct current ones.

The GISEL group is working to improve the energy exchange converters between direct current lines and alternating current ones. Specifically, they are working on new technology for converters known as Voltage Source Converters (VSC). Compared with conventional technology, “The VSC has many advantages; among others, it is easier to control the power that is transmitted, and that is very important on wind farms, for example. At the same time, given the fact that direct current has great economic advantages in underwater lines, it is very appropriate for them,” explained Larruskain.

But VSC technology has a number of drawbacks. First, its capacity to transmit energy is lower, the energy losses are greater and it does not respond well when problems arise. For example, if there is a short circuit, the system has problems. “That is why we’re working to minimize those problems,” she said.

The GISEL researchers want to make use of the advantages of both means of transmitting electricity to be able to address growing power consumption. Larruskain noted the fact that, “Even though more and more energy is being produced to meet the demand, problems may arise when it comes to transmitting that energy. It is not always possible to incorporate the surplus energy produced into the already existing lines owing to their limits. In these cases, the use of direct current could solve the problem of the alternating current lines already installed, because, among other things, the HVDC lines can transmit more electrical power.”

The GISEL research group has studied how to make the features of AC and DC lines compatible.

AC generated electric lines are three-phase making the number of conductors three, or multiples of that number. On the other hand, HVDC lines have two conductor poles: a positive one and a negative one.

“How are we going to divide two poles into three conductors?” wondered Larruskain. If we use one pole for each phase, one of the conductors of the original line will remain free and part of the power will be lost. That is why a line and a half of alternating current corresponds to each pole of direct current. Even though it looks impossible, there are various ways of making this redistribution.

Larruskain points out, “In the future HVDC grids are expected to coexist with alternating current grids, which are in the majority nowadays. Transmitting direct current via the currently existing lines could be a first step towards building HVDC grids.”

Larruskain is right, HVDC is coming simply because of the transmission efficiencies and the distances that are possible. The ideas and research into conversion of lines is a very smart move. In North America HVDC would take a lot of intermittency and demand problems and turn them back into opportunities.


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