Compact DC switchgear requires 90 percent less space

Gas instead of air: Thanks to a complete new design, it is now possible for the first time ever to construct compact gas-insulated switchgear for high-voltage direct current applications

In my most recent posting I wrote about high-voltage direct current (HVDC) transmission. This mature technology is highly important to the ongoing energy transition. While there are not yet any erected HVDC transmission lines in Germany, there will be in future. As an industrial nation, we need reliable, efficient power transmission systems in order for example to transport the environmentally friendly power generated off Germany's North Sea coast to consumer centers in the southern German states of Bavaria and Baden-Württemberg.

If we take a look at today’s direct current switchgear used in converter stations, which convert direct current back into alternating current, we can plainly see the next challenge facing us. Direct current (DC) switchgear has always required considerable space, as it is are engineered on the basis of air-insulating technology. Because air is actually a poor insulator, the distances between the individual components and that between live elements and the ground potential are very large in design. Such components can thus be up to five meters tall. This demands major space requirements for the overall switchyard - and space requirements play a particularly important role where space is expensive, such as on offshore power converter platforms.

Power converter platforms serving offshore wind farms

These giant platforms rising from the sea bundle the power generated by numerous offshore wind farms erected over 100 km from the coastline. The HVDC converter stations on the platform convert the alternating current generated by the wind turbines into direct current so that the electricity can be transmitted to shore with as little loss as possible. Once on shore, a second converter station converts the direct current back into alternating current, which is then fed into the existing transmission grids.

The BorWin2 platform that our company erected in the North Sea at the end of April 2014 northwest of the island of Borkum measures 73 meters long, 51 meters wide and 25 meters high. Its surface area is roughly that of half a football field.

From this platform, up to 800 megawatts of wind power is transported to shore by HVDC transmission at a voltage of 300 kilovolts (kV). This deliverable capacity is enough to cover the power demand of a major city of 800,000 households. In addition to its converter technology (called HVDC PLUS) and gas-insulated AC switchgear, the platform houses numerous auxiliary systems as well as the living and working quarters for on-board maintenance personnel.

Two complete air-insulated switchyards are also installed on board, which take up a lot of space. Cutting the space needed for this equipment would bring enormous advantages.

High voltages for direct current transmission onshore

Another factor now comes into play: The high demand for electric power in Germany will not diminish. So, in view of the long power line routes, it is foreseeable that direct current will be transmitted in future at higher voltages. This means that space demands for converter stations and hubs with switchgear will also continue to rise. Contrary to nations such as Australia, the United States or China, however, Germany is a densely populated country. Hence, the need for such installations to take up as little space as possible will play a role in this context as well.

Compact design, gas as insulating medium

Given these facts, it is plain to see that we need compact solutions both onshore and offshore - solutions that we have also found: using gas as the insulating medium instead of air. My department can once again rely on earlier work performed by our colleagues who for years have been successfully utilizing such solutions for alternating current (AC) applications.

Up until now it was not possible to construct compact gas-insulated systems for use with direct current, for it is indeed very difficult to control an electrical field under DC voltage conditions. Despite this, we have succeeded in developing an insulator with material capable of durably withstanding high-voltage direct current under long-lasting application.

We make use of what is called resin-impregnated paper (RIP) technology, which permits optimized field control. This has enabled us to develop a new design (with smaller equipment geometries) for gas-insulated 320-kV switchgear for direct current. As a result, our new systems require 90 percent less space in comparison to their air-insulated predecessors!

50-year service life

Since all live components are completely encapsulated, this equipment can be sited safely and reliably at any location irrespective of adjacent installations, and also be erected in outdoor settings. This has enormous benefits: Our new engineering will be able to offer its full advantages both offshore and on land.

We assess the service life of these switchgear to be up to 50 years, determined on the basis of long-term tests conducted jointly with the Technische Universität München (TUM). In coming articles I will be reporting on further steps to develop innovative direct current solutions.