Seven years after its first installation in Sweden, ABB’s HVDC Light technology has made a significant advance in its development. Instead of 300 MW or so it can now be rated as high as 1000 MW. The largest installation to date is a 350 MW underwater link between Estonia and Finland, scheduled to open in late November.

This upgrade may constitute a useful edge in the market for large capacity underground lines, given the general realisation that as permitting for OHL becomes more difficult T & D operators are turning more to underground cabling. The new technology is credited also with the ability to strengthen networks, on account of its very high power semiconductors which reputedly allow complete control of the power flow, both active and reactive. The additional capacity will increase the opportunities for using it in this way, says ABB’s Gunnar Asplund, who led the technology’s development.

“Changes to the electricity market so that power can be produced and consumed anywhere will show up a lot of bottlenecks that hinder trade,” Asplund said. “The other place where reinforcements will become necessary is in connecting offshore wind farms.” These are necessarily situated at the edges of networks where connections inevitably are weakest, and when, as is predicted, windfarms with total capacity in the order of 1000 MW are built, that problem will be exacerbated. According to ABB therefore the threefold increase in capacity extends the attractiveness of Light in several applications.

Bulk transfer

To reach the much higher power rating the current and voltage handling of a number of components had to be increased. The rating of the IGBT (insulated gate bipolar transistor) switching device in the valves has been extended from 1160A to 1740 A. The diode chip has been improved to enable faster IGBT turn-on, which has in turn reduced switching losses.

The larger current rating was achieved by paralleling a larger number of chips. It is not a trivial task, nor a quick one, to add more chips in parallel without losing capacity per chip, especially when increasing the switching speed. And the new, bigger, component then had to undergo long term reliability testing, such as tests for intermittent operating load (IOL) and short circuit failure mode (SCFM). Tests were also performed on a complete but scaled down converter to verify single position measurements and tests.

In respect of layout, the most dramatic shift was the change to a two-level converter design from the three-level converter, which enormously simplified the mechanical design, especially for higher voltage ratings (+/-300 kV DC). Instead of six separate valves per phase only two, albeit larger valves, were needed.

The cable system and its parts, primarily cable termination, joints and the cable itself, which is now 50% greater in diameter, can now handle 300 kV DC compared to the previous 150kV. A converter station has also been designed to accomodate the larger equipment needed for ±300kV DC.

Underground lines

Buried underground, HVDC Light is now a more serious alternative for utilities struggling to win planning permission for overhead lines. Its new larger capacity adds to its suitability for distances as great as 600 km, without reducing its environmental friendliness – DC currents do not of course radiate an electromagnetic field. It can also help city authorities to meet rising power needs without placing overhead lines and large items of power equipment in urban areas. And it makes talk of large offshore wind farms generating as much as 1000 MW a more realistic prospect.

Interest in putting power lines underground was evident at Cigré’s 2006 Paris session during a day of discussions and presentations focusing on cables. Hans-Ake Jonsson, head of ABB Cables, commented “Utilities are realising that they have to use cables because they are not getting anywhere with plans for overhead lines”.

The rise in DC transmission

In the 1930s the demand for more and more power brought back to life the prospect of high voltage DC transmission as an efficient medium for the conveyance of large power volumes from remote localities. This initiated the development of mercury arc converters and more than 20 years later in 1954 the world’s first commercial HVDC link based on mercury arc converters went into operation between the Swedish mainland and the island of Gotland. This was followed by mercury arc schemes of various sizes around the world. Then around 20 years later the thyristor semiconductor started to replace mercury converters.

HVDC Light is high voltage DC technology but based on voltage source converters (VSCs). Power ratings from a few tenths of a MW up to several hundred MW were feasible with the original configuration. One of the first underground links using HVDC Light, a 177 km connection between the states of Victoria and South Australian, was built in 2002.

Grid support

Switching devices in Light converters include insulated gate bipolar transistors (IGBTs) operating with high frequency pulse width modulation enbabling them to operate at high speed. VSC transmission system technology therefore has the advantage of being able to almost instantly change its working point within its capability and control active and reactive power independently. This can be used to support the grid with the best mixture of active and reactive power during stressed conditions. In many cases a mix of active and reactive power is the best solution compared to active or reactive power only.

VSC transmission systems can therefore give added support to the grid. Simulations carried out at ABB with VSC transmission systems have shown that for a parallel case (AC line and DC transmission) where the VSC transmission system is connected in parallel with an AC system, the VSC transmission system can damp oscillations 2-3 times more effectively than reactive shunt compensation. The benefits with a VSC transmission system during a grid restoration can be considerable since it can control voltage and stabilise frequency when active power is available at the remote end. The frequency control is then not limited in the same way as a conventional power plant where boiler dynamics may limit the operation during grid restoration.

HVDC Light valve enclosure Heavier Light cable for UG applications Simplified circuit of 3 phase two level HVDC light converter The StakPak IGBT in its two sub-module configuration