For well over 100 years, AC (Alternating Current) has been regarded as the natural choice for electrical power transmission. However, HVDC (High Voltage Direct Current) is now emerging as a practical and economical alternative that is not only very efficient for transmitting large amounts of power over long distances, it also offers an elegant solution for a number of reliability and stability issues associated with connecting sustainable energy schemes – especially wind farms – to the main power grid.

HVDC is also something that people involved in large scale CSP (Concentrating Solar Power) stations are tentatively looking at, as the potential to generate utility-scale solar generated electricity (and transport it vast distances) moves from a pipe dream to an area of serious consideration.

What is HVDC?

ABB pioneered HVDC, and developed the first commercial scheme over 50 years ago. This was a link between the Swedish mainland and the island of Gotland in the Baltic sea. The power rating was 20 MW and the transmission voltage 100 kV. Since then, a total of about 70,000 MW of HVDC transmission capacity has been installed in more than 90 projects worldwide.

A key advantage of HVDC is that long distance transmission is more efficient, as there is no need to charge the capacitance of a transmission line with the alternating voltage. In addition, the power flow can be controlled rapidly and accurately, as to both the power level and the direction. This possibility is often used in order to improve the performance and efficiency of the connected AC networks.

Originally, the HVDC converters were equipped with mercury arc valves. Later thyristor valves were introduced that made the design of HVDC systems more flexible, and increased the amount of power they could transfer.

Why use HVDC?

HVDC has a number of properties that make it different from AC transmission:

• The two converter stations can be connected to networks that are not synchronised, or do not even have the same frequency;
• Power can be transmitted over very long distances without compensation for reactive power; reactive power is power that does not add to the transmitted power, but is a by-product of AC transmission resulting from charging the line or cable capacitance at 50 or 60 times per second. Since HVDC operates at constant voltage it does not generate reactive power;
• Only two conductors are needed (possibly just one if the ground or sea is used as the return) compared with three conductors for AC.

HVDC Light characteristics

In 1997 ABB introduced a completely new converter and DC cable system called HVDC Light, based on transistorised voltage source converter (VSC) technology.

It offers many advantages for power transmission, especially in sustainable energy systems. Robust extruded XLPE underground (or submarine) cables provide the link between compact, modular converter stations with a very small installation footprint – and they are small enough to install on offshore platforms. The HVDC Light system provides exact control of active power transmission, ensuring contracted power can be delivered as requested. The power transmission can be combined with a frequency converter that varies the power to support the network frequency.

The HVDC Light converter controls reactive power, along with AC voltage control of the network connected to the converter station. Such rapid AC voltage control can also be used to improve the power quality through controlling flicker and transient disturbances.

 When connected to a passive network such as a wind farm, the HVDC Light transmission system can provide control functions for active and reactive power, so that both the voltage and frequency can be controlled from the converter station. This allows black starting by controlling the voltage and frequency from zero to nominal. HVDC Light also makes it possible to provide reactive power to the wind turbines during start up and fault-ride through conditions, assisting with grid code and stability issues.

Currently, ratings up to 350 MW are used for the HVDC Light stations, however continued improvements in semiconductor technology have resulted in 1,100 MW links now being available.

Where has HVDC light been used?

HVDC light capability has been proven in two wind farms:

Tjæreborg

In 1998, Eltra, the independent system operator and the transmission company in western Denmark, carried out a trial installation of a 7.2 MW HVDC Light system – at an existing wind farm at Tjaereborg (close to Esbjerg on the west coast) comprising four wind turbines with a total installed capacity of 6.5 MW.

Gotland

The excellent wind conditions on the Swedish Island of Gotland prompted the construction of a number of windpower facilities on the southern part of the island, reducing the need to import power. However, the AC network on the island was not designed for feeding power from the south to the central region. Normally, this would have been solved by constructing a new AC line, but as this would have passed through an area where bird life was protected, it was decided that overhead lines were undesirable. Instead, in 1999 an HVDC Light system with 70 km of underground cables was installed.

The Gotland HVDC Light system is rated at 50 MW and 65 MVA, and is connected in parallel with the existing 70 kV/30 kV AC grid. The total system has a peak load of about 160 MW, and there are now a total of 165 wind turbines with a total installed power of 90 MW, producing about 200 GWh.

The grid operator, Gotland Energi AB (GEAB) reported a number of benefits resulting from the HVDC Light installation:

• Flicker problems were eliminated and transient phenomena disappeared;
• System stability improved;
• Power flows, reactive power demands, voltage levels in the system and harmonics were reduced;
• Voltage stability during transient events has become much more predictable, which improves the output current stability from the asynchronous generators. This reduces the stresses on the AC grid and the mechanical construction of the wind turbines. The control of power flow from the converters makes the AC grid easier to supervise than a conventional AC network, and the power variations do not stress the AC grid as much as in normal networks;
• Voltage quality improved with increased windpower production).

The world's largest wind farm

Following on from those experiences above, in September 2007 E.ON Netz awarded ABB the contract to supply a complete 400MW HVDC Light transmission system, which will integrate the world's largest offshore wind farm into the German grid.

The Borkum 2 wind farm will be developed by BARD Engineering GmbH. It will consist of 80, 5 MW wind generators located about 130 km from the North Sea coast. The generators will feed power into a 36 kV AC cable system, which will be transformed to 154 kV for the HVDC Light offshore station. The receiving station will be located at Diele, 75 km from the coast where the power will be injected into the German 380 kV grid.

HVDC and Concentrating Solar Power (CSP)

Suitable locations for solar energy plants, especially CSP schemes, will almost certainly be in deserts, where they will operate at maximum efficiency and where the land is not used for agriculture, forestry or urban settlement. So it will become very important to be able to transmit very large amounts of power over long distances, from remote desert locations to population centres.

HVDC transmission over very long distances using overhead lines is already very well established. For example, in China the 3,000 MW link from the Three Gorges Hydroelectric Power Plant to the South China System, commissioned in 2004, is 940 km long.

The theoretical limit for HVDC transmission over overhead lines is around 3,500 km at present, which would accommodate almost any desert-based scheme. However, deserts are a particularly demanding environment for overhead lines due to the risk of salt contamination. Therefore, such a scheme would require the use of underground cables.

Currently, the world's longest high-voltage underground link is the 180 km Murraylink – a 220 MW HVDC Light project that connects the states of South Australia and Victoria. However, the 700 MW NorNed link, currently under construction between Norway and the Netherlands, is the world's longest underwater high voltage cables link, at 580 km in length. On the basis of this experience, we can be reasonably confident that existing HVDC technology, using the 320 kV transmission voltage could support a desert-based scheme up to 1,000 km. After this distance the transmission losses become too great, at around 8%.

Development work is already in hand to increase HVDC transmission voltages, which will enable the transmission distance to increase. And in the near future we expect to achieve practical cable transmission distances of 2,000 km. On that basis, if CSP schemes take off, then HVDC transmission technology will be ready and waiting to support them. It is even possible to imagine HVDC as providing the backbone for a very efficient European-wide transmission network, linking existing hydroelectric plants, wind farms on the costs and bulk solar energy from the Sahara desert.

Summary

HVDC is a proven flexible technology that can make a very significant contribution to the development of sustainable energy schemes, especially in cases where it would otherwise be difficult to connect the generator and the consumer. In addition to transmission efficiency, HVDC can enhance grid security and reliability.

The only barriers to an even greater take up of HVDC are probably market education and understanding. HVDC has a key advantage over AC because HVDC Light in particular has a low environmental impact due to its ‘invisible’ underground cables and compact converter stations. This can help to gain rapid project approval from the relevant authorities. There are a number of cases where HVDC might be seen initially as a more expensive option than AC. However, by enabling a sustainable energy scheme to be commissioned that generates revenues on a fast-track basis, HVDC could prove to be the most cost-effective solution in the long run.

About the Authors:

Peter Jones and Bo Westman work for ABB;