ABB has implemented several commercial HVDC power transmission schemes based on VSC (Voltage Source Converter) technology and is a pioneer in this field. Since the first project commissioned in 1997, its HVDC Light technology has evolved through three generations. It has now reached its fourth evolutionary stage, based on CTL (Cascaded Two Level) topology.
Dr. Heather Johnstone, Chief Editor
HVDC, which stands for high voltage direct current, is a well-proven technology used for transmitting electricity over long distances by overhead transmission lines or underground/submarine cables, with minimal losses.
In an HVDC system, electric power is taken from one point in a three-phase AC network, converted to DC in a converter station, transmitted to the receiving point by an overhead line or underground cable and then converted back to AC in another converter station and injected into the receiving AC network.
Typically, an HVDC transmission system has a rated power of several hundred megawattsand many are in the 1000–3000 MW range With an HVDC system, the power flow can be controlled rapidly and accurately in terms of both power level and direction. This capability is often used to improve the performance and efficiency of the connected AC networks.
ABB pioneered HVDC technology and is a world leader in the field. In total, around 140 GW of HVDC transmission capacity is installed in some 145 projects worldwide.
The first commercial HVDC scheme, based on mercury arc valves was commissioned in 1954. 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.
There was a significant improvement in HVDC technology in 1970 when thyristor valves were introduced in place of the mercury arc valves. This reduced the size and complexity of HVDC converter stations substantially.
The use of microcomputer control equipment in today’s projects has also contributed to HVDC’s current success as a powerful alternative to AC power transmission.
In 1995, ABB announced a new concept for HVDC converter stations, HVDC with Capacitor Commutated Converters (CCC), which further improved the performance of HVDC transmission systems. And, in 1997, a completely new converter and DC cable technology called HVDC Light was introduced.
The introduction of HVDC Light
The HVDC Light transmission technology deploys a new IGBT (Insulated Gate Bipolar Transistor) semiconductor-based converter in combination with XLPE (cross-linked polyethylene) DC cable systems. This innovation complements the traditional bipolar semiconductor-based converter technology and has proved to be a state-of-the-art power system with increased controllability.
HVDC Light is easy to install and offers a number of environmental benefits, including the use of underground or subsea power links, neutral electromagnetic fields, oil-free cables and compact converter stations. The technology also increases the reliability of power grids. The continuous development of HVDC Light has in the meantime increased the power range from tens of megawatts to 1200 MW at ±320 kV with cables.
HVDC Light has also opened up new applications for HVDC transmission, such as providing shore power supplies to islands and offshore oil & gas platforms, enabling city centre in-feeds and more recently the integration of offshore wind farms.
In future, this technology is likely to play an important part in the development of DC grids. (multi-terminal DC connections)
The next generation of HVDC Light
Initially HVDC Light had around 3 per cent losses per converter station, while in the latest generation it is now down to about 1 per cent. Reliability requirements have been driven by dynamic performance, harmonic generation and the flexibility to accommodate changing grid conditions.
HVDC Light converters today are based on CTL (cascaded two-level) converter topology, eliminating the need for AC filters and enabling a more compact converter design with a low level of harmonics and audible noise.
A special feature incorporated in HVDC Light is the IGBT module (StakPak), which incorporates a fail-safe short-circuit mode, allowing operations to continue, even after failure of an IGBT module, without auxiliary equipment. The latest generation of converter stations, comprising two 1000 MW and ±320 kV converters, has a footprint of 220 metres by 150 metres, which is substantially smaller than previous generations.
Multi-terminal HVDC Light
An important advantage of HVDC Light technology is that the power direction is changed by switching the direction of the current, and not by changing the polarity of the DC voltage. This facilitates the construction of multi-terminal HVDC Light systems. The terminals can be connected to different points in the same AC network or to different AC networks. The resulting DC grids can be radial, meshed or a combination of both.
Multi-terminal HVDC Light systems are particularly attractive for integration of large-scale renewable energy sources such as offshore wind farms and for reinforcement of interconnected regional AC grids.
DC grids fall into two categories: regional DC grids and interregional DC grids.
The powering of Norwegian oil company Statoil’s Troll A gas production platform via a DC link from the is the first time an offshore platform has been powered by such a link originating on land Source: ABB
A DC regional grid is defined as a system that comprises a single protection zone. Such a grid can be realized today with proven technology and DC breakers (to isolate the faulty part of the grid) are not needed. There are, however, some regulatory issues that need to be addressed.
An interregional DC grid is defined as a system that needs several protection zones. There are some technology gaps that need to be closed including: DC breakers, power flow control, automatic network restoration and DC/DC converters for connecting different regional systems
It is interesting to see how ABB’s MACH-2 control system can maintain the power balance in a multi-terminal system.
The power balance in a grid is maintained if the power input is equal to the power output, including losses. The power-balance indicator in a DC system is the DC voltage – if there is surplus power the DC voltage increases and if there is a deficit it decreases.
Following a power change, the DC voltage will fluctuate until the power balance is reached and then remain at the new steady-state value.
To keep the voltage at a pre-set value, i.e. to keep the energy balance, the MACH-2 system implements voltage control together with power control, where one terminal controls the DC voltage and the other terminal(s) is controlling the power. The rectifier is used to control the DC voltage and the inverters are used to control the power to the pre-set values at the two terminals respectively.
When a new power level is requested the DC voltage will change, and the DC voltage controller will act depending on the change and reduce or increase the power input to restore the voltage in the system.
Proven track record
ABB has a strong track record in deploying its HVDC Light technology with 16 projects either in operation or in progress. Examples of those in commercial operation include:
- Gotland, Sweden: a 70-km underground cable connection to a wind park
- Murraylink, Australia: a 180-km underground cable link between grids
- Cross Sound, US: a 40-km subsea cable link from Connecticut to Long Island
- Troll A, Norway: a 70-km subsea cable to feed power to an offshore gas production platform
- Eagle Pass, USA: an interconnection between US and Mexican regional grids (two 138 kV transmission lines)
offshore wind growth presents opportunities
Global wind power installations increased by 35.8 GW in 2010, according to the Global Wind Energy Council (GWEC), bringing total installed wind capacity up to 194.4 GW, a 22.5 per cent increase on 2009.
Offshore wind power in particular experienced a new record growth in Europe last year. According to the European Wind Energy Association (EWEA), 308 new offshore wind turbines were installed, representing an increase of 51 per cent on 2009’s installed wind power capacity. Approximately 883 MW of new capacity, worth in the region of €2.6 billion ($3.6 billion), was installed in 2010 in nine wind farms in five countries, making a total of 2964 MW.
The installed offshore wind power capacity now supplies the equivalent of 2.9 million average European Union households with electricity from a total of 1136 offshore wind turbines. In an average year these turbines are expected to produce 11.5 TWh.
EWEA forecasts continued strong growth with 1000-1500 MW of new offshore wind power capacity expected to be fully grid-connected in Europe during 2011.
Ten European wind farms are currently under construction representing a total of 3 GW, which will more than double the installed capacity of the 45 existing grid-connected offshore wind farms.
EWEA research shows that 19 GW of offshore wind capacity is currently consented and could generate 66.6 TWh of electricity in a normal wind year once operational – enough to supply 14 of the largest capital cities in Europe with electricity, including Paris, London and Berlin. Germany, the UK and several other countries around the North Sea have several large offshore wind energy capacity expansions planned.
Offshore wind is clearly an area that will see significant expansion in coming years and ABB expects its HVDC Light to be a key enabler in integrating power from remote locations into mainland grids to feed consumer needs.
One of the key characteristics of HVDC Light is its superior ability to stabilize the AC voltage at the terminals. This is particularly important for wind farms, where the variation in wind speed can cause severe voltage fluctuations.
BorWin1 offshore wind project
The first HVDC link to connect an offshore wind farm with an AC grid is the BorWin1 project in the North Sea.
Using HVDC Light technology, this 200 km link connects the Bard Offshore 1 wind farm located off Germany’s northern coast to the 380 kV high-voltage AC grid on the German mainland. The link has a capacity of 400 MW and a DC voltage of ±150 kV.
Once complete, the BARD Offshore 1 wind farm will comprise 80 wind turbines, each with a capacity of 5 MW. These will feed their power into a 36 kV AC cable system. This voltage will then be stepped up to 155 kV AC before reaching the HVDC Light converter station, which is located on a dedicated platform.
Here the AC power is converted to ±150 kV DC and fed into two 125-km subsea cables, which then continue into two 75-km land cables to the land-based converter station at Diele in Germany.
TenneT Offshore GmbH awarded ABB an contract to supply a 800 MW (±320 kV) transmission link that will connect offshore wind farms located in the DolWin1 wind farm cluster – the 400 MW Borkum West II wind farm, plus future wind farms – in the North Sea to the German grid.
The wind farms will be connected to an offshore HVDC converter station which will transmit electricity to the onshore HVDC station at Dörpen, on the northwest coast of Germany via 165 km of underwater and underground DC cables.
The Dörpen/West converter station will in turn feed AC power to the mainland grid. At 320 kV this will be the highest voltage level of extruded cable ever used for HVDC. Scheduled to be operational in 2013, this network of offshore wind farms is expected to avoid three million tonnes of carbon dioxide emissions per year by replacing fossil fuel based generation.
Germany currently meets in the region of 8 per cent of its electricity requirements with wind power and expects to double that by 2020.
The latest HVDC Light technology also has the potential to become the preferred system for power in-feeds to cities – for strengthening power networks in areas where new overhead lines are not a practical or desirable option.
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