New ultra high voltage direct current (UHVDC) transmission technology is helping power generation companies unlock the tremendous potential of hydropower by enabling the creation of power ‘superhighways’ that can deliver vast amounts of electricity over very long distances.

Bo Normark, ABB Grid Systems, Switzerland

Harnessing the energy provided by renewable sources, such as the natural forces of water flow, is one of the great hopes for meeting the world’s future power needs, especially in China and India, where reducing dependence on fossil fuels and cutting carbon dioxide emissions are growing priorities.

Extensive tests were conducted at extremely high, previously untried voltages. Above shows a discharge flash at over two million volts, and these flashes are more than 10 m in length
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However, to date, there has been a significant obstacle in the path to hydro energy expansion: distance. The best sources of hydro energy are often many hundreds of kilometres away from the centres of population and industry where demand for electricity is growing fastest. Until recently, carrying electricity over distances of much more than 1000 km or more from power plants has not been economically feasible, because of the transmission losses involved.

UHVDC technology promises to change all that by enabling the efficient transmission of electricity over distances of up to 3000 km.

Higher voltages, lower losses

UHVDC increases the transmission voltage to ±800 kV – a significant step up from the standard HVDC voltage of ±500 kV used in Asia, which was introduced more than two decades ago – and reduces transmission losses from typically ten per cent to seven per cent.

This huge reduction in transmission losses – equivalent to 192 MW on a 6400 MW line, or 86 x 2 MW wind turbines – makes it viable to produce electricity in remote regions of China, India, Brazil and Africa, where vast sources of hydropower have so far remained untapped.

The main beneficiaries from UHVDC technology will be China and India as these countries strive to secure reliable energy sources. India plans to build five UHVDC lines over the next ten years, each with a capacity of between 6000 MW and 8000 MW. China is planning one new UHVDC line for every year of the next decade, each with a capacity of between 5000 MW and 6400 MW – the first is scheduled to be in commercial operation by 2011. There are also plans to install 800 kV UHVDC lines in Brazil and southern Africa.

There are enormous potential benefits from the use of UHVDC technology. A 6400 MW UHVDC link can provide enough electricity to meet the needs of around 50 million people in India, or around 14 million in China, based on the average per capita electricity consumption of these countries.

What is more, UHVDC provides significant financial savings, around 30 per cent, compared with 500 kV DC and conventional 800 kV AC transmission technologies. These cost savings arise from the reduction in power line losses and the resulting savings in converter station and associated AC switchgear.

Another saving that should not be overlooked results from the significantly lower right-of-way needs of UHVDC – which can be less than half those of alternative transmission methods. With UHVDC, the width of the power line track, or transmission corridor, is minimal. To deliver the same capacity, alternative transmission methods would require two or more lines, with a much wider transmission corridor.

Unlike normal AC transmission lines, HVDC lines have an almost negligible oscillating magnetic field. This means that HVDC lines, unlike their AC counterparts, can easily satisfy the stricter magnetic field requirements (<0.4 microTesla) that are increasingly being enforced in developed markets.

It is worth noting that increases in AC electricity transmission capacity are also possible using another family of technologies known as FACTS (Flexible AC Transmission Systems). These can be used to maintain or improve voltage stability and grid reliability and reduce overall power losses.

Major Research Effort

The last significant increase in HVDC transmission voltage was made almost a quarter of a century ago, when ABB built a transmission line rated at 600 kV for Brazil’s Itaipu hydroelectric power plant. The main reason it has taken so long to move to 800 kV is that this development step required further basic research in several fields. The renewed interest in UHVDC as a commercial transmission method in recent years provided new impetus to restart this research.

The first area demanding some basic research was the development of new insulator materials for outdoor environments. With the use of higher voltages, there needs to be a wider gap between voltage and ground to isolate live parts.

Over the past 15 years, we have seen a move away from century-old porcelain to new polymer-based insulators, primarily silicone rubber, which provide very favourable properties for outdoor insulation. Refined measurement methods using advanced laser technology were needed to determine the characteristics of insulating materials and systems.

Second, the industry needed much more advanced computer-aided design tools to perform three-dimensional field calculations, mainly for transformer design. And third, advanced control systems for controlling the entire installation were needed.

Intensive Product Development

With the basic research elements in place, the past two years have seen an extensive product development project with the aim of making ±800 kV HVDC a reality. Much of this development work has been done at ABB’s R&D facilities in Sweden.

Extreme reliability is paramount with up to 9000 MW being carried on one pair of overhead lines, which could be serving potentially millions of people. This has demanded a completely new converter station design, with a mean time between failures of 25 years. Such high levels of availability have been achieved through extensive sectioning of both the main circuit and the auxiliary systems. In addition, all control and protection systems are duplicated, as are auxiliary systems such as emergency battery and auxiliary power systems.

ABB’s 800 kV UHVDC transformer is a key component for power links that can deliver vast amounts of electricity over very long distances
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The development programme for converter station equipment can be divided into two main areas: equipment with DC voltage grading and linear distribution; and equipment with oil/paper insulation and non-linear voltage distribution.

For the first category of equipment, which includes thyristor valves, the design may be scaled due to the fact that the voltage distribution is linear. Equipment in the second category includes the wall bushings and the converter transformers, and requires major research and develoment, and extensive testing.

New types of all the high-voltage devices subject to DC have been developed and fitted with newly developed polymer insulating materials. These include voltage dividers, bypass switches, radio interference capacitors, disconnectors and post insulators.

One of the main goals of the project was to develop a transformer and bushing for 800 kV, using the basic research as a foundation. Prototype bushings were produced and type-tested. For the transformer, a mock-up was built to simulate the parts of the transformer subject to high DC voltages.

Design criteria for air clearance have also been established, following extensive testing at extremely high, previously unused, voltages. The testing involved electrical discharges at over two million volts, which created flashes more than ten metres long.

The 800 kV UHVDC projects that are planned to be in operation in China before 2015
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To satisfy the extreme reliability demands of UHVDC, control and protection systems have been further developed. These are based on ABB’s proven Mach2 system. Furthermore, a new thyristor and thyristor valve have been developed to accommodate the increased voltage and current.

Special Endurance Testing

All critical devices have been put through extensive testing in a special endurance testing station built by ABB at STRI, an independent technology consulting company and accredited high-voltage laboratory in Ludvika, Sweden. The elevated voltage tests have extended over a year, in order to verify the long-term characteristics of the components used.

While operating conditions can be well defined for UHVDC converter stations, conditions for the UHVDC transmission lines may vary greatly along their length. A typical 2000-3000 km route will pass through all kinds of terrain, including areas of high altitude and high pollution levels, for example. ABB is gathering pollution data from existing high-altitude substations that can be used in the design of UHVDC converter stations and transmission lines.

China Leading the Way

The recent dramatic growth of the Chinese economy has led to double-digit growth in electricity consumption every year over the past ten years. China’s total installed power generation capacity has more than doubled over the past decade to around 650 GW currently, and by 2020 the government plans to have 1000 GW installed.

In order to meet this continued growth in demand, the Chinese Government has launched a programme to develop the country’s huge untapped hydropower potential in the southwest of the country. With the load centres located along the country’s east coast (i.e. Beijing, Shanghai and Guangzhou), highly efficient UHVDC bulk power transmission systems are needed to carry the electricity across vast distances.

The number and sheer scale of these projects makes China by far the biggest market for bulk power transmission in the world. China is planning for around ten UHVDC projects, each rated 5000-6400 MW, to be in operation before 2020, with transmission distances ranging from 2000 km to 3000 km. The first project is scheduled to be in commercial operation already by 2011.

The Xilodu–Xiangjiaba hydropower plant located 1000 km upstream from the Three Gorges Dam in western China has a planned ultimate power capacity of 18 200 MW, with 14 x 700 MW units on the river’s left bank and 12 x 700 MW units on the right bank. Four HVDC transmission links will make this power available to the load centres in eastern and southern China, together with several 500 kV AC lines.

One of these DC links will be used to transmit bulk power to Shanghai and will reinforce the interconnection of China’s central power region, to which the hydroelectric power plant will be connected synchronously, to the eastern power region. Power will be transmitted towards the east during the high-water season, and towards the central region in the low-water season.

Transformer milestone

A key milestone in the rollout of UHVDC was reached at the end of 2008 when ABB successfully tested an 800 kV transformer. This unit is destined for the UHVDC transmission corridor that will run between the Xilodu-Xiangjiaba hydropower plant all the way to Shanghai, some 2000 km to the east. When installed, this will be the world’s highest-voltage power link, and will have a record capacity of 6400 MW – capable of supplying over 30 million people.

The UHVDC transformer is the first of several to be deployed by the State Grid Corporation of China. It is a critical element of the systems needed to convert DC to AC, and back, and alter the voltage at each end of the UHVDC link. Among other challenges, raising the voltage to as much as 800 kV increases the technical requirements on a transformer’s insulation and on the design of critical parts such as the bushings.

China’s decision to deploy UHVDC transmission to connect the hydro-rich region of southwest China to the country’s main load centres – some 1500-2000 km to the east – underscores the feasibility of the technology. It seems the future of UHVDC will be assured as demand for electricity from renewable sources continues to grow.