Great leap forward leads to a Chinese revolution

The energy requirements of the expanding and developing economies of China, India, Africa and South America have presented new challenges for the world’s long distance bulk power transfer providers, mainly because the large fuel and hydro resources available in these regions have that are sited far from population centres. Increasing reliable power transfer over a distance, creating minimal environmental impact, reducing the frequency of power cuts, and increasing the efficiency of the existing grid are among these challenges. To meet them grid designers have pushed near to its theoretical limit the capability of current HVDC technology, and several national governments have published plans to utilise this technology at hitherto unknown voltages – 800 kV – a figure now generally acknowledged as belonging in the ultra-high voltage category.

Figure 1. Planned UHVDC transmission links in China

World’s longest power link

In 2007 China’s State Power Grid Corp (SGC) announced an ambitious investment plan that involved the expenditure of some yuan 220 billion ($28.6 billion) over a period of five years on developing the national transmission system. Most of this record level of investment was earmarked for line construction at an unprecedented rate, including 52 000 km of 110 kV transmission line capable of moving 230 GW around the system. During 2007 the corporation put into operation 48 000 km of 110 kV lines.

The plan also involves medium and high voltage transmission. China’s intention to connect the regional grids into a fully national system implies the construction of (for the most part) high voltage AC lines, while its need to move large amounts of power from the coal fields in the north (“coal by wire”) and from production sites such as the Three Gorges hydro project in central China to the major load centres in the east and south of the country has necessitated the construction of several bulk transfer lines over distances in the order of 1000 km, and the chosen technology for these lines has been HVDC, supplied mainly by ABB and Siemens, together with a large number of local suppliers.

Now the next logical stage has been reached, to exploit fully the voltage capacity of currently available HVDC technology to increase capacity and reduce power losses by raising its value from the current standard 500 kV to 800 kV, a level at the bottom end of the band of voltages now usually characterised as UHVDC. Increasing the voltage level also creates advantages for the environment, including the use of less land compared to conventional overhead lines.

Several lines have been planned (Figure 1), and the first of these, the 1400 km Yunnan – Guangdong 5 GW interconnector currently being built by the country’s second largest grid operator, China Southern Power Grid Corp, is due to come on line in 2010. The second, being touted as the country’s first power superhighway, is to carry 6.4 GW from the Xiangjiaba hydro power plant complex on the Jinsha river to Shanghai 2071 km (1286 miles) away on the country’s industrialised east coast. When completed in 2011 it will be the longest and most powerful HVDC link in the world, and will supply the power needs of around 31 million people.

Xiangjiaba -Shanghai

SGC, China’s largest transmission operator, has awarded the larger of its two main engineering and equipment supply contracts for the interconnector, worth $440 million, to ABB, which will supply system engineering comprising design, supply and installation of the two converter stations, and 800 kV UHVDC power transformers, switchgear and valves provided with newly developed 6-inch thyristors for the Shanghai (receiving) substation. It will also provide advanced control equipment for the substations and the power transmission system incorporating what it characterises as breakthrough technology. ABB will also co-operate with China's Tebian Electric Apparatus Stock Co to supply 18 converter transformers for the project.

The line’s power rating of 6400 MW will be more than double that of the most highly rated such line in operation today. Transmission losses are expected to be less than seven per cent.

The new six-inch thyristors and an advanced control system together allow the biggest capacity and efficiency leap in this field in 20 years, says ABB. The Xiangjiaba - Shanghai link represents a major breakthrough in the technology of electric power transmission, and it has broken two long standing records:

• The new system voltage is 33% higher than that of the Itaipu ±600 kV transmission link in Brazil, currently the world’s highest HVDC transmission voltage rating.

• The overhead line length, 2071 km, will be the longest overhead transmission in the world, compared with 1700 km for the Inga-Shaba HVDC transmission in Congo-Kinshasa, the current record holder.

Figure 2. STRI laboratory showing an 800 kV UHVDC test installation

Thyristor valves

Siemens Energy together with local partners is to deliver ten extra-high voltage converter transformers, five of them rated at 800 kV, as well as 6-inch thyristor valve technology for the Fulong converter station in Sichuan at the Xiangjiaba hydro-electric power plant end of the link. The order has been placed by SGC and has a value for Siemens of about r160 million.

The ten transformers will be built at the Nuremberg transformer factory which specialises in the manufacture of converter transformers, particularly the newly developed 800 kV HVDC transformers. ‘Last year Siemens became the first transformer manufacturer in the world to receive an order for 800 kV converter transformers [for the Yunnan-Guangdong interconnector]. This new order confirms that our customer is absolutely satisfied with our high performance technology’ commented Dr. Bertram Ehmann, head of the transformer business in the Siemens Energy division.

Siemens is equipping the valve towers of the Fulong station with the latest generation of 6-inch high-performance thyristors, without which, they confirm, the low-loss transmission of the very high DC voltages required in this installation would not be possible.

Advances

The increase in capacity and efficiency have been made possible by advances in research in a number of fields, including the development of new materials for outdoor insulators and advanced control systems with extremely high calculation capacity.

Transmission losses will be less than 7 %, significantly less than the losses (around 10 %) from conventional 500 kV HVDC transmission schemes. The savings from using UHVDC compared with HVDC on this line are equivalent to the annual power consumption of more than 900 000 people in China.

UHVDC transmission is a further development of HVDC, a technology pioneered by ABB more than 50 years ago. Its capacity increase was made possible by advances in basic research in a number of fields, among them the development of new materials for outdoor insulators and advanced control systems with extremely high calculation capacity. ABB has invested considerably in equipment development, manufacturing and testing facilities to enable this new technology to be used in commercial operation. The developed equipment has been under successful test operation at 850 kV DC since November 2006, at the STRI laboratory (Figure 2) in Ludvika, Sweden, where it has been inspected by representatives from SGC.