IN THE LAST COUPLE DECADES, THE CHINESE POWER INDUSTRY HAS WITNESSED TREMENDOUS GROWTH. From 1980 to 2005, China's installed capacity grew 8.6% per year. Since 1996, China has ranked second in both power production and annual consumption. And by the end of 2005, China's installed capacity had increased to 512 GW.
China has six regional grids: the North China Grid, Northeast Grid, Central China Grid, East China Grid, Northwest Grid and South China Grid. For the most part, interconnection between these grids has been accomplished.
China's system of 12.47 GW of ±500-kV dc makes it the most extensive in the world. The current 500-kV ac synchronized network connecting Northeast, North and Central China is a typical long-chain type of structure with an electrical distance of about 5000 km (3107 miles) from the Yimin thermal plant of Northeast China to the Ertan hydro plant in Sichuan Province. The low-frequency oscillation of 0.13 Hz with weak damping has greatly impacted system safety and reliability, which constrains the ability of operators to exchange power between regional networks.
Because the present system is not capable of meeting the future electricity needs of China, an ultrahigh voltage (UHV) grid will be built out. China is now developing a 1000-kV ac network supported by a series of ±800-kV dc projects. This UHV strategy will:
- Increase transmission capacity
One 1000-kV circuit transmits about 5 GW, approximately 5 times that of a 500 kV transmission line. An ±800-kV dc transmission line has the capacity of 6.4 GW, which is 2.1 times that of a ±500-kV dc power line.
- Extend transmission distance
A 1000-kV ac line can economically transmit power distances of 1000 km to 2000 km (621 miles to 1243 miles). An ±800-kV dc power line can economically transmit power over distances of 2000 km to 3000 km (1864 miles).
- Reduce transmission losses
If the conductor cross-sectional area and transmission power are held constant, the resistance losses of a 1000-kV ac power line is 25% that of the 500-kV ac power line. The resistance loss of ±800-kV dc transmission line is about 39% that of a comparable ±500-kV dc line.
- Reduce costs
The cost per unit of transmission capacity of 1000-kV ac and ±800-kV dc transmission scheme are 73% and 72% that of 500-kV ac and ±500-kV dc schemes, respectively.
- Reduce land requirements
A 1000-kV ac line power line saves 50% to 66% of the corridor area that a 500-kV ac line would require. An ±800-kV dc line would save 23% of the corridor area required by a 500-kV dc line.
In China, two-thirds of the coal reserves are located in the North and 80% of the hydropower reserves are located in the West. These energy sources are far removed from the energy demands located in the eastern areas 800 km (497 miles) to 3000 km away.
State Grid Corp. of China, the largest electric power provider in the country, will first build a 1000-kV transmission network covering North and Central China, and then expand to East China. State Grid will then tie the ac-dc UHV systems to Southwest China for a network with a capacity of 200 GW.
For the proposed UHV synchronized network connecting North, East and Central China, the total installed capacity will exceed 700 GW by 2020. With UHV technology, the electrical distance is shortened significantly, the grid structure is enhanced and the system stability is improved. Power-system simulation shows that the stability level will be high enough to transmit bulk power while protecting the system from high current faults. In case of a bipole failure of ±800 kV or a single transmission corridor failure of UHV ac, the system will be capable of maintaining system stability without experiencing serious low-frequency oscillation.
The high voltages and currents, long distances and variable power flows of the UHV system will make reactive power control even more challenging, requiring the reasonable installation of reactors and reactive equipment. Series compensation devices may also be required for UHV ac lines to transmit power over long distances.
State Grid is cooperating with research institutions and equipment manufacturers to conduct in-depth investigations on the design and manufacturing capability of UHV equipment. Over the past year, we have made breakthroughs in the development and manufacture of UHV equipment. The conceptual design for all 1000-kV devices has been completed. We have produced prototypes for 1000-kV porcelain circuit breakers and 1000-kV CVTs. The State Grid has certified type tests for 1000-kV disconnect switches, post insulators and polymer insulators. All other UHV devices are being produced to meet the requirements of the UHV demonstration project.
Development of the UHV network will meet the energy needs of China, while also becoming a major milestone in bringing the next generation of innovation and technology to our global power grid.
Yinbiao Shu is executive vice president of State Grid Corp. of China.