By Xie Ranhao

In this harvest season of the golden autumn, the 750kv transmission line and substation demonstrative project conducted by State Grid Corporation of China, which is the first highest voltage transmission line project in China, puts into production successfully.

As a demonstrative project, although the lines spanned in this 750 kv transmission and substation project by State Grid Corporation of China extend only 141 km from Guanting in Qinghai to east to Lanzhou in Gansu, its completion and operation symbolize that China's grid technology and manufacturing level of transmission and transformation equipment has stridden into the world advanced level, and has played an important role in accelerating the development of water and thermal power resources in northwestern China and promoting the optimized collocation of eastern and western energy resources.

Since the early 1970s, China has launched a 330kv transmission line and substation project in northwestern region while developing such large water power stations as Liujiaxia Hydropower Station and Longyangxia Hydropower Station at the upper stream of the Yellow River. After many years of construction, with the 330kv grid network covering Shaanxi, Gansu, Qinghai and Ningxia, a transmission network with the 330kv transmission line as the backbone has come into being in northwestern China. The construction and formation of the 330kv grid network in the northwest has effectively boosted the cascade development of hydropower stations on the upper stream of the Yellow River, strongly improved the compensative regulation between the cascade hydropower stations on the upper stream of the Yellow River and thermal power stations at northeast Gansu, Ningxia and Shaanxi, and greatly enhanced the hydropower efficacy of such completed large hydropower stations on the upper stream of the Yellow River as Longyangxia Hydropower Station.       

It's been proved in practice that the construction of the 330kv grid network in northwestern China has basically satisfied the electrical source development in northwestern China, especially the development of hydropower resources on the upper stream of the Yellow River, as well as the demand for power in the economic development and people's livelihood, making tremendous contribution to the economic development in northwestern China.   

However, as China's economic development has quickened its steps and the installation scale of power stations has expanded, the existing 330kv grid network in northwestern China has also presented a maladjustment to the rapid development of electrical source, which makes it difficult to satisfy the demands in the economic and social development in this region. 

As forecast by experts, by the year 2010, the transmission capacity of the 330kv grid network in the northwest will gradually tend to be saturated, the system short-circuit current will approach the breaking capacity of the circuit breaker, the line corridor will become tense, and the power transmission capacity will be restrained. Considering the requests for the co-existence of hydropower and thermal power and tidal current exchange within the network, the present 330kv net frame cannot meet the requirements anymore. Particularly, as newly started stations on the upper stream of the Yellow River like Gongboxia Hydropower Station and Laxiwa Hydropower Station will soon be completed and put into production consecutively, keeping transmission with a 330kv voltage will certainly lead to a high density of transmission lines on the upper stream of the Yellow River, the intensity of the line corridor, the overabundance of inter-provincial tie lines, a lower transmission efficiency, and even a resource waste because of the failure of transmission. To meet the requirements for large capacity power transmission and exchange of those stations ready for production, it is imperative to build transmission line projects with a higher voltage. Just against this background, the 750kv transmission line and substation demonstrative project of the State Grid Corporation is launched.     

The construction of the demonstrative 750 kv transmission and substation project project is also the requirement to realize the interregional transmission of power resources and the optimized collocation across China. Owing to the great nature, the west region boasts rich energy resources while the east with a relatively rapid economic development is indigent of them comparatively. Take northwestern China as an example. Despite the rich hydropower resources with the upper stream of the Yellow River as the representative, there are also luxuriant coal resources in north Shaanxi, Ningxia and Xinjiang; the known coal deposits in Shaanxi, Gansu, Ningxia and Xinjiang amount to 200 billion ton, accounting for 23.9 percent of the total amount in China. Moreover, the coal quality is very good in the coal bases in north Shaanxi and Ningxia with a low content of sulphur, reliable water resources and a relatively big environmental protection capacity. These coal bases are suitable for large-scale development and construction as power and coal bases and the power can be sent to the east through transmission lines, thus changing the resource advantage into an economic advantage, driving and improving the development of local economy. Doing that can be also propitious to relaxing the contradiction of China's long-time intense rail transportation.

However, if we keep on working with the current 330kv transmission lines and the interregional 500kv transmission lines across China, a new problem about how to reduce the transmission loss is to be encountered in our efforts to translate the resource advantage into economic advantage in the northwest.

According to the practices in developed countries, compared with 330kv and 500kv transmission lines, the transmission capacity of a long-distance 750kv transmission line is 4 times of that of a 330kv line and about twice that of a 500kv line whereas its transmission loss is obviously lower than that of a 330kv line or a 500kv line.

Moreover, the line corridor of a 750kv transmission line occupies a much smaller area than that of a 330kv line or a 500kv line. So it's beneficiary not only to improve the investment returns of grid network construction but to enhance the security of its operation. In order to reduce transmission line loss and improve the investment benefit of grid network construction, it is imperative to enhance transmission voltage.

It's known that with the 50 transmission line demonstrative project put into production in a favoring way, the State Grid Corporation of China will continue to strengthen the 750kv transmission line construction according to the plan for northwest grid network construction determined; by the year 2010, a backbone 750kv net frame with a shape of a reversed A will be shaped up in the northwest with the Qinghai Lawaxi Hydropower Station on the upper stream of the Yellow River as the apex and the Qianxian in Shaanxi and east to Yinchuan in Ningxia as the two feet. At that time, the 750 transmission lines will develop into an interconnection structure in the northwest starting from Yongdeng in Gansu, then passing through Zhangye and extending north to Anxi areas, where it will be connected to the Hami coal base in Xinjiang through 750 transmission lines.

Along with the shaping up of the 750kv backbone net frame by State Grid Corporation, the coverage of the grid network interconnection across China generally realized in mid June this year will further be extended to Xinjiang from Gansu; while vigorously improving the development and construction of hydropower bases and thermal power bases in the west, the optimized collocation of power resources will be actualized in a broader range. That is, to utilize the regulating function of large grid network to reasonably attemper the different electricity load characteristics and time difference so as to obtain comprehensive benefits such as staggering electro-peaks, avoiding electro-peaks, working as stand-by generator sets for each other, making hydropower stations coexist with thermal power stations, compensation across valleys and the improvement in the utilization of generator sets.

What's particularly worth mentioning is that according the requirement of independent innovation, the demonstrative 750kv transmission line and substation project, including equipment production, is the first high-voltage transmission line project that is designed, constructed and produced independently in China; through the cooperation of scientific and technological personnel in various departments to tackle key problems, crucial breakthroughs in many technologies key to the building of high-voltage grid networks have been made; at the same time, over 90 percent of the engineering equipment has been domestically produced.

Therefore, with the 750kv transmission line and substation demonstrative project by State Grid Corporation put into production, it can be called as a project that has opened a new page for either the acceleration of the development of 750kv grid networks in the northwest or the construction of strong state grid network with extra high-voltage grid network as the core, the improvement of the development of western energy resources and the promotion of the optimized collocation of eastern and western energy resources as well as the acceleration of the construction of first-class grid network in the world.