Features

Focus on UHV AC: China shows the way by energising 1,000 kV line [free access]

March 2, 2009

China has energised its first 1,000 kV ultra high voltage alternating current (UHV AC) transmission line, which has an electricity transfer capacity of 5,000 MW from the northern to the eastern and central parts of the country.

The Jindognan-Nanyang-Jingmen UHV AC transmission project began commercial operation in January 2009. The 650 km long line has two segments: a 360 km line from Jindognan in the northern province of Shaanxi to Nanyang, and a 290 km line from Nanyang to Jingmen in the central province of Hubei. The project includes a 3.7 km line crossing the Yellow river and a 2.9 km line crossing the Hanjiang river. The UHV AC has been built by the country's largest transmission utility, State Grid Corporation of China (SGCC), at an estimated cost of CNY5.7 billion.

The project, which began in 2006, took 28 months to complete and about 168 hours of test runs to be capable of commercial operation. China launched a UHV grid development strategy in 2004 and proposed a countrywide power grid with 1,000 kV AC and 800 kV direct current (DC) transmission lines as the backbone.

China, however, is not the first country to build UHV AC transmission systems. Since the late ‘70s, several countries considered constructing high voltage transmission lines for efficient power transfer. Among these, Russia and Japan are the only two major countries to have actually implemented the technology. In 1985, Russia energised a 500 km section of the 900 km transmission line at 1,100 kV between Ekibashtuz and Kokchetva. Currently, this line operates at 550 kV. In Japan, UHV AC studies were initiated in 1973 as a way to deal with the problems in expanding the 550 kV network. Construction of 1,100 kV lines was taken up by the Tokyo Electric Power Company (TEPCO). By 1999, TEPCO completed two such projects - one connecting a nuclear power station on the Sea of Japan to the metropolitan region (north-south route) and the other linking the power sources on the Pacific Ocean (east-west route). These transmission lines are now operated at 550 kV and will be upgraded to 1,100 kV by 2010.

Of late, there has been a keen interest in using UHV AC transmission in regions with high electricity demand and where the supply sources are far away from the demand centres. In India, an indigenous effort is underway to develop a transmission system based on 1,200 kV AC. The initiative is spearheaded by the country's central transmission utility, Power Grid Corporation of India Limited (PGCIL). As the first step, a test station is being set up in the state of Madhya Pradesh. The facility is likely to be commissioned by mid 2009. Amongst other countries, Brazil has been actively considering the UHV option for its estimated 192 GW of hydro power potential. It is planning to construct 2,700 km of UHV transmission lines with a transfer capacity of 12-15 GW. Subject to the load capability of the system, the expected UHV voltage could be 1,250 kV.

UHV AC technology offers many benefits. To begin with, a large quantity of power can be transferred over long distances at a very low current value, which means lower transmission line losses. Transmission line losses can be reduced to about 25 per cent and land requirements rationalised by 50-66 per cent as compared to standard extra high voltage (EHV) lines.

Second, transmission utilities can deliver a high amount of power without having to install multiple transmission lines and equipment. This lowers operational costs. The cost per unit transmission capacity of UHV AC lines is expected to be 73 per cent that of 500 kV AC lines. In addition, in high voltage lines, utilities look for stabilisation of the power system to withstand large generation and transmission outages.

The commercial application of such systems is, however, based on whether the increased transfer capacity can outweigh the cost of execution. Various studies have shown that UHV AC systems cannot be seen as a simple extension of the existing, lower voltage levels. Estimates show that the power transfer capacity of a 1,500 kV line is 4.2 times more than a 765 kV line, whereas its cost is greater by 3.5 times. These are only indicative estimates as the cost-benefit comparison for a specific utility is always determined by factors specific to the project.

A competing option in the high voltage segment is DC systems. UHV DC offers almost similar advantages for long distance transmission. However, AC technologies have more flexibility in operations as key equipment and components such as transformers have achieved significant technological maturity. Industry experience also shows that AC transmission lines can easily connect loads and generation points along the route. While DC lines do offer a competitive option, the cost of building tapping stations has often been a deterrent. Overall, the choice between AC and DC is based on the individual characteristics of the project being considered. Also, as China's planned UHV network shows, UHV AC and UHV DC can be used together to derive the best of both technologies.

In its pilot UHV AC project, China has made several breakthroughs in core technologies and equipment production to transmit power economically through the world's most complex UHV lines. SGCC formulated its own specifications for rated voltages through extensive grid studies and investigations of voltage coordination. The major breakthroughs include voltage standard, electromagnetism environment, over-voltage and insulation coordination, reactive voltage control and lightning-proof technology. As a result, China could domestically produce 90 per cent of the equipment needed for the project and now has intellectual property rights over UHV transmission lines.

 

Main rated values for UHV lines, based on SGCC's specifications

Rated voltage

1,100 kV

Nominal operating voltage

1,000 kV

Rated lightning impulse withstand voltage to earth

2,400 kV

Rated short-duration power frequency withstand voltage to earth

1,100 kV

Rated short-duration power frequency across the isolating distance

1,100+635 kV

Rated lightning impulse withstand voltage across the isolating distance

2,400+900 kV

Rated switching impulse withstand voltage to earth

1,800 kV

Rated switching impulse withstand voltage across the isolating distance

1,675+900 kV

Rated frequency

50 Hz

Rated normal current for feeder circuits

4,000 A

Rated normal current for main bus bar circuits

8,000 A

Rated short time withstand current

50 kA, 3s

Rated peak withstand current

125 kA

Source: Conference proceedings from GridTech 2009, India

 

The UHV transformer was developed indigenously by China. The transmission system uses 1,000 MVA/1,000 kV single-phase autotransformers. Shunt reactors have been used for compensating the large charging reactive power of the transmission line. The 960/720/600 MVAR shunt reactors and neutral grounding reactors have been installed because the long length of the transmission line imposes significant concerns of temporary over-voltages. With the use of the fixed shunt reactor and neutral-point reactor, secondary arc currents and recovery voltages are limited to a fairly low value.

The high insulation level of the UHV transmission line lowers the possibility of back flashover, caused by lightning strokes, to ground wires or tower tops. SGCC estimates that the lightning outage rate of the UHV line could be lower than that of a 500 kV line. The project makes use of a large cross-section and multiple conductor bundles, with an increased distance of the conductor to ground, to maintain the electromagnetic environment identical to that of 750 kV and 500 kV AC projects. The gas-insulated switchgear (GIS) for this system has been designed by ABB and the Chinese GIS supplier Xian Shiky.

Insulation level of UHV AC equipment (kV)

Equipment

Lightning impulse withstand voltage (Peak)

Switching impulse voltage (Peak)

AC (RMS)

Transformer and reactor

2,250

1,800

1,100 (5 min)

Circuit breaker and disconnector

2,400

1,800

1,100 (1 min)

CVT, TA

2,400

1,800

1,100 (1 min)

Clearance between open contacts of circuit breaker

2,400+900

1,675+900

1,100+635 (1 min)

CVT: Capacitor Voltage Transformer; TA: Transformer Ampere; RMS: root-mean-square

Source: SGCC

 

Electromagnetic indices of the UHV AC project

Environmental influence factor

Controlled value

Remark

Electric field strength

4

Inhabited houses nearby

7

Highway cross-over

10

Other areas

Magnetic induction intensity

100

-

Radio interference

55

0.5 Mhz, 20 metres away from the projection of the outer phases in fair days

Audible noise

Environmental protection requirements

-

Source: SGCC

 

 

SGCC's experience highlights the importance of having technical standards in UHV AC technology. The lack of standardisation of UHV equipment directly affects the techno-economic parameters of the project as utilities have to engage in elaborate research to achieve feasible solutions. For the transmission industry as a whole, technology standardisation helps achieve efficiency in the long run. Studies conducted by the International Electrotechnical Commission and CIGRE have revealed several phenomena which are different for UHV systems as compared to EHV systems. Many standard values differ from those currently stipulated for lower rated voltages. Thus, there is a need to develop international standards to ensure the safe and efficient use of this technology.

Encouraged by the progress in UHV AC transmission system development, China has announced the setting up of three new UHV AC systems for the national grid. Two of these lines will transmit electricity to Shanghai from power plants in the Anhui province and the Inner Mongolia Autonomous Region. The third line will link the northern Shaanxi province and Changsha city in Hunan. These lines are a part of the overall CNY100 billion investments planned for high voltage transmission networks by China.


China's UHV AC power grid by 2010

copy_of_ppt_zhenya_liu_e_copy_400