World’s Highest HVDC Line Delivers Power to Tibet
UTILITY PRACTICE & EXPERIENCE
Tibet
World’s Highest HVDC Line Delivers Power to
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T
he recently completed 750 kV/±400 kV Qinghai-Tibet Intertie certainly deserves to be included among China’s most ambitious network projects. Commissioned late in 2011, it connects with 750 kV AC lines running a distance of about 1500 km between the cities of Xining and Geermu, in central Qinghai Province. From a newly built converter substation in Geermu, it then continues as a ±400 kV DC line to Tibet’s capital, Lhasa, more than 1000 km away. Apart from these lines, the project also includes two 750 kV substations, a 750 kV switching station and the two ±400 kV converter stations. A 220 kV AC circle grid in Tibet is part of the same scheme which, in total, is said to have involved an investment equivalent to some US$ 2.5 billion.
and also the longest HVDC line ever built at such altitudes – quite literally along the ‘rooftop of the world’. For example, it traverses terrain that averages 4500 meters (nearly 15,000 ft) with the highest point reached at the dramatic 5300 m mountain pass at Tanggula – China’s gateway to Tibet (known in Chinese as Xizang). Apart from the obvious construction challenges across the vast, rugged Qinghai-Tibet plateau under extreme cold, permafrost, high UV and fragile local ecosystems, altitude impacted factors such as corona as well as electromagnetic field. Designing the most suitable line insulators for this environment and meeting performance requirements of external insulation on HV equipment also proved demanding.
What makes the new ±400 kV line portion of the INMR visits Geermu to report on aspects of this project particularly noteworthy is that it is among unique network project. the world’s highest overhead transmission lines
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Investments made in expanding China’s power grid over the past 10 years have been nothing short of extraordinary. Indeed, they are perhaps best put into context if one considers that, between 2005 and 2010 for example, the country added, on average, the equivalent capacity of the entire power system of the United Kingdom every single year.
Overview of recent UHV projects in China and location of Geermu–Lhasa ±400 kV line.
Prominent among these investments have been ground-breaking UHV projects – both AC and DC – such as the 640 km 1000 kV AC line between the provinces of Shanxi and Hubei (see INMR Q1, 2010) or the rapidly expanding 750 kV AC supergrid in Western China (see INMR Q4, 2010). When it comes to DC, China has also seen an increasing array of ambitious
Courtesy of State Grid Corp.
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All photos: INMR ©
750 kV AC lines from Xining arrive at Geermu Substation.
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Some 300,000 km of new lines with nominal voltage of 110 kV and higher are expected to have been added by the time China’s 12th 5 year plan ends in 2014.
projects such as the 1400 km long ±800 kV line between Chuxiong and Suidong Converter Stations in south China (see INMR Q4, 2009). And, in spite of a recent economic slowdown, the pace of growth in this sector seems only to be accelerating. Driving these investments is a commitment by the State Grid Corp. of China (SGCC) to increase electricity transmission as a proportion of total energy delivered. At present, energy sources in China are still transmitted mainly by road and rail, with as much as half of all railway capacity used just to move coal. Now, the SGCC intends to increase the share for electrical
transmission such that by 2020 the ratio of coal to energy transmission will drop significantly. Achieving this will require construction of numerous new lines and substations. Indeed, some 300,000 km of lines with nominal voltage of 110 kV and higher are expected to have been added by the time China’s 12th 5 year plan ends in 2014, transmitting a total of nearly 250 TW. Yu Jiajun is Sales Manager at Shenma Electric Power’s office in Beijing and his work focuses on new electrical infrastructure in China. He reports that the State Grid Corp. and the China Southern Power Grid will together invest more than RMB 2 trillion as part of the latest 5 Year Plan to strengthen and optimize the country’s power grid and that much of this will involve EHV and UHV projects. For example, he says that a dozen UHVDC projects (most at ±800 kV but with one at ±1100 kV) are on the drawing boards over the coming years. Among
Suspension and tension towers (bottom and left) on 750 kV lines leaving Xining. All photos: INMR ©
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Typical 40 m high suspension tower along ±400 kV line to Tibet. Height of towers determined by maximum permitted electro-magnetic field in populated areas.
these are the Hami–Zhengzhou and Xiluodu–Zhejiang ±800 kV DC projects due for completion this year. Shenma is among China’s largest suppliers of hollow and solid core composite insulators for substation equipment applications.
More recently, the company, based in Jiangsu Province near Shanghai, has expanded to also offer overhead line insulators and has already supplied several key EHV and UHV projects. Shenma was also awarded the contract to supply about 60% of the
composite insulators installed on the new ±400 kV Geermu–Lhasa line. Professor Liang Xidong of Tsinghua University’s Dept. of Electrical Engineering and now serving as President of Qinghai University points out that the project to connect Tibet with the rest of China is vital to the future economic growth of the region. Power shortages in winter, when river levels are low, are common. Another advantage of the new Intertie will be to increase the reliability of the Tibetan power grid. For example, a major outage occurred several years ago when Yanghu Hydropower Plant, which supplies a high proportion of
All photos: INMR ©
Double string on highway crossing. Each suspension insulator 8 meters long and offers some 28 m total creepage.
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Tension tower shows comparative arcing distance of porcelain cap & pin strings and silicone long rods. In China, silicone insulators are typically dimensioned with 75% of the creepage distance normally required for ceramic equivalents.
the province’s total power, had to interrupt operation because of an accident. The new ±400 kV line, notes Liang, will also permit power flow between Qinghai and Tibet to be adjusted to meet the needs of better exploiting the region’s rich natural resources. Apparently, the original intent was for the line to be designed for ±500 kV. But constraints in terms of external insulation at these altitudes combined with the realization that
power demand in Tibet probably did not require such a system voltage resulted in the decision to scale the project down to ±400 kV. Wu Guangya, an insulator specialist from the High Voltage Research
Institute in the central Chinese city of Wuhan, served as UHV expert to the State Grid Corporation on this project. Wu explains that one of the key considerations when it came to specification of insulators and hardware such as fittings and corona
Key considerations in selection of insulators and related hardware involved the high altitude, perpetually high UV, frequent cold and remoteness of the area. All photos: INMR ©
Corona rings on single and double ±400 kV strings have to cope with increased potential for corona activity at high altitude.
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Cap and pin porcelain insulators on gantry tower to Geermu are among substation’s major sources of corona noise.
have been able to satisfy the line’s pollution performance criteria.
rings involved the extreme altitude. Other factors included perpetually high UV and frequent cold in the area through which the line runs. Wu says that the decision to select silicone composite insulators for all suspension applications on the new line was based on
an assessment of their relative advantages and disadvantages in relation to porcelain or glass cap & pin strings. He remarks that both ceramic technologies have a history of successful application on Chinese power systems and that a variety of possible shed designs might
However, he also notes that regulations in China require that porcelain discs be checked regularly for loss of dielectric. Conducting such inspections annually on every tower across remote regions of Qinghai would represent a real maintenance challenge. “Moreover,” notes Wu, “should the number of zero value insulators in any string become critical due to insufficient inspection, this could quickly lead to line flashover.
All photos: INMR ©
Wall and converter transformer bushings at Geermu Substation all equipped with silicone housings formulated to resist high UV exposure.
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As for glass, Wu observes that higher thunderstorm activity south of the Yangtze River (the demarcation line between north and south China) has made toughened glass a more popular choice than porcelain. Experience with glass in the north, by contrast, has been less and he says that replacing any discs that self-shatter across the rugged terrain in Qinghai and Tibet would also present a maintenance headache. Wu also notes that dust and pollution problems in north China mean that double shed discs on insulator strings are preferred, but there is still not sufficient operating experience with such designs in the case of glass.
Some silicone insulators used on 750 kV AC lines in northwest desert regions of China have experienced tearing of large diameter sheds under continuous high wind shear.
Given the decision to specify composite insulators for suspension applications on the new ±400 kV line, Wu points out that there were still two key issues to resolve given the service environment: possible premature ageing under the almost constant high UV and risk of reduced performance under frequent cold temperatures. According to Shenma’s Yu, the issue of UV resistance of silicone is one
Operating experience in China suggests that hydrophobicity of silicone is reduced at ambient temperatures of between -2 and -10°C.
that has been addressed by the Chinese insulator industry through incorporation of fillers that prevent breakdown of the chemical bonds in the silicone polymer by high-energy UV rays. Moreover, he claims that the silicone rubber used for this line is a unique formulation of high quality in respect to tear strength of the shed material, which is greater than 20 kN/m versus the more typical 7 kN/m. This apparently was another issue to resolve since recent experience with high winds along 750 kV AC lines in north-western desert regions has demonstrated that wide sheds can be susceptible to tearing under the impact of constant wind shear.
All photos: INMR ©
Wu addresses the impact of sustained cold on performance of composite insulators by noting that operating experience in China suggests that hydrophobicity of silicone is reduced at ambient temperatures of between -2 and -10°C. “For example,” he says, “we have identified flashover behaviour of such insulators at 220 kV, typically
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±400 kV insulator geometry has three different shed diameters spaced 40 mm apart, with small size sheds on both sides of mid size sheds and also on both sides of large size sheds.
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consecutive large sheds amounts to 160 mm (less than the 180 mm spacing Wu says is normally required in China for UHV applications).
Table: Design Parameters of Composite Insulators for ±400 kV & 750 kV QinZang Intertie Lines* Voltage Level
±400 kV
750 kV
SML
Height mm
kN
Insulation Length
Creepage Distance
mm
mm
160
8000
7490
28030
210
8000
7490
28030
300
8000
7490
28030
210
7150
6840
23530
420
7300
6890
23530
* Source: China EPRI
on rainy mornings when temperature falls into this critical band. Research in Wuhan then confirmed that this was due to a relative loss of hydrophobicity.” Wu goes on to emphasize that a related finding from the same investigation was that water on composite insulators turns to ice not a 0°C but rather at -3°C. Overcoming this potential risk of reduction in hydrophobicity of silicone rubber under persistent cold temperatures, according to Wu, involved developing the most suitable shed geometry for the line’s pollution environment. In the case of the ±400 kV Qinzang Project insulators, this turned out to be three different shed diameters: small, midsize and large, each separated by a distance of 40 mm along the shank
“There has been a lot of controversy whether tension strings on UHV lines should be composite and, if so, what proportion to use for the purpose of testing.”
and in a configuration where there is a small shed on either side of both the mid-size and the large sheds. As such, the distance between any two
Wu explains that there are five main pollution classes in China – ‘A’ being the lowest to ‘E’, the highest. Unified specific creepage distance (USCD) then varies from a typical low of 22.0 mm/kV to as much as 53.7 mm/kV for ‘E’ type service environments. This line, he states, has been classified as passing through a type ‘C’ service environment with normally prescribed USCD requirement of 34.7 mm/kV. Allowing for the fact that the line is DC and that about 88 percent of its total length traverses areas with an altitude exceeding 4000 m, a correction factor of 1.28 was applied, resulting in a final USCD of somewhat more than 44 mm/kV. While suspension insulators on the new ±400 kV Geermu–Lhasa Line are silicone composite type, porcelain cap & pin strings dominate most tension applications. A small number of porcelain long rods were also selected for use on a handful of towers in order to obtain operating experience with how well these perform in terms of self-cleaning in such a service environment.
All photos: INMR ©
Single phase 750/330 kV AC transformers, 330 kV GIS side and spare transformer for converter station (right).
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All photos: INMR ©
Disc insulators on discontinued line near salt lake demonstrate impact of local dust and pollution. Paving stones at Geermu Substation show presence of salt deposits from constant truck activity outside.
According to Wu, the use of ceramic disc strings for tension is typical of what is found on most UHV AC and DC lines in China where composite insulator technology has been dominant – but only in suspension. This is due mainly to concern about
possible damage to insulators from linemen walking across strings for conductor maintenance or repair. Says Wu, “there is still a lot of controversy here as to whether tension strings should ever be composite and, if so, what proportion
to use for the purpose of evaluating how well this works. While some experts recommend as much as half of all strings, others suggest only 5%. My own view is that 20% would be a better target for testing purposes.”
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All exterior porcelain at Geermu coated with RTV silicone. Booster sheds added along 750 kV GIS bushings.
All photos: INMR ©
As part of the final process to confirm design parameters of the new ±400 kV line in terms of tower clearances, on site testing was carried out in the summer of 2011. The findings allowed researchers to compare lightning and impulse switching data obtained from simulations performed before construction at the UHV test laboratory in Beijing (at 54 m) as well as at the new 4300 m high test station in Tibet. This work was carried out by the China Electric Power Research Institute (CEPRI) on a simulated tower structure located near Tanggula Mountain, the highest point along the line (some 5000 m). According to CEPRI, minimum length here would have to be 12 m (with 14 m the longest) versus the more typical 9.75 m for an insulator used in UHVAC. These findings applied to silicone insulators and would be longer in the case of porcelain.
Main applications for composite hollow core insulators at Geermu are converter transformer and wall bushings.
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