Design of UHV AC transmission line in China

Authors


SUMMARY

Ultra High Voltage (1000 kV; UHV) AC transmission technology is the most advanced AC transmission mode with the highest voltage until now. Based on the construction experience of 1000 kV Jindongnan–Nanyang–Jingmen UHV AC Pilot Demonstration Project, the major design principles and the key technical solutions of UHV AC transmission line (including large crossing) are introduced, such as the choosing of conductor and earth wire, overvoltage and insulation coordination, lightening protection and grounding, insulator string and fittings, clearance to obstacles, tower and foundation. Copyright © 2011 John Wiley & Sons, Ltd.

INTRODUCTION

Thousand kilovolts Jindongnan–Nanyang–Jingmen UHV AC Pilot & Demonstration Project is the only one in commercial operation which carries the highest voltage all around the world. This world leader transmission and distribution project is also completely self-designed, self-constructed, self-manufactured, self-commissioned, self-operated, and independent intellectual property rights are owned.

The overall length of this project is 639.847 km, including the large crossing section of 6.607 km. The altitude along the route varies from 0 to 1400 m. The line is single-circuit with 1275 towers, running through plains, hills and mountains, crossing Yellow River and Han River. The type of conductor is ACSR 8×LGJ-500/35.

Ever since 1960s, former Soviet Union and Japan has built 1150 kV single-circuit UHV AC line and 1000 kV double-circuit line. However, those lines are operated under 500 kV due to economic reasons 1–3. Since the main load centers are far away from energy base in our country, it is an effective measure to solve uneven energy distribution by UHV transmission technique 4–10.

The key design principles and technical solutions for 1000 kV single-circuit UHV AC pilot transmission line are mainly introduced in this paper.

ROUTING

The route of transmission line should be selected technically and economically from multi-proposals, taking into account of the route agreements, environmental protection, hydrometeorology, landform and geology. Meanwhile, utilizing technologies of satellite photography, aerial photography, Helava digital photogrammetry technology, geological remote sensing, and GPS survey, to optimize the main route so as to build a project which is economical, secure, reliable, environmental friendly and conveniently to construct and maintain.

WEATHER CONDITIONS AND STRUCTURE RELIABILITY

The primary weather factors influencing the design of overhead transmission line are wind speed and covered-ice thickness. Reasonable determination of the weather condition level (related to return period) and the relevant load values is the basis of construction goal which is pursuing “safe and reliable, economical and rational.”

According to the records of the weather stations and the design and work experience about the 500 kV transmission line near the selected route, the wind speed for design the 1000 kV line is defined as the average wind speed during 10 minutes, at the 10 m above ground, with 100-year return period 11–13. This principle is also a compromise of the reliability and the cost, and at least higher than 27 m/second for safety.

Considering the importance of UHV transmission line in power grid, the structure importance coefficient is adopted as 1.1 in normal condition and accidental condition while calculating the tower strength.

By means of evaluating the influence of various load assumptions acted to the weight of the tower, the estimating principle of loads is constituted in view of both the safety and economy.

From the principles mentioned above, structure reliability target value β of UHV tower is more than 3.7 which fulfills the qualifications of the first safety class building, with a higher level than 500 kV transmission line (β ≥ 3.2, meet the requirement of the second safety class building).

Based on the investigated data along the route, with a view to experiences of the existing transmission line, the designed cover ice thickness is evaluated as 10 mm.

CONDUCTOR SELECTION

Radio interference (RI) and audible noise (AN) caused by corona discharge of UHV conductors become critical factors on the selection of bundles.

According to the design and operating experiences of 500–1150 kV transmission lines around the world, the limited values of corona performance generated from 1000 kV UHV power lines are determined as follows. The RI level with a measuring frequency of 0.5 MHz should not be more than 55 dB at the location of 2 m above ground and 20 m horizontal from an outside phase conductor in fair weather condition. The AN level should not be more than 55 dB(A) at the location of 20 m horizontal from an outside phase conductor in wet conductor condition. These limits are basically equivalent with those of extra-high voltage transmission lines in China 14, 15.

In order to satisfy the above consideration, the split number and the cross-section area of sub-conductor need to be more than 8 and 500 mm2 separately. Considering construction investment, operation loss and cost of fund, 8×LGJ-500/35 aluminium conductor steel-reinforced with a diameter of 30 mm is proved to be the most economical configuration for UHV phase conductor by using the method of minimum annual cost. The AN level of transmission lines should meet the requirements of type 0 AN area through the National Macaque Nature Reserve in Henan Province. It means that the AN level should not be more than 50 dB(A). As a consequence, 8×LGJ-630/45 is recommended in the nature reserve section.

A UHV test line with the same conductor configuration has been operated since February 13, 2007. The test results show that RI and AN level are in agreement with the predicting value. Moreover, corona characteristic experiments are also performed on the 1000 kV UHV demonstration transmission line which is operated on January 6, 2009. The results verify that RI and AN level of the engineering satisfy the electromagnetic environment requirements authorized by Environmental Protection Administration of the People Republic of China.

GROUND WIRE SELECTION

Corona discharge is a governing factor for selecting the diameter of ground wire of 1000 kV UHV power lines. For the purpose of limiting corona inception, the ratio of ground wire surface gradient to corona onset gradient needs to be not more than 0.75, Em/E0 ≤ 0.75, and the diameter of ground wire should be more than 17 mm.

By considering each aspect of factor roundly, one ground wire is recommended to be a JLB20A-170 type aluminium sheathed stranded conductor (ASSC), the other is an Optical Power Ground Wire(OPGW) with similar characteristic and the monofilament diameter of outer layer should be not less than 3 mm.

In the areas of tower height restricted, ground wires are recommended to be a JLB30-400 type ASSC and a corresponding OPGW.

To reduce the electric energy loss, the ordinary ground wire is operating in the mode of subsection insulation and single-point grounding and the OPGW is grounded on each tower 9, 10.

INSULATOR SELECTION

Principles

The type and quantity of insulator should assure the safe and reliable operation of transmission line under conditions of operating voltage, switching overvoltage, and lightning overvoltage.

According to pollution investigation and actual measurement, the pollution level is determined along the line. Based on insulation design and operation experience of transmission lines, contamination performances of different insulators are compared and withstand voltage tests of full-scale insulator string with different types are performed under contamination condition 16. The quantity of insulators is determined by using specific creepage distance method and polluted withstand voltage method. Moreover, the cases of switching and lightning overvoltage are also verified.

Investigation and division of pollution level

There are several coal mine factories, power plants, steel manufactories, cement mills, and quarries in Shanxi and Henan Provinces where the 1000 kV UHV Demonstration Project pass through. Therefore, the air pollution is heavy. The division result of pollution level along the line is shown in Table I.

Table I. Division of pollution level along the line.
Pollution levelESDD (mg/cm2)Specific creepage distance (cm/kV)Length of line (km)Percentage (%)
Level II0.06–0.102.00–2.50187.3829.29
Level III0.10–0.252.50–3.20341.86853.44
Level IV>0.253.20–3.80110.48517.27

Type of insulator

The calculation result of exterior load shows that mechanical intensity of suspension string is 300–550 kN and single or twin insulator strings are used generally. As for the strain insulator string, the mechanical intensity need to be more than 3 × 400 kN and 2 × 550 kN. To simplify the model number, 300, 420, and 550 kN insulators are recommended in the 1000 kV UHV Demonstration Project.

The contamination flashover characteristics of ordinary-type, double-shed, and tri-shed disc insulators are compared in different ESDD (equivalent salt deposit density) cases by research institute. Results show that double-shed and tri-shed disc insulators are more excellent at various aspects, such as higher flashover voltage and better performance in high altitude area. Double-shed and tri-shed disc insulators can be applied to Levels II and III pollution area, respectively. However, composite insulators with good contamination flashover resistance are applied in the UHV Project.

Finally, twin 550 kN disc porcelain or glass insulator is used for the tension string in the 1000 kV UHV Demonstration Project.

Quantity of insulator

Figure 5a shows the contamination withstand voltage test result of long string with CA590-EZ, CA887-EZ, FC300/195 type of insulators. From Figure 1, it can be seen that contamination flashover voltage is in linear relation with length of string and the previous theory for insulator configuration is still suitable for UHV case.

Figure 1.

Relation between length of string with flashover voltage.

Furthermore, test result shows that the contamination withstand voltage of double-shed porcelain insulator is 5% higher than that of ordinary-type and the contamination withstand voltage of V-string is 5–20% higher than that of I-string. The contaminated withstand voltage of twin V-string with a separation of 600 mm or more is corresponding with that of single I-string.

The quantity of insulators is determined by using specific creepage distance method and polluted withstand voltage method. In Levels II and III pollution areas, 54 double-shed and 54 tri-shed porcelain insulators can be used, respectively. Finally, composite insulators with 9750 mm unit spacing and 30 000 mm creepage distance are applied in both Levels II and III pollution area in the project. The insulator quantity of tension string is determined by the same principle as that of suspension string.

The above quantities of insulators are verified to satisfy the requirements of switching and lightning overvoltage cases.

LIGHTNING PROTECTION AND GROUNDING

Operation experience of Japan and former Soviet Union shows that shielding failure is the main reason of lighting trip-out of 1000 kV UHV transmission line.

During lightning protection design of the UHV Project, the thunderstorm day is selected as 40 and the back flashover current is considered to be more than 200 kA. The main measures for lightning protection are shown as follows 9–11.

  • (1)Double ground wires are allocated on the whole line except the section of 0–2 km away from substation.
  • (2)The shielding angle of ground wire to outside phase conductor should not more than 5° and −5° in plain country and mountain area, respectively.
  • (3)In midspan, the distance between the phase conductors with the ground wire, S1, should satisfy the following requirement.
equation(1)

where, Um represents the maximal operating voltage, kV. l represents the span length, m.

  • (4)The shielding angle of ground wire to outside phase conductor should not more than −5° in the section of 0–2 km away from substation. Moreover, a third ground wire is allocated at the middle of transmission line.
  • (5)Each tower is equipped with a horizontal grounding device.

With the above measures, the predicted lightning trip-out rate of UHV power lines is about 0.1 time/100 km.a, which is 70% of 500 kV transmission lines.

INSULATOR STRING AND FITTINGS

An 8-subconductor bundler is arranged as a regular octagon with circumcircle diameter 1045 mm and the distance between the adjacent subconductors 400 mm to form the conductor of one phase. In order to dwindle the top hamper and the cross arms in size, the suspension insulator sets comprising one string or a double-string set (composite insulator set comprising only double string) on intermediate tower are arranged as IVI form (V-type string of the center phase and I-type string of the side phases). The mechanical strength of suspension sets includes 300, 420 kN mainly. The mechanical strength of suspension sets on jumpers is 210 kN.

In the macaque-protection nature reserve, due to the adoption of 8×LGJ-630/45 steel reinforced aluminum conductor, the tension insulator sets comprises 3 × 550 kN ceramic insulator strings. In other spans, the tension insulator sets comprises 2 × 550 kN ceramic insulator strings. To utilize the natural-cleaning effect of wind and rain adequately, the multi-string sets are all arranged horizontally.

To ameliorate the electric field distribution alone a string to restrain the corona development on the fittings, a precise simulation has been implemented. On the basis of the study conclusion, the size of grading ring is fixed, and the grading ring at the end of a double-string set is shaped as athletic track. As the set is composed of disc insulators, the grading ring should be placed between the second and the third insulator. A double-string tension set should match two circular grading rings. Typical insulator string sets in practical project are shown in Figure 2.

Figure 2.

Typical insulator string set.

The gap between two insulator strings in one set is 600 mm.

Because there is no grading ring appending onto the suspension strings, all the suspension clamps must be qualified to meet the requirement of corona restraint.

To improve the insulating reliability of air spacing and lessen the weight of tension tower, rigid jumpers are adopted in the project 17. Two kinds of rigid aluminum tube jumper, respectively, with suspension insulator strings and with tilted ladders are applied according to the various installation positions. Typical rigid aluminum tube jumpers are shown in Figure 3.

Figure 3.

Typical rigid aluminum tube jumper

The EB type bolts (Joint hung plate) are adopted to connect a suspension set with the tower, and the GD type ones (Twisted strap) are adopted to connect a tension set with the tower. Both of them have the highest strength among the fittings in the insulator set.

AIR GAPS

Clearances of 1000 kV AC UHV tower are determined by the maximum of operating voltage and switching overvoltage and the flashover characteristic of air gap. The insulation co-ordination methods include the standard of DL/T 620–1997 “Overvoltage protection and insulation coordination for AC electrical installations” and design methods of Japan and former Soviet Union 18–24.

According to the standard of DL/T 620-1997, the 50% flashover voltage of air gap between phase conductor with tower, U50%, should satisfy

equation(2)

where, K is statistical co-ordination coefficient of operation voltage of air gap, K = 1.40 in general and K = 1.50 in V-string case, equation image.

As for air gap under switching overvoltage condition, K is statistical co-ordination coefficient of switching overvoltage of air gap, K = 1.10 in general and K = 1.25 in V-string case. U is statistical switching overvoltage of phase-to-ground, Us. Switching overvoltage multiple is assumed to be 1.70 p.u.

According to the design method of Japan, the maximum of power frequency overvoltage is not more than 1.1 times of phase voltage. Operation voltage clearance should satisfy

equation(3)

where, k0 is coefficient of altitude correction. σ is standard deviation, equation image. As for air gap under switching overvoltage condition, Um is statistical switching overvoltage, Us, and switching overvoltage multiple is 1.6–1.7 p.u. Standard deviation equation image.

According to the design method of former Soviet Union, the co-ordination value of 50% flashover voltage, U50.1.c, can be calculated by

equation(4)

where, equation image and equation image are coefficients of variation for multiple gaps in parallel and single gap, respectively. As for the power frequency overvoltage case, equation image, equation image, equation image, Z = 3. As for the switching overvoltage case, U = Us, equation image, equation image, Z = 2.45.

Finally, method of IEC60071-2 is recommended to be applied for altitude correction.

equation(5)

where, H represents altitude height, m. m represents coefficient of correction. As for operation and lightning case, m = 1.0.

The final value of air gap on the tower is shown in Table II, which is determined by the air gap flashover test data on the full-scale UHV tower.

Table II. Air gap on the tower (units: m).
Altitude (m)Operation voltage case (side phase I-string)Switching overvoltage caseLive working case
Side phase I-stringMiddle phase V-stringSide phase I-stringMiddle phase V-string
  1. Note: Switching overvoltage multiple is assumed to be 1.70 p.u. The value in the bracket represents the clearance to upper cross arm.

5002.75.66.7 (7.9)5.66.2
10002.96.07.2 (8.0)6.06.7
15003.16.47.7 (8.1)6.47.2

Live working case is not a factor that controls size of tower and only to be a verification case. In the live working case, switching on non-loaded line is not considered and the home range for workman is assumed to be 0.5 m.

The lightning overvoltage clearance is not specified as for single-circuit AC UHV transmission lines. The lightning protective measures are determined by the calculation result of lightning trip-out rate.

TRANSPOSITION

According to the Quality of Electric Energy Supply Admissible Three-Phase Voltage Unbalance Factor (GB/T 15543-1995), the allowed unbalance factor of public joint in power system is 2% under working voltage and less than 4% in short term.

The unbalance factor of this transmission line is 5.19%, over the allowed value. So a full circle transposition is suggested to be carried out in Jindongnan–Nanyang section and Nanyang–Jingmen section separately. In this way, the voltage unbalance factor can fall down to 0.06%, so that the three-phase voltage can be deemed in balance. The transposition schematic diagram can be seen in Figure 4.

Figure 4.

Transposition schematic diagram.

CLEARANCE TO THE EARTH AND THE OBSTACLES

Clearance from the conductor to the earth and the obstacles is determined on the basis of electrical field intensity limit, electrical insulation strength under switching overvoltage and the requirement of technical agreement between the power department and other industries. Unlike to the EHV lines, with the voltage improved, some spacing determined by insulation strength originally under EHV becomes to be determined by electrical field intensity in UHV 25, 26.

The limit of the electric field strength is determined as follows 27: (1) The limit for the area that people can be easily come to and the road that overhead line crossed is 7 kV/m; (2) The limit for the farm field that overhead line crossed is 10 kV/m; (3) The limit for the area near the domestic structure is 4 kV/m.

When the minimum spacing determined by insulation strength (clearance to brae, crag, transmission line, tip of mast, etc.), the insulation strength derived mainly from switching overvoltage. The gap determined by switch overvoltage is 7 m, and some additional margin is necessary.

On demand of the electrical field limits mentioned-above, calculation process was executed aiming at the typical tower, and the conclusions are drawn as the clearance of the conductor. The clearances to the earth are shown in Table III, and the ones to the obstacles are shown in Table IV.

Table III. Clearance to the earth (units: m).
Traversing area1000 kV line
Non-residential area22
Residential area27
Slope available to step13
Cliff unavailable to step11
Table IV. Clearance to the obstacles (units: m).
 Obstacle1000 kV line
RailwayRail surface27
   Power line of electrified railway16
   Road surface27
Transmission lineMiddle span10
 Top of tower16
 Communication line18
 Uninhabited building15.5
   Forest14
   Fruiter16

TOWER TYPE SELECTION AND PLANNING

Guyed tower can greatly reduce the tower weight. However, it is rarely used in transmission line currently because it occupies large area and affects mechanized farming. Therefore, this project adopts just self supporting type of towers.

To minimize the window dimension and length of cross arm, and considering electromagnetic environment and tower weight, IVI-type configuration is used for most suspension towers in the whole line 28. Cup-shape tower is adopted in mountain lines for its better performance in lightning protection and lighter weight, whose shielding angle is kept as −5° or so, as shown in Figure 5a. Cat-head-shape tower is adopted in plain and hilly lines for its narrow corridor, whose shielding angle is less than −5° as shown in Figure 5b. The shape of tension tower is as shown in Figure 5c. Gantry tower and split tension tower are specially designed to solve problems of aviation limited area and goaf area, respectively, as shown in Figure 5d and e.

Figure 5.

Typical tower type.

Based on the actual situation and weather condition of the whole line, as well as horizontal and vertical loads on towers, height, deflection angle, configuration of the towers, two sets of tower types are planned for this project. They are towers used in plain and hilly area with wind speed of 27 m/second and towers in mountain area with wind speed of 30 m/second. Among each set, there are five series of suspension tower and three series of tension tower whose deflection angles are, respectively, 0–20°, 20–40°, and 40–70°. Unequal leg extensions are used for mountain towers to meet requirements in environmental protection.

FOUNDATION TYPE SELECTION

Considering the topography and characteristics of geology along the line, as well as economic indicators and environmental protection, different foundation types are adopted according to soil type of the tower spots.

Clay soil: Bell-foundation in undisturbed soil is preference in mountain and hilly area while pad and chimney foundation is in plain and open area. Special foundations like stage-foundation, slab foundation with upright stem and manual-dig pile can also be used according to local conditions.

Gravelly soil: pad and chimney foundation is preference while stage-foundation and manual-dig pile could be used according to local conditions.

Rock: rock socketed foundation could be used according to local conditions and forces on foundation. Only a few foundations of mini-pile anchored in rock were used.

Soft soil: bored pile foundations should be used when foundation reaction is significant, or alternative types of foundation, in accordance with geology characteristics as well as economic comparison between slab foundation and bored pile foundation, may be used.

Dedicated researches like 3D geophysical prospecting and foundation deformation are performed to colliery influenced area. Slab foundation and soil grouting are adopted to ensure foundation stability.

Further measures like using higher exposed chimney foundation, vegetation coverage slope, and improved drainages are adopted along the whole line to enforce environmental protection and water and soil conservation.

WIDTH OF CORRIDOR

During the designing stage, the corridor width of transmission line is defined to clarify the demolition range. Demolition range near 1000 kV transmission line is defined as two principles as below:

  • (1)Horizontal distance between the dwelling house and the outer conductor is 7 m at least (500 kV line corresponding to 5 m, 750 kV line corresponding to 6 m); otherwise the house ought to be demolished.
  • (1)The electrical field intensity at dwelling house is 4 kV/m at most; otherwise the house ought to be demolished.

According to the principles as above, the corridor width is about 45–72 m for the cat-head-type tower and 65–96 m for cup-type tower 29.

DESIGNING LARGE-CROSSING LINE

There are two large-crossing sections in the UHV Demonstration Project. One is Xihuagong Yellow River large span in Henan Province, the other is Yanshantou Han River large span in Hubei Province. The main technical characteristics of each large span are shown in Table V.

Table V. Main technical characteristics of each large span.
Crossing sectionLength of strain section (m)Span (m)Crossing schemeFull height of crossing tower (m)Operating voltage clearance (m)Switching overvoltage clearance (m)Lightning overvoltage clearance (m)Suspension stringQuantity for each stringTension stringQuantity for each string
  1. ST, tension tower; SU, suspension tower.

Xihuagong Yellow River large span3651450–1220 –995–986ST-SU-SU -SU-ST122.82.75.6 (6.2)7.54 × 420 kN616 × 550 kN56
Yanshantou Han River large span2956706–1650–600ST-SU-SU-ST181.82.75.6 (6.2)8.24 × 550 kN466 × 550 kN46

The conductor pattern of large span section is determined by the delivery power under the heat condition and the mechanical performance. Furthermore, it also needs to meet the electromagnetic environment requirements. Finally, 6×AACSR/EST-500/230 type aluminum alloy conductor steel reinforced is selected to be the conductor of large span section and two ground wires are provided 30. One is a JLB20B-240 type ASSC, the other is an OPGW with similar characteristic.

Insulation co-ordination consideration of large span section is the same as that of common section. I-type string with disc insulator is used on the suspension crossing tower. The pattern of insulator string and the value of air gap are shown in Table V.

The suspension crossing tower is Cup-type steel-pipe tower and the tension tower is “equation image” type angle tower. To be more reliable, the design wind speed of large-crossing section is increased by 10% more than that of general section. Thus, the reference design wind speeds are enhanced to be 31 and 30 m/second in the Yellow River and Han River large span, respectively. Ice thickness is recommended to be 15 mm, which is 5 mm more than that of adjacent section.

TECHNICAL AND ECONOMIC COMPARISON WITH TRANSMISSION LINE OF OTHER VOLTAGE CLASS

The differences of primary characteristics between 1000 kV UHV AC Pilot Demonstration Project and the typical 330–750 kV single circuit transmission line with conductor configured horizontally is shown as below.

The wave impedance of a 330 kV line with 2 × 400 mm2 conductor, a 500 kV line with 4 × 400 mm2 conductor, a 750 kV line with 6 × 400 mm2 conductor and a 1000 kV line with 8 × 500 mm2 conductor is 302, 259, 241, and 245 Ω, respectively, and the corresponding electrical current density is 0.829, 0.733, 0.787, and 0.620 A/m2. The natural transmission power of 1000 kV line is 4–5 times that of 500 kV line. Caused by the corona effect, the bigger conductor cross-section of UHV line leads to the more dilute current density which reduced the resistance loss.

If the natural transmission power of 1000 kV line and 500 kV line is 5000 and 1000 MW, respectively, the cost per unit natural power of 1000 kV line is 56% that of 500 kV line. And the corridor width per unit natural power of 1000 kV line is 40% that of 500 kV line 31–33.

CONCLUSIONS

Referring to the designing experience of 1000 kV Jindongnan–Nanyang–Jingmen UHV AC Pilot Demonstration Project, the design principles of single circuit UHV overhead transmission line is summarized in this paper. The project is put into production on January 6, 2009. It has been run for 1 year and a half until now, and no failure has been found. According to the criterions described as above, the cost and corridor width per unit natural power of 1000 kV line both are much less than 500 kV line. This phenomenon indicates that the 1000 kV UHV AC transmission line is feasible in technology and reasonable in economy.

Ancillary