Design of 500 kV AC Double-Circuit Transmission Line on the Same Tower  Located at 20 mm Heavy Icing Area

Design of 500 kV AC Double-Circuit Transmission Line on the Same Tower 

Located at 20 mm Heavy Icing Area

ZHANG Haiping, ZHANG Chi, WANG Jiangtao, ZHAO Qingbin, REN Deshun, GOU Jie

(Sichuan Electric Power Design & Consulting Co., LTD., Chengdu 610016, Sichuan Province, China)

ABSTRACT:During the construction of the first 500 kV AC double-circuit transmission line on the same tower located at 20 mm heavy icing area in China, the dynamic response of the transmission lines under various ice-shedding cases are analyzed by numerical simulation, the engineering formula for calculating ice-shedding jump height and swing distance are obtained, and also variation of dynamic load on the tower. Based on those conclusion, routing, insulation coordination, tower head design, tower load and tower type of 500 kV AC double-circuit transmission line on the same tower located at 20 mm heavy icing area are researched. The summarized design method and thought can provide an important reference for the double-circuit transmission line on the same tower located at heavy icing area.

0 Preface

The western Sichuan, Yunnan, and Tibet regions are rich in hydropower resources, but these areas have a harsh natural environment and a complex human and social environment. With the increase of water and electricity transmission lines, the resources of the corridors are becoming scarcer, so that the high-altitude and heavy-ice lines that have been traditionally erected in a single circuit have also been required to be erected in the same tower. The Maoxian-Mao County II500 kV double-circuit line (referred to as the Maomao 500 kV line) has been put into operation. It is the first double-circuit line on the same tower in the 20 mm heavy ice area in China. Restricted by factors such as Maoxiang County Maoxiangping Industrial Development Zone, Baodingshan National Nature Reserve and established or planned high-voltage line corridors, the route path of the 20 mm heavy ice area crossing the land ridge section is the only one,  which must be designed according to the double-circuit transmission line on the same tower.

There is no precedent for the application of the double-circuit line on the same tower in the heavy ice area. The main technical bottleneck is the design of the double-circuit tower on the same tower in the heavy ice area.Because it is impossible to accurately grasp the dynamic response law of wire deicing under different working conditions, it lacks the guiding principles for the design of the tower head size and internal stress of the double-circuit line on the same tower in the heavy ice area.In recent years, domestic and foreign scholars have carried out a lot of research on the dynamic response of wire deicing by simulation test and numerical analysis [1-17], and some conclusions with engineering practical value are obtained. Especially after the ice disaster in southern China in 2008, domestic scholars conducted a more in-depth study on the dynamic response law of wire deicing in order to explore the mechanism of ice disaster accidents: Chen Kequan et al [14] used ABAQUS software to establish a finite element model of a 500 kV transmission line, and obtained the influence of different factors on the height of the wire depilation jump;Yi Wenyuan [15] used ABAQUS software to analyze the deicing dynamic response of UHV 8 split conductor, and gave a simplified formula of ice jump height and yaw;Yan Bo et al. [16] used numerical simulation method to analyze the numerical results of wire dynamic response under various deicing conditions, and obtained a practical simplified calculation formula for the maximum height of wire deicing jump.Yang Fengli et al. [17] established a deicing jump analysis model for UHV lines in heavy ice areas, focusing on the dynamic response law of overhanging towers, and obtained the unbalanced tension and vertical loads of overhanging towers in heavy ice areas. Recommended value.The above models all adopt a simplified model of 3-degree-of-freedom system that ignores the bending and torsional stiffness of the wire, and most of them do not consider the influence of the tower constraint. However, the research methods and ideas lay a good foundation for grasping the dynamic response law of wire de-icing dynamic response.

Based on the actual situation of the Maomao 500 kV heavy ice area line project, this paper uses Ansys software to establish a refined finite element model including iron tower, grounding wire, insulator string, spacer bar, de-icing load, wind load, etc. The simulation method is used to study the dynamic response law of de-icing of transmission lines in heavy ice area, and then carry out research work on path selection, insulation configuration and tower design of 500 kV double-circuit line on the same tower in 20 mm heavy ice area.

1. De-icing dynamic response of transmission line in 20 mm heavy ice area

1.1 Transmission line finite element model

The wire is twisted from a steel core with high strength and an outer aluminum wire. It has certain bending and torsion resistance and cannot be simply considered as a cable unit.Therefore, the space beam Beam4 unit can be used to more realistically simulate the stress state of the wire.

The insulator string is also simulated by the Beam4 unit. For the disc insulator, the connection structure is considered by releasing the angular displacement coupling of the node every 0.17 m.The spacer bars are simplified to the coupling of the unit degrees of freedom of the unit nodes and a concentrated force converted from the weight of the spacer bars.

The tower member is also simulated by the Beam4 unit. The multi-speed tower-line coupling model is established according to the tension tower at both ends and the middle straight tower.

1.1.2    Line load

In view of the lack of mechanical parameters of ice coating, the mass method is used to simulate, that is, increase the density of the wire to simulate ice coating:

 Where:ρci the equivalent density of the wire after ice coating;ρc、Δc the density and cross-sectional area of the wire; ρi、Δithe density and cross-sectional area of the ice coating.

The de-icing load is simulated by changing the density method, that is, at the moment of deicing, the wire density is changed from the equivalent density after ice coating to the actual density of the wire,at the same time, the wire defrosting range and the ice removal rate are controlled by controlling the range of the density unit.

The horizontal wind load of the grounding line is calculated according to the method in Design Code for 110 kV~750 kV Overhead Transmission Lines (referred to as Design Code) [18], and the wind load is simulated by applying concentrated force at the element node.Concentration force is calculated as follows:

F=PL_p

Where: F is the unit node concentration force; Lp is the unit length.

1.2  脱冰跳跃高度公式

1.2   De-icing jump height formula

In order to facilitate engineering design, the engineering community has been working on a simplified calculation formula for the height of the wire ice jump.In [16], the engineering simplified formula based on the two kinds of static state arc Δ before and after the wire deicing is given. For the multi-speed transmission line, Δ can not be directly calculated, and only the arc drop δ before and after the ice can be obtained. Through a large number of simulation calculations, the ice jump height of the wire is proportional to the ice thickness, the gauge distance, the ice removal rate, and inversely proportional to the wire cross section, and according to the calculation results, the calculation formula of δ-based deicing height based on δ is obtained by fitting the continuous five-speed tensile section model [19].

Where: δ is the arc sag before and after icing; Ɯ is the icing rate; r is the wire type coefficient, the value is shown in Table 1; η is the gear distance correction coefficient, η=[(L-500)/100]* 0.06, where L is the span.

Tab. 1  Value of g

According to the calculation of Table 2, the calculation error of formula (3) is within 8%, which can meet the requirements of engineering application, and the formula can achieve good scalability according to the values of different parameters.

Tab. 2  Difference of ice-shedding jump height between numerical simulation value and calculated pressure

1.3 De-icing jump yaw distance formula

The simulation results show that the wire icing yaw distance is proportional to the ice thickness, wind speed, horizontal span and de-icing rate, inversely proportional to the wire cross-section, and is related to the insulator string type, and fits the 5 mm heavy ice zone for 5 consecutive times. The yaw formula of the tensile-resistance model wire when it is 100% detached.

Where: B is the traverse distance of the wire; v is the wind speed at the same time as deicing; l is the horizontal gear; m, t is the coefficient related to the wire type and the insulator string type, and its value is shown in Table 3.

Tab. 3  Value of m and t

It can be seen from Table 4 that the calculation error of equation (4) is within 9%, which can meet the requirements of engineering application, and the formula can achieve good scalability according to the values of different parameters.

Tab. 4  Difference of ice-shedding swing distance between numerical simulation value and calculated pressure

1.4   Change law of dynamic load of tower during deicing

The load at the connection point between the insulator string and the tower is decomposed into three directions of X, Y, and Z. FX, FY, and FZ represent the forces in the direction of the line, the direction of gravity, and the direction of the horizontal, respectively. During the deicing process, the dynamic load changes with time, and the change process of the dynamic load in different directions can be obtained through the observation window (taking the FX direction dynamic load value as an example).

Comparing the dynamic load observations under different working conditions with the static standard load or design load, the following rules are obtained:

1) During the deicing process, the force in the X direction of the linear tower and the tensile tower in the continuous gear is less than the static standard load calculated according to the Technical Specification for Heavy Overhead Transmission Line Design (referred to as the Heavy Ice Regulation) [20]. And the tension difference on both sides of the tower increases as the gear difference increases.

2) During the deicing process, the forces in the Y and Z directions of the linear tower and the tensile tower will be greater than the static standard loads calculated according to the heavy ice regulations, but still less than the load design value after considering the dynamic amplification factor.

3) The unbalanced tension of the tensile towers with large differences in the spans on both sides may exceed the static standard load calculated according to the maximum used tension percentage of the heavy ice regulations, and the values shall be checked according to the corresponding de-icing rate specified in the regulations.

2   Principle of path selection for double-circuit lines on the same tower in heavy ice area

From the perspective of safety and economy, the lines in the heavy ice area should be routed in a single line. Therefore, the line selected by the same ice tower in the heavy ice area must have uniqueness on the path channel. Combined with the conclusions of the de-icing dynamic response of the transmission line on the 20 mm heavy ice area in the previous section, the route selection of the heavy ice area is supplemented on the basis of the heavy ice regulation:

1) In view of the fact that the ice-skiping jump height and the yaw distance in the heavy ice area are proportional to the continuous number, the number of continuous lines in the heavy ice area should not be too much, and should not exceed 5 files. The resistance section should not be too long and should not exceed 3 km.

2) In view of the fact that the ice-skiping jump height and the yaw distance in the heavy ice area are proportional to the gear distance (horizontal gear distance), the unbalanced tension increases with the increase of the difference between the two sides of the tower, so the line distance of the heavy ice area should not be too large. The distance between adjacent gears should not be too large, and the recommended range of continuous gears should not exceed 500 m.

3) Since the horizontal and vertical forces have power amplification during the deicing process, the number of turns in the heavy ice zone should not be too large, and the conditions of the tower must be controlled.

4) The ice boundary is the weak link of the line. When the 20 mm ice area is separated from the 10 mm ice area, the 20 mm ice area should extend to the 10 mm ice area for 1 to 3 or the 15 mm ice area for transition.

5) Pay attention to the investigation and discrimination of the micro-topography micro-meteorological zone, carry out the verification of extreme meteorological conditions when necessary, and appropriately strengthen the structural strength of the ground wire suspension system and the tower structure according to the verification results.

3   High altitude heavy ice area insulation configuration

3.1    Stain division and insulator type

Judging from the 500 kV lines already built in Sichuan, heavy ice areas are generally accompanied by high altitudes. The pollution level is not very serious and does not exceed c grade, but the Maomao 500 kV line has special characteristics. The heavy ice section crosses the industry. In the park, the pollution situation is obviously increasing. Therefore, depending on the heavy ice section of the project, insulation should be arranged according to the d-class pollution area. At the same time, considering the characteristics of poor terrain and inconvenient operation and maintenance of the heavy ice area, the overhang and tensile insulators are made of high quality stain-resistant glass disc insulators.

3.2    Number of insulators

The Mao-Mao heavy ice area has the characteristics of high altitude, heavy ice area and heavy pollution area. The selection process of insulator number selection is very representative. According to the provisions of 9.9.1 of the heavy ice regulation: “The insulation coordination of the transmission line shall enable the line to operate safely and reliably under various conditions such as power frequency voltage, operating overvoltage, lightning overvoltage, etc. The repeated ice line shall also be in accordance with the insulator string. The power frequency and wet compressive strength after icing is checked. Therefore, the insulation configuration of the double-circuit line on the same tower in the heavy ice area is developed.

1) Select the number of insulators according to the power frequency creepage distance.

According to the design specifications 7.0.5 and 7.0.8: the creepage ratio method (λ>3.2 cm/kV) is used to select the number of insulators and the high altitude correction is performed,the number of insulators selected according to the power frequency creepage distance is shown in Table 5.

Tab. 5  Number of the insulators

2) Operation overvoltage verification.

According to the method of "Overvoltage protection and insulation coordination of AC electrical equipment" [21], verify the positive polarity of the insulator string. The 50% discharge voltage of the surge voltage wave meets the requirements of the operating overvoltage. At the same time, it is corrected by high altitude: At an altitude of 2,200 m, 26 XP-160 (structure height 155 mm) standard insulators can meet the operating overvoltage requirements. It can be seen that the operating overvoltage does not control the number of insulators.

3) Lightning overvoltage verification.

From the perspective of the operation of the double-circuit line project on the same tower at home and abroad, the reverse-phase sequence is used to balance the high-insulation design, and the lightning strike rate is significantly reduced. Therefore, the reverse-phase sequence is used to balance the high-insulation method. See Table 5 for the selection results of the number of insulators according to the requirements of the lightning protection level of the tower.

4) Ice withstand voltage verification.

In view of the fact that the insulation strength of the insulator string in the heavy ice area will be significantly reduced under the condition of ice coating, the insulation flashover occurs under the condition of power frequency voltage, and several 500 kV and 220 kV lines in the southwestern region have experienced such tripping accidents in the heavy ice section. Therefore, the determination of the number of insulators in the heavy ice zone requires verification of the power frequency ice pressure resistance capability based on the above principles. Its calculation formula [20] is as follows:

  M=Um/HUn

Where: m is the number of insulators per string; Um is the highest operating phase voltage of the system, kV; Un is the ice pressure resistance, kV/m; H is the height of each structural insulation, m.

According to the experience of insulation design of heavy ice area in China, in the area where the pollution is not serious, the power frequency withstand voltage gradient verification standard of iced insulator string is 70 kV/m. The two-wire and Pusi lines designed according to this method have been in operation for many years, and there has been no incident of insulator-chain ice-flashing. Considering that the application engineering is the coexistence of heavy ice areas and heavy pollution areas, the ice withstand voltage should be appropriately increased, and the calibration standard value is 60 kV/m.

In addition, the ice flashover discharge voltage decreases with increasing altitude, and the relationship [20] is as follows:

Where: Uo is the ice flash voltage at standard atmospheric pressure, kV; Uh is the ice flash voltage at altitude h (unit m), kV; P is the standard atmospheric pressure, 101.325 kPa; Ph is the atmospheric pressure at altitude h (unit m) , kPa; n is the characteristic index, according to the relevant research conclusions of Chongqing University [20], the recommended characteristic index is 0.7.

After the altitude correction, the ice withstand voltage calibration standard value is 50 kV/m, so the number of insulators after the ice withstand voltage verification is calculated is shown in Table 5. It can be seen from Table 5 that the number of insulation insulators is selected to be controlled by ice withstand voltage.

3.3    Air gap

Relying on the engineering heavy ice area line at an altitude of 1 700~2 200 m, the value of the tower head and the phase air gap corrected by the altitude are shown in Table 6.

Tab. 6  Air gap of 500 kV AC double circuit transmission line on the same tower located at heavy icing area

Note: The electrification maintenance should also consider the human activity range of 0.5 m.

4   Design of the double tower of the same tower in the heavy ice area

4.1  Tower size control condition

Considering the safety and economy of the tower, it is recommended to use a vertically arranged drum tower in the double-circuit tower of the same tower in the heavy ice area. The straight tower adopts the "VIV" arrangement, and the argument of the tower size control condition is developed.

According to the heavy ice regulation 10.0.1: "In order to reduce or avoid the flashover accident between the wires, if the non-horizontal arrangement is adopted, there should be sufficient vertical line spacing and horizontal displacement on the tower to meet the wire and ground wire or wire. Static and dynamic clearance clearance requirements between deicing during different periods.The static approach distance should not be less than the gap value of the operating overvoltage; the dynamic approach distance should not be less than the gap value of the power frequency voltage. Therefore, the gap design of the double-circuit tower tower in the heavy ice zone is based on this.

1) Horizontal offset distance.

According to the project data, the maximum gear distance in the continuous section of the heavy ice section is 454 m, and the maximum horizontal span is 316 m. According to the general calculation of mechanics, the arc drop before and after ice coating is 1.19 m. According to the ice jump height formula (3), the maximum ice jump height is 24.34 m and 25.49 m, respectively, when the ice removal rate is 80% and 100%. Since the height of the ice-skating jump increases with the increase of the span, it is uneconomical to consider only increasing the height of the tower to meet the dynamic safety distance of the ice jump, and the safety and reliability of the tower will also be reduced. If the lower conductor is allowed to jump to the upper conductor or the ground line that is not de-iced, in order to avoid phase-flashing or phase-flashing, it must be ensured that the power-frequency phase-to-phase gap is satisfied between the conductors or between the grounding conductors under the dynamic conditions of the deicing jump. Or the power frequency phase gap, this requirement is mainly guaranteed by the horizontal offset between the grounding lines and the wires. According to the heavy ice procedure:

Horizontal offset value ≥ working voltage gap value + combined wire radius + maximum offset distance during wire ice jump.

The working voltage gap value is shown in Table 6. The combined wire radius is 0.225 m (split spacing is 0.45 m), and the maximum yaw distance of the wire at a horizontal span of 316 m is 3.55 m according to the de-icing yaw formula (4). Therefore, the minimum horizontal offset between the double-circuit iron tower wires and the grounding wire in the heavy ice area of this project is shown in Table 7.

Tab. 7  Distance of horizontal displacement

2) The distance between vertical lines.

   According to the provisions of 10.0.1 and 10.0.4 of the heavy ice regulations, the vertical arrangement of the heavy ice area in the same tower double-circuit tower design needs to verify the static between the upper and lower layers when the wire is separated from the ground between the conductors and the ground wire. Close to the safety distance, this distance should not be less than the gap value of the line operating voltage. At the same time, the verification conditions are given in the procedure, which is specifically: deicing in the middle of the continuous gear, and the remaining gears and grounding are not de-icing. The de-icing rate of the intermediate gear should be determined according to the operating experience. When there is no data, the heavy ice zone of 330 kV and above can be selected to be no less than 80% of the designed ice weight [20].

   Static proximity may be expressed as an absolutely static state after deicing in different periods, or it may be expressed as a relatively static state between different layers of grounding lines during different periods of deicing jumps, so to ensure safety, Regardless of the horizontal displacement distance when calculating the distance between vertical lines, then:

Between the wires (between the guide wires) the distance between the vertical lines = the upper layer guide (ground) line uneven ice static sag - the lower layer conductor uneven ice static sag + phase (ground) operation overvoltage gap value.

The interphase (ground) operation overvoltage gap value can be seen from Table 6. The grounding line unevenness ice static sag calculation is essentially the calculation of the uneven ice grounding line stress and the suspended insulator substring offset value, which can be calculated according to the method in [22]. Calculated. Therefore, the calculation results of the vertical line distances of the towers based on the planning of the project are shown in Table 8.

Tab. 8  Vertical dimension of different layer of typical tower

3) The size control conditions of the double-circuit tower head in the same tower in the heavy ice area.

According to the results of the tower head arrangement, the following conclusions are drawn: the height of the lower phase layer, the height of the tensile tower and the height of the ground support are controlled by the distance between the vertical lines, and the height of the upper and middle layers of the straight tower is controlled by the gap circle; The length of the cross-arm in the tension tower is controlled by the horizontal displacement distance, the length of the upper and lower phase cross-arm is controlled by the gap circle, and the length of the ground support is controlled by the lightning protection angle.

4.2   Iron Tower Load Study

The safety level of the double-circuit tower of the same tower in the heavy ice area was determined to be one level, and the structural importance coefficient was taken as 1.1. Combined with the relevant provisions of the load value and load combination of the double-circuit iron tower in the single-circuit and light-middle ice areas of the heavy ice area, the load combination of the double-return in the same area of the heavy ice area should not be lower than that of the light and medium ice areas, and the load value should not be It is lower than the relevant regulations of the single-circuit heavy ice area.

1) The value of the tower load.

At present, the domestic single-circuit line in the heavy ice area has obtained a wealth of operational experience, which indicates that the design value of the tower load in the heavy ice area is safe according to the provisions of the heavy ice regulations. Therefore, the double-circuit tower of the heavy ice tower is on the load value. There is no difference with single loop, and will not be repeated here.

2) Load combination study.

The double-circuit iron tower in the heavy ice area should calculate the normal operation of the line, the broken line (longitudinal unbalanced tension when splitting the conductor), the uneven icing condition and the load combination under the installation condition. If necessary, check the earthquake and rare ice load. Waiting for working conditions. The main load combination conditions are as follows.

(1) Normal operation.

A. Basic wind speed, no ice, no broken line (including minimum vertical load and maximum horizontal load combination).

B. Maximum ice coating, corresponding wind speed and temperature, no disconnection.

C. Minimum temperature, no ice, no wind, no disconnection (applicable to terminal and corner tower).

(2) Ice-breaking condition: Calculated according to the broken line, -5 °C, ice, no wind load.

Suspended pole tower: In the same gear, single conductor breaks any two-phase conductor (the split conductor has any longitudinal unbalanced tension); in the same gear, one ground wire is broken, and one conductor breaks any phase conductor (any one of the split conductors) The phase conductor has a longitudinal unbalanced tension

Tension resistant tower: In the same gear, break any two-phase conductor and ground wire; if there is any ground wire and any phase conductor in the same gear.

(3) Uneven ice coating: Calculated according to unbroken line, -5 °C, uneven ice, and wind speed of 10 m/s.

A. All guides and ground wires have unbalanced tension in the same direction, so that the tower can withstand the maximum bending moment.

B.All the guide and ground wires have unbalanced tension at different times, so that the tower can withstand the maximum torque.

4.3 Tower type research

In the heavy ice area, the double-tower straight tower of the same tower can be arranged in three ways: horizontal, triangular or vertical. The comparison of the tower types of the three arrangements is shown in Table A1 of the Appendix. Relying on the heavy ice area of the project, it is located in the high mountain terrain. The mountain slope is steep. It can be seen from Appendix A1 that the shortest vertical alignment tower is the least affected by the terrain under the premise of meeting the ground distance requirement. At the same time, due to the uneven ice and ice in the heavy ice zone, the tower is subjected to large bending moments and torque, and the tower generates large longitudinal deformation. In order to ensure safety, the longitudinal deformation is controlled in the design of the tower in the heavy ice zone to become the tower design. The important goal. Therefore, the double-circuit iron tower in the heavy ice area prioritizes the vertical arrangement in the selection.

In addition, the tower heads of the vertically arranged straight towers can be further divided into "V-I-V" and "3V" arrangements, as shown in Appendix Table A2.

According to the above calculation results, in the case of the same edge distance, the "VIV" arrangement linear tower calculation tower weight is slightly heavier than the "3V" arrangement linear tower, but because the length of the middle crossarm is less than the "3V" arrangement, the wire hanging point The displacement is much lower than the "3V" arrangement straight line tower. The heavy-ice area is the same as the "V-I-V" vertical arrangement.

4.4 True Tower Test

The double-circuit design of the same tower in the heavy ice area is the first in China. To verify whether the overall structure of the tower is reasonable and safe, and whether the transmission line is consistent with the theoretical design, it needs to be verified by the true test of the tower. With the support of relevant scientific research units, the test tower successfully passed the loading and overload test. During the test, the iron tower did not produce excessive displacement, and the bearing capacity and stiffness could meet the load requirements. The experimental results were basically consistent with the theoretical analysis [23].

5   Engineering applications

The length of the 20 mm heavy ice area of the first 500 kV same-tower double-circuit line Maomao Line in China is 2×7.581 km. The entire line of heavy ice area is designed according to the same tower, and the wire is made of 4×LGJ-500/45 steel core. Aluminum stranded wire and ground wire adopt OPGW-150 optical cable. The planned and designed heavy ice area of the same tower double-return series tower contains four types of towers, of which the linear tower and the tensile tower are each two types of towers. A total of 28 bases of double-circuit iron towers with the same tower of heavy ice are built in the project, among which the linear tower is 10 bases and the tower is 18 bases. The terrain of the heavy ice area is Junling and Gaoshan Daling, and the elevation is between 700 and 2 200 m. The project was put into operation in December 2013 and remained safe and stable during the severe period of local ice coating.

6  Conclusion

Based on the Mao-Mao line of the 500 kV double-circuit line on the same tower in the 20 mm heavy ice area in China, based on the in-depth analysis of the dynamic response law of the de-icing of the transmission line in the heavy ice area, the 500 kV double tower of the 20 mm heavy ice area is the same. The circuit design ideas and design methods are studied, and the following conclusions are drawn for the reference design of the follow-up heavy ice tower double-circuit line:

1) The wire skating jump height and yaw distance are proportional to the continuous gear number, the gear distance, the icing rate, the ice thickness, and the inverse of the wire cross section.

2) The unbalanced tension during the deicing process is less than the static standard value specified in the regulations. The horizontal and vertical loads are smaller than the load design value, and the tower load can be taken according to the heavy ice procedure.

3) When selecting the path of the double-circuit line on the same tower in the heavy ice area, attention should be paid to the factors such as the number of continuous gears, the length of the tensile section, the span, the difference of the span, the conditions of the tower, the boundary of the ice and the micro-meteorology.

4) Ice withstand voltage may become the control condition for the selection of the number of line insulators in the high-altitude heavy ice area.

5) The horizontal offset distance and the vertical line distance are the control conditions for the size of the tower double-circuit iron tower head in the heavy ice area.

6) The calculation standard of the tower load of the double-circuit line on the same tower in the heavy ice area should not be lower than the double-circuit line of the single circuit and the light and medium ice area in the heavy ice area.

7) The arrangement of conductors on the double-circuit line of the same tower in the heavy ice area is recommended to be vertically arranged, and the straight line tower is preferably arranged in the “V-I-V” arrangement.

8) The rationality and safety of the structural arrangement of the tower were verified by the true tower test.

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Tab. A1  Comparison of three different arrangement mode tower

Tab. A2  Comparison of “V-I-V” and “3V” type tower

 By: JUCRO Electric (Focus on Vacuum Interrupter, Vacuum circuit breaker, Vacuum contactor and Switchgear)

                                                                                                                                                                  2018.11.6