Research on General Design and Application of Double-circuit T-Connection Tower In 110kV

Based on the investigation results and the current general design of state grid, this paper reasonably planned the design conditions of the tower. Taking the general design module as the basic tower type, a kind of double loop T-connection tower in 110kV with high generality was proposed, then carry out tower head planning and structural internal force analysis. Finally, the electric field finite element simulation is carried out to demonstrate the safety and reliability of the T-connection tower of cross arm arrangement. The tower can cover the T-connection conditions of 110kV double-circuit line in most areas of the country, and draw the tower diagram, which can be included in the general design of state grid transmission tower for reference of line designers. The cross arm of tower head is arranged in a crisscross way, which greatly simplifies the the double-loop T-connection scheme. One tower can replace the traditional multi-tower to complete the T-connection of the line, which reduces the cos t and occupation of land, also achieves good economic and environmental benefits.


1.Introduction
At present, the T-connection of 110kV transmission line is to use strain tower as the T-connection point, use 2-3 base tower to drill through the original line and merge with the T-connection line on the other side to form a double circuit. This traditional T-connection mode leads to waste of land resources and high project cost [1]- [2] .
In order to solve the above problems, taking the general design module of State Grid Corporation of China as the basic tower type, a 110 kV double-circuit T-connected tower with high versatility is proposed. The cross arm of the tower head is arranged in a crisscross manner to realize the T-connection of the main line with one tower. This kind of T connection tower effectively reduces the project investment, reduces the project area, shortens the construction period, and can bring huge social and economic benefits.

2.Tower design condition planning
According to a number of engineering design conditions involved in the investigation, the model of grounding wire of 110kV transmission line is as shown in Table 1 According to the survey and general design, 1×JL/G1A-300/40 is selected for the conductor and JLB20A-100 is selected for the ground wire (the performance of JLB20A-100 can cover GJ-80).
The icing thickness and wind speed of 110 kV line design conditions are shown in table 2 and table  3. According to the investigation, the icing thickness of 110kV line is 10 mm, and the wind speed is 29 m/s, which can cover the T-connection conditions of the wire in most parts of the country.
According to table 1-3 and combined with the state grid general design, the design conditions planning of double-circuit T-connection tower in 110kV is as shown in table 4.

Tower head planning
Taking the general design [3] module of State Grid Corporation as the basic tower type, the cross arm of the tower head of 110kV double circuit T-connection tower is arranged in a crisscross way. The tower head planning is shown in figure 1.

Calculation method of sag of direct lead jumper
Calculation formula of inclination angle  of strain insulator string [2] (1) In the formula: Gv-Tension insulator string gravity, N; g 1 -Gravity per unit length of conductor wire, N/m; l, h-Calculate the side span and height difference, m; T-Horizontal tension of conductor under calculation conditions, N The wind deflection angle  of strain insulator string is calculated as follows: In the formula: P, G-Wind load and self weight load of conductor, N The solution method of minimum allowable sag f min [4] (graphic method): The minimum allowable sag refers to the minimum sag between the upper cross arm components and the tension string grounding components under various clearance inspection conditions after the jumper wire is over tensioned and wind deflection is generated. The minimum sag is the largest of the minimum sag under various inspection conditions, which is generally controlled.
Maximum allowable sag f max solution method [5] : when calculating the maximum allowable sag, it is necessary to calculate the average horizontal offset e cp and average vertical displacement d cp of suspension points on both sides of jumper wire under lightning overvoltage, switching overvoltage and power frequency voltage. The maximum allowable sag f cp of jumper wire is calculated, and the minimum sag is taken as the maximum allowable sag f max of jumper wire.

3.2.2.Calculation results of direct lead jump wire
According to the tower head planning and the above formula, the wind deviation angle and construction sag of jumper about each crossbar is shown in the

3.3.Drawing of tower head clearance circle
In the layout of the tower, the structural margin corresponds to the selection of angle steel alignment, and the structural margin of 110kV tower is taken as 150 mm [6] . Draw the tower head clearance circle [7] of T-connected tower through the wind deflection angle and construction sag under the maximum wind speed, operating over-voltage, lightning over-voltage and live working conditions obtained   The distance between the conductor and the ground wire on the tower shall be calculated according to the following formula: In the formula: S-Distance between conductor and ground wire(m); L-Span(m). According to the tower head plan, the distance between the conductor and the ground wire is S = 7.74m>0.012×300 +1 =4.6m, meeting the requirements.

3.4.2.Cross arm size verification
1)The equivalent distance between horizontal lines of triangular arrangement of traverse [8] is calculated according to the following formula: In the formula: D x -Equivalent distance between horizontal lines of triangular arrangement of conductors(m); D p -Horizontal projection distance between conductors(m); D z -Vertical projection distance between wires(m); 2)For T-connected tower, the distance between horizontal lines is calculated according to the following formula: According to the specification requirements, D X >D. According to the tower head plan, the 110kV double circuit T connection tower can meet the

Design conditions of T-connected tower
The design conditions of T-connected tower in 110kV are shown in table 6.

4.2.The most unfavorable load combination of double-circuit T-connected tower
The load combination of normal operation, disconnection, uneven icing and installation of T-connection tower shall be calculated [9]- [10] . For T-connection tower, the most unfavorable load is combined with the overhead view, as shown in table 7.

4.3.Slope optimization and root span determination
The slope of 110kV double-circuit tower (below the head of tower) is generally 9% ~ 15%. When the root opening increases, the stress of the main members decreases, and the member specification decreases. But when the root span is too large, the geometric dimensions of the diagonal and auxiliary members increase, and the member structure layout becomes complex, so that the tower weight increases correspondingly [11]- [12] . The optimal tower slope of 110kV double-circuit T-connection tower is determined by Dao Heng's function about optimization slope. As is shown in  It can be concluded that the optimal slope of 110kV double circuit T-connection tower is 12.2%. According to the variable groove width through the string center calculation, the tower root span size corresponding to each nominal height can be obtained, as shown in the table 9.

.Electric field and potential distribution near T-connected tower
The distribution of electric potential and electric field is calculated by the method of finite element simulation when the initial phase of conductor voltage is zero [13] , as shown in the figure 5-8.

4.4.2.Electric field intensity at 1.5m above the ground
In the code for design of electric power industry in China, it is required to control the undistorted electric field no more than 4kV/m at 1.5m above the ground for the crossed non long-term residential buildings and adjacent houses. Select the section of 1.5m above the ground to calculate the electric field strength. The calculation results are shown in Figure 9. The maximum value is 1.32kV/m, which is less than the standard of 4KV/m, meeting the requirements.

4.5.Full stress calculation
Using Dao Heng calculation software to calculate the full stress of the 110kV double-circuit T-connection tower after the load is applied. By controlling the index of slenderness ratio and stress ratio, the member specifications and arrangement forms of the 110kV double-circuit T-connection tower are adjusted repeatedly, and finally the T-connection tower structure with resource conservation and reasonable stress distribution is completed.
The stress ratio and slenderness ratio of the whole pole of 110kV double circuit T-connection tower meet the requirements, as is shown in figure 10 and figure 11. Most of the stress ratio colors of tower main members are in the yellow and green range (stress ratio:70% -95%), and the stress ratio is controlled within 0.95. The stress ratio of diagonal and auxiliary members is controlled within 0.98, and most of them are in the green and blue range (70% -85%). Therefore, it can be concluded that the model is reasonable and the stress of the members can be effectively allocated.  Figure 10. Stress ratio distribution Figure 11. Slenderness ratio distribution

5.Fabrication of tower processing drawing
According to the general design mode, the design conditions、 supplementary instructions、 root-opening size and foundation force of 110kV double circuit T-connection tower are indicated.
After the command diagram output by the T-connection tower model, the processing diagram of 110kV double circuit T-connection tower is drawn according to the command diagram, which can be used as a direct reference by the design unit according to the use conditions.

6.Engineering application and economic comparative analysis
The pilot application is based on the 110kV transmission line project of An Tuo-Jing Ye No.2 substation T-connected to Ling Shou No.2 substation in Shijiazhuang. The following three T-connected plans are proposed for comparative analysis of the project, as shown in table 10.  The total investment of plan A is 38.8% and 59.4% lower than that of plan B and plan C respectively, and the floor area is 0.6 Mu lower than that of plan C, saving 66.7% of the floor area. As plan C adopts cable duct arrangement, the total investment is high, and it is not easy to form a general CISAT 2020 Journal of Physics: Conference Series 1634 (2020) 012169 IOP Publishing doi:10.1088/1742-6596/1634/1/012169 10 design for popularization and application. Therefore, the project adopts one 110kV double circuit T-connection tower instead of the traditional T-connection scheme, which effectively reduces the project investment and achieves good economic benefits.

7.Conclusion
Through reasonable planning of tower design conditions, the crisscros 110kV double-circuit T-connection tower can meet the T-connection conditions of most of the plain areas in the country, and realize the goal that one tower completes T-connection of main line. The T-connection tower can be used as a direct reference for the design unit according to the use conditions.
The T-connection tower has been successfully applied to transmission project of An Tuo-Jing Ye No.2 substation T-connected to Ling Shou No.2 substation, which has effectively reduced the land occupation, reduced the cost and brought good economic and environmental benefits.