Study on the minimum polarity distance of suspension tower for UHVDV transmission line in ultrahigh altitude area

The design experience of ± 800 kV UHVDC transmission line in 3500 m and above is less. Based on the basic design conditions of ± 800 kV DC transmission line project, the minimum polarity distance of the suspension tower in an ultrahigh altitude area is investigated and the effects of the electromagnetic environment, insulator string length, V-string angle string and air gap on polarity distance are analyzed. Finally, the minimum polarity distance of the suspension towers under different altitudes, V-string angle string and icing area conditions are calculated, which provides technical support for the tower design of UHVDC transmission lines in ultrahigh altitude areas.


Introduction
The Tibet Autonomous Region is rich in exploitable energy, with about 140 million kilowatts of hydro energy [1], about 700 million kilowatts of solar energy, and about 180 million kilowatts of wind energy.Hydro energy and new energy have great potential for development, and the demand for the outward transmission of Tibetan electricity is becoming increasingly urgent [2].
Located in the southwest of the Qinghai-Tibet Plateau, Tibet is known as the "roof of the world" with an average altitude of more than 4,000 meters.The ultrahigh altitude is one of the biggest characteristics of Tibet, which has brought great challenges to the insulation, electromagnetic and lightning protection, etc. of ultra-High voltage direct current (UHVDC) transmission line projects in the Tibet area [3][4][5].At present, the maximum altitude of the designed UHVDC transmission lines in China has reached 4000 m, but there is relatively few research on UHVDC transmission technology above 4000 m or even 5000 m.
In the investment of the UHVDC transmission line project, the tower investment accounts for a large proportion, and the size of the tower in the ultrahigh altitude area will be even larger, resulting in a larger proportion of investment.According to previous engineering design experience [6][7], a 10% increase in polarity distance increases the tower weight reduction by about 5%.Under the premise of meeting the engineering insulation configuration and electromagnetic interference, reducing the polarity distance as much as possible is the key to controlling the engineering cost of UHVDC transmission lines.
Combined with the research results of electromagnetic environment and insulation coordination of ultrahigh altitude ± 800 kV UHVDC transmission lines, the analysis and research on the value of

Limit value for electromagnetic environment 1) Synthetic electric field and ion current density
In general non-residential areas, the limit value of ground synthetic electric field strength under DC transmission lines is 30 kV/m.and other synthetic electric field strength and ion current density limits are detailed in Table 1.When the line is adjacent to a residential building, the undistorted synthetic electric field limit on the ground where the building is located under wet conductor conditions is 15 kV/m.2) DC magnetic field The Control Value of the Electromagnetic Environment for ± 800 kV DC Overhead Transmission Line (Q/GDW145-2006) stipulates that the limit value of Magnetic flux density generated by DC transmission line current is 10 mT.The magnetic field intensity of the DC transmission line is far less than the limit value, which is generally not the design control condition.
3) Radio interference At an altitude of 1000 m and below, the radio interference at 0.5 MHz frequency shall not exceed 58 dB (μV/m) at 20 m outside the ground projection of the positive polarity conductor of a DC overhead transmission line, 80% of the time and 80% of the confidence level.

4) Audible noise
At an altitude of 1000 m and below, the audible noise (L50) generated by the corona shall not exceed 45 dB (A) at 20 m outside the ground projection of the positive polarity conductor of the DC overhead transmission line.When the altitude is greater than 1000 m and the line passes through sparsely populated areas, the audible noise generated by the corona shall not exceed 50 dB (A).

Insulator string length and V-string angle
The length of the insulator string and the V-string angle string have a significant impact on the polarity distance of the suspension tower of the ± 800 kV DC transmission line, and the required minimum polarity distance needs to be calculated based on the conclusions of the study on the insulation configuration of ultrahigh altitude ± 800 kV DC transmission lines [8][9].

Air gap
The polarity distance suspension tower of the ± 800 kV DC transmission lines also needs to meet the requirements of the air gap at the tower head, as shown in Table 2. Based on the research conclusion of the air gap of ± 800 kV DC transmission lines at ultra-high altitudes, the minimum polarity distance required for the air gap is calculated.Ultrahigh altitude areas are generally sparsely populated areas.According to the limit of 50 dB (A) considerations, when the 6×1000 mm 2 conductor is used, the minimum polarity distance satisfying the audible noise limit at different altitudes is 16-18m.When the 6×1250 mm 2 conductor is used, the polarity distance is not controlled by the electromagnetic environment limits.

Minimum polarity distance required for insulator string length
According to the research results of insulation coordination for ± 800 kV DC lines, composite insulators are used for suspension insulator strings in light to medium icing areas, while outer umbrella disc insulators are used for suspension insulator strings in heavy icing areas.The recommended parameters are shown in Tables 3 and 4. The minimum polarity distance required for the length of insulator strings is calculated by using the following formula:


where s is the polarity distance, l is the length of the insulator string, d is the horizontal distance from the suspension point of the connecting plate to the center of the connecting plate, and w is the width of the tower body.
The length of the hardware part of the insulator string is 1.16-2.24m, the horizontal distance from the suspension point of the connecting plate to the center of the connecting plate is 0.25 m, and the width of the tower body corresponding to the wire is 3.8-4.4m.
According to the design basic wind speed values and tower construction planning, the V-string angle in light and medium icing areas is set at 85°-95°, and the V-string angle in heavy icing areas is set at 75°.The minimum polarity distance is shown in Tables 5 and 6.From Tables 5 and 6, it can be seen that the V-string angle has a significant effect on the interpolarity distance, and the larger the V-string angle is, the larger the corresponding polarity distance is.

Minimum polarity distance for air gap requirements
The operating over-voltage multiple of ± 800kV DC lines is generally 1.4 -1.6 p.u., according to the actual operating over-voltage distribution curve of the project, differential design is considered based on the operating over-voltage multiple of 1.52 p.u. and 1.6p.u.The gap of hot-line work is the control condition for the tower head gap, and the minimum polarity distance is shown in Table 7.
The minimum polarity distance required for the air gap is calculated by the following formula: where s is the polarity distance, dgap is the minimum gap distance for hot-line work, drc is the distance from the equalizing ring to the center of the split conductor, and w is the width of the tower body.
The distance from the equalizing ring to the center of the split conductor is taken as 1.5 m, and the width of the tower body corresponding to the conductor is taken as 3.8-4.4m.
Considering only the factor of the air gap, the minimum polarity distance corresponding to ± 800 kV DC line operating over-voltage multipliers of 1.52p.u. and 1.6p.u. are shown in Table 7.

Polarity distance selection
The polarity distance of the suspension tower is mainly controlled by the electromagnetic environment, minimum air gap, insulator string length, and V-string angle [10][11][12].The relationship between the polarity distance of the suspension tower and the electromagnetic environment limit, air gap, insulator string length, and V-string angle is shown in Figure 2.  From Figure 2 and 3, it can be seen that the polarity distance of suspension towers in light to medium icing areas is greatly affected by an air gap, insulator string length, and V-string angle, and it is necessary to consider the operating over-voltage multiplier, altitude and population level when planning and designing of the tower.While the pole spacing of suspension towers in heavy ice areas is mainly controlled by the insulator string length, and the planning and designing of the tower can be carried out according to the altitude and grading.

4. Polarity distance study 4 . 1 Figure 1
Minimum polarity distance required for electromagnetic environmentThe ion current density, synthetic electric field strength, audible noise and radio interference of the DC line are all related to the distance of the polarity conductors of the suspension tower.Audible noise at 20 m outside the projection to ground.According to the calculation conclusions of the electromagnetic environment, the ground ion current density and the ground synthetic electric field strength control the selection of the height of the polar conductor, and the audible noise controls the selection of the polarity distance.

Figure 1 (
a) and (b) lists the audible noise values at 20 m outside the ground projection of the positive polarity conductor at different polarity distances when 6×1000 mm 2 and 6×1250 mm 2 conductors are used, and the audible noise corrections are made in accordance with the conclusions of the Chinese Academy of Electrical Sciences (CAES) true-type and analogue test line section tests at altitudes of 0-4300 m.

Figure 2
Figure 2 Minimum polarity distance at different altitudes located in a light to medium icing area with a V-string angle of 85°.

Figure 3
Figure 3 Minimum polarity distance at different altitudes located in a heavy icing area with a V-string angle of 75°.

Table 1
Limits for synthetic electric field strength and ion current density.

Table 3
Length and creepage distance values of composite insulators.

Table 4
Recommended configuration for disc insulators in heavy icing areas.

Table 5
Minimum polarity distance required for the length of insulator strings in light to medium icing area.

Table 6
Minimum polarity distance required for the length of insulator strings in heavy icing area.

Table 7
Minimum polarity distance for air gap requirements.