1. Introduction
The 750 kV transmission lines are an important part of the State Grid, which connects the northwest load centers and the major energy bases. At present, there is a problem of land occupation in important transmission channels in the northwest region. Therefore, it is necessary to study the transmission technologies with larger transmission capacity and less land occupation. The multi-circuit transmission technology of the same tower has high capacity and less land occupation in the same tower, which has high technical feasibility.
Composite insulators are used in 750 kV multi-circuit transmission lines on the same tower. To keep the safe operation, the corona characteristic must be considered in the design of transmission line. The two ends of insulators, especially the high voltage end, are seriously distorted on electric field, and corona discharge is easy to occur and cause the electric erosion of insulators. Therefore, it is necessary to install the grading ring at the high voltage end and ground end. The corona characteristic of the grading ring has important relationships with its structure size and installation location. Therefore, its structure must be optimised to control the electric field strength of grading rings within a reasonable range.
In the aspect of structure optimisation of grading rings, related researches have been carried out. The electric field distribution of insulator strings and fittings of transmission line with different voltage levels are analysed by finite element model, by which the influence of structural parameters on the electric field strength is analysed, and the corresponding grading ring optimisation structure is put forward. For the transmission line with 750 kV voltage level, combined with the experiment and finite element simulation, the optimisation structure of grading ring with 750 kV double-circuit-compact circuit on the same tower is proposed. A three-dimensional electric field simulation model for 750 kV transmission line tower insulators is built in, and the research shows that the grading ring with the pipe diameter of 110 mm and ring of 800 mm can effectively improve the electric field distribution of high voltage ends of insulator strings.
The 750 kV transmission lines that have been built and connected in China are mainly concentrated in Northwest China, with high altitude and serious corona condition. The electric field strength on the surface of the hardware fittings may be higher than the limit, which can easily lead to corona. Therefore, the design of the fitting structures must be optimised, and the corona control of the fittings should be within a reasonable range so as to satisfy the requirement of safe and reliable operation of the line. Compared with the double-circuit line on the same tower, the tower of four-circuit transmission line is taller and the structure is more complicated. At present, there is no research on the grading ring parameters of the 750 kV four-circuit transmission line on the same tower. Therefore, a three-dimensional finite element model of 750 kV transmission line with six layer cross-arms is built in this paper, and the influences of pipe diameter, ring diameter and installation position of grading ring are analysed. The structure parameters that meet the need of corona control are proposed, which can provide references for the grading ring design of 750 kV four-circuit transmission lines.
2 Electric field calculation of transmission line on six cross-arms tower
2.1 Calculation model and initial parameters
Six layer cross-arms arrangement can be used in 750 kV four-circuit transmission line on the same tower. The preliminary design arrangement is shown in Fig. 1 a, and the phase sequence of the conductors is shown in Fig. 1 b.
Fig. 1
Diagrams of six cross-arms arrangement and conductor phase sequence
(a) Six layer cross-arms arrangement, (b) Phase sequence diagrams of conductors
The conductors used for 750 kV transmission line are six bundled conductors in regular hexagonal arrangement, with split spacing of 400 mm, sub-conductor type is JL1/G1A-500/45, and the outer diameter is 30 mm. The suspension string adopts the ‘I’ composite insulator string, and the composite insulator contains two kinds of shed. The spacing between the sheds is 70 mm, with the large shed diameter of 200 mm, the small 160 mm, and the length of the insulator string is 7190 mm.
Using C 4 phase as analysis phase, the whole three-dimensional finite element model of transmission line is built. The whole and local models are shown in Fig. 2 . Considering the calculation efficiency, only the analysed phase is modelled completely, including insulators, grading rings, yoke plate, conductors and connection fittings. The rest of the phase models only contain the conductors. Three ring structures are adopted in the model. There are two large and small grading rings at the high voltage end, and a small grading ring at the ground end. The parameters and installation locations of the grading rings are shown in Table 1 . The grading ring arrangement is shown in Fig. 3 .
Fig. 2
Simulation model
(a) Whole model, (b) Local model
Table 1 Parameters of the grading rings (mm)
Parameters of the grading rings (mm)
| Type of fittings | Pipe diameter | Ring diameter | Installation position |
|---|---|---|---|
| large ring on the high voltage end | 50 | 800 | 120 |
| small ring on the high voltage end | 30 | 270 | 0 |
| small ring on the ground end | 30 | 270 | 0 |
Fig. 3
Arrangement diagram of the grading ring
The air truncation boundary is a half column with 400 m radius and the length of the column is 24 m. In the analysed phase, the grading rings, conductors, yoke plate and other hardware fittings on high voltage end are loaded with the maximum operation phase voltage of 750 kV AC power system U m = 750 × 1.1√2/√3 = 673.6 kV. For other phase conductors, A, B phase conductors load U m/2 = −336.8 kV, and C phase conductors loading voltage is U m. Grading rings and hardware fittings at the ground end are loaded with zero, so as to the tower and the outer air boundary.
The electric field distribution on the surface of grading ring and end fitting on high voltage end is calculated, which are, respectively, shown in Figs. 4 a and b. The local electric field distribution of the composite insulators is shown in Fig. 4 c, in which the grey area in the cloud map is the area where the electric field strength exceeds the maximum value of the high voltage end fitting. The maximum electric field strength, which is 27.19 kV/cm, appears at the surface of the large grading ring on high voltage end, and the maximum electric field strength of the high voltage end fitting is 7.71 kV/cm.
Fig. 4
Contours of electric field distribution (kV/m)
(a) Large grading ring on high voltage end, (b) Local electric field distribution of the composite insulators
2.2 Analysis of calculation results
The change of air density caused by altitude changes is one of the important factors that affect the corona onset voltage of fittings [ 2 ]. The higher the altitude, the easier of the corona discharge occurs. An altitude correction formula for corona onset field of the fitting surface is proposed in Q/GDW 551-2010 ‘Design and construction guide for reducing audible noise of transmission lines’ [ 11 ]:
(1)
E=E0/(K1K2)E=E0/(K1K2)
where E is the correction corona onset field of hardware fittings, E 0 is the maximum corona onset field at zero altitude, which has a 40 kV/cm value, K 1 is an altitude correction factor and K 2 is safety margin coefficient value which is 1.4.
According to ( 1 ), the correction of the corona onset field on hardware fitting surface under different altitudes should be made. The altitude of the line running range is 0–1500 m, so the altitude correction coefficient is 1.17, and the corona onset field is revised to 24.42 kV/cm at this height. Taking 85% of the correction value as the control field strength of the fitting surface, the value is 20.76 kV/cm.
Comparing the calculation results of the initial model parameters, it is found that the maximum electric field strength on the surface of the large grading ring is higher than the working control field value. The IEEE working group proposed that the maximum field strength of the end fitting surface should not be higher than 4.5 kV/cm (rms) [ 12 ], which is converted to peak value of about 6.36 kV/cm, so take 6 kV/cm as a reference value. However the calculation result of the maximum field strength of end fitting surface is 7.71 kV/cm, which is 28.5% higher than the reference value. Therefore, the structure of the grading ring should be optimised in order to meet the requirements of the electric field strength on the hardware fitting surface.
3 Influence of the grading ring parameters on the electric field distribution
3.1 Influence of pipe diameter
The pipe diameter of grading ring is one of the key factors to control the field strength on the hardware fitting surface. Keeping the ring diameter and installation position of the pressure ring invariable, the electric field distribution is calculated with the pipe diameter of 80, 100 and 120 mm. Compared with the initial calculation result with the pipe diameter of 50 mm, the summary of the maximum field strength in each concerning area is shown in Table 2 .
Table 2
Electric field calculation results under different pipe diameters of the grading rings (kV/cm)
| Pipe diameter, mm | Ground end | High voltage end | |||
|---|---|---|---|---|---|
| Grading ring | End fitting | Small ring | Large ring | End fitting | |
| 50 | 3.52 | 2.37 | 14.07 | 27.19 | 7.71 |
| 80 | 3.54 | 2.52 | 11.72 | 21.27 | 6.90 |
| 100 | 3.49 | 2.49 | 10.22 | 19.09 | 6.24 |
| 120 | 3.50 | 2.50 | 9.52 | 17.43 | 5.96 |
From Table 2 , it is known that the maximum electric field strength of the fittings on high voltage end has a decreasing trend with the increase of pipe diameter, and the values of fittings on ground end have little change. When the pipe diameter of the large grading ring is increased from 50 to 120 mm, the maximum field strength on the surface of the large grading ring is decreased by 35.9%, and the value of high voltage end fitting is reduced by 25.3%. The simulation results show that increasing the pipe diameter of the grading ring can effectively control the field strength on hardware fitting surface. When the pipe diameter is 100–120 mm, the field strength of large grading ring can be controlled below 20.76 kV/cm, and that of high voltage end fitting can be controlled below 6.36 kV/cm.
3.2 Influence of installation position
Keep the ring diameter as 800 mm as well as the pipe diameter as 100 mm and change the installation position of large grading ring, the maximum field strength results in each concerning area are shown in Table 3 .
Table 3
Electric field calculation results under different installation position of grading rings (kV/cm)
| Installation position, mm | Ground end | High voltage end | |||
|---|---|---|---|---|---|
| Grading ring | End fitting | Small ring | Large ring | End fitting | |
| 100 | 3.50 | 2.49 | 10.95 | 18.94 | 6.64 |
| 150 | 3.51 | 2.50 | 9.84 | 19.29 | 5.81 |
| 200 | 3.52 | 2.51 | 10.10 | 19.55 | 5.35 |
| 250 | 3.54 | 2.52 | 11.13 | 19.85 | 5.14 |
Table 3
Electric field calculation results under different installation position of grading rings (kV/cm)
From the calculation results in Table 3 , it can be seen that the installation position of the large grading ring has a great influence on the surface field strength of the high voltage end fitting. When the installation position is increased from 100 to 250 mm, the surface field strength of the insulator's high voltage end fitting is reduced from 6.64 to 5.14 kV/cm, which is reduced by 22.6%. When the installation position is 150–250 mm, the surface field strength of the insulator's high voltage end fitting can meet the limit value of 6 kV/cm. Take the installation position of 200 mm, for example, the local electric field distribution of composite insulators is shown in Fig. 5 , and the electric field curve from high voltage to ground end 1 mm outside the insulator sheath is shown in Fig. 6 . The maximum field strength of the high voltage end fitting is 5.35 kV/cm. The electric field strength of the insulator sheath reaches the maximum at 44.5 cm from the end, and the maximum value is 4.44 kV/cm. The results show that the grading ring can protect the composite insulators effectively, as it can reduce the surface field strength of insulator and end fittings.
Fig. 5
Electric field distribution of the composite insulator and grading rings (kV/m)
Fig. 6
Electric field distribution along the sheath of insulator with the distance of 1 mm
In addition, the maximum electric field strength of the large grading ring is increased by 4.8% when the installation position is increased from 100 to 250 mm, but it is still lower than the field strength limit value. The values of the small grading ring and insulator end fitting on ground end are also increased by 1.1 and 1.2% respectively. The increase of the surface field strength of the three kinds of fittings is due to the increase of grading ring's installation position, which makes the shortest distance between the high voltage and grounding electrodes decreased and therefore leads to the increase of the field strength on the hardware fitting surface. The surface field strength of the small grading ring at the high voltage end is reduced first and then increased. The field strength is the smallest at the installation position of 150 mm, which is 10.1% lower than that at the installation position of 100 mm. It can be seen that the surface strengths of the four kinds of hardware fittings have changed little when the installation position is changed, and it is still in the acceptable range.
3.3 Influence of ring diameter
The ring diameter can also affect the electric field distribution of hardware fittings. Keep the installation position of 200 mm and the pipe diameter of 100 mm, calculate the maximum field strength of hardware fittings when the ring diameter is 700, 900 or 1000 mm. The comparison of the calculation results is shown in Table 4 .
Table 4
Electric field calculation results under different grading ring diameters (kV/cm)
| Ring diameter, mm | Ground end | High voltage end | |||
|---|---|---|---|---|---|
| Grading ring | End fitting | Small ring | Large ring | End fitting | |
| 700 | 2.50 | 3.51 | 9.84 | 19.82 | 4.95 |
| 800 | 2.51 | 3.52 | 10.10 | 19.55 | 5.35 |
| 900 | 2.53 | 3.55 | 10.96 | 19.41 | 5.77 |
| 1000 | 2.54 | 3.57 | 11.80 | 19.38 | 6.12 |
Table 4
Electric field calculation results under different grading ring diameters (kV/cm)
In Table 4 , it can be found that when the ring diameter increases from 700 to 1000 mm, the field strength of large grading ring on high voltage end decreases by 1.44%, which has changed little, but that of the end fitting is increased from 4.95 to 6.12 kV/cm, which is increased by 23.6%, and the field strength of the small ring on high voltage end is also increased. The increase of electric filed strength is mainly because of the shielding effect of large grading ring. When the ring diameter is short, the distances between large grading ring and other high voltage fittings are also short. As the high voltage fittings have the same potential as the grading ring, the short distance of fittings makes the electric field distribution more uniform, so the electric field strengths of other high voltage fittings will decrease as the ring diameter reduces.
To sum up, when the pipe diameter is 100 mm with the installation position of 150–250 mm and the grading ring diameter of 700–900 mm, the electric field strength on large grading ring and high voltage end fitting can meet the reference value.
4 Conclusion
In this paper, a three-dimensional electrostatic field model of the four-circuit transmission line on the same tower is built. The influence of the grading ring structural parameters on the electric field distribution is analysed by simulation. The following summarises the research results:
The maximum electric field strength appears on the large grading ring surface at the high voltage end, which is greatly influenced by the pipe diameter of the grading ring. When the pipe diameter is 100–120 mm, the field strength on grading ring surface can be controlled below 20.76 kV/cm. Increasing the pipe diameter can also decrease the field strength on high voltage end fitting surface. The value can be controlled under 6 kV/cm when the pipe diameter is 120 mm.
The installation position and ring diameter will also affect the surface field strength of grading ring and high voltage end fittings. With the increase of installation position and the decrease of ring diameter, the field strength of the high voltage end fitting will be reduced, and the surface filed strength of grading ring will be increased, which is not obvious. The field strength of hardware fittings at ground end has little change when the parameters of grading ring changes.
According to the calculation results, it is recommended that the pipe diameter is 100 mm, the installation position is 150–250 mm, and the ring diameter is 700–900 mm, so that the electric field strength can be the requirement and have a certain tolerance.
It can be seen that the parameters of four-circuit transmission line are close to that of two-circuit transmission line which have 850 mm ring diameter, 100 mm pipe diameter and 340 mm installation position [ 9 ]. The reasonability of recommended parameters can be proved by the comparison.
5 Acknowledgments
This work was supported by Science and Technology Project of SGCC: Research on The Key Technology of 750 kV AC Power Transmission Line with Four-circuit on The Same Tower (Grant No. GY 71-17-018).
6 References
- LiuZ.Y.ZhangQ.P.: ‘ Study on the development mode of national power grid of China’, Proc. CSEE, 2013, 33, (7), pp. 1–10 .
- WeiM.T.HuoF.RuanJ.J. et al.: ‘ Study on electric field distribution and grading ring's parameters design of backward V-type insulator string on UHV AC transmission line’, Adv. Technol. Electral. Eng. Energ., 2016, 35, (10), pp. 23–28 .
- WangH.Y.XiaJ.X.HuangD.C. et al.: ‘ Corona characteristics of insulator string grading ring for 500 kV substation at ultra high altitude’, High Voltage Eng., 2013, 39, (3), pp. 661–667 .
- MeiH.W.PengG.M.DaiH.Q. et al.: ‘ Installing insulation jacket to improve outdoor insulation performance of composite insulator’, IEEE Trans. Dielectr. Electr. Insul., 2012, 18, (6), pp. 2004–2012 .
- DoshiT.GorurR.S.HuntJ.: ‘ Electric field computation of composite line insulators up to 1200 kV AC’, IEEE Trans. Dielectr. Electr. Insul., 2011, 18, (3), pp. 861–867 .
- IlhanS.OzdemirA.IsmailogluH.: ‘ Impacts of corona rings on the insulation performance of composite polymer insulator strings’, IEEE Trans. Dielectr. Electr. Insul., 2015, 22, (3), pp. 1605–1612 .
- M'HamdiB.TeguarM.MekhaldiA.: ‘ Optimal design of corona ring on HV composite insulator using PSO approach with dynamic population size’, IEEE Trans. Dielectr. Electr. Insul., 2016, 23, (2), pp. 1048–1057 .
- HuangD.C.XieX.J.HuangZ.F. et al.: ‘ Grading ring parameter design and corona characteristic test arrangement of 1000 kV Compact Transmission line’, High Voltage Eng., 2013, 39, (12), pp. 2933–2942 .
- ChenY.XieX.J.XieL. et al.: ‘ Experimental study on optimizing of grading rings in 750 kV compact two circuits transmission line’, High Voltage Eng., 2008, 34, (11), pp. 2390–2394 .
- BoX.W.BianX.M.WangL.M. et al.: ‘ Optimal design of grading rings for 750 kV AC composite insulator’, High Volt. Appar., 2010, 46, (12), pp. 5–8 .
- Q/GDW 550-2010: ‘Design and construction guide for reducing audible noise of transmission lines’ , 2010.
- PhillipsA.J.KuffelJ.BakerA. et al.: ‘ Electric fields on AC composite transmission line insulators’, IEEE Trans. Power Deliv., 2008, 23, (2), pp. 823–830 .