Research on vibration response of 550 kV GIL isolation compensation unit against earthquake loading

To analyze the vibration response of Gas Insulated Transmission Lines (GIL) of urban power grid under different seismic intensity levels, based on the actual structure of the standard isolation compensation unit (isolation unit and compensation unit) of 550kV GIL, A fine 3D model of isolation compensation unit segment of 550 kV GIL was established, and the actual vibration response of each standard unit segment of GIL under different seismic conditions was studied by response spectrum analysis. This study clarified the action mechanism and law of GIL vibration characteristics under seismic conditions of urban power grids, provided the relative support for the research of shock absorption of GIL, and was great significance to the actual project of GIL.


Introduction
With the rapid increase of electricity consumption in urban areas, urban power transmission needs to overcome many challenges such as small footprint and safety to establish a new power transmission mode.GIL is a gas insulated transmission line, and its insulating medium is generally SF6 gas or N2 and SF6 mixed gas, which has a wide development space and application prospect [1][2][3].Because GIL uses compressed gas as the insulating medium, its application is not affected by the external environment, and it has become the first choice for long-distance transmission in the complex operation environment of urban power grids.
Earthquake load is a great threat to the safe operation of power grid, so the research on the seismic performance of electrical equipment has received extensive attention [4][5][6].Recently, the domestic research on GIL may focuses on the electrical performance, and the research on its seismic performance is relatively few.Zhang Lihong et al. used Vibration-type decomposition reaction spectroscopy and Power time course analysis to analyze earthquake response, providing reference for GIL earthquake response analysis modeling method [7].Li Qingmin et al. used the response spectrum analysis method to carry out relevant research on the seismic performance of 220 kV GIL, and further analyzed and summarized the seismic response rules of the weak parts of GIL [8].Li Yuting et al. analyzed the structural seismic response under the excitation of different seismic response spectra by response spectrum analysis [9].Li Xiaoxuan et al. took GIL of a converter station as the research object to study the vibration response of GIL under earthquake excitation in different directions, and obtained the seismic response characteristics and seismic weak position of GIL [10].Research on the vibration response of 550 kV GIL is scarce, so was great significance to study the vibration response rule of 550 kV GIL under different seismic conditions for the the actual project of GIL.
Therefore, aiming at the vibration response problem of 550 kV GIL operating under seismic conditions, a three-dimensional solid model of GIL standard isolation compensation unit segment was established, which was calculated and analyzed by using finite element software based on response spectrum analysis method, to master its seismic weak parts and study its seismic response characteristics.

Finite element model building
GIL transmission pipeline is a coaxial cylinder system.Based on the actual structure size of 550 kV GIL transmission pipeline, this paper constructs a finite element model of 550 kV GIL 14m isolation compensation element segment.The GIL standard isolation compensation unit consists of a conductor, fixed three-post insulator, sliding three-post insulator, bellows, basin insulator and housing.In order to reflect the vibration response characteristics of GIL under earthquake excitation as much as possible, the model is simplified reasonably considering the calculation accuracy.The material is mainly aluminum alloy, and the specific material structure parameters are shown in Table 1.The isolation compensation unit segment model and part model are shown as follows (Figure 1), (a) refers to the Isolation compensation unit, (b) refers to the Basin insulator, (c) refers to the bellows:

Analysis of GIL response spectrum under different seismic intensity
Based on the modal analysis results, the seismic response spectrum excitation is applied to GIL standard isolation compensation unit for response spectrum analysis.After identifying the relevant standards, the seismic response analysis of GIL is carried out in Shenzhen area as an example.The seismic intensity level in Shenzhen area is 7 degrees, the designed basic seismic acceleration is 0.1g, and the designed earthquake group is the first group.In order to better reflect the change of vibration response of GIL under seismic intensity, this paper selects 7, 8, 9 earthquake grades for response spectrum analysis and calculation.The seismic wave contains shear wave component and longitudinal wave component.In this paper, the vertical excitation is 0.65 times the horizontal excitation amplitude.
Where Y is the direction perpendicular to the axis and X is the horizontal axis direction. 3

Analysis of GIL response spectrum at magnitude 7 earthquake intensity
The acceleration spectra in X and Y directions at magnitude-7 seismic intensity are applied to the response spectrum module, and the total deformation and equivalent stress of the compensation isolation unit are analyzed.The comparison of GIL vibration response under X and Y direction seismic excitation input is shown in Figure 2, Figure 2  Under the action of seismic acceleration spectrum in the X direction, the maximum deformation occurs on the conductor, the dx is 0.3821mm, the fx is 0.4786MPa, and the minimum equivalent stress is 0.1125Pa.Under the action of Y-direction seismic acceleration spectrum, the dy is 0.10161mm, the fy is 1.7878MPa, and the minimum stress is 2.887Pa.The dy occurs at the bellows, and the fy occurs at the contact between the three-pillar insulator and the housing.Under seismic excitation, dx>dy, and fy > fx.

Analysis of GIL response spectrum at magnitude 8 earthquake intensity
The acceleration spectra in X and Y directions with magnitude 8 seismic intensity are applied to the response spectrum module, and then the total deformation and equivalent stress of the linear element are analyzed.The comparison of GIL vibration response during seismic excitation input in X and Y directions is shown in Figure 3, Figure 3 (e) refers to the total deformation dx in the X direction, Figure 3 (f) refers to the equivalent stress fx in the X direction, Figure 3 (g) refers to the total deformation dy in the Y direction, Figure 3  Under the action of X direction seismic acceleration spectrum, the dx is 0.76419mm, the maximum fx is 0.95693MPa, and the minimum stress is 0.22756Pa.Under the action of Y-direction seismic acceleration spectrum, the dy is 0.20699mm, the fy is 3.6315MPa, and the minimum is 5.8545Pa.The maximum displacement is consistent with the response rule under the maximum stress and magnitude 7 seismic intensity.The dy occurs at the bellows, and the fy occurs at the contact between the three-pillar insulator and the shell.Under seismic excitation, dx>dy, fy > fx.

Analysis of GIL response spectrum at magnitude 9 earthquake intensity
The acceleration spectra in X and Y directions at magnitude-9 seismic intensity are applied to the response spectrum module to analyze the total deformation and equivalent stress of the linear element.The comparison of GIL vibration response under X and Y direction seismic excitation input is shown in Figure 4, Figure 4 (u) refers to the total deformation dx in the X direction, Figure 4 (v) refers to the equivalent stress fx in the X direction, Figure 4 (w) refers to the total deformation dy in the Y direction, Figure 4 (z) refers to the equivalent stress fy in the Y direction: Under the action of X direction seismic acceleration spectrum, the dx is 1.5284mm, the maximum fx is 1.9139MPa, and the minimum stress is 0.45014Pa.Under the action of Y-direction seismic acceleration spectrum, the dy is 0.40643mm, the fy is 7.1514MPa, and the minimum is 11.548Pa.The fy occurs at the contact between the three-pillar insulator and the shell, and the maximum displacement response occurs at the conductor.Under seismic excitation, dx>dy, fy > fx.

Comparative analysis of GIL vibration response under different seismic intensities
Figure 5 (a) and (b) show the maximum displacement and maximum equivalent stress data of GIL equipment under different seismic excitations.In the vibration response caused by external motivation in all directions, the maximum vibration response occurs at the contact between conductors, insulators and conductors, housing and bellows, and the basin insulators are relatively stable.Considering the order of magnitude of mechanical strength of all materials, it can be judged that insulators and conductors are the weak points in the seismic response of Gill structures.The results of vibration response under different seismic intensities are consistent.The maximum stress occurs at the contact between insulator and shell/conductor in the Y direction, and the maximum displacement occurs at the conductor.When GIL structure is stimulated by earthquake, dx>dy and fy>fx are obtained.The main factors affecting vibration response of isolation compensation unit are the parameters of insulator, conductor and bellows, that is, the elastic modulus and so on.

Conclusions
In this paper, three-dimensional numerical simulation is used to analyze the response spectrum of the seismic response of the isolation compensation unit of 550 kV GIL in actual engineering, and the vibration response rule of the isolation compensation unit is obtained.The research shows that: (1) Vibration response results are consistent under different seismic intensities.The maximum stress occurs at the contact between insulator and shell/conductor in Y direction, and the maximum displacement occurs at conductor; (2) When GIL structure is stimulated by earthquake, dx>dy and fy>fx; (3) The main factors affecting the vibration response of the isolation compensation unit are the parameters of the insulator, conductor and bellows, that is, the elastic modulus and other factors.

Figure 1 .
Figure 1. Isolation compensation unit structure model

Figure 2 .
(a) refers to the total deformation dx in the X direction, Figure 2 (b) refers to the equivalent stress fx in the X direction, Figure 2 (c) refers to the total deformation dy in the Y direction, Figure 2 (d) refers to the equivalent stress fy in the Y direction: Isolation compensation unit deformation and equivalent stress under magnitude 7 earthquake excitation.

Figure 3 .
Figure 3. Isolation compensation unit deformation and equivalent stress under magnitude 8 earthquake excitation

Figure 4 .
Isolation compensation unit deformation and equivalent stress under magnitude 9 earthquake excitation.

Figure 5 .
Figure 5.Comparison of GIL vibration response under different seismic conditions.