Gas Insulated Metal Enclosed Transmission Line (GIL) Seismic Response Analysis

The seismic response analysis of a 550 kV gas-insulated metal-enclosed transmission line (GIL) is carried out to address the seismic performance issues in the design of the GIL. Firstly, the GIL is modeled in three dimensions, and the pre-stressed modal analysis is conducted to obtain the vibration characteristics of the GIL structure. Secondly, the seismic response spectrum analysis is conducted according to the national standard, and the seismic characteristics are obtained to meet the requirement of resisting an earthquake of magnitude 9, but there are problems of relatively large stress distribution and small safety coefficient of the brace. Finally, it is proposed to change the cross-section of the bracket to strengthen the stiffness of the bracket to improve the seismic performance of the GIL equipment, and simulations are carried out to verify the results, which show that increasing the cross-section of the bracket can improve the seismic performance of the equipment.


Introduction
Transmission lines are the vital lifeline of modern power systems, so the use of safe and efficient transmission methods and transmission lines has become crucial.Since the 1970s, gas-insulated metalenclosed transmission lines have been gradually put into use around the world.Gas-insulated metalenclosed transmission line (GIL) is a kind of high-voltage, high-current power transmission equipment insulated with SF6 gas or SF6/N2 gas mixture.It has the advantages of large transmission capacity, low environmental impact, high operational reliability, low unit loss, and land saving [1][2][3].
GIL is mainly used in hydroelectric power stations, thermal power stations, pumped storage power stations, nuclear power stations and other power stations, the high voltage side of the main transformer to the GIS connection or GIS outlets, the bus contact within the substation, avoiding important buildings, attractions, mountains and rivers and other places instead of overhead lines or power cables.Due to its excellent performance, GIL can be adapted to the application scenarios of urban comprehensive pipeline corridors, power tunnels, cross-river and cross-highway, and high vertical drop.Depending on the application, GIL can be laid in different ways.The laying methods of GIL in existing projects include tunnel laying, surface laying, overall burial, overhead laying, vertical or inclined shaft laying, and direct burial laying, etc. [4].Earthquake is one of the natural disasters that seriously threaten the safe operation of electric power systems, and electric power facilities have suffered serious damage in earthquakes over the years, so the seismic performance of electrical equipment needs to be focused on [5].During the actual operation of the ultra-high voltage (UHV) GIL, it may face the influence of harsh environments such as extreme temperature, high altitude, windy and sandy, humidity, etc., and is also threatened by earthquakes.If the internal structure of GIL is not reasonably set up, it may cause the entire long-distance pipeline to be plastically and irreversibly deformed due to excessive stress leading to serious damages such as air leakage or even pipeline rupture [6,7].Compared with the overhead transmission tower-line system, GIL is more similar to oil and gas transmission pipelines in terms of appearance and application environment, however, GIL is a coaxial cylindrical structure consisting of inner conductor, support insulator, and outer shell, which makes its vibration response special, and it is difficult to apply the vibration response distribution characteristics of the overhead tower-line system and the conventional pipeline structure as well as the research results to GIL directly [8].Li [6] established the modeling of 550kV GIL equipment and performed seismic response analysis by response spectrum method, which obtained that the GIL equipment has a certain safety coefficient under the action of 7-degree earthquake.
In this paper, we will establish a refined finite element model of the selected section of 550kV GIL line, calculate its seismic response spectrum analysis by using ANSYS Workbench, analyze its weak parts and explore the improvement measures to enhance the seismic performance of GIL equipment.

Modeling of GIL Structure
In this paper, the application scenario of GIL is to simulate the application scenario of underground pipeline corridor, such as figure 1 for a GIL pipeline corridor project.
In this paper, the 550 kV GIL equipment consists of typical structures such as linear unit (split-phase), gas isolation unit (split-phase), compensation unit (split-phase), and corner unit (split-phase), which specifically include components such as 18m linear unit, 16m compensation unit, 12m linear unit, basin insulators, and brackets.The overall model of GIL is established through SOLIDWORKS, as shown in figure 2. The total length of the model is 60.57m, the width is 13.79m, and the three-phase line is arranged vertically with a single GIL line, which is fixed by the bracket.

Finite Element Analysis Setup
Before performing the seismic response spectrum analysis, the prestress modal analysis should be conducted.The GIL model is imported into the static analysis module in ANSYS Workbench, and the material properties of each part of the GIL are set, as shown in table 1.The analysis model is imported externally, and the beam unit and shell unit are no longer used in ANSYA for simplification, and the purpose of reducing the error can also be achieved by directly using the imported model for calculation.
The boundary conditions and connections in the model are mainly three parts [9]: firstly, the connection between the bottom of the bracket and the ground; secondly, the connection between the hoop pads on the bracket and the GIL busbar barrel; thirdly, the connection between the GIL busbar barrel and the three-pillar type insulator.The modeling of these three connections is described in detail below, with all other contacts defaulting to fixed contacts.
Referring to the actual application scenario, the connection between the bottom of the bracket and the ground is directly set as binding contact.The bracket of GIL can be divided into two categories, fixed bracket and sliding bracket, according to the connection method with the enclosure.Fixed brackets are generally distributed at the corners or ends of the GIL, while sliding brackets are generally distributed along straight lines in the GIL lines [10].There are 11 brackets in the model, which contain 3 fixed brackets and 8 sliding brackets, of which the fixed brackets are marked by red boxes in figure 2 and the rest are sliding brackets.The fixed bracket holds the GIL busbar barrel directly by bolting and the connection between them is set up for binding contact.While the sliding bracket retains the degree of freedom of the GIL line along the axial direction, so the connection between them is set up as a frictional contact with a friction coefficient of 0.15.The three-pillar insulators in the model are considered as fixed three-pillars, thus the connection between the GIL busbar barrel and the three-pillar insulators is directly set up as a binding contact.Once the contact setup in the model is complete, the next step is to set up the boundary conditions of the model.Considering the role of the earth's gravitational force, the standard gravitational acceleration is first applied to the GIL model.Then, displacement constraints are applied to the ends of the conductive rods and busbar barrels at both ends of the GIL model to simulate the actual working conditions in which the ends of the GIL equipment are constrained.In addition, the mass of SF6 gas that should be filled in the model is about 1500 kg, and the mass of SF6 gas is added to the busbar barrel in the form of distributed mass.The mesh delineation and the calculation of the static structure are carried out after the boundary conditions are set.

Prestress Modal Analysis of GIL Model
The modal analysis setup requires attention to the selection of the modal order.The method for determining the modal analysis order n in ANSYS Workbench is the modal mass participation factor method, where the modal mass participation factor   is defined as the ratio of the effective modal mass participating in the combination of modes to the total mass of the structure: where   is the jth order effective modal mass; and   is the overall model mass.
In engineering, when   ≥0.80, it is considered to meet the requirements of modal analysis.After several modal analysis calculations of the model, it is found that the mass participation coefficient of the model in the X and Z directions is greater than 0.85 when n=300, and the mass participation coefficient in the Y direction is close to 0.8.Each vibration mode of the GIL model exhibits bending deformation along the X, Y, and Z directions, respectively.For example, the 19th-order vibration mode, 21st-order vibration mode and 53rd-order vibration mode have significant bending deformation in the X, Y and Z directions, respectively, are shown in figure 3. The natural frequency range of the model's first 300 orders is 5.35Hz~67.56Hz,which satisfies the experimental excitation frequency range required by "Seismic Requirements for High Voltage Switchgear and Control Equipment" [11] should be in the range of 0.5 HZ to 35 HZ.

Seismic Wave Selection
Currently, the main methods for seismic analysis of electrical equipment are time course analysis and response spectrum analysis [12].The response spectrum method can reflect the influence of seismic dynamic characteristics and structural dynamic characteristics on structural seismic response, and can determine the maximum response value of the structure [13].The GIL structure studied in this paper is relatively complex, and the response spectrum method can be used to quantitatively grasp its dynamic characteristics and seismic response rules.
According to the provisions of the load spectrum in GB/T13540-2009, the horizontal ground acceleration is selected as AG5: ZPA = 5m/s 2 (0.5g), and the damping ratio is 0.05, as shown in table 2. For the seismic level in the vertical direction, it is calculated and verified according to the standard as 0.5 times in the horizontal direction [14].Response spectrum analysis with a combination of input excitation directions in X+Y direction and Z+Y direction is chosen to analyze the structural dynamic characteristics and seismic response of the GIL.

Analysis of Seismic Response Spectrum Results
According to the calculated modal frequency, it can be determined that the vibration mode of the GIL model belongs to the dense mode, and the CQC modal combination method and multi-point excitation input should be adopted [15].The damping ratio of the model is selected to be 5 %, then the mass damping coefficient α of the model can be calculated to be 0.2806, and the stiffness damping coefficient β is 0.00889.

EPES-2023
Journal of Physics: Conference Series 2731 (2024) 012021 The working load of structural seismic response spectrum calculation is divided into X + Y seismic condition and Z + Y seismic condition.The load under X + Y seismic conditions includes X-direction horizontal load spectrum, Y-direction vertical load spectrum and structural weight.The load under Z + Y seismic conditions includes Z-direction horizontal load spectrum, Y-direction vertical load spectrum and structural weight.The response spectrum analysis results of GIL structure by ANSYS Workbench include displacement response and stress response, as shown in figure 4. The stress safety factor of GIL lines under seismic loading is 10, which indicates that it is safe under the standardized AG5 seismic standard loading spectrum specified in the standard, and thus it can be determined that the GIL equipment can resist earthquakes of magnitude nine [16].However, the stress distribution of the braces in the GIL equipment is relatively large, and the safety factor is just 2.35.Therefore, the focus of improving the seismic performance of the GIL equipment can be placed on the strengthening and improvement of the braces.

GIL Seismic Performance Analysis
The seismic capacity of GIL depends on the strength, elasticity and damping capacity of the equipment structure and materials.Measures to improve the seismic capacity of GIL include: 1) Reduce the quality of the equipment; 2) Reduce the center of gravity and the total height of the equipment as well as reduce the mounting height of the equipment; 3) Adopt suitable mounting brackets, such as adjusting the height of the bracket and the bracket cross-section; 4) Install additional vibration dampeners and dampers between the base of the equipment and the mounting pedestal.
According to the analysis of GIL seismic response spectrum results, this paper enhances the seismic capacity of the equipment by changing the cross section of the bracket.The original equipment bracket is an I-beam structure, and now the bracket profile is changed from I-beam to square tube, as shown in fgure 5.This measure improves the cross-sectional area and flexural stiffness of the bracket, which makes the overall stiffness of the GIL equipment rise, thus reducing the vibration response of the equipment under seismic action.It can be seen by comparing figure 6 (a) with figure 4 (a) that the overall displacement of the GIL equipment was reduced after the bracket was changed from I-beam profile to square tube structure.Under the seismic condition in the X+Y direction, the vibration displacement response of the GIL equipment is reduced by 16.94%, 19.41%, and 11.87% in the X, Y, and Z directions, respectively, and the stresses in the GIL line and bracket are reduced by 16.75% and 27.44%, respectively.Under the seismic condition in Z+Y direction, the vibration displacement response of the GIL equipment is reduced by 15.88%, 13.27% and -9.66% in X, Y and Z directions, respectively, and the stresses in the GIL line and bracket are reduced by 29.78% and 8.12%, respectively.The slight increase in displacement in the z-direction may be due to the fact that the inherent frequency of the overall model changes after the bracket structure is changed, resulting in a certain inherent frequency close to the seismic spectral frequency, which makes the z-direction displacement results slightly increased.In conclusion, it is of great practical significance that the seismic performance of the brace can be significantly improved by increasing its cross-section size.

Conclusion
In this paper, 550kV GIL lines are modeled and analyzed by static and modal analysis.On this basis, the mechanical properties of GIL lines under seismic loads are studied, and measures to improve the seismic performance of GIL equipment are proposed and verified.The main conclusions are as follows: (1) The stress and deformation values of the GIL lines are within the permissible range under the standardized AG5 seismic loads.It shows that the overall structure of the GIL equipment is reasonable, and the bracket has a good restraining effect on the equipment, which effectively reduces the deformation of the long GIL busbar.
(2) Observing all the GIL simulation results, it can be realized that the maximum displacements are basically concentrated in the middle of the GIL equipment at the conductive rods and busbar barrels, while the maximum stresses are all concentrated in the brackets.Therefore, these two parts are the weak parts of the seismic resistance of GIL equipment and should be focused on.
(3) The analysis of the stress results of the bracket can be realized that the maximum stress is in the bracket near the connection between the crossbeam cantilever and the vertical frame, which is the stress concentration area can be strengthened, the rest of the parts are safe.
(4) Changing the structure of the bracket profile and increasing the bracket cross-section area, the maximum displacement and maximum stress of the GIL line are decreased.Therefore, increasing the bracket cross-section can improve the seismic performance of the equipment.
This conclusion is derived from the simulation calculation of GIL simplified model, which has certain limitations, and will be further investigated in future research to address this problem.

Figure 1 .
Figure 1.A GIL integrated pipe corridor project.Figure 2. The overall model of GIL.

Figure 2 .
Figure 1.A GIL integrated pipe corridor project.Figure 2. The overall model of GIL.

Figure 4 .
Figure 4. Displacement and stress response cloud diagram of GIL structure under seismic actions: (a) displacement cloud diagram of GIL equipment; (b) The stress cloud diagram of GIL line; (c) The stress cloud diagram of the support.The maximum displacements of the GIL structure in the X, Y and Z directions under the X+Y seismic condition were 12.718 mm, 4.3878 mm and 0.8238 mm, respectively, and the maximum displacements in the X, Y and Z directions under the Z+Y seismic condition were 2.4916 mm, 1.2578 mm and 2.2879 mm, respectively.The maximum stresses of the GIL line in the X+Y seismic condition were 17.453 MPa and 7.6433 MPa in the Z+Y seismic condition.The maximum stresses of the braces in the X+Y seismic condition were 100.99 MPa and 7.6433 MPa, respectively.The maximum stress of the GIL line is 17.453 MPa in X+Y direction and 7.6433 MPa in Z+Y direction, and the maximum stress of the brace is 100.99MPa in X+Y direction and 16.462 MPa in Z+Y direction.The stress safety factor of GIL lines under seismic loading is 10, which indicates that it is safe under the standardized AG5 seismic standard loading spectrum specified in the standard, and thus it can be determined that the GIL equipment can resist earthquakes of magnitude nine[16].However, the stress distribution of the braces in the GIL equipment is relatively large, and the safety factor is just 2.35.Therefore, the focus of improving the seismic performance of the GIL equipment can be placed on the strengthening and improvement of the braces.

Figure 5 .Figure 6 .
Figure 5. Bracket cross-section: (a) Original model bracket cross-section (I-beam); (b) Modified bracket cross-section (square tube); (c) Bracket with modified cross-section.In order to compare the results with the previous ones, the seismic excitation spectrum and the direction combination of loads are not varied.The load combinations in X+Y and Z+Y directions are still used, and the results of the response spectrum calculation are shown in figure6which lists the rate of change of each response after the modification of the bracket cross-section.

Table 1 .
GIL main material parameters.