Research on Isolation Design of Ultra-High Voltage Converter Transformer

To assess the performance of the bushings in UHV converter transformers, a finite element time analysis was performed on a ±800 kV UHV converter transformer with or without the implementation of horizontal seismic isolation devices. The simulation results show that the maximum stress response value of the root of the alternating current bushing is 68.96 MPa, when the horizontal seismic isolation device is not installed, and the stress response values of the root of the two direct current bushing are close, with 26.06 MPa and 25.89 MPa respectively; when the direct current bushing is not vertically arranged, the angle between the direct current bushing axis and the horizontal X-direction is smaller, the relative displacement response is smaller in the X-direction; After installing the horizontal with seismic isolation device, the stress at the root of the bushing is significantly reduced compared with the without seismic isolation results, and the peak stresses at the root of the alternating current bushing, direct current bushing-1 and direct current bushing-2 are reduced with seismic isolation: 72.17%, 51.76% and 51.49%, respectively, all of which meet the requirement of seismic isolation efficiency greater than 50%. The research results provide technical support for the seismic resistance of UHV converter transformers and promote seismic research and the seismic isolation level of power facilities.


Introduction
The transformer, as the key electrical equipment in converter stations, whose damage occurred under earthquakes, will produce a large area of power supply interruptions in the power system, resulting in serious economic losses.Transformer equipment damage is mainly in the form of tilting and displacement of the body, or fracture or dislocation of the high-voltage bushing, especially if the bushing earthquake vulnerability is high.Therefore, the study of seismic isolation techniques that are applicable to the transformer category is of great importance, which can guarantee the secure functioning of the power grid in the event of an earthquake catastrophe.
Extensive and comprehensive research on seismic isolation technology for transformers has been carried out by scholars both domestically and internationally.Ersoy et al. conducted research on the simultaneous effects of high-voltage transformers under earthquakes.They utilized finite element simulation and elastic time analysis to determine the seismic response time of multiple components of transformers and their bushings [1] .Zhang and Zhang gave a detailed solution for the main transformer of a domestic Ultra-High Voltage substation in terms of installing seismic isolation devices [2] .After the prototype of the 750 kV transformer adopts foundation isolation, Li et al. found that the average seismic damping efficiency of the equipment body was not less than 60%, and the average seismic damping efficiency of the bushings was not less than 80% by the finite element software [3] .Ma et al. conducted a study on the effectiveness of the composite seismic isolation bearing system in reducing the acceleration and strain response of a 220 kV transformer-bushing system during an isolation shaking table test.The results showed that the isolated system experienced less than half the acceleration and strain response of the un-isolated system.However, there was some amplification of the displacement response in the isolated system [4] .Xie et al. used ±200 kV and ±800 kV converter transformers as subjects, and the strain response of the bushing was reduced by about 50% after using friction pendulum isolation, and the displacement response was not amplified [5] .The University of Buffalo conducted seismic isolation tests on a steel braced-bushing system using a triple repetition friction pendulumvertical spring damping element system and found that the use of friction pendulum bearings-vertical damping elements improved the safety of the bushing under seismic conditions [6] .
The research findings have led to the development of a finite element model for a ± 800 kV converter transformer of the true type.This model has been utilized for performing seismic and seismic isolation calculations, and the data results after comparing the simulation calculations were used to analyze the seismic isolation effect of horizontal seismic isolation devices on the UHV converter transformer equipment.Thus, it is to provide some technical support for the research of seismic and seismic isolation technology of UHV converter transformers.

Equipment Introduction
The subject of analysis pertains to the equipment utilized for a ±800 kV UHV converter transformer, which weighs 542.2 tons in total.It consists of the body, heat sink, oil pillow, two direct current (DC) bushings, one alternating current (AC) bushing and other parts.Figure 1 gives a schematic diagram of a typical ±800 kV converter transformer structure.

Finite element analysis
According to the seismic fortification level in China, the maximum acceleration of the substation in the existing seismic design code is 0.4 g.The horizontal acceleration of 0.4 g is also used in the finite element calculation, and the combination ratio of horizontal XY seismic action is 1:1.

Seismic analysis
Figure 2 illustrates the finite element model of the converter transformer, and the X-direction and Ydirection are indicated.When 0.4 gX+0.4 gY horizontal bidirectional combined seismic wave is applied, the maximum stress value at the root of each bushing and the peak relative displacement value in each direction at the top of the bushing are extracted by simulation calculation.Among them, the stress time history at the AC bushing root is depicted in Figure 3, and Table 1 shows the peak stress value at the root of each bushing and the peak relative displacement in each direction of the top horizontal direction.The peak stresses at the roots of the AC bushing, DC bushing-1 and DC bushing-2 are 68.70MPa, 26.04 MPa and 25.39 MPa, and the peak stress values at the roots of the two DC bushing are close; the peak values of relative displacements in the X-direction and Y-direction of the AC bushing are 26.08 mm and 33.36 mm, which indicates that the relative displacements in the Y-direction of the AC bushing are more sensitive for seismic waves; The peak relative displacement values in the X-direction and Ydirection at the top of valve side bushing-1 are 75.74mm and 213.99 mm, and the maximum displacement value in the Y direction is 2.83 times that in the X direction; The peak relative displacement values in both directions at the top of valve side bushing-2 are 28.63 mm and 214.16 mm, and the peak displacement value in the Y direction is 7.48 times that in the X direction.This indicates that when the DC bushing is not vertically arranged, its Y-direction is more sensitive to seismic waves.While for the peak relative X-direction displacement, when the angle between the DC bushing axis and the horizontal X-direction is smaller, the relative displacement response is smaller.

Seismic isolation analysis
From the results of the seismic calculation analysis, a seismic isolation device needs to be installed at the bottom of the converter transformer box to reduce the stress response.A total of 32 rubber isolators with an effective diameter of 200 mm are symmetrically arranged at the bottom of the box, and the equivalent stiffness of 250% horizontal shear deformation is selected.The parameters of the rubber isolator are listed in Table 2.The isolation finite element model is shown in Figure 4.A horizontal artificial seismic wave is applied to the finite element model for Figure 4, which is extracted the maximum stress value at the root of the bushing and the maximum value of relative displacement at the top.The maximum stress value of each bushing at the root and the maximum value of relative displacement at the top are shown in Table 3.As can be seen from the above chart, after the installation of the vibration isolation device, the peak stress values of the AC bushing, valve side bushing-1 and valve side bushing-2 are 19.19MPa, 12.57 MPa and 12.56 MPa, respectively, and that in the X-direction and Y-direction of the AC bushing is 89.08 mm and 142.74 mm, respectively; the peak values of X-direction and Y-direction displacements of DC bushing-1 are 101.72mmand 224.01mm respectively; the peak values of X-direction and Ydirection displacements at the top of DC bushing-2 are 89.76mm and 223.86mm, respectively.
Figure 5 and Figure 6 show the comparison of seismic results and seismic isolation results for the bushing root stress in the converter transformer.Table 4 shows the isolation efficiency for the peak stress and the amplification factor for bushings.
where Ș is the seismic isolation efficiency; Y V is the peak stress of the bushing with isolation; N V is the peak stress of the bushing without isolation.
In the equation, Į is the amplification factor; Y ' is the peak relative displacement of the bushing with isolation; N ' is the peak relative displacement of the bushing without isolation.As can be seen from the above graphs, the stresses at the root of the bushing are reduced significantly compared to the seismic results after the seismic isolation device is used.The peak stress at the root of the AC bushing, DC bushing-1, and DC bushing-2 are reduced by 72.17%, 51.76%, and 51.49%, compared with the seismic results.All meet the requirement of seismic isolation efficiency greater than 50%.The variation range of the ratio (amplification factor) between seismic isolation results and seismic results for the maximum relative displacement is between 1.05 and 4.38 at the top of the AC bushing, DC bushing-1 and DC bushing-2.That is, for the peak relative displacement of the bushing, the seismic isolation calculation outcomes exhibit varying degrees of amplification effect in contrast to the calculation results.

Conclusion
By conducting finite element analysis on the ± 800 kV Ultra-High Voltage converter transformer model with or without seismic isolation devices, the following conclusions can be derived: (1) Under the excitation of combined horizontal seismic waves, the stress of the AC bushing is the largest, and that of the two DC bushings are small, and the values are close to each other.
(2) The Y-direction relative displacement of each bushing is more sensitive to the seismic wave.When the DC bushing is not vertically arranged, the angle between the bushing axis and the horizontal X-direction is smaller, the X-direction relative displacement response is smaller.
(3) After installing the seismic isolation device, the stress at the root of the bushing is significantly reduced compared to the seismic calculation results.The peak stress at the root of the AC bushing, valve side bushing-1, and valve side bushing-2 are reduced by 72.17%, 51.76%, and 51.49%, compared to the seismic calculation results.The requirement of isolation efficiency meets not less than 50%.
(4) After the installation of the seismic isolation device, the relative displacement results are magnified to varying degrees, compared to the seismic calculations.The amplification factor varies between 1.05 and 4.38.

Figure 1 .
Figure 1.The structure diagram of ± 800 kV converter transformer

Figure 4 .
Figure 4.The isolation Finite element model

Figure 5 .Figure 6 .
Figure 5.Comparison of the AC bushing stress with/without isolation

Table 1 .
Calculation results of each bushing.

Table 2 .
Parameters of the rubber isolation device

Table 3 .
Seismic isolation calculation results of each bushing

Table 4 .
Table of seismic isolation efficiency and amplification factor