1. Introduction
Electric power systems involve all aspects of people’s lives and are the main energy supply system in modern society [
1]. Once a power system is damaged, it has a major impact on daily life and productivity and it even endangers the national economy and social security. In electric power systems, a substation is a part of an electrical transmission and distribution system that can transform (raise or lower) voltage and perform other important functions. A typical substation structure system includes transformers, bushings, lightning arresters, switches, circuit breakers, and other devices that may be interconnected by air-insulated bare conductors strung on support structures. Among them, a high-voltage (HV) bushing is an outlet device that guides the lead lines of the high-voltage and low-voltage windings in a transformer to the outside of the fuel tank, and a line entrance surge arrestor protects substation equipment. In substations, HV bushings and lightning arrestors are often connected to in-station jumpers by “λ”-shaped down lead transmission lines (DLTLs) (
Figure 1). HV bushings and lightning arresters not only insulate DLTLs against the ground but also immobilize them, so the working performance of HV bushings and arrestors directly affects the long-term reliable operation of the whole transformer substation system.
In practical engineering, HV bushings, lightning arrestors, and the other electric devices mentioned above feature relatively heavy, high, and large structures and are mostly made of brittle ceramics, which are easily damaged under the action of earthquakes [
2]. In recent years, damage to substation facilities caused by earthquakes has occurred from time to time [
3]. In 2007, the Niigata earthquake in Japan caused a transformer to catch fire and affected the operation of the nuclear power plant nearby. In 2008, a large amount of porcelain switchgear equipment was damaged in the Wen Chu earthquake [
4]. In 2010, earthquakes in Mexico and New Zealand caused serious damage to electrical equipment such as broken insulators. In 2011, the Tohoku earthquake caused a total of 621 electrical equipment damage events [
5,
6]. On 20 April 2013, a 7.0-magnitude earthquake occurred in Lu Shan County, Ya’an City, Sichuan Province, China and caused serious losses, including damage to a total of 626 substations. It was necessary to shut down 34 substations of 35 kV and above [
7]. On 15 November 2019, an earthquake of magnitude 5.2 occurred in Jing xi City, Guangxi Province, China, which caused displacements between the porcelain jacket and flange of a main transformer HV bushing [
8]. Oil seepage occurred at the neutral point of the high- and medium-voltage sides of the bushings [
9]. Therefore, earthquake-induced failure of HV electrical equipment has become a common issue around the world.
Researchers in China and other countries have performed experimental research and theoretical analyses regarding the dynamic response of HV electrical devices under seismic loading. Gilani et al. [
10] conducted static and seismic simulator tests on two 230 kV, U-shaped transformer bushings to evaluate the seismic performance of the bushings. The Pacific Earthquake Engineering Research (PEER) Center, headquartered at the University of California, Berkeley, evaluated the standard industry procedures for seismic identification of substation equipment based on seismic simulator analysis and triaxial tests and gave some suggestions on the suitability of in-service HV transformer bushings. Wilcoski et al. proposed a Construction Engineering Research Laboratory (CERL) equipment fragility and protection program, CERL Equipment Fragility and Protection Procedure (CEFAPP), for large-scale power transformer bushing tests. Ersoy et al. simulated the interactions between transformers and bushing structures in detail using the finite element (FE) method and noted the problems warranting attention in practical engineering applications. Bellorini et al. conducted on-site dynamic tests and a numerical FE analysis on a 160 MVA 230/135 kV power transformer, evaluated the coefficients (K and R) defined by IEC 61463 and calculated the natural frequency and seismic response of the entire transformer under different seismic loads. Whittaker et al. experimentally studied the seismic performance of 196, 230, and 550 kV porcelain transformer bushings and noted the difference between the good performance of HV bushings in the laboratory and the poor performance of outdoor bushings in service. Filiatrault et al. [
11] conducted a study on four transformer bushing models through a shaking table experiment and an FE simulation and noted that HV bushings installed on transformer cover plates were more susceptible to seismic loads than those installed on a rigid foundation. This conclusion is consistent with the conjectures and research results of many other scholars. Wang et al. [
12] used a shaking table and a group of artificial seismic waves to experimentally study a certain model of a 1100 kV ultra-high voltage (UHV) gas-insulated switchgear porcelain bushing based and obtained the acceleration, displacement, and strain responses of key parts of the porcelain bushing. Sun et al. [
13] conducted a shaking table test on a 1100 kV UHV bushing and obtained the acceleration and stress responses at the most unfavorable position of the casing. Li et al. [
14,
15] studied the seismic response of surge arresters by considering the influence of the interaction of 220, 500, 750, and 1000 kV ceramic surge arresters with steel supporting structures. Jean-Bernard et al. [
16] proposed a simple and practical method for estimating the expected displacement of the interconnection points of substation equipment under earthquake action. Ghalibafian H et al. [
17] experimentally studied the dynamic characteristics of flexible conductors and the interactions between conductors and interconnection equipment using the shaking table test. Ma et al. [
18] clarified the importance of dynamic interactions between high-voltage bushings, turrets, and power transformer tanks through numerical simulations.
These studies are of great importance for the seismic analysis and optimal design of HV substation structures. However, the existing research has mostly focused separately on the dynamic characteristics or seismic responses of HV bushings, HV bushing-transformer systems, and other devices, which means that those studies do not consider the adverse impacts of coupling vibrations on HV bushings and lightning arresters in whole systems under seismic loadings. Therefore, even when HV bushings, arresters and other substation devices meet standards for design and manufacturing, their mechanical characteristics and fatigue properties may not tolerate actual complex stress states in engineering practice. However, due to the limits of laboratory test conditions and the structural complexity of the actual HV bushings, arresters, and DLTL-connection systems, the current shaking table test studies of the interactions between flexible conductors and interconnection substation equipment are based on some simple test models that are quite different from actual structures. To date, there have been very few studies on the specific characteristics and influence of the coupled vibration responses of HV bushings, lightning arresters, and DLTL (HVB-LA-DLTL) systems.
To address the topics discussed above, this study takes a 1000 kV UHV substation in Central China as an engineering example and uses ANSYS software to build an FE model of the HVB-LA-DLTL system. The time–history responses of the system under different seismic intensities are analyzed based on two groups of actual seismic records and one set of artificial earthquake waves. The results for the HVB-LA-DLTL system are compared with those for the corresponding single bushing and arrester under seismic loads. On this basis, the influence of the coupling vibration of the upper DLTL on HV bushing and lightning arrester is discussed. The purpose of this study is to provide technical support for the optimal design, maintenance, and strengthening of HVB-LA-DLTL systems in UHV substations.
Targeting the shortcomings of the abovementioned research status, this paper mainly studies the bearing performance of a certain high-voltage casing lead structure system under earthquake action. Previously, relevant research was conducted on a high-voltage casing system under wind loads on the basis of the actual engineering of a 1000 kV UHV substation in Central China. This article continues to use the same engineering background from the literature [
19], selects El Centro waves, Taft waves, and artificial standard time–history waves from the PEER database, applies X, Z one-way and three-way input. The acceleration response, displacement response, and stress response of the key parts of the high-voltage casing system under the action of three different peaks of 0.07, 0.2, and 0.4 g, are used to determine the safety factor and compare the safety factor with the seismic design code for power facilities GB50260 required in 2013.
6. Conclusions
HVB-LA-DLTL systems are complex coupling systems. The coupling effect between the HV bushings, lightning arresters, and DLTL under seismic loadings has a great influence on the force applied on the HV bushings and lightning arresters. However, the existing studies do not consider the adverse impacts of coupling vibrations on HV bushings and lightning arresters in whole systems under seismic loadings.
This paper analyzes the bearing performance of a certain high-voltage casing lead structure system and single structure electrical equipment under seismic loads. El Centro waves, Taft waves, and artificial standard time–history waves were selected for one-way and three-way input tests. The PGA is obtained from the response of the structure under six working conditions: 0.035, 0.1, 0.22, 0.07, 0.2, and 0.4 g. The following conclusions are drawn regarding the load-bearing performance:
(1) Under the action of earthquake loading, the down conductor of the upper part of the structural system has a very strong adverse effect on the lower electrical equipment. As the seismic intensity increases, the structural displacement increases, and a lack of redundancy of the down conductor causes the high-voltage casing and the arrester to pull against each other and quickly amplifies their respective dynamic responses in a nonlinear form;
(2) The down conductor has a greater impact on the high-voltage casing than on the arrester. When the structure is under the influence of the PGA = 0.4 g earthquake, the maximum stress at the root of the high-voltage casing in the system increases by approximately 13.02% compared to that of the single body. The maximum stress at the root of the arrester increases by approximately 7.80%. The down line has a considerably greater effect on the magnification of the structure in the X-direction than in the Z-direction. Sufficient redundancy during the design of the down conductor is recommended;
(3) When the PGA is 0.4 g, the safety factor of the root stress of the high-voltage casing under horizontal input is less than the 1.67 specification required by the standard GB50260-2013. According to the particular features of the high-voltage casing lead system and local seismic fortification requirements, the adoption of appropriate dampers and other shock-absorbing equipment are recommended;
(4) In response to the application of one-way and three-way seismic inputs in the X- and Z-directions, the most vulnerable location in the structure under earthquake action is at the root of the high-pressure casing. Comparing the three-way and the one-way inputs shows that the response of the structure greatly increases under three-way input, and the structure deforms substantially. In the design process, three-way and one-way inputs should be calculated separately for envelope design.