Transformer Bushing Research Articles

Regularity analysis of the temperature distribution of epoxy impregnated paper converter transformer bushings

The converter transformer epoxy impregnated paper bushing is a multi-media electrical equipment, and the main materials include the internal conductor, the external conductor, transformer oil, SF6 gas, the air inside internal conductor, the main insulation material epoxy impregnated paper, sheath, and fittings. In actual operating, the converter transformer bushing is undertaken by the harmonic current with large number of high harmonics and the temperature distribution of the epoxy impregnated paper converter transformer bushings under actual current waveform needs to be researched. What's more, the temperature distribution of the epoxy impregnated paper converter transformer bushings is affected by several factors and to provide basis for the optimization of bushings it's necessary to research about the impact of these factors on the temperature distribution of the epoxy impregnated paper converter transformer bushings. In this paper, on the basis of our previous paper [8], the 3-D temperature distribution of the 400 kV epoxy impregnated paper converter transformer bushing under actual current waveform with large number of harmonics has been simulated by using the computational fluid dynamics (CFD) and finite element method (FEM) analysis. The temperature distribution regularity of the 400 kV converter transformer bushing under different influencing factors has been computed and analyzed. The dissipation structure of the bushing has been optimized and the DC steady-state electric field distribution of the 400 kV converter transformer bushing has been obtained considering the temperature gradient in epoxy impregnated paper.

Heat analysis of the power transformer bushings in the transient and steady states considering the load variations

Bushings are considered as one of the main components of the power transformers. This is due to the fact that the reliable and efficient performance of the bushings significantly influences the reliability of the transformers which, in turn, are counted as the fundamental part of the power systems. The thermal behaviour of the bushings, as a consequence of the power dissipation, significantly affects the bushing’s performance and design. In this paper, a novel algorithm is proposed for the heat analysis of the bushings. This algorithm aims at specifying the hot spot temperature and the temperature distribution of the bushing. Therefore, the problem is formulated using the Joule Heating equations as the partial differential equations for both transient and steady states. Then, the finite element method is employed to solve the problem. The algorithm is implemented on an oil-impregnated paper bushing. In the steady state, the temperature distribution of the whole bushing is determined. In addition, the effects of different currents on the location and magnitude of the hot spot temperature have been investigated. In the transient state, three overload scenarios are simulated two of which are from the IEEE C57.19.100 and IEC 60076-7 standards. In addition, the effects of the wind speed and electric load on the bushing’s temperature distribution for 48h have been investigated.

Application of digital image processing to detect transformer bushing faults and oil degradation using FRA polar plot signature

Frequency Response Analysis (FRA) technique is commonly used to assess the mechanical integrity of power transformer core and windings. A few papers investigating the ability of FRA technique to detect bushing faults and transformer oil degradation can be found in the literatures. However, in all of these papers, only magnitude of the FRA signature along with visual analysis was used for fault identification and quantification which may lead to inconsistent conclusions for the same FRA signature when interpreted by various personnel. As such, there is an essential need to standardize and automate the FRA interpretation process to improve its reliability and accuracy. This paper is taking a step forward toward this goal through presenting a new FRA interpretation approach by incorporating the magnitude and phase angle plots of the measured FRA signature into one polar plot that comprises more features than the conventional magnitude plot currently used for fault diagnosis. Moreover, the proposed polar plot facilitates the application of digital image processing techniques (DIP) to automate and standardize the whole process. The new proposed technique is assessed through its application to detect minor transformer bushing faults and insulating oil degradations. In this regard, two transformers of different ratings, winding structure and physical dimensions are simulated using 3D finite element analysis to implement various levels of transformer bushing faults and oil degradation. The obtained FRA polar plots of the two investigated transformers under various bushings and oil health conditions are manipulated using the developed DIP codes to investigate the impact of each fault type / level on the proposed polar plot signature. Also, Practical measurements are performed to validate simulation results and evaluate the feasibility of the proposed technique. Results show the ability of the proposed technique to automate the detection of minor levels of bushing faults and insulating oil degradation with a high degree of accuracy.

3-D coupled electromagnetic-fluid-thermal analysis of epoxy impregnated paper converter transformer bushings

The temperature distribution is vital to the lifetime and reliability of converter transformer bushings. However, the temperature distribution of epoxy impregnated paper converter bushings under a certain current has usually been calculated through the two dimensional model, without considering the heat convection and radiation effects. In this paper, using the computational fluid dynamics (CFD) and finite element method (FEM) analysis, a 3-D electromagnetic-fluid-thermal analysis method has been proposed. With the considering of heat conduction, heat convection and radiation, the temperature distribution of the converter transformer bushing has been simulated according to the actual operating environment. The temperature and velocity distribution of the converter transformer bushing has been obtained. Moreover, the simulation results have been compared with the experimental results to verify the accuracy of the simulation method. The simulation results, especially for the inner conductor, external conductor and the epoxy impregnated paper, are very close to the experimental results and the maximum temperature difference percentage is within 10 %, which indicates the accuracy and effectiveness of the electromagnetic-fluid-thermal analysis method proposed.

Monitoring of transient overvoltages on the power transformers and shunt reactors – field experience in the Croatian power transmission system

Power transformers and shunt reactors may be subjected to various dielectric stresses such as lightning and switching overvoltages. Since the exposure of equipment to overvoltages during operation and the overvoltage amplitudes are usually unknow, an on-line overvoltage transient recorder is used with the ability to sample, analyze and store transients at transformer terminals in real-time. In this paper, transient overvoltage monitoring system is presented. Overvoltages are measured on the outside measurement terminal of the shunt reactor and transformer bushing. Field experience regarding the application of monitoring system in Croatia is described including different cases of lightning and switching overvoltages. Lightning overvoltages recorded by monitoring system are correlated with data from the lightning location system (LLS). Switching overvoltages recorded on the shunt reactor are compared with numerical simulations in EMTP-RV software. Collected data about overvoltage stresses can be used as the basis for the assessment of the transformer and shunt reactor insulation condition and estimation of health index.

Effect of transformer oil on room temperature vulcanized silicone rubber

Concerns over the operation safety of power transformers have triggered an application of room temperature vulcanized (RTV) silicone rubber on transformer bushings. However, transformer oil may cause a severe deterioration in the performance of RTV silicone rubber coatings due to oil leakage. Little attention has been paid to the degradation of RTV silicone rubber due to transformer oil. This paper presents an experimental investigation to explore the deterioration of RTV silicone rubber in transformer oil. An oil immersion experiment is performed. Mass, hardness and hydrophobicity are analyzed to research the macro properties of RTV silicone rubbers. FTIR, XPS and SEM are employed to investigate their microstructures. TG analysis is performed to evaluate their thermostability. Besides, GC-MS method is used for the analysis of the constituents of transformer oil. The results reveal that the RTV silicone rubbers deteriorate irreversibly. After immersion, the mass of RTV silicone rubber increases and the hardness decreases significantly. The hydrophobicity of RTV silicone rubber decreases after immersion and then increases with fluctuation during the standing time, but the surfaces keep hydrophobic all the time. FTIR investigations display clearly the decrease of Si-CH3, Si-O-Si, Si(CH3)2 and Si(CH3)3, and the increase of C-H. Besides, the change of O-H bond is very slight. XPS verifies the broken chemical bonds. The SEM photographs describe the surface structure change from the shape of chunks and spheres to uniform wrinkles and bulges. TG curves imply that some products with poor temperature resistance are generated. GC-MS analysis shows that silicone-containing molecule appears in transformer oil after immersion, which proves the deterioration of RTV silicone rubber indirectly.

Latest Trends of Transformer Bushing Seismic Analysis

SUMMARYIn Tohoku Region Pacific Coast Earthquake, many of the power equipment (especially center clamping type bushings for power transformers) were damaged due to the seismic motion that exceeds the design criteria. The problem of the conventional seismic analysis became clear as the actual damage was revealed. Since elaboration of seismic analysis and evaluation technology was called for, the elaboration possibility of seismic analysis was verified by carrying out comparison with various test results and the analysis result by the latest technique.

Impact analysis of the capacitive coupling sensor on bushing external insulation

Online impulse frequency response analysis (IFRA) method is proposed and developed as a new technique to detect the winding mechanical deformation of a running transformer. The preliminary application of IFRA has been realised by using the bushing capacitive coupling sensor (CCS). However, whether the installation of the sensor affects the normal operation of the transformer is still unknown. To study above problem, this study first establishes a simulation model for transformer bushing based on the finite-element method and discusses the influence of sensor installation on the electric field distribution of capacitive bushing. The simulation proves that the installation of the CCS has minimal effect on the safe operation of the bushing. Second, the bushing withstand voltage tests are conducted in a lab, which proves that the installation of CCS meet the requirement for safe operation of capacitive bushing. Finally, the CCSs are installed on running power facilities, and the long and safe running experience indicates that the bushing and related facilities are unaffected by CCSs. The simulation analysis, withstand voltage test, and field application verify that the installation of CCS would not threaten the safe operation of high-voltage bushing, laying the foundation for engineering application of IFRA.

Measurements, Observations, and Implications of Moving Electrical-Arc Behavior and Effect of Reclosure Events on Overhead Lines and Worker Protection

Characteristics of an electric arc in industrial and utility electrical equipment is dependent on many factors such as fault-current level, duration of the arc, geometry of feeding conductors and electrodes, proximity of panels, and other items affecting the direction or focus of the arc energy. The voltage rating of the system will affect the overall dimensions of conductor spacing, which will result in higher released energy for the same fault conditions. Another factor of importance is the type of heat energy (predominately radiant or predominately convective). This can be generalized into four types: 1) moving high-heat flux arc in open air of overhead lines or substation buswork; 2) stationary directional high-heat flux arc in open air at the end of overhead line or substation buswork or transformer bushing; 3) ejected directional high-heat flux arc in enclosed medium-voltage switchgear or cable-splice failure; and d) directional hot air exhaust from low heat-flux arc in low-voltage switchboards' cabinets and motor control centers. The results of experimental testing for type 1 arc behavior and incident heat energy measurements of a moving and stationary open-air arc are presented and discussed. Work practices for mitigating the effect of electric arc are recommended for overhead lines.

An Intelligent On-Line Inspection and Warning System Based on Infrared Image for Transformer Bushings

This paper presents an on-line inspection and warning system based on infrared image to detect the thermal fault of transformer bushings in the early stage. The network infrared thermal imager combined with the omnidirectional PTZ platform is the key device for the surveillance task. BRISK (Binary Robust Invariant Scalable Keypoints) algorithm is applied for the automatic object recognition and localization. In addition, the software based on B/S architecture is designed and realized for remote data transmission, so the remote users can acquire the real-time information through the private network. This system realizes the remote, real-time online monitoring and analyzing of transformer bushings, the comparison results validate the accuracy and efficiency of the recognition method, and the evaluations of the measured data verify the practicability of the system in the 110kV substation. Keywords: BRISK, infrared image, object recognition, on-line monitoring, transformer bushing.

Heat analysis of the power transformer bushings using the finite element method

Reliability of power transformers significantly depends on the performance of their bushings and bushing lifetime is mainly dependent on its hottest spot temperature, which is caused by the power dissipation in the bushing's conductor. In this paper, the thermal analysis of the bushing is firstly performed in two-dimensional space using the differential equations. Then, the Finite Element Method (FEM) is employed to determine a bushing's temperature distribution and specify its hottest spot. The effects of increasing the temperature on the bushing's conductor are also investigated and the results are compared in different currents. The proposed method calculates the temperature distribution in all parts of bushing including conductor surface, paper, porcelain and oil. It is useful for manufacturer and utilities to evaluate the insulation design, temperature distribution and loss of insulation life.

Detection of power transformer bushing faults and oil degradation using frequency response analysis

Frequency response analysis (FRA) has been globally accepted as a reliable tool to detect mechanical deformation within power transformers. However, because of its reliance on graphical analysis, interpretation of FRA signature is still a challenging area that calls for skilled personnel, as so far, there is no widely accepted reliable standard code for FRA signature identification and quantification. While several papers investigating the impact of various mechanical winding deformations on the transformer FRA signature can be found in the literature, no attention was given to investigate the impact of various bushing faults and transformer oil degradation on the FRA signature. This paper introduces a detailed simulation and practical analyses to elaborate the impact of bushing faults as well as transformer oil degradation on the transformer FRA signature. In this regard, the physical geometrical dimension of a three phase power transformer is simulated using 3D finite element analysis to emulate the real transformer operation. Various bushing faults have been emulated on the studied model and oil degradation is implemented through changing oil permittivity. Practical FRA test is conducted on a three phase 132 kV, 35 MVA power transformer to validate the simulation results. Results show that bushing faults and oil degradation can be visibly detected through FRA signature.

Dielectric response measurement by impulse stimulus on AC: Measurement considerations, and laboratory testing on a bushing

Dielectric response (DR) measurement is commonly used for condition assessment of insulation systems of power components. A method for measurement of dielectric response using the stimulus of natural transients such as lightning and switching impulses was proposed in earlier work. Its desirable features include the ability to make measurements online over a range of frequencies, without requiring a voltage source. This article presents a laboratory demonstration of the method on a 150 kV service-aged transformer bushing, where the stimulus was a standard lightning impulse voltage superimposed on an AC voltage. Several aspects of the measurement and data processing that affect the results are studied experimentally and numerically. The results are compared with low-voltage frequency domain spectroscopy (FDS). Reasonable accuracy for monitoring changes in DR can be achieved by suitable choice of measurement circuit and data acquisition. The study suggests an approach for online monitoring of dielectric properties of power transformer bushings, and diagnostics of defects that affect the high frequency region of DR, such as moisture content.

The bubble effect in bushings – investigations on models

This paper refers to the bubble effect in transformer bushings. The bubble effect consists in releasing water from moistened cellulose insulation after exceeding a critical temperature. The most dangerous result is a pressure increase that leads to an explosion and fire. Research was conducted using laboratory models of OIP (Oil- Impregnated Paper) and RBP (Resin-Bonded Paper) types of bushings immersed in mineral oil and synthetic ester. For all of the investigated systems we determined the dynamics of water adsorption upon contact of the insulating material with air and the temperature characteristics of bubble effect initiation depending on water content in cellulose. In the experiment we investigated the dynamics of changes of water content in dielectric liquid, water content in air over the surface of the dielectric liquid, and air pressure over the surface of the dielectric liquid. In the future, the technical possibilities will be analyzed of using the dynamic changes of the three quantities investigated here to detect the bubble effect in bushings.

Development of On-Line Monitoring for Power Transformer Bushing

This paper presents the development of on-line monitoring for condenser type bushing of power transformer in order to detect a degradation of bushing and to provide the alarm before bushing failure. For the bushing degradation analysis, the capacitance and power factor of bushing internal insulation are on-line monitored and measured by installing the sensing device at the test tap of bushing. The changing of internal insulation capacitance leads to the change of voltage across tap capacitor and the change of leakage current through bushing insulation at the test tab. Such changes result in the change of power factor or phase angle of leakage current. Thus, the leakage current is additionally detected by installing the Hall Effect sensing device to observe the trend of bushing failure. A total of eight cases of both normal and degraded insulation are investigated. The internal insulation capacitance as well as the magnitude and phase angle of three phase leakage current were measured and compared with the commissioning value or observed the increasing trend to set the alarm level. In case of degraded bushing, the leakage currents changed and caused the summation of leakage current value to be greater than the standard value. Then the alarm is activated. This knowledge was used to develop the detection and decision making algorithms in LabVIEW program to determine the condition of transformer bushing.

Calculation of Nonlinear Electromagnetic Fields in the Steel Wall Vicinity of Transformer Bushings

Successful analytical formulas have been previously proposed to calculate the losses in tank regions of transformers assuming linear permeabilities in the analyzed boundary-valued problem. This has resulted in easy-to-implement and low-cost computational design procedures from a transformer factory economical point of view. However, designers and analysts of transformers are constantly seeking new ways of reducing transformer losses in actual power networks with thousands of transformers. As a result, this paper has focused on proposing new analytical formulas to determine the electromagnetic field in bushing regions of transformers, taking account of the true nature of the nonlinear permeability behavior of the tank wall. This way, the nonlinear Maxwell’s equations in the regions surrounding the bushings are solved using an integral equation formulation that properly includes boundary conditions. A practical iterative procedure is thus proposed to solve the resulting nonlinear equation. The iterative scheme shows excellent numerical convergence properties with a very low computational demand as compared with finite-element (FE) nonlinear models. A comparison between our analytical results and those of 3-D FE simulations reveals a close match for a wide range of conductor currents. Hence, our new formulas can be used to improve the design of transformers, increasing their efficiency.

Compensation method for improving capabilities of AC current test equipment

Testing of electrical equipment with AC currents is a common test in both high power and high voltage laboratories. It implies driving currents, sometimes several tens of kilo-amperes high, through a test object loop conductor. Test transformers are normally used as current sources, but the limiting factor is usually their relatively low rated voltage. Voltage on the test transformer is determined by product of loop impedance and test current. Proposed compensation method deals with reduction of loop impedance which consequently reduces voltage on test transformer and improves test capabilities.Experimental verification of the method was successfully carried out on two test objects: 110kV cable and 600kV transformer bushing.

Transformer Bushing Online Monitoring Method Based on the Method of Relative Angle Difference

Transformer is the main equipment of power system, its operation reliability is directly related to power system security and reliability of power supply. In order to guarantee the safe operation of power system, must strengthen the monitoring of the main transformer.With the development and popularization of intelligent station, the detection of transformers insulation is faced with the new problem that PT signal is not convenient to get and even unable to get. This paper proposes the relative method which is used to calculate and monitor dielectric loss of transformer bushing and designs the on-line monitoring device, through the data acquiring form the on-line monitoring instrument which are installed in the transformer of substation shows that this method has high precision, strong anti-interference ability.

The Investigation of Replacing Ceramic Bushings with Silicon Bushings in the Distribution Transformers

In recent years, the deficiencies of ceramic insulators along with their high maintenance costs have resulted in the replacement of ceramic insulators with silicon type in the pollution area. This idea has been employed for more than two decades in the polluted areas. Humidity of the weather in the south of Iran and the presence of pollutants in the air have made special conditions for construction and maintenance of some equipment including transformer bushings. Usual ceramic bushings, due to their ability in absorbing pollution and the rapid reduction of creepage distance (in a limited time period), have reduced transformer disconnection severely as a result of earth fault. Additionally, they are costly to be washed regularly. Therefore, using intelligent materials in designing bushing can increase the reliability of network and consequently reduction of costs. In this regard, this paper investigates the use of silicon bushings in the distribution systems and proposes operational ideas for the optimal operation of these devices in the polluted areas. The sample bushing was evaluated based on the IEC60137 standard test.

Dielectric response of oil-impregnated paper by utilizing lightning and switching transients

On-line monitoring of insulation systems in power components such as power transformer bushings has significant value for their reliability. Dielectric spectroscopy is known as a powerful tool in condition assessment of power components’ solid insulation. However, it is difficult to apply it on-line. Therefore, a new approach of intended use for on-line diagnosis of oil-impregnated paper, as a major insulation component of power transformers and bushings, is presented. In this technique, natural transients such as lightning and switching impulses are used as stimuli for on-line dielectric response measurements. The wide frequency range of these transients is their advantage as stimuli for dielectric spectroscopy. The influence of moisture content and temperature on the dielectric response of oil-impregnated paper has been investigated for performance evaluation of the proposed technique. It is shown that this technique can track the changes caused by moisture content and temperature in the dielectric response of oil-impregnated paper. The results of both low and high voltage study cases have been verified by comparison with Frequency Domain Spectroscopy (FDS). According to the well-matched results with FDS, the proposed technique can offer a valuable approach for frequent monitoring of dielectric properties of oil-impregnated paper insulated systems such as those commonly encountered in power transformer bushings. However, the validity range of the results depends on the bandwidth of the applied transients and other discussed measurement considerations.