Condenser Bushings Research Articles

Application of a Rogowski Coil Sensor for Separating Internal and External Partial Discharge Pulses in Power Transformers

Partial discharge (PD) measurement is an essential tool for evaluating the health condition of a transformer insulation system. The Conventional PD measurement systems have a major drawback since they cannot discriminate the internal PD pulses and external pulse-shaped interferences, especially in online and on-site conditions. Various de-noising techniques (e.g. wavelets) have been also used in the literature, which is effective for extracting the PD signal from the noisy signals or classifying the PD types. However, the PD pulses and some of the randomly external pulse-shaped interferences have similar time-frequency characteristics, which makes them difficult to separate. In this paper, a new cost-effective and simple logic method is proposed for the separation of these interferences. This uses the polarity comparison of the output voltages of the Rogowski Coil (RC) sensor, mounted on the bushing, and measurement impedance, connected to the test tap. The same polarity represents the internal pulse, while the opposite polarity indicates the external pulses. A distribution transformer ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$20/0.4\ {\bm{kV}},\ 500\ {\bm{kVA}})$</tex-math></inline-formula> and a condenser bushing ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$72.5\ {\bm{kV}})$</tex-math></inline-formula> were considered to evaluate the performance of the proposed system. The results confirm the effectiveness of the proposed PD measurement system for separating the pulses.

Study on Lifespan Prediction of Oil–Paper Bushing Considering the Influence of Moisture-Accelerated Aging

The moisture-dependent parameter in the aging kinetic model is inaccessible due to the fluctuation of moisture during the aging period of bushing insulation. This significantly restricts the adoption of aging kinetics-based approaches for predicting transformer bushing insulation lifespan. Given this issue, an improved aging kinetic model is proposed in this article, where the influence of moisture fluctuation is interpreted by the moisture acceleration coefficient. To obtain the involved parameters in the improved model, the humidity of bushing insulation is determined by recognizing the moisture-related feature, which can be extracted from the logarithmic-derivative method derived from frequency domain spectroscopy (FDS). Finally, the improved aging kinetic model and logarithmic-derivative method are then used to provide a method for predicting the lifespan of bushing insulation. According to verification experiments carried out with this model, the relative errors of the re-created laboratory samples are 8.12%, 8.89%, and 25.15%, respectively. In that regard, this article is likely to serve as a model for predicting the lifespan of oil–paper insulation on genuine condenser bushings.

EFFECT OF ALUMINUM FOILS NUMBER AND ITS LENGTH IN IMPROVEMENT OF ELECTRIC FIELD DISTRIBUTION OF HIGH VOLTAGE CONDENSER BUSHING

High voltage condenser bushing is one of the important component that is widely used in the high voltage system. At high voltage levels more than 52kV the distribution of electric field in condenser bushing is irregular between the lead conductor and the grounded metallic flange. This paper studied the effects of changing in both: the number layers of aluminum foils and Oil Impregnated Paper (OIP), increasing the length of aluminum foils layers, and also increasing the thickness of OIP layer on the distribution of electric potential and electric field in condenser bushing by using Finite Element Method (FEM) and built the bushing model in ANSYS software. The harmonic analysis was performed of the bushing model at maximum value of withstand voltage test at 50Hz, from the analysis results are obtained the maximum value of electric field on the inner and outer surface of the bushing, the obtained electric field values were good and acceptable compared to the permissible electrical stress values of the dielectric insulators. This work can also aid in the design of high voltage bushing stress control, a knowledge of the electrical field distribution in bushing geometry. Moreover, the results of analysis are shown as contour plots, graphs plotted, and tables.

Ageing characteristics and lifetime model of oil–paper insulation for oil‐immersed paper condenser bushing

Ageing characteristics and lifetime model of oil–paper insulation for oil‐immersed paper condenser bushing

Lumped RC Thermal Network Method Applied to Transient Temperature Calculation of High Voltage Bushing and Its Insulation Life Assessment

To analyze how the internal transient temperature of the high voltage condenser bushing varies with its short-time current carrying capacity and long-aging properties, lumped RC thermal network model applicable to the structure of HV bushing is proposed. This model considers three heat transfer modes of the conduction, the convection and the radiation, and the changing relationship between material properties and temperature and the the thermal RC network topology is realized based on the MATLAB/SIMULINK. In the example of 550kV oil-SF6 bushing, the calculation results of transient temperature thermal network are in good agreement with the actual temperature rise test results which proves effectiveness and practicality of lumped RC thermal network. We used the thermal network to confirm the periodic transient variation at the hottest spot of HV bushing and analyze the shot-time current carrying capacity by step load curve fitting circadian cycle change load. Finite element method and thermal network method are used to get the electric and thermal stress respectively and HV bushing long-aging properties analysis was conducted based on classic ZHURKOV and CRINE electric and thermal combined aging model. This paper applies lumped RC thermal network method to transient temperature calculation and internal insulation life assessment of the HV bushing, reinforced classic bushing design method only considering rated steady-state operating conditions and has theoretical guidance on assessing short-time current-carrying capacity and long-aging properties in high voltage bushings.

Online Monitoring of Partial Discharges in Power Transformers Using Capacitive Coupling in the Tap of Condenser Bushings

Failures in power transformers are one of the most serious occurrences in a power system. Thus, the monitoring of transformers and their ancillary equipment, such as bushings, is of great importance to improving the operational efficiency of these assets. In this context, this paper presents the development of a monitoring system for the measurement of partial discharges (PDs), which are a key parameter in the analysis of insulation condition. PD measurements were performed using the electrical method. For this purpose, a capacitive coupling device was developed for bushings that works as a sensor for high-frequency signals and also as a protection apparatus to guarantee the integrity of the bushings in cases of extreme events, such as lightning surges. In addition, a computational routine is presented that applies a digital filtering process followed by a proposed step for differentiating PDs from noises. For validation, the proposed system was subjected to laboratory tests and field applications, from which the viability of the project and the efficiency in detecting PDs were verified.

Dielectric and Thermal Conductivity of Epoxy Resin Impregnated Nano-h-BN Modified Insulating Paper.

Epoxy resin-impregnated insulation paper (RIP) composites are used as the inner insulation of dry condenser bushing in the ultra-high voltage direct current (UHVDC) power transmission system. To improve the dielectric properties and heat conductivity of RIP, hexagonal boron nitride (h-BN) nano-flakes are added to the insulation paper at concentrations of 0–50 wt % before impregnation with pure epoxy resin. X-ray diffraction (XRD), scanning electron microscopy (SEM) observations, thermal conductivity as well as the typical dielectric properties of direct current (DC) volume conductivity. DC breakdown strength and space charge characteristics were obtained. The maximum of nano-h-BN modified heat conductivity reach 0.478 W/(m·K), increased by 139% compared with unmodified RIP. The DC breakdown electric field strength of the nano-h-BN modified RIP does not reduce much. The conductivity of nano-h-BN modified is less sensitive to temperature. As well, the space charge is suppressed when the content is 50 wt %. Therefore, the nano-h-BN modified RIP is potentially useful in practical dry DC bushing application.

Dielectric Properties of Epoxy Resin Impregnated Nano-SiO₂ Modified Insulating Paper.

Epoxy resin-impregnated insulation paper (RIP) composites are used as the inner insulation of dry condenser bushing in the ultra-high voltage direct current (UHVDC) power transmission system. To improve the dielectric properties of RIP, nano-SiO2 is added to the insulation paper at concentrations of 0–4wt % before impregnation with pure epoxy resin. X-ray diffraction (XRD), scanning electron microscopy observations as well as the typical dielectric properties of relative permittivity, DC volume conductivity, DC breakdown strength, and thermally stimulated depolarization current (TSDC), were obtained. The effects of trap parameters on the breakdown field strength and volume conductivity were investigated. The DC breakdown electric field strength of the sample increased as the trap level increased. The maximum DC breakdown strength of nano-SiO2-modified RIP was increased by 10.6% the nano-SiO2 content of 2 wt %. The relative permittivity and DC volume conductivity were first decreased and then increased with increasing nano-SiO2 content. These changes occurred near the interfaces between nano-SiO2 and RIP. The increased DC breakdown strength was mainly attributed to the increased trap level.

Insulation Assessment of Oil Impregnated Paper Condenser Bushings using Dielectric Frequency Response Technique

Objectives: Investigation of insulation to judge the health of the condenser bushing with different techniques has been the area of interest for the researchers in recent past. Power transformers and their bushings are manufactured with oil impregnated paper insulation but their insulation integrity is comprised due to absorbed, residual or liberated moisture/ water which leads to further deterioration and catastrophic failure. Methods: Conventional measurement technique taken at rated power frequency only gives relative indication of specific dielectric properties and health of the condenser bushings. This article introduces a new approach for insulation assessment of the condenser bushings where OIP insulation as a key component has been investigated in a wide frequency response spectrum and to confirm the validity of this approach, measurements were taken on several condenser bushings of same type and age. Findings: The advance technique is a nondestructive method which evaluates the water concentration of the test object. The resulting curve is highly determined by the behavior of solid insulation and it is also sensitive to oil conductivity and insulation geometry of the bushing. Furthermore, the interpretation scheme related to dielectric response is explained. Applications: The signatures achieved helps in reliable data analysis and can be used in qualitative comparisons of bushings which compliments the available quality assurance techniques in the factory. This approach can also be used for frequent monitoring and effective moisture management at site.

Quantitative diagnosis of moisture content in oil‐paper condenser bushing insulation based on frequency domain spectroscopy and polarisation and depolarisation current

Dielectric response techniques have been proven useful for evaluating the moisture content (K m.c) of oil-paper insulation. However, an accurate model for oil-paper condenser bushing has not been presented. This study designed a laboratory oil-impregnated paper (OIP) bushing model to simulate moistened defects in oil-paper bushings. Moreover, a 10 kV OIP bushing model was simulated using COMSOL multiphysics AC/DC to confirm that the distribution of potential meets the design requirements. Furthermore, this work investigates the effect of moisture content on the dielectric frequency response and polarisation and depolarisation current (PDC) of models of oil-paper bushings. A group of characteristic parameters was extracted from the dielectric dissipation factor curves and the DC conductivities of different moisture contents were computed by charge difference method from PDC curves. The obtained results suggest that the dissipation factor and the capacitance increase with the increase of moisture content in the whole frequency range, and the quantitative relationships among the characteristic parameters, DC conductivities, and the moisture content were established and follow an exponential law.

Detection of conductive layers short circuit in HV condenser bushings using Frequency Domain Spectroscopy

Frequency Domain Spectroscopy (FDS) is mainly used to determine the moisture in the paper-oil insulation systems in power transformers and Oil-Impregnated-Paper (OIP) bushings. Another application of this method is detection of puncture developing in the paper layer between the adjacent conductive foils in the condenser core. The paper presents theoretical calculations of tanδ frequency response in the case of short circuit of conductive layers in bushing's condenser core and their experimental verification, with the help of paper-oil insulation capacitor. Furthermore, examples of measurements of real bushings in operation that agree with the proposed idea are described.

DC Ultrahigh Voltage Insulation Technology for 1 MV Power Supply System for Fusion Application

To realize dc 1 MV power supply system for neutral beam injector in ITER, long-pulse dc ultrahigh-voltage (HV) insulation technologies have been developed. For a dc 1 MV insulating transformer, a composite bushing has been newly developed to overcome the manufacturing difficulty of a conventional condenser bushing (CB). The composite bushing has a small-sized CB and fiber reinforced plastic tube to sustain 1 MV stably inside and outside of the bushing. The mock-up of the insulating transformer has fulfilled the ITER requirement of dc 1.2 MV insulation for 3600 s. An HV bushing to transmit multiple voltages simultaneously has also been developed through a careful experimental study on vacuum insulation characteristics with multiple gaps between large electrodes. It has been found that the voltage holding is dominated not by number of the electrodes but by a total cathode surface area of electrodes. Based on this result, the vacuum insulation inside the HV bushing with five electrostatic screens has been designed to realize a stable vacuum insulation of 1 MV.

Remote detecting and locating partial discharge in bushings by using wideband RF antenna array

It is bushing that performs an essential function in power transformer. Since the existing on-line partial discharge (PD) test methods for bushings require the current from the test tap, it is difficult to be applied to in-service bushings, especially to some old bushings without voltage taps. In order to detect PDs without changing grounding circuit, a novel non-contacting on-line approach to detecting and locating PDs in bushings using time delay of arrival (TDOA) of radio frequency (RF) antenna array signals is proposed in this paper. Based on a real 126 kV oil-impregnated paper (OIP) condenser bushing, four typical PD defect models were made, which were suspended discharge on bushing's top cap, poor connection in bushing tap, surface discharge on oil-side porcelain insulator and suspended discharge near the oil-side grading ring. The detection and location of PDs induced by these defects were carried out and different features were obtained. For the defects of suspended discharge on bushing's top cap and poor connection in bushing tap, this approach gives good location accuracy and the error is less than 20 cm. For the defects inside the bushing housing, the RF signals generated from PDs still can be detected and located, but the calculated location results are all focused around the bottom of the air-side porcelain insulator. In order to reveal the test results, the propagation of electromagnetic (EM) waves in the bushing housing was simulated, which indicated that the coaxial gap between the condenser core and the fixing flange allows various modes of EM waves to propagate with a attenuation from 40 to 60 dB. In the on-site tour-inspection tests, a real PD fault in an 110kV cable terminal bushing was located in a substation, which verified the availability and effectiveness of this approach.

Potential of determining moisture content in mineral insulating oil by fourier transform infrared spectroscopy

Transformer oils are vulnerable to moisture, which can have adverse effects including a dramatic reduction in dielectric strength, accelerating the aging of the solid insulation (cellulose depolymerization) and the formation of decompositional gasses at high temperatures. Moisture may be generated inside the transformer due to degradation of the oil-paper insulation or there may be ingress of moisture due to free-breathing arrangements. A moisture content exceeding 50% of oil saturation greatly reduces the breakdown voltage, resulting in catastrophic failures as well as the potential for transformer fires [1]. The detrimental effect of water within a transformer is not only limited to the breakdown voltage of the oil. The rate of degradation of paper insulation increases in direct proportion to the water content [2], [3]. Moisture also has a decisive effect on the maximum temperature that the transformer can run at before moisture-induced failure occurs. Consequently, transformer life expectancy can be calculated from water content and temperature [3]. A failure of transformer accessories such as condenser bushings sometimes leads to a transformer failure and long-term outage. Often, moisture is a cause of explosion for service-aged bushings [4].

Study of the Impact of Initial Moisture Content in Oil Impregnated Insulation Paper on Thermal Aging Rate of Condenser Bushing

This paper studied the impact of moisture on the correlated characteristics of the condenser bushings oil-paper insulation system. The oil-impregnated paper samples underwent accelerated thermal aging at 130 °C after preparation at different initial moisture contents (1%, 3%, 5% and 7%). All the samples were extracted periodically for the measurement of the moisture content, the degree of polymerization (DP) and frequency domain dielectric spectroscopy (FDS). Next, the measurement results of samples were compared to the related research results of transformer oil-paper insulation, offering a theoretical basis of the parameter analysis. The obtained results show that the moisture fluctuation amplitude can reflect the different initial moisture contents of insulating paper and the mass ratio of oil and paper has little impact on the moisture content fluctuation pattern in oil-paper but has a great impact on moisture fluctuation amplitude; reduction of DP presents an accelerating trend with the increase of initial moisture content, and the aging rate of test samples is higher under low moisture content but lower under high moisture content compared to the insulation paper in transformers. Two obvious “deceleration zones” appeared in the dielectric spectrum with the decrease of frequency, and not only does the integral value of dielectric dissipation factor (tan δ) reflect the aging degree, but it reflects the moisture content in solid insulation. These types of research in this paper can be applied to evaluate the condition of humidified insulation and the aging state of solid insulation for condenser bushings.

Dielectric properties of multi-layer epoxy resinimpregnated crepe paper composites

Epoxy resin-impregnated crepe paper (RIP) composites have been used as the inner insulation of dry condenser bushing in the ultra-high voltage direct current (UHVDC) power transmission system. In order to study the dielectric properties of pure epoxy resin and RIP composites, four kinds of samples (pure epoxy resin, RIP composites with 1, 2, 3 layers of crepe paper respectively) were prepared via the vacuum casting process. The Fourier transform infrared spectroscopy (FTIR) as well as three typical dielectric properties, i.e. complex permittivity, dc conductivity, thermally stimulated depolarization current (TSDC) have been investigated. It shows that the addition of crepe paper nearly brings about larger values of relative permittivity as well as dielectric loss factor than the pure epoxy resin. Samples containing crepe paper also have larger dc conductivities and TSDC currents. These differences are related to the abundant hydroxyl groups in the cellulose paper. The hydroxyl groups are polar groups and easy to absorb water or other impurities which resulting in larger permittivity and dc conductivity. Moreover, the crosslinking degree of the epoxy resin matrix decreases with the introduction of crepe paper. Then, the polarized dipole moments are easier to relax, which is responsible for the decrease of the trap depth corresponding to the process of glass transition.

Inner insulation structure optimization of UHV RIP oil-SF6 bushing using electro-thermal simulation and advanced equal margin design method

The iso-margin method based on the classical analytical equations is widely used in the development of extra high voltage (EHV) bushing. Therefore, its effectiveness and feasibility have been fully validated. However, with the theoretical formula, it is difficult to evaluate the influence of stray capacitance and temperature on the internal electric stress of the ultra high voltage (UHV) bushing. It's necessary to further improve the traditional iso-margin method. Firstly, the basic principle of iso-margin method has been briefly described. After that, the E-field distribution of condenser bushing was investigated on the Finite Element Method (FEM) computing platform. Then, the mathematical model of the advanced equal margin design method was established. Furthermore, this paper presents the program flow of optimization which combines the simulation of electro-thermal coupling process and the particle swarm optimization (PSO) algorithm. The proposed methodology was applied to the design of the prototype of the UHV resin impregnated paper (RIP) oil-SF 6 bushing, which realizes the uniform axial E-field distribution of the bushing condenser. Meanwhile, the partial discharge margin between adjacent foils can be equal. Moreover, the hot-spot temperature is lower than the operation limiting temperature of the RIP material. The bushing condenser was fabricated according to the optimal structure design. The prototype of bushing has passed through all the type tests. In this paper, the FEM electro-thermal coupling simulation and the advanced equal margin design method were applied to the inner insulation structure optimization of the UHV RIP oil-SF 6 bushing. Meanwhile, the proposed methodology provides some theoretical guidelines for the future research and development of other types of bushing on the UHV level.

A dielectric frequency response model to evaluate the moisture content within an oil impregnated paper condenser bushing

Increased moisture content within an oil-paper insulation system can significantly reduce its life expectancy, and for oil impregnated paper condenser type bushings, it is the most common cause of failure. Distinction between moisture contents using traditional power frequency tests of dissipation factor and capacitance can be difficult, particularly at ambient temperatures. Dielectric frequency response is becoming an established technique to measure the dissipation factor and capacitance of a bushing, but to date an accurate model to evaluate the condenser moisture content has not been presented using this technique. In this study, a dielectric frequency response bushing model is proposed and finite element method software is used to simulate the variation in dissipation factor and capacitance of a bushing with varying moisture content, as a function of frequency and temperature. The modelled results are compared with measurements reported in the literature and from the authors’ own field measurements, where a good agreement is demonstrated. It is shown that the distinction in dissipation factor between moisture contents at frequencies <0.1 Hz is in excess of ten times greater than at 50 Hz. The proposed model can be used by practitioners to evaluate moisture content within the condenser insulation and distinguish between moisture contents ranging from 0.2 to 4.0%.

Electrical insulation design and evaluation of 60 kV prototype condenser cone bushing for the superconducting equipment

A cryogenic bushing is an essential component to be developed for commercial applications of high voltage (HV) superconducting devices. Due to the steep temperature gradient of the ambient of cryogenic bushing, general gas bushing adopting SF6 gas as an insulating media could not be directly used due to the freezing of SF6 gas. Therefore, condenser type bushing with special material considering cryogenic environment would be better choice for superconducting equipment. Considering these circumstance, we focused on the design of condenser bushing made of fiber reinforced plastic (FRP). In case of the design of the condenser bushing, it is very important to reduce the electric field intensification on the mounted flange part of the cryostat, which is the most vulnerable part of bushings. In this paper, design factors of cryogenic bushing were analyzed, and finally 60kV condenser bushing was fabricated and tested. In order to achieve optimal electric field configuration, the configuration of condenser cone was determined using 2D electric field simulation results. Based on the experimental and the analytical works, 60kV FRP condenser bushing was fabricated. Finally, the fabricated condenser bushing has been tested by applying lightning impulse and AC overvoltage test. From the test results, it was possible to get satisfactory results which confirm the design of cryogenic bushing in cryogenic environment.

Insulation design of cryogenic bushing for superconducting electric power applications

Recently, the superconductivity projects to develop commercial superconducting devices for extra high voltage transmission lines have been undergoing in many countries. One of the critical components to be developed for high voltage superconducting devices, including superconducting transformers, cables, and fault current limiters, is a high voltage bushing, to supply high current to devices without insulating difficulties, that is designed for cryogenic environments. Unfortunately, suitable bushings for HTS equipment were not fully developed for some cryogenic insulation issues. Such high voltage bushings would need to provide electrical insulation capabilities from room temperature to cryogenic temperatures.In this paper, design factors of cryogenic bushings were discussed and test results of specimen were introduced in detail. First, the dielectric strength of three kinds of metals has been measured with uniform and non-uniform electrodes by withstand voltage of impulse and AC breakdown test in LN2. Second, puncture breakdown voltage of glass fiber reinforced plastics (GFRPs) plates has been analyzed with non-uniform electrodes. Finally, creepage discharge voltages were measured according to the configuration of non-uniform and uniform electrode on the FRP plate. From the test results, we obtained the basic design factors of extra high voltage condenser bushing, which could be used in cryogenic environment.