Air Gap Defects Research Articles

A Partial Discharge Detection System for XLPE Cable Terminals and Acoustic Wave Sensing Characteristics Based on Fiber Optic Mach-Zehnder Interferometer

Monitoring the cable terminal’s partial discharge (PD) is of great significance to ensure the cable line’s safe and stable operation. To improve the anti-interference ability and real-time performance of partial discharge detection, this paper develops a partial discharge Mach-Zehnder interferometer (MZI) detection system for cross-linked polyethylene (XLPE) cable terminals. The optical fiber is implanted into the 10 kV XLPE cable terminal, and airgap defects of different volumes are set in the main insulation. The optical fiber inside the cable terminal serves as the sensing head of the MZI signal arm. PD test was carried out on the samples, and the test results of fiber optic MZI and high-frequency current transformer (HFCT) were compared and analyzed. The results showed that the 10 kV XLPE cable terminal does not produce PD due to optical fiber implantation. The partial discharge of 196 pC can be recognized by the fiber optic MZI for the cylindrical airgap defect sample with a base radius of 1.5 mm and a height of 3 mm. For the cylindrical airgap defect sample with a base radius of 1.5 mm and a height of 1 mm, the sensitivity of partial discharge detection by fiber optic MZI is higher than that by HFCT. The sensitivity of partial discharge inception voltage (PDIV) detection is increased by 27.5%.

Influence of typical composite insulators defects on axial temperature based on simulation analysis

In this paper, the finite element analysis software comsol is used to design two different defect models on the short-string composite insulator model, namely, the air gap defect and the mandrel-damp defect. The thermal field of composite insulators with two kinds of defects is simulated, and comparing the axial temperature gradient changes of mand-sheath interface under normal condition and under the condition of defects. The results show that local heating does not occur when the composite insulator contains an air gap defect without partial discharge.

Terahertz Imaging Method for Composite Insulator Defects Based on Edge Detection Algorithm

Composite insulator is prone to internal defects such as air gaps which seriously endanger the safety of power systems. To detect the internal defects in composite insulators, a terahertz (THz) imaging method based on edge detection algorithm was proposed in this article. First, air gap defect sample consisting of planar multilayers was imaged based on the envelope area of the THz reflection waveform. Then, Canny operator was applied to extract the edges of the defects. After that, the time interval of defect characteristic pulses was used to calculate the defect depth and then 3-D imaging of defects was, therefore, obtained. The experimental results showed that the imaging method based on envelope area was very effective in determining the edges of air gap defect and it had better performance compared with those using pulse amplitude or time interval as parameter for defect imaging. According to the results, the accuracy was quite satisfactory when the defect depth was above 0.339 mm. To verify the effectiveness of the proposed method, the developed THz imaging system was also applied to the rod of composite insulator. It was found that the relative error was smaller along the axial direction of the rod than the lateral direction.

Detection of Internal Defects of Full-Size Composite Insulators Based on Microwave Technique

In this article, a detection method based on microwave technique is proposed to detect the internal defects of composite insulators. Based on this method, an automatic detection system is developed for full-size composite insulator. In the experiments with 110-kV composite insulators, three types of defects, namely the air-gap defect, carbonization defect, and conductive defect, are artificially made, and they are located exactly by the developed automatic defect detection system. It is found that, due to the dielectric constant and magnetic permeability properties, the conductive defect can be detected easily, whereas the carbonization defect is the most difficult one to be detected among all three defects. In addition, a real defective 500-kV composite insulator from transmission line after years of operation is also detected by the developed system and the results are consistent with the infrared imaging method. According to the test results, the developed system is efficient, labor-saving, and robust. It can have a great potential in the quality control of composite insulators.

Corrigendum: Partial discharge characteristics of an air gap defect in the epoxy resin of a saturable reactor under an exponential decay pulse voltage

Corrigendum: Partial discharge characteristics of an air gap defect in the epoxy resin of a saturable reactor under an exponential decay pulse voltage

Partial discharge identification of DC cross-linked polyethylene cables based on GA-BP algorithm

In this paper, XLPE DC cables were used to construct three kinds of defects of inner semiconducting layer breakage, internal air gap defect, and insulating surface scratching defect. And their DC PD characteristics were tested under different voltage amplitudes and polarities. The study found that these three defects exhibit different DC partial discharge characteristics and change with voltage amplitude and polarity. Subsequently, two methods were used to identify the type of defect. The recognition rate of the traditional defect type identification method reached 97.1%, and the recognition rate of the optimized defect type identification method based on GA-BP was increased to 98.9%, which solved the problem that the traditional method was affected by the voltage level and polarity, and improved the recognition rate.

Partial discharge characteristics of an air gap defect in the epoxy resin of a saturable reactor under an exponential decay pulse voltage

Saturable reactor insulation is currently stressed by an exponential decay pulse voltage under normal operating conditions. The partial discharge (PD) characteristics of epoxy resin under an exponential decay pulse voltage were studied here and were compared at 25 and 110°C. In addition, this study compares these PD characteristics with those under a sinusoidal voltage to better measure the insulation design margin of the saturable reactor under an exponential decay pulse voltage. Finally, this study explains the PD mechanism based on the three-capacitor circuit model and space charge accumulation. Compared with the sinusoidal voltage, a higher amplitude, a higher inception voltage and fewer PDs are obtained under the pulse voltage. The reason may be related to the accumulation of space charge. Due to the duality of the space charge effect, the promotion effect of space charge accumulation on the PD under the pulse voltage is dominant, and an increase in temperature will weaken the promotion effect. In contrast, the inhibitory effect of space charge accumulation on the PD under the sinusoidal voltage is dominant. The experimental results can provide a basis for the optimal design of saturable reactor insulation under an exponential decay pulse voltage.

Study on Insulation Characteristics of Scratch Defects in 35kV XLPE Cable Terminals

In order to study the influence of different ring-shaped scratches on the insulation characteristics of 35kV Crosslinked polyethylene(XLPE) cable terminations, the model of air-gap defects caused by ring-shaped scratches was introduced at the end of the outer semi-conductor shielding layer, and the finite element simulation calculation was used to calculate the model. The electric field distribution characteristics inside the cable terminal were simultaneously made into cable end samples of different types of scratch defects, and the discharge characteristics were tested by a partial discharge test circuit. The results show that the air gap defect caused by the annular scratch defect increases the degree of electric field distortion. The maximum electric field strength is much larger than the breakdown field strength of the air, and it is prone to partial discharge activity. The Phase Resolved Partial Discharge(PRPD) of the cable terminal with ring scratch defects The spectrum shows a typical “rabbit ear” shape. As the scratch defects deepen, the PRPD spectrum begins to gradually transition to a short-feeling “cat ear” shape.

Detection of Small Defects in Composite Insulators Using Terahertz Technique and Deconvolution Method

After a long-term operation, defects may occur in composite insulators and threat the safe operation of power transmission. To secure the power transmission system, it would be beneficial to detect the defects in their initial stage. In this article, the terahertz technique combined with a deconvolution method was proposed, and a terahertz time-domain spectroscopy system was built. To validate the method proposed, three types of defects, namely, single air gap defect, inclusion defect, and double-layer air gap defect were artificially prepared and tested. According to the test results, it was found that compared with the traditional terahertz method, the air gap size that could lead to pulse overlapping was reduced from 1.22 to 0.57 mm. After the deconvolution process, air gap defect as small as 0.173 mm could be distinguished due to the shape characteristics of unipolar pulse. It was also found that when the defect was damp, the obtained waveform near the defect became smooth. The results of this article demonstrate that terahertz time-domain spectroscopy (THz-TDS), combined with the deconvolution method, can have high potential in the detection of defects in composite insulators.

THz wave detection of gap defects based on convolutional neural network improved by residual shrinkage network

Internal air gap is a kind of serious defect in the insulation equipment, which threatens the safe operation of the power grid. In order to diagnose the position and thickness of the internal air gap, this paper proposed terahertz wave detection method based on wavelet analysis and CNN(convolution neural network) model. According to the time-frequency characteristics of the wavelet cluster, the calculation method of air gap depth was proposed. To determine the thickness of internal air gap, the performance of several classification methods such as waveform feature analysis, Bayes, MLP (Multi-layer Perceptron), SVM(Support Vector Machine) and CNN was compared. The results showed that CNN modified by residual shrinkage network and SVM(CNN-RSN-SVM) had the best performance. By adjusting the parameters, the classification accuracy of the CNN-RSN-SVM model was elevated to 98.91%. Furthermore, 3D imaging method of air gap defect based on wavelet analysis and CNN-RSN-SVM classification model was formed.

Quantitative Detection of Interfacial Air Gap in Insulation Equipment Based on Terahertz Wave Contrast Method

The nondestructive testing of internal defects in power grid equipment has always been a spotlight of research. Since terahertz wave has good application in the nondestructive detection fields, we apply terahertz wave to detect the interfacial air gap of insulation equipment. At present, terahertz wave is difficult to measure the thickness of the internal air gap. Hence, this paper presents a method for quantitative measurement of internal air gap based on the waveform database. In addition, the propagation characteristics of the terahertz wave in medium are analyzed. Furthermore, based on the value of $n(\omega)$ and $\kappa (\omega)$ , the calculation formula of the transmission and reflection coefficients is simplified. After obtaining the reflected wave of the single outer layer material, we quantitatively calculate the time-domain reflection wave of interface. According to the principle of terahertz detection based on photoconductive antenna, the detection of multiple reflection waves is analyzed. To verify the superposition characteristics of multiple waves, the terahertz reflection waves of samples with interfacial air gap are tested. By comparing the measured waves and the artificially calculated waves, we prove that the superposition of the waveform is approximately linear. Based on the good coincidence between artificially generated waveform and measured waveform, we propose a method of quantitatively diagnosing the interface air-gap defect. Indeed, the deviation between the test results and the actual air-gap size is small.

Partial Discharge Detection and Discharge Type Identification of Gas Insulated Switchgear Based on Photoelectric Detection

Gas insulated switchgear (GIS) is a relatively common high-voltage substation equipment. Due to the internal complex structure and integrated switch, it is easy to cause insulation defects, burrs and partial discharge of metal particles, thereby reducing the insulation of equipment performance, which has attracted the attention of many researchers. In this study, aiming at the problem of partial discharge in GIS, a detection device of partial discharge based on photomultiplier instrument is proposed. In this device, Zemax software is used to simulate a front-end optical signal receiving module that enhances the photosensitive surface and enhances the optical signal aggregation effect. In terms of signal processing, the output voltage signal of the photomultiplier instrument is filtered, and the corresponding signal is extracted for amplification. The partial discharge detecting device designed in the experiment is used to judge the internal insulation defects of the GIS, and to discriminate the metal protrusion defects, the metal particle defects on the insulator surface, the free metal particle defects, and the air gap defects between the insulator and the high voltage conductor, respectively. The partial discharge signal ϕ-u-v spectra of different defect models are constructed. Through the image simulation of partial discharge signals corresponding to defects, it is verified that the detection device can distinguish the partial discharge caused by different defects, which is helpful to prevent the occurrence of malignant discharge accidents and maintain the stable operation of GIS equipment.

Decision tree rules for insulation condition assessment of pre-molded power cable joints with artificial defects

A decision tree method is presented that can judge the initial and final stages of insulation degradation for pre-molded power cable joints with air gap and void defects. According to the partial discharge (PD) of eight experimental subjects, it is validated that the discharge phase changes toward the voltage zero-crossing area and becomes wider. This well-known phenomenon can be applied to assess the transition of insulation status by the cluster theory. Subsequently, the Gini index coefficient and information gain of the decision tree algorithm yield the insulation status diagnostic rules. The result indicates an overall recognition rate of 85% in training data. Furthermore, the proposed diagnostic method has successfully detected the alert of the final stage prior to insulation breakdown for two additional experimental subjects. These decision tree rules are feasible and economical for an online PD monitoring device to achieve condition-based maintenance in distribution power cables.

Determination of optimal taper in continuous casting billet mould using thermo-mechanical models of mould and billet

An industrial continuous casting process of steel billet is analysed using numerical models for the heat transfer and mechanical deformation in the mould and billet. Elastoplastic thermo-mechanical model, including creep and thermal stresses, is developed to determine the temperature and stress fields, and the resultant deformation in the mould. A three-dimensional, turbulent flow model is also developed to simulate fluid flow and solidification of the molten steel as it flows inside the mould. The heat transfer at the mould-billet interface is decoupled using a heat flux profile obtained from inverse heat transfer calculations, developed in an earlier work. Temperature measurements made in the industrial billet mould have been used for this purpose. Important results from the models are discussed to gain understanding of the physical aspects governing the casting process. The distortion profile of the mould and the thermal shrinkage of the billet are used to determine an optimal taper for the mould. Two separate taper profiles are suggested for the corner and off-corner regions around the mould periphery, due to the different nature of the heat transfer in these regions. Finally, these two profiles are compared with the existing single tapered profile for the operational mould in the industry. It is observed that the existing profile has significantly less taper in the meniscus region which can lead to higher air gap and subsequent defects in casting.

岸电电缆绝缘气隙热源等效模型及载流量的影响研究

岸电电缆绝缘气隙热源等效模型及载流量的影响研究

Influences of Different Ratios of AC-DC Combined Voltage on Internal Gas Cavity Discharge in Oil-Pressboard Insulation

The degradation of insulation would be accelerated by the partial discharge (PD) incurred by an internal air-gap defect in oil-pressboard insulation. At different bridge rectifier locations in the converter transformer, the oil-pressboard insulation typically endures various ratios of combined ac and positive dc voltages. Therefore, the dielectric status of the converter transformer is closely correlated to the ratios of ac to dc voltages. It is practically significant to conduct the study of the correlation between discharge characteristics and ac-dc ratios. At the same time, it enhances the diagnosis of faults in converter transformers. According to the actual internal operation conditions of converter transformers, the study established a test platform. Furthermore, the conventional pulse current method was adopted to compare the discharge characteristics at the initiation and close-to-breakdown stages. Under these ratios, test results display distinctive PD dynamics. The PD of the air gap generates surface charges. These charges, in turn, establish an inverse electric field. As a result, the proportion of dc in the ac-dc combined voltage increases, and total initial voltage and total breakdown voltage increase continuously, but the recurrence rate of discharge pulse and discharge magnitude decrease at the initiation and close-to-breakdown stages. As the positive dc proportion increases, the discharge recurrence rate in the negative half cycle gradually exceeds that in the positive one. These research findings provide useful criteria for the fault diagnosis of converter transformers.

Numerical Simulation for the Cold Extrusion Process of the Worm Wheel Blank

The development of worm gear wheel blanks cold extrusion forming technology which can both reduce the manufacturing costs of the worm gear and enhance the Market competitiveness of products .Numerical simulation of cold extrusion forming technology of wheel blank was built by using Qform3D forging simulation software. That is, after the quantitative analysis of the distribution of stress fields and strain fields in the forming technology, determining the technology plan and conducting technology test, then the reasonable and feasible technology is designed. Cold extrusion can avoid the presence of the air gap, shrinkage and other tissue defects in the casting process, meanwhile raw materials can be saved 47% approximately than cutting.

Transmission of terahertz wave through one-dimensional photonic crystals containing single and multiple metallic defects

We investigated numerically and experimentally the transmission of terahertz (THz) waves through single and multiple metallic defects created in a one-dimensional (1D) photonic crystal (PC) by inserting single metallic wires or arrays of parallel metallic wires into the air-gap defect of the 1D PC. The transmission properties of the metallic defect modes generated in the photonic bandgap (PBG) were characterized by using THz time-domain spectroscopy. For single metallic defects, it was found that the appearance the defect mode depends not only on the diameter of the metallic wires but also on the polarization of the THz wave. For transverse magnetic (TM) polarized waves whose electric fields are parallel to the metallic wires, the incident THz wave is generally split into two identical parts. In sharp contrast, the excitation of surface plasmon polaritons (SPPs) with enhanced field intensity is observed for transverse electric (TE) polarized waves whose electric fields are perpendicular to the metallic wires. In both cases, two resonant modes with reduced transmittance are observed in the PBG. While the resonant mode related to SPPs is found at the long-wavelength side of the original defect mode, the resonant mode without the excitation of SPPs appears at the short-wavelength side. Numerical simulation based on the finite-difference time-domain (FDTD) technique revealed that the electric field of SPPs is more tightly confined at the surface of the metallic wire when it is placed in the PC, implying that the confinement of a THz wave in the propagation direction will facilitate the localization of SPPs in the transverse direction. For two parallel metallic wires, the defect mode was found to depend on the separation between them. If they are widely separated, then the excitation of SPPs is similar to that observed in single metallic wires. However, the excitation of dipole-like SPPs does not occur for two closely packed metallic wires because of their large lateral size. It was also revealed that two parallel metallic wires with a small diameter and a narrow separation could be employed to achieve a significant enhancement, as large as 21.6, for the electric field in between them. More interestingly, the enhancement factor becomes larger when the confinement of the electric field in the propagation direction is increased. For an array of four widely separated wires whose lateral dimension is wider than the diameter of the THz beam, only one resonant mode is observed at the long-wavelength side of the original defect mode. The experimental observations are in good agreement with the simulation results based on the FDTD technique. The enhanced concentration of the electric field of SPPs at the surfaces of metallic defects may be useful for focusing and sensing of THz waves.

Tunable multichannel optical filter based on silicon photonic band gap materials actuation

A Si-based tunable omnidirectional reflecting photonic band gap structure with a relatively large air gap defect is fabricated and measured. Using only one device, low-voltage tuning around two telecom wavelengths of 1.55 and 1.3 μm by electrostatic force is realized. Four widely spaced resonant modes within the photonic band gap are observed, which is in good agreement with numerical simulations. The whole process is at low temperature and can be compatible with current microelectronics process technology. There are several potential applications of this technology in wavelength division multiplexing devices.