Transactions of China Electrotechnical Society updated on May 16, 2024

Transactions of China Electrotechnical Society is a comprehensive academic journal in the field of electrical engineering sponsored by the China Electrotechnical Society, reporting international and domestic leading academic and scientific research achievements in basic theoretical research and engineering application. Ma Weiming, academician of the Chinese Academy of Engineering, is the director of the editorial board of the Journal and the editor-in-chief of the journal.

A scheme for improving the performance of current differential protection of flexible low-frequency transmission system based on control and protection coordination

Author: Zheng Tao, Kang Heng

Abstract: Low-frequency transmission is a new type of transmission technology that has the advantages of traditional AC and DC transmission technology, and has a good application prospect in scenarios such as offshore wind power transmission and flexible interconnection of urban power grids. When the low-frequency line of the flexible low-frequency transmission system is asymmetrically faulted, the modular multi-level matrix converter (M3C) usually adopts the fault control strategy to achieve the corresponding control goal, which affects the characteristics of the fault current and then affects the performance of the line protection.

In this paper, the causes of M3C capacitor voltage imbalance are analyzed, and in order to solve the problem that the capacitor voltage equalization control limits the degree of freedom of M3C negative sequence current, a capacitance voltage equalization strategy based on circulation control is adopted to realize the mutual decoupling of M3C capacitor voltage equalization control and negative sequence current control. Secondly, based on the action characteristics of current differential protection, a scheme to improve the protection performance of flexible low-frequency transmission system based on control and protection coordination is proposed.

With the goal of improving the performance of current differential protection, the negative sequence current control scheme is determined by integrating the M3C fault control strategy, and the negative sequence current reference value is selected in combination with the M3C bridge arm overcurrent problem. Finally, the corresponding simulation model of flexible low-frequency transmission system is built in PSCAD/EMTDC, and the feasibility and effectiveness of the proposed scheme are verified by a large number of simulation experiments.

Electrical impedance imaging algorithm of grounding grid based on elastic norm

Authors: Zhang Weizhe, Zhang Chengqian, Liu Jiansheng, Zhang Liang, Ma Wenbo

Abstract:As a non-invasive imaging method, electrical impedance imaging (EIT) has become one of the research focuses of grounding grid corrosion diagnosis, but the image reconstruction effect under the existing algorithms is not ideal. In order to accurately diagnose the corrosion of the substation grounding grid, an L1-L2 elastic norm algorithm is proposed to modify the EIT inverse problem model of the grounding grid, so as to obtain a solution of the resistivity of the grounding grid that is closer to the real value, so as to improve the accuracy of corrosion diagnosis.

Firstly, the basic principle of EIT technology in the corrosion diagnosis of grounding network was introduced. Secondly, the simulation model of the grounding grid under the action of four algorithms, including L1 norm, L2 norm, one-step Newton method (NOSER) and L1-L2 elastic norm, was analyzed, and the galvanized steel model was built for experimental verification. Finally, the evaluation criteria such as image correlation coefficient and image relative error were introduced to evaluate the reconstructed grounding grid images under the four algorithms.

The results show that the proposed algorithm has a good effect in improving the pathology and undercharacterization of the EIT technology of the grounding network, and compared with the other three algorithms, the image correlation coefficient is increased by 32.19% on average, and the relative error is reduced by 22.54% on average. The proposed algorithm enhances the application of EIT technology in the corrosion diagnosis of grounding grid.

Study on the effect of moisture content on the low-frequency dielectric relaxation process of epoxy impregnated paper

Authors: Zhang Hongliang, Huang Ning, Liu Peng, Jin Hai, Peng Zongren

Abstract:Epoxy composite material is the main insulating material of the capacitor core of UHVDC bushing, which will have a great impact on the bushing when exposed to moisture. In order to clarify the effect of moisture content on the low-frequency dielectric relaxation process of epoxy composites, epoxy resin and epoxy impregnated paper were prepared, and the water absorption and low-frequency dielectric properties of the two samples were measured under different humidity conditions. The dielectric spectrum information was fitted and analyzed by using the Havriliak-Negami model, compound AC conductivity and complex electrical modulus, and the dielectric characteristics of the cellulose σ polarization and interface polarization in epoxy-impregnated paper were obtained.

The results show that the moisture content has little effect on the low-frequency dielectric properties of epoxy specimens, but has a significant effect on the low-frequency dielectric properties of epoxy-impregnated paper specimens. Moisture will reduce the σ polarization activation energy of epoxy impregnated paper specimens and improve the σ polarization dielectric relaxation strength. The increase of moisture content will also increase the DC conductivity of epoxy impregnated paper, reduce the interfacial polarization time constant, and enhance the dielectric relaxation intensity of interfacial polarization. In this paper, the influence degree and mechanism of moisture content in the process of low-frequency dielectric relaxation of epoxy-impregnated paper are quantitatively analyzed, which provides a theoretical basis for the evaluation of the damp state of UHVDC bushing.

Research on epoxy performance of liquid crystal for high-voltage and high-power IGBT (1): electrical insulation characteristics

Authors: Wang Zhengdong, Cao Xiaolong, Yang Ganqiu, Luo Meng, Zhou Yuanhang

Abstract:With the development of insulated gate bipolar transistors (IGBTs) in the direction of high voltage and high power in industrial applications, higher requirements are put forward for their heat dissipation, heat resistance and insulation performance, and it is urgent to develop a high-performance insulating potting material. In this paper, it is proposed to construct liquid crystal domains by using biphenyl epoxy monomer and benzidine curing agent double liquid crystal molecules, focusing on the control of the thermal and electrical properties of the materials.

The series of papers (1) studies its related thermal properties, and the results show that by using liquid crystal benzidine curing agent and biphenyl liquid crystal epoxy to construct a cured product with π-π stacked liquid crystal domain and high cross-linking density network structure, the glass transition temperature Tg can reach 247 °C, which is 76.4% higher than that of the traditional bisphenol A type epoxy, and the thermal conductivity is increased to 0.351 W/(m· K), compared with BPA epoxy, increased by 64.0%. In addition, the microscopic parameters and macroscopic properties of cross-linked polymers, such as free volume, Tg, and thermal conductivity, were analyzed by molecular dynamics simulation.

Research on the epoxy performance of liquid crystal for high-voltage and high-power IGBT (2): electrical insulation characteristics

Authors: Wang Zhengdong, Cao Xiaolong, Yang Ganqiu, Luo Meng, Zhou Yuanhang

Abstract:The insulation failure of potting materials for high-voltage and high-power IGBTs is one of the key factors restricting their rapid development. The series of two papers aims to synergistically improve the thermal conductivity, heat resistance and electrical insulation properties of epoxy insulating potting materials through the research ideas of constructing liquid crystal domains.

The results show that the liquid crystal domains constructed by π-π self-assembly can effectively reduce the free volume after epoxy curing, thereby inhibiting carrier migration, and exhibit a higher breakdown field strength (126.72 kV/mm) and lower partial discharge (1.37 pC) than bisphenol A epoxy. The study of thermally stimulated depolarization current shows that the biphenyl epoxy-biphenyl curing agent double liquid crystal crosslinking material has a high trap energy level and enhanced electron capture ability.

In addition, the parameters of the cross-linked system, such as the free volume, mean square displacement under electric field, and breakdown field strength, were simulated and calculated by molecular dynamics simulation software, and the electrical properties of the materials were analyzed. The research results can provide a certain reference for the engineering application of new epoxy resin in the field of high-voltage and high-power IGBT potting insulating materials.

GIS partial discharge optical signal detection technology and optical image diagnosis method

Authors: Li Ze, Qian Yong, Liu Wei, Zang Yiming, Sheng Gehao, Jiang Xiuchen

Abstract:Photometry is an effective partial discharge detection technique. In order to study the optical characteristics of typical partial discharge and the diagnostic method of optical image, an experimental platform for joint detection of gas-insulated partial discharge was built to realize the synchronous acquisition of optical signals and electrical signals. Then, the light guide column method and the fluorescence fiber method were used to measure and statistically analyze the typical discharge optical signals in SF6 gas, and the discharge characteristic parameters of the two optical detection methods were compared and studied, including the initial discharge voltage, phase distribution, pulse amplitude and quantity, etc. On this basis, a fault diagnosis method for partial discharge optical images based on Pyramid Gradient Histogram (PHOG) feature was proposed by using the typical partial discharge signals collected by the light guide column. The results show that the diagnostic accuracy of the model is 90.4%, which verifies the reliability of partial discharge optical detection and diagnosis based on light guide.

A high-precision detection method of SO2 content in GIS equipment based on the principle of non-dispersive infrared

Authors: Chen Yukun, Chu Jifeng, Ruan Zhuoyi, Li Haoyuan, Yang Aijun, Yuan Huan, Rong Mingzhe, Wang Xiaohua

Abstract:The internal discharge of gas-insulated switchgear (GIS) will lead to the decomposition of SF6 gas into characteristic products such as SO2, and the severity of discharge fault can be inverted according to the SO2 content. However, the existing SO2 gas sensors cannot take into account the detection accuracy, manufacturing cost, equipment volume, and cross-sensitivity. Moreover, the operating environment temperature of GIS equipment is complex and changeable, which can easily lead to sensor drift failure.

Therefore, a dual-channel SO2 detection device based on the principle of Non-Dispersive Infrared (NDIR) is designed, and a wide-range migratory temperature (0~40°C) compensation method based on polynomial fitting coefficient correction is proposed, which can accurately detect SO2 in the background of SF6 within the range of 1 000 μL/L, and the average detection error is less than ±2.8%.

By simulating the SF6 gas discharge of different intensities, the SF6 decomposition products with a concentration of 27~118 μL/L SO2 were obtained. According to the benchmarking results of the gas chromatograph, the average detection error of the designed SO2 detection device is ±3.8 μL/L, which proves the great potential of the SO2 detection device in the field of online monitoring of discharge faults of GIS equipment.

Experimental exploration of dielectric barrier discharge plasma treatment of transformer waste insulating oil

Authors: Zhang Guozhi, Wang Wenxiang, Zhang Lei, Shao Tao, Zhang Xiaoxing

Abstract:Waste insulating oil for transformers not only endangers human health, but also threatens the ecological environment, and it is of far-reaching significance to dispose of waste insulating oil safely and effectively. Based on this, this paper attempts to use dielectric barrier discharge (DBD) plasma technology to treat transformer waste insulating oil, and experiments to explore the characteristics of flat plate DBD and coaxial DBD degradation treatment of waste insulating oil under different background gases.

The experimental results show that the bonded dehydrogenation reaction of the waste insulating oil treated by DBD plasma decreases and the gas is generated, and the coaxial DBD reaction under the background of argon is more sufficient. In the argon background gas, the proportion of H2 in the gas product is more than 50%, and in the air background gas, the proportion of H2 and CO2 in the gas product is basically the same, both close to 50%. In addition, the thermogravimetric characteristics of the waste insulating oil after DBD plasma treatment have changed severely, and they are transformed into a small number of alkane molecules with longer carbon chains and a large number of shorter alkane molecules. The research results of this paper provide a new solution for the treatment of transformer waste insulating oil.

DC bias voltage generation conditions under typical faults in distribution network and PT fuse analysis caused by them

Authors: Wen Weijie, Hu Tiewei, Chen Xiaolong, Wang Lei, Li Bin

Abstract:The problem of frequent fuse blowing of high-voltage side protection fuses of electromagnetic voltage transformer (PT) is widely found in the neutral ungrounded distribution network of 10~35kV, which increases the operation and maintenance burden of the power grid and threatens the reliability of power supply.

In order to clarify the deep mechanism of frequent blowing of PT high-voltage side fuse, this paper analyzes the no-load transient voltage of the system under typical faults based on the superposition principle and numerical calculation method, and finds that the voltage presents a special form of AC/DC composite - power frequency voltage plus DC bias voltage, and summarizes the formation conditions and mechanism of DC bias voltage as follows: when the fault disappears, the net charge of the three-phase stray capacitor is not zero (Qn≠0), and the DC bias voltage is formed by Qn without a discharge path.

Under the influence of different factors such as line length, PT impedance, load and reactive power compensation device, the influence of Qn bleeder path on DC bias voltage is studied, and it is found that PT deep saturation under DC bias voltage is the ultimate reason for triggering fuse blowing. Finally, the 10kV distribution network model in PSCAD/EMTDC and the 380V/150V physical dynamic simulation test platform were simulated and tested.

Gas gap switch trigger failure analysis and life improvement method

Authors: Dong Bingbing, Meng Yan, Guo Zhiyuan

Abstract:The jet plasma-triggered gas gap switch has a fast response and simple structure, which may meet the action requirements of rapid fault isolation of the commutation branch of the hybrid high-voltage DC circuit breaker for multi-terminal flexible power grid. However, the analysis of the trigger deterioration process caused by the cumulative effect of repeated trigger discharge is insufficient, and the research on the trigger life improvement method considering the influence of multiple factors is not clear.

Increasing the pulse capacitance voltage of the trigger loop can effectively reduce the jitter of the discharge delay, but the impact on the life improvement can be negligible. The stepped increase of the charging voltage of the energy storage capacitor can significantly improve the trigger life and contact stability. After polytetrafluoroethylene (PTFE) is doped with inorganic filler, it can effectively improve the plasma injection characteristics, but aggravate the ablation and deterioration of the trigger cavity, and the trigger life is PTFE>PTFE+0.2%MoS2>PTFE+0.5%Cu powder, and the research results can provide theoretical reference and engineering application guidance for the failure judgment and life improvement method of gas gap switch.