In this paper, power electronic transformer (PET)-based railway traction systems are comprehensively reviewed according to the unique application features and requirements. By comparing PET and conventional line frequency transformer (LFT)-based systems, their pros and cons are summarized. By further reviewing all kinds of PET-based designs from the early concepts to the latest ones in the order of their publication dates, the developing trends are highlighted. By synthetically considering the requirements and the state of the art, the key challenges and opportunities are identified and discussed. It shows that although PET-based systems are still developing and far from mature, they are already superior to LFT-based systems in terms of system weight, efficiency, and functionalities especially for 15-kV/16.7-Hz applications. With the advancements on wide bandgap power devices, medium frequency transformers, and converters, PET systems will be even more promising and available for all types of railway tractions in the near future.

Power Electronic Transformer-Based Railway Traction Systems: Challenges and Opportunities

In this study, a simulation model of surface charge accumulation has been established. The model considers three accumulation ways, i.e. electrical conduction within the gas, through insulator volume and along the insulator surface. The generation, diffusion, drift and recombination of charge carriers are also taken into account. Based on it, the influence of polarity reversal, reversal time on surface charge and electric field distribution on a basin-type insulator are studied. The polarity of the surface charges and the direction of the electric field change after the voltage polarity reversal. When the preload voltage is equal to reversal voltage, the surface charge and the electric field distributions at steady state before and after voltage polarity reversal are all the same with opposite sign, and not affected by the reversal time. However, the time to reach the steady state varies with different reversal time. The steady-state surface charge and electric field increased with the rise of reversal voltage. The transient normal and tangential electric field would not exceed the value of the steady state, which means voltage polarity reversal has no additional influence on insulation performance. This research can provide guidance to the design and manufacture of DC GIS/GIL.

Dynamics of surface charge and electric field distributions on basin-type insulator in GIS/GIL due to voltage polarity reversal

Theoretical model of absorption coefficient of an inhomogeneous MPP absorber with multi-cavity depths

Vegetable oils (VOs) are based on natural ester oil that is considered as an alternative to mineral oil for transformer applications. The dispersion of nanofillers into the VO is expected to improve the physicochemical and electrical performances. Surface-modified insulating nanoparticle is dispersed into the VO to prepare the VO-based nanofluid (NF). An open-beaker oxidative aging study is carried out at 115 °C for VO and NF at three distinct aging durations such as 100, 300, and 500 h. Alterations in the molecular level of aged VO and NF are studied through the various spectroscopic and physicochemical analyses such as nuclear magnetic resonance, Fourier transform infrared, UV-vis absorbance, fluorescence, color, acid number, interfacial tension, and flash point. The fluorescence-based analysis is carried out for VO and NF at fresh and aged condition to monitor the comparative aging degradation. Oxidative aging of VO and NF results in the formation of flammable and harmful gases, which are analyzed using dissolved gas analysis technique. A comparative analysis of the electrical properties such as ac breakdown voltage, dielectric constant, and dielectric dissipation factor for fresh and aged sample of VO and NF is studied.

Condition Assessment of Aged Ester-Based Nanofluid Through Physicochemical and Spectroscopic Measurement

With the aim of exploring an alternative method of nano-doping in cellulose paper and improving flashover strength of oil-impregnated paper, radio frequency (RF) magnetron sputtering method is introduced into the structural modification of insulating paper for converter transformers. In this experiment, insulating paper was treated with ZnO sputtering for 0, 7.5, 15 and 30 min, respectively. The surface morphology of dry paper was observed with a scanning electron microscope. The bonding mode of sputtered ZnO particles with cellulose matrix was investigated via attenuated total reflection infrared (ATR-IR) spectroscopy and mechanical property of the sputtered samples were studied. Surface and volume conductivities of the oil-impregnated sputtered paper were measured. Moreover, the charge dissipation characteristics of sputtered insulating paper was investigated by means of surface potential decay. In addition, trap distribution and carrier mobility of specimen were further obtained. Finally, the DC flashover strength were tested. The results showed that ZnO magnetron sputtering had a distinct influence on the structure of the insulating paper, resulting in the formation of hydrogen bond and chemical bond and an increase of the surface and volume conductivities. ZnO sputtering was found to decrease the initial potential and accelerate charge decay. Moreover, appropriate sputtering enhanced the surface flashover strength.

Effects of ZnO magnetron sputtering on surface charge and flashover voltage of oil-impregnated paper

Although the amorphous alloy core distribution transformers (AMDTs) have the advantage of lower no-load losses over orientation silicon–steel distribution transformers, a high noise level is inevitable. The vibration and noise for different positions of cores are still unknown. Understanding the vibration and noise behaviors of cores under operation is vital to reduce the noise of AMDT. In this paper, two cores from 10 kVA AMDT and 24-turn coils were used as the testing objects, and the ac voltage of 54.3 V was applied to the coils until the magenetic flux density attained 1.337 T. The finite-element method was used to analyze the cores’ magnetic-field distribution. An attempt was made to find the largest amplitude position of the cores by vibration sensors. Sensors were arranged on the surface of the cores according to the magnetic flux density distribution. A personal computer data acquisition platform was set up for signal measurement. Fast-Fourier transform and wavelet packet transform were integrated to analyze the vibration signals. The vibration amplitude related with time and frequency was discussed at different positions of the cores, and experimental results indicated that the position with the largest amplitude was in the middle of the core’s yoke and limb at 200 Hz. The experimental results suggested that magnetostriction played a major role in exciting noise and vibration. A filler of silicon rubber and foam under the cores can lessen the vibration energy. This performs well in reducing noise. All the above testing results and analysis are helpful to optimize the design and improve the manufacturing technology of AMDT.

Dynamic Behavior of Magnetostriction-Induced Vibration and Noise of Amorphous Alloy Cores

Transformer oil, insulating paper, high voltage direct current (HVDC) bushings and on-load tap changer (OLTC) are essential parts of converter transformers. When the power flow of the HVDC system is reversed or a sudden outrage occurs, the charge field will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation damage. In this case, it is necessary to carry out the characterization of the oil–paper interface to avoid these consequences. This paper analyses the interface charge accumulation and dissipation characteristics of an oil–paper insulation system under DC and polarity reversal voltages. In this paper, induced electrification was used to charge the composite oil–paper insulation. The amplitude of DC and polarity reversal voltages applied in this paper was 4 kV, and paper with thicknesses of 0.08 and 0.13 mm was used. This research offers a theoretical exploration to optimize the transformer oil– paper insulation structure and prevent damage to its insulation systems. From the research on different layers and thicknesses of paper, it can be concluded that the decay is fast in the initial period and then becomes slower with the lapse of time until finally the decay curve tends to become flat. With an increase in the number of the paper layers, the interface charge density decreases. So in real life multilayer paper is used to avoid failures owing to its structural weakness. Regarding the DC polarity reversal effect, increasing the thickness of the paper is equivalent to adding paper layers. The dissipation rate of the positive interface charge is quicker than that of the negative one. In the polarity reversal test, the dissipation rate becomes smaller as the reversal time gets longer.

Interface charge behavior of multi-layer oil–paper insulation under DC and polarity reversal voltages

Oil-paper insulation system is an extraordinarily essential system of converter transformers as well as HVDC bushings. When the system changes its state, such as switching the thyristors, operating over-voltage appears, and the charge will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation failure, so charge accumulation and dissipation on oil-paper insulation under pulse voltage need more investigation. This paper studies the surface charge behavior on double-layer oil-paper insulation under pulse voltage and inducing electrification was used to charge the oil-paper composite. Studies of various voltage amplitudes, pulse frequencies, numbers and polarities showed that the decay is fast initially, and then becomes slower with the time. Eventually the dissipation of charge behaviors tends to be flat. With the increase of voltage amplitudes, the surface potential rises, and less positive charges are accumulated on the surface than negative charges. The increase of the pulse frequency causes the initial values of surface potential a little smaller and the charge to dissipate more quickly. Also, with the increase of pulse numbers, the initial value gets bigger and the surface charge would be slowly dissipated. From the tdV/dt and decay time curves, the figures of positive voltage usually have two peaks, while there is only one peak in most of the curves under the negative one. With the increase of voltage amplitudes, the charge will dissipate more quickly. Additionally, the peak value and the characteristic time increase with the reduction of pulse frequency and the addition of numbers.

Dynamic behavior of surface charge on double-layer oil-paper insulation under pulse voltage

Noise pollution is an important issue in modern society. This paper is mainly concerned with noise suppressing of amorphous metal alloy core superconducting distribution transformers (AMSDTs). A microperforated sound absorption structure based on the electroacoustical equivalent circuit model was designed and installed in the inner sides of the box circumference. Up to now, there have been few investigations and reports of its application in superconducting transformer. In this paper, single and double microperforated panels (MPPs) were installed in the inner sides of the box circumference when the box was filled with air. To understand the reduction in vibration noise behaviors of AMSDTs installed with MPPs under normal operation, an AMSDT model was used as the test object, and a single MPP, double MPPs, and different backed cavities filled with the air experimentally and compared when rated voltages were applied to the AMSDT model. Tests were also carried out on the AMSDT model with different temperature air cavity and MPPs made with different materials. An attempt was made to find the largest amplitude and its corresponding frequency on the box surface using vibration sensors, which were arranged on the surface of the box. A data-acquisition platform was set up for signal measurement. The results validate the proposition that the MPP absorber effectively reduces the audible noise of the AMSDT, that the performance is improved by partitioning the backed cavity into differently sized sections, and that the performance is affected when the cavities are filled with air and overexcitation currents or voltages are applied to the AMSDT model. The test results are helpful for optimizing the design and reducing the noise of AMSDTs.

Suppressing Noise for an HTS Amorphous Metal Core Transformer by Using Microperforated Panel Absorber

Although the cores made with amorphous metal alloy have lowered no-load losses compared with the orientation silicon steel cores, a high cost and noise level are inevitable because the magnetostriction for amorphous metal alloy strip is larger than the ordinary one. This study mainly focused on the experimental systems and methods for magnetic properties measurement, vibration characteristics and noise level of amorphous alloy strip and combinations. The vibration characteristics and noise level of amorphous alloy composite strip were also studied. The influence of the annealing process on magnetic energy, vibration characteristics and noise level of amorphous alloy core were analysed in detail as well. All the testing results and analysis above are helpful for the transformer manufacturers, and they can replace the expensive amorphous alloy metal strip with the combinations to reduce the noise and cost of the cores.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-epa.2019.0137

Daosheng Liu

Papers

Dynamic Behavior of Magnetostriction-Induced Vibration and Noise of Amorphous Alloy Cores

Interface charge behavior of multi-layer oil–paper insulation under DC and polarity reversal voltages

Dynamic behavior of surface charge on double-layer oil-paper insulation under pulse voltage

Suppressing Noise for an HTS Amorphous Metal Core Transformer by Using Microperforated Panel Absorber

Magnetic properties and vibration characteristics of amorphous alloy strip and its combination