Siwei Liu

Diagnosis of transformer winding faults based on FEM simulation and on-site experiments

Power transformers are the most expensive and important component in the power system, and may be subjected to breakdown due to mechanical and/or electrical faults. It is difficult to obtain the condition of the power transformer by only one test method, so transformer fault diagnosis based on different experimental and simulation methods is widely recommended. In this paper, the transformer winding faults diagnosis based on FEM simulation and on-site experiments is described. Based on the FEM simulation, it is necessary to analyze the effect of the winding faults on the change of the electrical parameters. The typical types of winding faults, such as axial displacement, and radial deformation, etc. are taken into consideration. Different fault degrees are investigated in the simulation analysis. Using the parameters calculated in the FEM model, an equivalent circuit of the power transformer is present in ATP/EMTP to study the performance of typical on-site experiments, such as frequency response analysis (FRA) and short-circuit impendence (SCI) test. The frequency bands, which might be treated as the diagnostic bands in the FRA, are simulated and compared. It is found that the resonances in the frequency bands of 200–250 kHz and 350–450 kHz are able to describe the situation, where the displacement percentage of the winding is varying from 0% to 9%. The SCI values under different fault severity are also obtained and compared. Therefore, the diagnosis method can directly bridge the types and degrees of the winding faults with the experiment results. The operating states of several 110 kV power transformers are tracked, and it can be found that the simulation results show a better consistency with the on-site experiment results, and the proposed diagnosis method will help to discriminate the winding faults.

Pulse Magnetic Properties Measurement and Characterization of Fe-Based Nanocrystalline Cores for High-Voltage Pulse Magnetics Applications

Fe-based nanocrystalline (FE-N) cores are widely used in power electronics and pulsed power applications. In pulsed power applications, it always requires high excitation current (up to tens of kiloampere) to excite large-volume cores (up to hundreds of cubic centimeter) with fewer number of winding turns, and it is hard to use the traditional test scheme to obtain the pulse magnetic properties in this case. The principles and scopes for the application of pulse magnetic properties measurement circuits, including the single-winding test circuit and double-winding test circuit, are analyzed and presented. Although the double-winding test circuit is classical to test the hysteresis loop, the single-winding test circuit is not totally useless in some cases. Based on the double-winding test circuit, the initial magnetization curves and maximum hysteresis loops of toroidal FE-N cores with high and low remanence are obtained under a relative constant magnetization rate with the order of 0.7-7 T/μs, and an equivalent frequency from 49 to 650 kHz. The unsaturated permeability is calculated based on the experimental results and fitted with the magnetization rate. The loss densities and permeability are fitted and compared. A design procedure of the saturable pulse transformer is illustrated for the application of the testing results. It is proved that the initial energy loss and unsaturated permeability are used for the design of the pulse transformer, and the traditional core loss is suitable for the estimation of power loss under the repetitive operation condition.

Optimization Design of a Repetitive Nanosecond Pulse Generator Based on Saturable Pulse Transformer and Magnetic Switch

Saturable pulse transformer (SPT) and magnetic switch (MS) are widely used together in pulse generators (PGs). It is easy to obtain a desirable output with a high voltage and low rise time when choosing the suitable PG topology. A compact, unipolar, and repetitive high-voltage nanosecond PG, which is based on SPT and MS, is designed and constructed. The operation principles of the resonant charging based on SPT and MS are described. The design and performance optimization of the magnetic components including SPT and MS are presented. The minimum volume of the SPT is derived and verified in the experiment. The core volume and turns of the MS are determined both considering the charging process and the output characteristic. The saturation times of the pulse transformer and MS can be optimally determined based on the B-H curve, which is obtained under the same working conditions. It reveals that the design and performance optimization methods of the SPT and MS are reasonable and can be used to guide the design of pulse transformers and MSs in pulsed power, power electronics, and industrial electronics.

Modeling analysis of pulsed magnetization process of magnetic core based on inverse Jiles-Atherton model

The J-A (Jiles-Atherton) model is widely used to describe the magnetization characteristics of magnetic cores in a low-frequency alternating field. However, this model is deficient in the quantitative analysis of the eddy current loss and residual loss in a high-frequency magnetic field. Based on the decomposition of magnetization intensity, an inverse J-A model is established which uses magnetic flux density B as an input variable. Static and dynamic core losses under high frequency excitation are separated based on the inverse J-A model. Optimized parameters of the inverse J-A model are obtained based on particle swarm optimization. The platform for the pulsed magnetization characteristic test is designed and constructed. The hysteresis curves of ferrite and Fe-based nanocrystalline cores at high magnetization rates are measured. The simulated and measured hysteresis curves are presented and compared. It is found that the inverse J-A model can be used to describe the magnetization characteristics at high magnetization rates and to separate the static loss and dynamic loss accurately.

Application of Jiles-Atherton model in description of temperature characteristics of magnetic core

The Jiles-Atherton (J-A) model is widely used in magnetic characteristic modeling in alternating field. A dynamic J-A model, which is applied under high-frequency condition, is established. A pointwise iterative calculation method is used, and an approximate calculation method of the anhysteretic magnetization intensity and its derivative is proposed. The solving procedure of the J-A model is optimized. The platform for the magnetization and temperature characteristic test is designed and constructed. The hysteresis curves of ferrite cores at different temperatures are measured. The variation of parameters of magnetic properties with temperature is analyzed, including maximum magnetic flux density, remanence, coercivity, and core loss. Based on the particle swarm optimization combined with natural selection, optimized parameters of the J-A model are obtained. Their variation is analyzed, and their fitted curves with temperature are obtained. It is found that J-A models modification in temperature can be used to guide the modeling analysis of electrical equipment containing magnetic cores working in high-temperature environment and to predict magnetic properties at high temperatures.

Study on underwater subsonic electrical discharges: Streamer morphology and development

This paper focus on the streamer behaviors of underwater subsonic electrical discharges. The influence of applied voltage on the development mode and morphology of the underwater subsonic streamers is studied. The discussion on the development mechanism of the underwater subsonic streamer is given. Finally, the polarity effect on the underwater subsonic streamer morphology and development is analyzed. The results show that the positive streamers develop in two modes: bubble cluster mode (positive subsonic mode) at low applied voltage and tree-like streamer mode (positive supersonic mode) at high applied voltage. Meanwhile the negative streamers develop in tree-like streamer mode (negative subsonic mode) but with numerous filaments. The large discrepancy between mobility of ions and electrons is supposed to be the main cause of polarity effect.

Influence of voltage polarity on intensity of shock waves induced by underwater pulse discharges

The shock wave generated by high-voltage pulse discharges in liquid has been widely used in industrial applications. In this paper, an integrated experimental platform of underwater pulse discharges is established, a needle-plate electrodes system with the gap distance of 10 mm is used. The influence of voltage polarity on the process, morphology and physical characteristics of streamer development are discussed, based on the analysis of energy transfer process, the influence of voltage polarity on the shock intensity is studied. The results show that for the subsonic streamer, the use of negative polarity voltage can significantly improve the energy conversion efficiency of the shock wave. For the positive polarity, the supersonic streamer mode can significantly reduce the energy loss of the pre-breakdown process, increase the rate of expansion of the plasma channel, and obtain a higher intensity of shock wave. Using the negative polarity voltage, or controlling the voltage of the positive polarity to make streamers develop in the supersonic streamer mode, can significantly improve the intensity of the shock wave.

Effect of electrical breakdown discharge modes on shock wave intensity in water

Based on the underwater pulsed discharge platform, the effect of the electrical breakdown discharge modes on the shock wave intensity in water is discussed in the pin-to-plate configuration. Based on the analysis of discharge images captured by a high-speed camera, it can be concluded that the discharge modes are classified as subsonic bush-like streamers or supersonic filamentary streamers. Experimental results demonstrate that the amplitude of applied voltage have a great influence on the discharge modes as well as the shock wave intensity. In the experimental conditions a threshold value of 22.5kV is necessary for the subsonic streamers turning into supersonic streamers, which is a remarkable measure to reduce the energy loss and improve the energy conversion efficiency and eventually enhances the shock wave intensity.

Underwater positive streamer propagation with different insulation modes

Application of the high voltage on the electrode system in water can lead to the induction of the streamer and the propagation of the streamer to the opposite electrode. In this paper, an underwater electrode system with a gap distance of 10mm is constructed. It is convenient for this system to investigate the development of the streamer. Whether the tip of the positive needle electrode is directly exposed to water is called direct liquid discharge or dielectric discharge, respectively. It is shown that during the pre-breakdown process, the positive streamer generated by direct liquid discharge is always induced near the positive needle electrode. The lower leakage current is beneficial to induce the breakdown voltage and improve the propagation speed. When the voltage across the gap is low, the gray bush-like subsonic streamers in the beginning is eventually turned to the bright filamentary supersonic streamers. In addition, the influence of the non-uniformity level of the electric field on the development of the initial streamer is compared and analyzed. Besides, one way to effectively reduce the pre-breakdown leakage energy is put forward.

General diagnosis of transformer winding axial displacement faults based on FEM simulation and on-site experiments

Structural faults detection of power transformers are quite significant, and general diagnosis based on different test and simulation methods is commonly used to detect mechanical and/or electrical faults in power transformers. In this paper, the diagnosis of the transformer winding axial displacement faults based on FEM simulation and on-site experiment is carried out and presented. Based on the FEM simulation, it is necessary to analyze the effect of the axial displacement faults on the change of the electrical parameters. The whole windings or the single disks axial displacement are under consideration, and different fault levels are taken into consideration in the simulation analysis to investigate the influence of axial displacement on the electrical parameters. Using the calculated parameters, a distributed model of the power transformer based on ATP/EMTP is present to study the performance of frequency response curve on the axial displacement with different fault levels. The operating states of several 110 kV power transformers are tracked, and a typical transformer after the short-circuit impact is tested. The FRA test results are obtained before and after the short-circuit impact. The simulation results and the on-site test results are compared, and they show a better consistency. It can be concluded that under the typical impact of the axial displacement, the frequency response curve starts an obvious variation from 150 kHz, and the resonances and anti-resonances are shifted to the high frequency direction. The overall fault detection results show that the proposed method can discriminate the winding axial displacement fault with the high hit ratio.