Search

In this paper, the electron-molecule collision ionization is added to field emission under non-vacuum conditions, and the change of emission current caused by vacuum adjustment in field emission of carbon nanotubes is explained. The field emission current density equation under non-vacuum conditions is established. Through the theoretical analysis and the processing of experimental data, it can be concluded that when other variables are controlled unchanged, the change of pressure will affect the concentration of gas molecules in the air and the collision probability with electrons, then the density of emission current is changed. The study has a certain reference value for the application of field emission in low vacuum and atmospheric pressure.

According to the incomplete technical features of the traditional low-frequency transformer (LFT) and the existing power electronic transformer (PET), especially in terms of the function, the voltage level, and the cost, the core flux modulation of the hybrid transformer was achieved through the establishment of auxiliary windings combined with connection of the power electronic converter (PEC). For the hybrid transformer, a three-port converter topology is proposed and the strategy applicable for the dc bias compensation by connecting the converter is presented. By utilizing the dc modulation magnetic flux, the dc bias problem could be directionally compensated. The exciting current could be improved significantly and maintained at a steady state. The method proposed does not change the connection of the transformer external circuit and does not increase the peripheral device of the transformer, which provides a new way to solve the problems of magnetic state fluctuation and excitation current distortion.

Power transformer iron core is mainly composed of electrical sheet steel, which is easily affected by vibration and noise. Therefore, it is necessary to measure the magnetostriction influence of electrical sheet steel. This paper analyzes the performance of vibration and noise in the power transformer iron core considering the magnetostriction influence of electrical steel sheet under different dc biases. First, the butterfly loops of magnetostrictive and magnetic hysteresis loops of electrical steel sheet measured by using a laser displacement magnetostriction measurement system, and the effect of dc bias on magnetostrictive and hysteresis characteristics was analyzed. Second, modeled to single-valued curves along the rolling direction and transversal direction for the finite-element method. Finally, the simulation results of a single-phase power transformer are verified by the experimental results under the dc bias.

The nonlinear ac-dc composite electric field in the winding area of the converter transformer was studied by considering the frequency and temperature dependence of the insulation materials in this paper. First, the relative permittivity and conductivity of vegetable oils, mineral oils, and insulation papers were measured at different frequencies and temperatures. Then, the oil-paper insulation structure model of one 500 kV converter transformer valve-side winding was established. The nonlinear composite electric field of the converter transformer insulation with vegetable oils and mineral oils was analyzed. The maximum voltage of the converter transformer with mineral oils is 524.9 kV, at -20 °C and that with vegetable oils is 497.1 kV. The results show that the calculation method proposed in this paper is more accurate than the traditional superposition method.

In this paper, a bearing current release circuit based on carbon nanotube field emission is proposed. By combining the carbon nanotube field emission equivalent circuit with the motor bearing current equivalent circuit, the equivalent circuit of motor bearing current release based on carbon nanotube field emission is obtained. The relationship between the parameters is analyzed, and the circuit model of bearing current equivalent circuit based on carbon nanotube field emission is established. A non-contact field emission bypass is set between the shaft and the housing of the motor to eliminate the bearing current. This method can avoid material wear and is a new way to restrain bearing current.

DC bias in power transformers leads to strong core saturation and thus nonlinearly inferior operation. This paper analyzes the ability of power transformers to withstand the dc bias by novel analytical criteria. The proposed criteria are obtained from the nonlinear equivalent circuit, which considers the effects of nonlinear inductances. The validity of the proposed analytical criteria is verified by the finite-element analysis results and experimental results, which indicates that the proposed analytical criteria are effective to evaluate the ability to withstand the dc bias of power transformers.

This paper designs three distribution devices for the smart grid, which are, respectively, novel transformer with dc bias restraining ability, energy-saving contactor, and controllable reactor with adjustable intrinsic magnetic state based on the magnetically tunable nanocomposite material core. First, the magnetic performance of this magnetic material was analyzed and the magnetic properties processing method was put forward. One kind of nanocomposite which is close to the semihard magnetic state with low coercivity and high remanence was attained. Nanocomposite with four magnetic properties was processed and prepared using the distribution devices design. Second, in order to adjust the magnetic state better, the magnetization and demagnetization control circuit based on the single-phase supply power of rectification and inverter for the nanocomposite magnetic performance adjustment has been designed, which can mutual transform the material's soft and hard magnetic phases. Finally, based on the nanocomposite and the control circuit, a novel power transformer, an energy-saving contactor, and a magnetically controllable reactor were manufactured for the smart grid. The maintained remanence of the nanocomposite core after the magnetization could neutralize the dc bias magnetic flux in the transformer main core without changing the transformer neutral point connection mode, could pull in the contactor movable core instead of the traditional electromagnetic-type fixed core, and could adjust the reactor core saturation degree instead of the traditional electromagnetic coil. The simulation and experimental results verify the correctness of the design, which provides reliable, intelligent, interactive, and energy-saving power equipment for the smart power grids safe operation.

Purpose The purpose of this paper is to solve the reactive power adjustment and the overvoltage suppression problems in the extra high voltage (EHV) long distance grid, which often appears transient overvoltage, light load loss and other issues. Design/methodology/approach One 800 kV EHV magnetically saturation controllable reactor (MSCR) used self-power supply control system is designed. The structure and the working mechanism of the novel MSCR are analyzed in detail. Then the control and capacity step adjustment characteristics are obtained by experiments. The harmonic characteristic is studied by theoretical analysis and low voltage test. Findings To solve the problem of harmonics in the working current of nets windings, the fifth and the seventh filers are equipped between the compensation windings and the control system. The effectiveness of the harmonic suppression method is proved by simulation and experiments. Originality/value It proves that the 800 kV EHV MSCR design in this paper can achieve the purpose of the reactive power continuous linear adjustment, and the capacity adjustment is sensitive. After filtering, the harmonics level of the working current meets the standard of the EHV grid.

The Gauss-Seidel preconditioned conjugate gradient method applying to element-by-element finite-element method (EBE-FEM) is derived and the calculation process is presented in this paper. The EBE-FEM is a parallel numerical algorithm, in which a conjugate gradient (CG) type iterative solution scheme can be entirely decomposed into computations on the element level without assembling the global system matrix. The validity and accuracy of the proposed method have been verified through the applications to magnetic field analysis of a long straight current-carrying wire and a three-phase electric motor. Compared with the Jacobi preconditioned CG method applying to EBE-FEM, less computational time and cost are needed for the proposed method with the same accuracy. The proposed method can be applied to electromagnetic field analysis of power transformer and electric motor, which may be related to large-scale numerical computation.

In recent years, with the continuous updating of power electronic devices, mathematical signal processors and high-quality rare earth permanent magnet materials, permanent magnet AC servo systems have been widely used. And modern permanent magnet synchronous motors mostly use closed-loop vector control drive systems, which need to detect the rotor position and phase winding current value in real time. However, the excess number of sensors increases the cost of the drive system and is not conducive to improving the reliability of the system. In this paper, the improved current reconstruction technology and SMO-based position sensorless control technology are combined to realize the vector control of permanent magnet synchronous motor by using single current. Current reconstruction is achieved by measuring only the voltage source inverter DC link current.

This paper introduces the mathematical model of a double boost circuit for the condition that the short circuit to ground happens to one phase of inverters, taking into account the impact of leakage inductance of power transformers. The corresponding energy transmission modes are analyzed, mainly focusing on the reason that the rectifier diode reverses voltage and the DC bus voltage rises. Then a novel method for rectifier determination, considering the short circuit to ground in one phase of inverters, is proposed. The effectiveness of the proposed method is verified by both simulation and experiment.

An improved analysis method to calculate the eddy-current losses in large power transformer tank walls is presented. In this model, the perpendicular magnetic flux density has been obtained at discrete points on the surface of the tank by the finite-element method (FEM). A double Fourier series is adopted to express the obtained magnetic flux density by analytical formulas. The coefficients of the analytical expressions are determined by a least mean error with a curve fitting technique and an optimization algorithm. Based on the electromagnetic theory and the Maxwell equations, the eddy-current losses are obtained by the analytical formulas. The validation of this method is verified by TEAM Problem 21 (Model B). The calculation results are in good agreement with the measured results. Then, it could be extended to calculate the eddy-current losses in the metal structural parts of the power transformer.

The magnetostriction of electrical steel is one of the main sources causing deformation and vibration in rotational machine and transformer cores. The presence of dc bias or higher harmonics of magnetic field in the electrical apparatus cores makes the vibration and noise of cores more serious. Therefore, it is necessary to identify how different magnetization patterns contribute to the magnetostriction noise. This paper performs the measurement of magnetostrictive strain in non-oriented and grain-oriented electrical steel sheets under sinusoidal excitation, one with a dc bias and the other with a third harmonic component, respectively. An investigation of the effect of dc biased magnetic field on the magnetostrictive property is carried out. The influences of a third harmonic of magnetic field with a variety of amplitudes and phase delay on magnetostriction are studied and the acoustic noise level is calculated due to the presence of the second (100 Hz) and fourth (200 Hz) harmonics of magnetostrictive strain. The research results are important for evaluating the noise of electrical machine and transformer core effectively.

The modular multilevel converter (MMC) has a very good development prospect, however, still has a lot of problems and challenges, one of the essential problems is more intense computational burden with the voltage level of the MMC increasing. An improved model this method is reduced to decrease computational burden. The proposed improved method is verified by simulation system based on 11-level MMC using MATLAB/Simulink.

The insulation system of one 330kV Inverted-type Oil-immersed Current Transformer (IOCT) has broken down recently. In order to find the cause of the accident, the dielectric parameters of the IOCT insulation materials with different moisture content were tested at different temperature. Then, the electric field distribution of the IOCT insulation system was calculated using the data measured. The strongest electric field strength of main insulation was analyzed, and the extreme operating environment of IOCT was obtained.

This paper studies the dc bias phenomenon and integrated magnetic technology in a single-phase transformer. A new transformer with the function of dc bias elimination based on the nano-composite magnetic material is designed, and a controllable reactor is integrated into the transformer by integrated magnetic technology. The mathematical model of hysteresis is established, and the correspondence relationship between coercivity and remanence of this material is determined; dc bias magnetic potential in the transformer core can directionally be eliminated using the conversion of coercivity and remanence of the nano-composite magnetic material. The integrated controllable reactor has the notable characteristics of decoupling with other windings, high linearity, less extra covering area, and energy saving. The prototype experimental results verify the correctness of the design.

High-frequency current in the bearings of variable-frequency motors coupled with their shafts is a common issue that will shorten the bearings lifetime ultimately. A new method, which could be used to suppress high-frequency discharge bearing current, was proposed in this paper. In this method, an electromagnetic shielding slot wedge (ESLW) was used to reduce the capacitive coupling between the stator windings and the rotor. The electromagnetic coupling resulted from the common-mode voltage could be blocked by this electromagnetic shield layer. In addition, the mathematical model of the suppression method was given and the high-frequency electromagnetism performance of the motor was analyzed. Finally, an experimental platform was setup and a motor was modified to evaluate the suppressing effectiveness. The shaft voltage, bearing currents, and the general features of the modified motors, with and without ESLW, were measured and compared.

This paper proposes a novel controllable reactor with a function of intrinsic magnetic state regulation based on the nanocomposite magnetic material. First, in order to analyze the conversion mechanism and preparation technology of this material, one mathematical model of hysteresis is established, and the corresponding relationship between the coercivity and the remanence of this material is determined. Second, the nanocomposite magnetic material is implanted into the traditional controllable reactor. Then, the structure design and electromagnetic design of the controllable reactor are completed based on the reluctance of the magnetic circuit theory. Finally, effective control parameters and control system for the controllable reactor are designed. Simulation and experimental results show that the inductance value can be regulated continuously and rapidly, and the reactor can be used for reactive power regulation and harmonic suppression which provides intelligent and energy-saving power equipment for intelligent electric network safe operation.

In this paper, a novel topology of two-level voltage-type inverter is proposed. The proposed inverter has three bridge arms while each bridge is made up of 2 thyristors, 1 IGBT and 4 diodes. Thyristors complete the phase positioning of the inverter, IGBT completes the modulation of different modulation modes such as SPWM, SVPWM, and SHPWM, and the diodes complete the commutation of the bridge arms. Compared to the traditional voltage-type inverter with 6 IGBTs, the proposed voltage-type inverter using 3 IGBTs can achieve the same function with highly reduced cost. The principle of the proposed two-level inverter is explained in detail. The simulation and experiment results demonstrate the performance and effectiveness of the proposed inverter-type inverter.

Magnetostriction in grain-oriented (GO) electrical silicon steel arises vibration and noise of the transformer core, especially when operating under direct current (dc) bias. In this paper, the anisotropy of magnetostriction of GO silicon steel sheets along the rolling direction (RD) and other directions deviated from RD was measured based on a standard single-sheet measurement system. The effect of applied stress on magnetostriction was also measured and analyzed. The magnetostriction under alternating magnetic field plus dc biased field was measured and a constitutive equation considering single-valued curves of magnetostriction under dc bias when incorporated with magnetic finite-element analysis was investigated. Considering a simple model as an example, the effectiveness of the proposed modeling method was verified by the comparison of simulation data and measured ones.

This paper presents a complex-valued model when considering rotational magnetic properties of an electrical steel sheet from the viewpoint of improving the computation efficiency. The effective magnetic reluctivity and magnetic hysteresis coefficients in this model were interpolated from measured data based on a 2-D rotational magnetic property measurement system, in which the effect of higher harmonic terms of magnetic field intensity after saturation was considered. To reduce the computation error to some extent resulted from complex approximation of magnetic field quantities, the effective magnetic coefficients were expressed as the function of magnitude and direction of magnetic flux density. Considering transformer and three-phase induction motor core models as examples, the magnetic field distribution and core loss were calculated and the effectiveness of the proposed model was verified with the experimental results.

Purpose – A simplified method for calculating iron loss of three-phase transformer is proposed in this paper. The rotating iron loss measured from 2-D vector magnetic property measurement system of gain-oriented silicon steel sheet can be taken into account in this method. The paper aims to discuss these issues. Design/methodology/approach – The finite element analysis formulation is combined with the magnetic reluctivity model expressed by diagonal tensor for 2-D nonlinear and anisotropic magnetic problem, while the iron loss is computed in terms of the interpolation of rotational loss curves measured under various loci of controlled magnetic flux density B. Findings – The iron loss of three-phase transformer is obtained by the proposed method. And the calculating iron loss is verified with experimental results. Originality/value – The method presented in this paper enables the iron loss of three-phase transformer to be more accurately calculated and more easily applied, considering the rotational iron loss.

Purpose – The magnetostriction of grain-oriented electrical silicon steel sheet is studied for the magnetic field direction along the rolling direction and deviating from it. The method of calculating the vibration of transformer is developed through COMSOL. The paper aims to discuss these issues. Design/methodology/approach – Measurements of signals of magnetostriction and magnetic polarization, and calculation through software. Findings – The angle between the magnetic field direction and the rolling direction does a great influence on magnetostriction strain. Originality/value – The maximum λ p-p of transversal magnetostriction is above 30 times more than the value when the angle is 0°. The transversal magnetostriction is a main reason of vibration increasing at the corner of transformer.

The graphic processing unit (GPU) acceleration of 3D eddy current losses calculation by parallelizing the element by element (EbE) finite element (FE) method is proposed in the paper. Two distinctive characteristics of EbE method included, one is not to assemble the system matrix and the other is inherent parallel nature. So we can fulfill the parallelism directly on the elements level based on the GPU platform. Considering the ill-conditioned character of 3D eddy current problem, a novel preconditioner named generalized Jacobi (GJ) is used to accelerate the conjugate gradient (CG) iterative solution scheme. A large Transformer is taken as an example to compute the eddy current losses to validate the proposed method, the numerical experiments is carried out on the GPU.

Gauss-Seidel preconditioned conjugate gradient (GS-PCG) method based on element-by-element (EBE) is derived and the calculation process of finite element method (FEM) is presented in this paper. The corresponding FEM program are developed and implemented on the magnetic field calculations which produced by straight conductor carrying current and 3-phases induction motor respectively. The validity and accuracy of the method have been verified by comparing the result with the analytical solution. Compared with Jacobi preconditioned conjugate gradient (J-PCG) method based on EBE-FEM, less computational time and cost are needed for the proposed method with same accuracy. The proposed method can be applied to electromagnetic field analysis of transformer and motor, which may refer large scale numerical computation.

This paper introduces the mathematical model of a double boost circuit for the condition that the short circuit to ground happens to one phase of inverters, taking into account the impact of leakage inductance of power transformers. The corresponding energy transmission modes are analyzed, mainly focusing on the reason why the rectifier diode reverses voltage and the DC bus voltage rises. Then a novel method for rectifier determination, considering the short circuit to ground in one phase of inverters, is proposed. The effectiveness of the proposed method is verified by both simulation and experiment.

This paper proposes a new approach to eliminate dc bias occurred in transformer based on nanocomposite magnetic material. The dc bias flux in transformer main core can be eliminated directionally using the remanence of the nanocomposite core. According to the material characteristic of the rapid response to an external magnetic field, a magnetization and demagnetization circuit for the nanocomposite was designed based on rectification circuit combined with a capacitor charging circuit. The decaying ac filed used for demagnetizing the material from the ac side can be achieved, while the steady dc field output by rectification circuit can be used for magnetizing the material. The state of the nanocomposite core could be transformed effectively.

This paper design three distribution devices for the strong and smart grid, respectively are novel transformer with function of dc bias restraining, energy-saving contactor and controllable reactor with adjustable intrinsic magnetic state based on nanocomposite magnetic material core. The magnetic performance of this material was analyzed and the relationship between the remanence and coercivity was determined. The magnetization and demagnetization circuit for the nanocomposite core has been designed based on three-phase rectification circuit combined with a capacitor charging circuit. The remanence of the nanocomposite core can neutralize the dc bias flux occurred in transformer main core, can pull in the movable core of the contactor instead of the traditional fixed core and adjust the saturation degree of the reactor core. The electromagnetic design of the three distribution devices was conducted and the simulation, experiment results verify correctness of the design which provides intelligent and energy-saving power equipment for the smart power grids safe operation.