Zhiye Du

3-D Coupled Electromagnetic-Fluid-Thermal Analysis of Oil-Immersed Triangular Wound Core Transformer

This paper presents a 3-D coupled electromagnetic-fluid-thermal analysis method for temperature rise prediction in oil-immersed triangular wound core transformer. The temperatures rising inside the transformer are mainly determined by the heat sources and the heat convection effect of the transformer oil. The heat sources which include the electric power losses of transformer core, high-voltage windings, and low-voltage windings can be investigated by electromagnetic analysis using the finite element method in no-load and rated load running situations, respectively. Then, the calculation results of electric power losses are brought into the fluid-thermal field analysis as the heat sources. The temperature distribution inside the transformer and the velocity of transformer oil are analyzed by the finite volume method in the fluid-thermal field. To improve the accuracy of calculation results, the influence of temperature on the electric power losses and the parameters of transformer oil are considered in this paper. Using iterative computations, the power losses of windings are updated in accordance with the thermal field calculation result until the maximum difference of temperature between two adjacent steps is less than 0.01 K. At the end of this paper, the simulation results of coupled analysis used in the 10 kV oil-immersed triangular wound core transformer are compared with analytical formula results proposed by the International Electrotechnical Commission standard, and the good agreement of hot-spot temperature indicates the validity of coupled analysis.

Performance Improvement of a Coil Launcher

In order to improve the performance of a coil launcher, various cores and shells are considered in the structure design in this paper. Different geometry shapes and materials are loaded on the cores and the shells. A single-stage coaxial induction coil launcher is analyzed for simplicity based on Ansoft 2-D simulations. The conclusion can be summarized as follows. A magnetic core will improve the performance of a coil launcher greatly. Coil current and resistance loss of the system will be reduced while kinetic energy conversion rate and efficiency will be increased. A conductive core will reduce the performance because of eddy-current effect. A magnetic conductive core combines magnetization and eddy-current effect. Geometry of a core will also influence the performance of a coil launcher. In general, the greater the cross-sectional area and the length is, the better the performance. A shell takes little effect on the performance of a coil launcher, no matter what material and geometry is used. A magnetic nonconductive core with large cross-sectional area and enough length is suggested to be used in a coil launcher design. A detailed study will be presented in this paper.

Highly Stable Upwind FEM for Solving Ionized Field of HVDC Transmission Line

The ionized field produced by corona discharge from a high-voltage direct current (HVDC) transmission line has a great influence on the electromagnetic environment. In this paper, a highly stable iterative algorithm based on an upwind finite-element method is introduced to analyze the ionized field of HVDC transmission line in the presence of the wind. The Kaptzovs assumption is introduced on the conductor surface as a boundary condition. In the iterative procedure presented by Takuma, the controlling method is added to guarantee the convergence of the iteration, which has been tested to be effective. The impact of the wind on the ground-level electric-field intensity and ion current density of a bipolar HVDC transmission line is analyzed, and we find the wind has a significant influence on the ionized field which has to be considered in the engineering design.

Helicopter Live-Line Work on 1000-kV UHV Transmission Lines

Live-line work methods using helicopters in the vicinity of 1000-kV transmission lines were simulated with finite-element modeling of the electric-field distributions. The effect of the approach path of the helicopter live-line work platform toward the energized conductor was computed. Predicted electric fields on the line worker head, body, arms, and legs were compared with measured results from a single-phase ultra-high-voltage (UHV) mock-up, and with calculation results from the simulation of an extra-high-voltage (EHV) 500-kV transmission-line geometry. The UHV conductor-to-work platform discharge occurred at a distance of about 2 m, in line with standard flashover models. Additional shielding efficiency of about 4 dB is recommended for the electrically conductive clothing when working on the UHV lines in China, compared to EHV lines.

Analysis of Excess Loss in SiFe Laminations Considering Eddy-Current Dominated Domain Wall Motion

To develop efficient silicon steel plates and electric machines using silicon steel laminations, the evaluation of excess loss in silicon steel plates is important. In this paper, investigation on excess loss calculation using the finite element method is carried out. The excess loss, which is mainly due to two factors, lack of flux penetration and domain wall pinning, is considered by the proposed approach using the domain wall bowing degree (DWBD), which depends on the above two factors. The DWBD is obtained using nonlinear eddy-current analysis. The correlation between the excess loss and the DWBD can be expressed as an exponential equation. Using the equation, the excess loss can be predicted under harmonic frequency in the range of 50-2000 Hz. These results can be applied to iron loss estimation, especially, when there are harmonics in the power supply.

Analysis of Interference Current for High-Voltage Arresters Based on Resistance–Capacitance Network

The iterative algorithm based on resistance-capacitance (RC ) network model is put forward to eliminate the interphase interference in measuring the total leakage current of high-voltage zinc oxide arresters; this method can calculate the total leakage current and phase angle difference (the angle between voltage and current) of arresters precisely. The finite-element method is applied to calculate the stray capacitances of arresters by establishing 3-D arresters models. The total leakage current, resistive leakage current, and phase angle difference of arresters can be obtained through the calculation. The actual leakage currents and phase angle differences of 220 and 500 kV arresters in the substation were measured and compared with the calculation results. The comparison results demonstrated the effectiveness and correctness of the proposed method. The interference currents can be obtained by analyzing the one-phase and three-phase results. Then, using the iterative algorithm, the interphase interference of arresters can be eliminated effectively. Compared with some other measurement methods, this method is advantageous for its simplicity and generality.

Performance Analysis of a Coil Launcher Based on Improved CFM and Nonoverlapping Mortar FEM

Performance analysis of a coil launcher is very important for experimental research and electromagnetic optimization design. To check the effect of the improvement and analyze electromagnetic transient in a coil launcher, a field coupling circuit simulation method is introduced in this paper. Initially, the current of drive coils is calculated by the circuit model based on improved current filament method, and then the exciting current is loaded in 2-D axisymmetric field model. The field model is built based on nonoverlapping mortar finite-element method (NO-MFEM). NO-MFEM divides the whole domain into two subdomains: one contains the movable part (armature), and the other contains the source current (coils). The two subdomains are discretized independently and the two sets of meshes are nonconforming on the interface. When the movable part changes its location, it is necessary only to change the node coordinates in movable subdomains and information of mortar nodes and elements. In this paper, performance analysis of a three-stage coaxial induction coil launcher is carried out based on the proposed field-circuit method. The correspondence of the simulation results proves the validity of the field-circuit method.

Calculation of distribution parameters for Research on Propagation Characteristic of PD in Transformer Winding

A simulation model of a 180 turns continuous disc type transformer winding was used to study the propagation characteristics of partial discharge (PD) based on multiconductor transmission line (MTL) theory. In order to obtain distribution parameters of unit length of windings, two 2D axial symmetric finite element(FE) models are set up under ANSYS environment. The electromagnetic FE model is used to calculate the capacitance distribution parameters (K) and the time harmonic magnetic FE model used for inductance(L) and resistance(R) parameters. In calculation of K matrix, a modified method is proposed which decrease distinctly computer cost without more loss of accuracy. For the evaluation of L and R parameter of transformer with iron core in high frequency, the skin effect of wire and core is taken into consideration. The FE meshes are controlled subtly in accordance with skin depth. The frequency-dependent parameters are calculated accurately by time harmonic magnetic field analysis. The proposed methods of calculating parameters are helpful to research on propagation characteristic of PD pulses in electric equipments.

Wave process in scale-down model of UHVDC converter transformer winding under the lightning impulse voltage

An experimental scale-down model of UHVDC converter transformer is designed and manufactured to study the wave process in the winding under impulse voltage. Nine outgoing lines are set up on the different winding coil of the scale-down model. The voltage waveforms of different outgoing lines are obtained by lightning impulse test. The simulation model of scale-down transformer is established to calculate the distributed parameter matrices by the finite element method. The equivalent circuit of transformer winding is proposed to calculate the node voltage waveforms and the potential gradient distribution. The comparative analysis of test and simulation results shows that the oscillating voltage occurs in the winding and the simulation results can reflect the wave process characteristics.

Analysis of Voltage Distribution Characteristics in UHVDC Converter Transformer Winding Based on the Reduced-Scale Model

Based on the similarity theory, a reduced-scale model is proposed to study the voltage distribution characteristics in ultrahigh-voltage direct current converter transformer windings under the impulse voltage. Through comparing the distribution characteristics of electric field intensity and magnetic flux intensity of the scale model with that of the original model, the effectiveness of the reduced-scale method is verified. Furthermore, the actual production design is also considered in the model. The matrices of the inductance, capacitance, and resistance parameters for the reduced-scale model are obtained by finite element method. Then, the multi-conductor transmission line model is set up to study the propagation process of impulse voltage along the windings of the converter transformer. The voltage distribution curve is obtained under a standard lighting impulse voltage and the turn-to-turn fault is taken into consideration as well. The simulation results are the basis for the following experimental study.