Jiangjun Ruan

Calculation of Temperature Rise in Air-cooled Induction Motors Through 3-D Coupled Electromagnetic Fluid-Dynamical and Thermal Finite-Element Analysis

This paper investigates a 3-D coupled-field finite-element method (FEM) used in simulation of temperature distributions in air-cooled asynchronous induction motors. The temperature rise in motors is due to Joules losses in stator windings and squirrel cages, and heat dissipation by air convection and solid conduction. The Joules losses calculated by 3-D eddy-current field analysis are used as the input for the thermal field analysis, which is deeply dependent on accurate air fluid field analysis. Moreover, a novel multi-component fluid model is proposed to deal with the influence of rotor rotation upon the air convection. A test prototype is designed and manufactured. The good agreement of the temperature distributions between the simulated and measured results validates the proposed methodology.

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.

A prediction method for breakdown voltage of typical air gaps based on electric field features and support vector machine

Breakdown voltage of the air gap is of vital importance for the design of the external insulation in high-voltage transmission and transformation projects. In this paper, a new prediction method for the breakdown voltages of typical air gaps based on the electric field features and support vector machine (SVM) was proposed. According to the finite element calculation results of static electric field distribution, the electric field values in the whole region, discharge channel, surface of the electrode and the shortest path were extracted and post-processed, which constituted the electric field features characterizing the gap structure. Then, the breakdown voltage prediction model of the air gap was established by using electric field features as the input parameters to SVM, and whether the gap breakdown would happen as the output parameters of SVM, which changing the regression problem to a binary classification problem. This model was applied to predict the power frequency breakdown voltages of different short air gaps including sphere-sphere gaps, rod-plane gaps, sphere-plane gaps and sphereplane- sphere gaps. The power frequency breakdown voltages of longer air gaps which are affected by corona, and the 50% positive switching impulse breakdown voltages of long sphere-plane gaps and rod-plane gaps were predicted as well. The predicted results agree well with experimental values and simulated results of the published models, which validate the effectiveness of the proposed model. This method supplies a new possible way to obtain the breakdown voltage of air gaps.

Finite Element Modeling of Heat Transfer in a Nanofluid Filled Transformer

In order to investigate the heat transfer characteristics of transformer oil containing nanoparticles, an electric-thermal-fluidic analysis using the FEM is carried out in this paper. The temperature and velocity distribution of oil and density of nanoparticles in a nanofluid filled transformer under natural and forced convections are obtained. The results show that generally the heat transfer efficiency is better under the forced convection. In addition, the distribution of nanoparticles in the oil depends on the temperature and velocity of the oil. This indicates that the local density and electrophoresis of nanoparticles must be considered for nanofluid application in transformers because of their potential influence on the temperature and velocity of the oil. Therefore, improvement of the simulation model taking into account of the local density of nanoparticles using the effective thermal conductivity had been carried out. Furthermore, electrophoretic force term had been implemented in Navier-Stokes equation, whereas the effective dielectric permeability was obtained by a homogenization technique using FEM.

Scaling Study in a Capacitor-Driven Railgun

This paper focuses on the scaling study of a capacitor-driven railgun. Railgun is an important kind of electromagnetic launcher. Although the basic principle is relatively simple, due to complicated technologies and extreme conditions, railgun is still far away from practical application. Scaling method is widely used in different field projects, including railgun systems. Subscale-systems test results and experiences can be extremely valuable for the actual system. This paper derives the circuit-scaling relationships based on circuit equations and momentum equations in a capacitor-driven railgun system. Four circuit linear-scaling methods (LSMs) are introduced, and a further approximate LSM is discussed. Finally, numerical verifications of the three models were given, including an original, a full LSM model, and an approximate full LSM model. It is shown that the performances of the full LSM model can be extrapolated perfectly to the original. The results of the full approximate LSM model can get almost the same result as the full LSM model, although some circuit equations are violated. All of the analyses demonstrate the flexibility and exactness of the circuit-scaling method. A detailed derivation and numerical validation are presented in this paper.

Armature Performance Comparison of an Induction Coil Launcher

This paper focused on the verification of the current filament method and the performance comparison of different armatures. Armature has as much influence on the performance of a coil launcher as any other major subsystem. The design of an armature is influenced by a large set of highly coupled parameters. The sleeve armature is a common choice for experiment and analysis of a coil launcher. The current filament method is widely used to analyze the sleeve armature. However, due to skin effect, the current distribution will be inconsistent, and excessive heating will be caused in the sleeve armature. How to use the current filament method accurately should be considered. Another good choice is the solenoid armature, which can make current and temperature distribution uniformly. To verify the current filament method and compare the performance between a sleeve armature and a solenoid armature, several kinds of projectiles were constructed and tested, including a sleeve projectile, a 24-turn copper-ring projectile, a 20-turn wire-ring projectile, and a 20-turn solenoid projectile. Then, 2-D finite-element simulations based on the experiments were taken to further study the problems. It is shown that in the sleeve armature (including multiturn ring armature) most of the induced current tend to distribute unevenly in the armature. Over concentration of the current will cause excessive heat and limit the material, structure, velocity, and efficiency of the sleeve armature. The sleeve armature must be divided into enough number of filaments; otherwise, the current filament method may be invalid. A solenoid armature can solve these problems by distributing the induced current evenly. However, fabrication of the solenoid armature becomes another difficulty. Further analysis of the current density in the solenoid armature shows that the skin effect will work in the wire of the solenoid, but it has little influence on the system performance. A detailed description of the experiments and simulations will be presented in this paper.

Study on Static and Dynamic Voltage Distribution Characteristics and Voltage Sharing Design of a 126-kV Modular Triple-Break Vacuum Circuit Breaker

The static and dynamic voltage distribution characteristics and voltage sharing design of a 126-kV modular triple-break vacuum circuit breaker (VCB) was discussed in this paper. The finite-element method and power frequency tests were used to calculate and verify the static voltage distribution ratios and distributed capacitance parameters of multi-break VCBs, respectively. A model combining the post arc current model and equivalent capacitance model was proposed for simulations of dynamic transient recovery voltage (TRV) distribution characteristics of the triple-break VCB. The simulation results indicated that besides the stray capacitances, the TRV sharing design should take the influence of residual charge (RC) into account, and the influence of RC on the TRV distribution is relevant to the differences of RC parameters among the series interrupters. Therefore, the appropriate value of grading capacitors should meet the requirement of the worst case. Based on the investigation of this paper, a 126 kV U-shaped triple-break VCB prototype with 1000 pF grading capacitors has been produced, and the successful large current breaking tests indicate the good breaking capacity and suitable voltage sharing design of the triple-break VCB.

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.