大高度的非晶变压器磁芯制备及其性能研究

研究了一种制备非晶变压器磁芯的新方法,该方法首先将高度低的铁基非晶磁芯热处理后在两侧面用树脂封装,然后将多个封装后的磁芯叠加成高度能满足非晶变压器需求的组合磁芯。研究结果表明,采用宽度为(50±0.2)mm、厚度(33±2)μm的Fe78Si9B13非晶带材,经过420℃×2h退火处理后,制备成高度为200mm的电力变压器磁芯,该磁芯的磁性能与高度为50mm的原装磁芯的磁性能相近,在f=1kHz,组合磁芯的矩形比为0.5～0.7,最大饱和磁感应强度1.4～1.5T。组合磁芯表现出良好的软磁性能,可以满足电力变压器要求。     

Application of a new orthogonal-flux-type amorphous core tohigh-frequency parametric inverters and three-phase measuringtransformers

The fundamentals of a parametric oscillator using an orthogonal-flux-type amorphous core are described, and several applications are proposed. The parametric oscillator circuit has a power-conversion efficiency of 42% and automatic overcurrent protection. The flux distribution calculated by the finite-element method (FEM) suggests that the discharge of a tuning capacitor generates a circulating flux in a leg of the core. The parametric oscillator, a self-oscillating invertor using a Royer circuit, shows an efficiency of 47% at 18.5 kHz and offers overcurrent protection. As a separately controlled parametric inverter, it operates at a frequency of 74 kHz and shows an efficiency of 14% at the maximum output power of 11 W. It can also be used as a potential transformer and as a current transformer in a three-phase system     

Transformer Core Parameter Identification Using Frequency Response Analysis

We present a novel model-based approach for parameter identification of a laminated core, such as magnetic permeability and electrical conductivity, of power transformers on the basis of frequency response analysis (FRA) measurements. The method establishes a transformer core model using the duality principle between magnetic and electrical circuits for parameter identification with genetic algorithms. We use reference input impedance frequency responses, calculated by a well-known lumped parameter model of a three-phase transformer and finite-element computations, to analyze identification accuracy of the method. The results verify the ability of the approach to accurately identify the core lamination parameters with respect to the reference values. The approach can be used for parameter identification of a demagnetized core with known geometrical parameters when the core lamination samples are unavailable for experimental tests. The approach can also be employed for transformer core modeling and FRA result interpretation at low frequencies.     

A highly accurate, hand-held clamp-on current transformer

The design, operation, and performance of a 1000:5 A highly accurate, hand-held, clamp-on-type current transformer are presented. This precision current transformer uses a sensitive magnetic circuit to detect transformer ampere-turn unbalance between the primary and secondary circuits. The unbalance represents an error in the ratio and phase angle of the transformer. The largest of the errors is due to the core magnetization current and magnetic reluctance caused by the cutting of the core material. After sensing the errors, electronic feedback through a magnetic circuit is used to provide an error-correcting current. This reduces the overall errors dramatically. These types of devices are referred to as “active” current transformers because of the use of electronic amplifier and feedback circuits. The device described has a novel feature of an openable, split core. This “clamp-on” capability enables use of the transformer on a bus or cable without the complications or need to open the current-carrying circuit to be measured. Commonly used “clamp-on”-type current transformers generally have uncertainties from about 1 to 5% at full-scale rated current. This paper describes a commercialization of active current transformers having a ratio uncertainty of less than ±0.05% over a current range from full-scale rated to 1% of full-scale. Additionally, this product has a small phase angle which is an important consideration when measuring electric power, energy, and power factor. It is intended to be used by electric utilities, standards laboratories, testing laboratories, and in applications where high measurement accuracy and the split-core, clamp-on feature are attractive considerations     

Flux distribution and power loss in the mitered overlap joint in power transformer cores

The design of the joints in a power transformer core has a marked influence on the efficiency of the core as a whole. Two experimental cores have been built such that the lengths of their 45° mitered overlaps could be varied from 0 to 2.0 cm. The larger core, with 1.0 m limbs, was used for investigating flux and loss variations and the second, smaller core was used to study how the results might be affected by changes in dimensions. For the larger core, the power loss was measured in two ways for a range of overlaps: first, using the localized power loss technique, and then by measuring the total power loss with a precision wattmeter. In both cases, a minimum power loss was found when the core was built with an overlap of 0.5 cm. The overlap length was varied in the smaller core and again a minimum power loss was found, but with a 1.0 cm overlap. A change in loss of over 20% was found for the range of overlap lengths used, and the optimum overlap length was independent of flux density over the range from 1.0 T to 1.8 T. The special flux distribution was determined from an array of search coils. The variations in flux distribution enables a qualitative explanation of the occurrence of the minimum in power loss.     

Pulse Shape Improvement in Core-Type High-Voltage Pulse Transformers With Auxiliary Windings

High-voltage pulsed power technologies are rapidly emerging as a key to efficient and flexible use of electrical power for many industrial applications. One of the most important elements in high-voltage pulse-generating circuit technology is the transformer, generally used to further increase the pulse output voltage level. However, its nonideal behavior has significant influence on the output pulse shape. The most attractive winding configuration for high-voltage, the core-type transformer with primary and secondary on different core legs, is seldom used in pulsed applications, because of its weak magnetic coupling between windings, which would result in a slow-rising output voltage pulse. This paper shows that auxiliary windings, suitably positioned and connected, provide a dramatic improvement in the pulse rise time in core-type high-voltage pulse transformers. The paper derives a mathematical model and uses it to describe the observed behavior of the transformer with auxiliary windings. It discusses experimental results, obtained from a high-voltage test transformer associated with a high-voltage pulse generating circuit, and the simulation results obtained from the numerical evaluation of the developed differential equations implemented in Matlab and taking into account the measured transformer parameters.      

Application of probabilistic neural network for differential relaying of power transformer

Investigations towards the applicability of probabilistic neural networks (PNNs) as core classifiers to discriminate between magnetising inrush and internal fault of power transformer are made. An algorithm has been developed around the theme of conventional differential protection of transformer. It makes use of the ratio of the voltage-to-frequency and the amplitude of differential current for the detection of the operating condition of the transformer. The PNN has a significant advantage in terms of a much faster learning capability because it is constructed with a single pass of exemplar pattern set and without any iteration for weight adaptation. For the evaluation of the developed algorithm, transformer modelling and simulation of fault are carried out in power system computer-aided designing PSCAD/EMTDC. The operating condition detection algorithm is implemented in MATLAB     

Power density of piezoelectric transformers improved using a contact heat transfer structure

Based on contact heat transfer, a novel method to increase power density of piezoelectric transformers is proposed. A heat transfer structure is realized by directly attaching a dissipater to the piezoelectric transformer plate. By maintaining the vibration mode of the transformer and limiting additional energy losses from the contact interface, an appropriate design can improve power density of the transformer on a large scale, resulting from effective suppression of its working temperature rise. A prototype device was fabricated from a rectangular piezoelectric transformer, a copper heat transfer sheet, a thermal grease insulation pad, and an aluminum heat radiator. The experimental results show the transformer maintains a maximum power density of 135 W/cm(3) and an efficiency of 90.8% with a temperature rise of less than 10 °C after more than 36 h, without notable changes in performance.     

Computation of Parameters of Power Transformer Windings for Use in Frequency Response Analysis

We present a 3-D model for calculating magnetic fields in a power transformer and the effective parameters (inductance and resistance) of its windings. The transformer is representative of large transformers with power ratings ranging from hundreds of kilovolt amperes to hundreds of megavolt amperes. The model accounts for anisotropic frequency-dependent properties of the laminated transformer core and eddy currents in the steel sheets. We discuss the results of the calculations performed in the frequency range 10 Hz-10 MHz, and show that the largest variations of both the magnetic field and parameters of the windings take place at frequencies below ~10 kHz and, at frequencies higher than 1 MHz, the magnetic core does not significantly affect the variation of the effective parameters     

An enhanced fiber-optic temperature sensor system for powertransformer monitoring

The paper deals with a sensor for temperature measurements in fluids. The sensor is based on a fiber-optic sensing element and on microcontroller-based signal processing hardware. The physical principle behind its operation is briefly reviewed, and its application to power transformer hot-spot temperature measurement is reported. Laboratory experimental results are given for both sensor characterization and measurements carried out on a 25-kVA oil-cooled power transformer     

Multiscale Computations of Parameters of Power Transformer Windings at High Frequencies. Part I: Small-Scale Level

We propose a hierarchical multiscale approach to modeling of high-frequency phenomena in power transformers. The method is based on considering high-frequency effects on two spatial levels. Part I of the paper focuses on a small-scale level where eddy-current as well as geometrical and proximity effects in conductors of transformer windings are examined. It develops a computer model that accounts for anisotropic frequency-dependent diamagnetic properties of transformer windings, which are represented as composite media with periodic structure. Calculations of effective characteristics of the laminated transformer core take into account anisotropic magnetic properties of steel sheets. The frequency-dependent parameters of transformer elements thus obtained will be further used in Part II for computing effective inductances and resistances of windings and integral losses on a large-scale level, i.e., for the whole transformer.     

小型高压充电实验装置中高压变压器设计

小型高压变压器作为该脉冲功率实验装置的核心器件,其正确的设计与制作是该实验装置能否正常运行的关键,因此本文着重研究了小型高压脉冲功率实验装置中小型高压变压器的设计方法,根据设计方法制作了样品,样品实验结果表明,该变压器满足了小型脉冲功率实验装置的要求.     

一种汽油变频发电机组中的辅助直流电源设计

针对汽油变频发电机组中逆变器所需多组低压直流电源问题,设计了一种多路输出的开关辅助电源, 结构简单紧凑、输出电压纹波小、体积小、成本低;通过分析高频变压器的设计原理,设计了一台功率为１５W 的四路直流输出的开关辅助电源;实验证明了该辅助电源的可行性。     

The structure and optical properties of silicon nanowires prepared by inductively coupled plasma chemical vapor deposition

Silicon nanowires were prepared by vapor-liquid-solid (VLS) mechanism at a growth temperature as low as 380 °C in an inductively coupled plasma chemical vapor deposition system. The nanowires consist of crystalline core surrounded by a thick amorphous silicon shell. An increase in plasma power produces dense and long nanowires with thick amorphous shell, accompanied with a thick uncatalyzed amorphous silicon film on the silicon substrate. Small catalyst nanoparticles are easier activated by plasma to grow the dense and thin nanowires in comparison with the large-size nanoparticles. Moreover, an enhanced optical absorption is achieved due to the strong light trapping and anti-reflection effects in the thin and tapered silicon nanowires with high density.     

Modelling, simulation and measurement of fast transients in transformer windings with consideration of frequency-dependent losses

For the specification of winding insulation of transformers, it is important to know the electrical stresses to which the winding can be exposed during fast transient oscillations. These oscillations occur during switching operations performed by circuit breakers, or when gas-insulated substations (GIS) are used. Therefore one of the priorities is to use a high-frequency transformer model capable to simulating fast transient oscillations in the windings. The model presented requires only information about the geometry of the winding and the core, as well the electrical and magnetic parameters for the used materials. In the transformer model, the frequency-dependent core and copper losses are included. Numerical computations are performed with and without the core losses being taken into account. Two types of measurement are taken to verify the validity of the model. First, the voltage transients are measured and computed by the application of a step impulse with a rise time of 50 ns. Then, the transformer is switched by a vacuum circuit breaker, and the multiple reignitions, which contain oscillations with a wide frequency range, are analysed. The results verify that the model is suitable to simulate the voltage distribution in transformer windings over a wide frequency range.     

Sensitivity studies on power transformer ferroresonance of a 400 kV double circuit

The ability to predict ferroresonance significantly relies on the accuracy of the transformer model and the power system's parameters. The accomplishment of a suitable simulation model allows the sensitivity studies to be performed to determine the degree of influence of various components and parameters of the ferroresonance phenomenon such as line length, point-on-wave switching and transformer core loss. The modelling work carried out in ATP (commercially available software) on a 1000 MVA 400/275/13 kV power transformer model is described and the simulation with field ferroresonance test recordings is verified. The maps that define the boundaries between safe and ferroresonant (fundamental, subharmonic, chaotic) regions as a function of system parameters are created through the sensitivity studies performed.     

Problems in the design of power transformers

The power loss of transformer cores can still be considerably reduced by improved joint design based on new knowledge of the localized flux distribution, both in the corners and in the limbs. Rotational flux has been measured in the T-joints of experimental cores, and it has been found to cause localized high loss regions dependent on the joint design. Circulating harmonic fluxes have been found in individual laminations in the limbs of three-phase cores. The path of the harmonics is independent of joint design although their magnitudes are dependent. Circulating 3rd harmonics as large as 0.3 T have been found in a core magnetized sinusoidally at 1.6 T. These cause additional losses calculated to be 20% of the nominal core loss, and great savings would be possible if the circulating harmonics could be reduced or eliminated.     

Equivalent circuit for high-voltage partial-core resonant transformers

An equivalent circuit model for the partial-core resonant transformer is presented. A model which omits core losses is first introduced and is verified on a sample air-core resonant transformer. Partial-cores are then introduced into the transformer and the resonant characteristics are re-measured. The model is modified to account for the core losses and then verified against the measured data. Comparisons and transformations between the model and the Steinmetz 'exact' transformer equivalent circuit are given. A procedure for measuring the equivalent circuit inductances is described and then used to investigate the inductance variation with core displacement characteristics of the sample transformer in both axial-offset and centre-gap arrangements. In a capacitive load test the sample transformer was linear to a significantly higher voltage when using the centre-gap arrangement.     

Micro-mechanisms, mechanical behaviour and probabilistic fracture analysis of PA 66 fibres

The micro/macro structural evolution of polyamide 66 fibres during tensile loading and failure initiation have been studied by coupling multi-scale measurements such as wide angle X-ray diffraction (WAXD) with profile fitting, birefringence, differential scanning calorimetry (DSC), micro-Raman spectroscopy and mechanical testing at two strain rates. A large dependence of mechanical properties, including strain rate effects, on the behaviour of both oriented and random amorphous regions has been shown with important contributions from the isotropic amorphous domains. The results indicate the presence of compressive residual stresses beneath the skin, showing a skin/core sub-structure. Statistical fracture treatments have been applied using a time-dependent weakest link Weibull model, in order to give an evaluation of the dispersion of defects. Local damage was taken into account using a crack growth propagation law as a function of stress intensity factor near the defect. The paper shows how both experimental and theoretical fracture toughness results are in quite good agreement.     

Modeling and analysis of dual-output piezoelectric transformer operating at the thickness-shear vibration mode.

In our previous study, the multioutput piezoelectric transformer operating at the thickness-shear vibration mode was proposed and experimentally investigated. By designing a new construction of support and lead wire connection, a power density of 52.7 W/cm3 and a total output power of 169.8 W were achieved at a temperature rise less than 20 degrees C. In this work, a theoretical model was developed for the dual-output piezoelectric transformer operating at the thickness-shear vibration mode. The equivalent circuit parameters of the piezoelectric transformer were derived. Based on this, the impedance characteristics, equivalent inductance, capacitance ratio, voltage gain, and efficiency of the piezoelectric transformer were calculated. The theoretical results were verified by experimental data. Furthermore, the effect of the transformer size on the voltage gain, efficiency, output power and power density, and the effect of the load of one output on the voltage gain of another output were analyzed. Some useful guidelines were achieved by these analyses.