全桥CLLC谐振变换器过谐振工况下电流等效简化模型

谐振电流的有效值和峰值对变换器的效率分析和器件选型尤为重要。针对计算谐振电流时使用基波分析法准确性太低,而使用时域分析法计算又太过复杂的问题,在工作频率大于谐振频率工况下对电路谐振电流波形进行分析和理论推导,同时对电流合理地假设和等效,构建简化的谐振电流模型。实验结果表明,简化电流模型比基波分析法的准确性更高,且计算简单。

LLC直流变压器同步整流与临界连续过谐振控制

采用固定开关频率的LLC谐振式直流变压器已广泛应用于各类隔离式两级结构拓扑中。针对应用于宽电压输入、大电流输出、工作于完全谐振状态且采用同步整流的LLC直流变压器,分析了其副边受同步整流管寄生结电容的影响而无法实现临界导通模式CCM的现象与原因,提出了无需额外传感器的同步整流数字控制计算方法;利用LLC直流变压器输出电压不控的特点,提出了基于效率优化的临界连续过谐振控制策略,分析了该控制策略在轻载时优于传统的完全谐振开环控制策略的原因;最后,搭建了1台实验样机,验证了该控制策略的正确性和可行性。

半桥型LLC谐振变换器的损耗分析

对于半桥型LLC谐振变换器,建立了一种改进的损耗估算模型。该损耗模型可为LLC谐振变换器的效率优化设计提供参考。此处改进了励磁电流的估算公式以减小与实际值的偏差。根据变压器和谐振电感各自磁通波形的特点,具体分析计算了磁芯损耗。在开关损耗部分,对关断时间的选取进行了详细地讨论介绍。基于所建立的损耗模型分析比较了 LLC谐振变换器在欠谐振区域和过谐振区域工作时的损耗特性。设计制作了两台分别工作在欠谐振区域和过谐振区域的1 kW样机,验证了损耗估算模型的准确性。

Frequency response of a new kind of silicon nanoelectromechanical systems resonators

Diffraction effects will bring about more difficulties in actuating resonators,which are electrostatically actuated ones with sub-micrometer or nanometer dimensions,and in detecting the frequency of the resonator by optical detection.To avoid the effects of diffraction,a new type of nanoelectromechanical systems（NEMS） resonators is fabricated and actuated to oscillate.As a comparison,a doubly clamped silicon beam is also fabricated and studied.The smallest width and thickness of the resonators are 180 and 200 nm,respectively.The mechanical oscillation responses of these two kinds of resonators are studied experimentally.Results show that the resonant frequencies are from 6.8 to 20 MHz,much lower than the theoretical values.Based on the simulation,it is found that over-etching is one of the important factors which results in lower frequencies than the theoretical values.It is also found that the difference between resonance frequencies of two types of resonators decreases with the increase in beam length.The quality factor is improved greatly by lowering the pressure in the sample chamber at room temperature.

The Mechanism of Eliminating Harmonic Resonance with Controllers and Application to the Power Systems

The phenomenon of anti-symmetrical bifurcation of periodic solutionsoccurring near an integral manifold is the intrinsic cause resulting in harmonic resonanceover-voltage in power systems. Due to this discovery, the principle of eliminating resonance byusing anti-bifurcation technique is presented, which makes that the theoretical bases of verymeasure to eliminate resonance are unified firstly from a point of view of basic theory. Ourdiscussion models depend on a class of nonlinear control model. Using the direct Lyapunov method, acomplete theoretical proof is given in accordance with the measure of eliminating resonance byconnecting nonlinear resistor in series to the neutral point of P. T., and the feedback control lawbeing applied. It comprises the action of parameters of resistor to eliminate resonance and theactual process of eliminating resonance, i.e., to go against bifurcation process which forces thebig harmonic solutions to retreat to the integral manifold gradually and disappear eventually, whichby using the nonlinear controllers. This makes it sure that the intrinsic cause of resonance iseliminated thoroughly. The obtained theory results and computing results are better than thepresented results.

Dual-Band Coupled-Line Bandpass Filter with Independently Tunable Bandwidths

This paper reports a novel dual-band coupled-line bandpass filter.Comprising a quadruple-mode coupled-line resonator,this proposed filter could filter signals on two frequency bands.After introducing two varactors,one of the four resonances could be continuously altered by changing the capacitance of the varactors,thus the two frequency bandwidths could be independently tunned.In this paper,the detailed explanation for its operation is given,and the agreement between the expected and practical performance sufficiently confirms the robustness and effectiveness of this proposed filter.

Study on Over-Voltage of 220 kV Transformer＇s Neutrals and Protection Strategy

In order to limit short-circuit current and satisfy the need of relay setting, only part of the 220 kV power transformer is grounded, so on the neutrals of ungrounded transformers will appear the over-voltage. Currently the value of over-voltage on transformer neutral point and the corresponding protection strategy is based on theoretical formula. This article uses the PSCAD/EMTDC software to calculate the over-voltage on the neutral point of 4 ungrounded power transformers in Chongqing 220 kV power grid. The result shows that the power frequency transient over-voltage on the neutrals may reach 178 kV. If the single-phase grounding fault occurs in ungrounded power system, the power frequency over-voltage on the neutrals will be more serious and rise to 138.6 kV. Non-full phase operation may cause serious ferro-resonance over-voltage on the neutrals of no-load transformer, which may last for seconds and may rise to 723.7 kV, causing serious threat to the transformer neutrals and line-side equipment. The article also studies the gap parameter which should be taken on the neutrals of 220 kV transformers at the end.

Ferroresonance at Open Phase Operating Conditions of Power Transformers

The reliability of electric supply to consumers is one of the most important factors that determine the requirements imposed on modem utility companies. This paper presents the results of investigation by computer software of the overvoltages resulting from a ferroresonance conditions in MV networks at open phase operating condition with and without connection to earth on source and load sides of distribution transformer. This overvoltage may reach 4.2 pu on one of the HV side of transformer unswitched phases. The results of the study show that ferroresonance overvoltage may be controlled by replacing fuses with circuit breakers on HV side to ensure switching-off all phases. Insertion of resistor or reactor in the neutral of source and loadsides of the transformer with 5% active load will help in suppressing overvoltages.

Detection of Blade Mistuning in a Low Pressure Turbine Rotor Resulting from Manufacturing Tolerances and Differences in Blade Mounting

For a serious prediction of vibration characteristics of any structure, a detailed knowledge of the modal characteristic is essential. This is especially important for bladed turbine rotors. Mistuning of the blading of a turbine rotor can appear due to manufacturing tolerances or because of the blading process itself due to unequal mounting of the blades into the disk. This paper investigates the mistuning of the individual blades of a low pressure turbine with respect to the effects mentioned above. Two different rotors with different aerodynamic design of the blades were investigated. The blades were mounted to the disk with a so-called hammer head root which is especially prone to mounting irregularities. For detailed investigations, the rotor was excited with a shaker system to detect the forced response behavior of the individual blades. The measurements were done with a laser vibrometer system. As the excitation of rotor structure was held constant during measurement, it was possible to detect the line of nodes and mode shapes as well. It could be shown that the assembly process has an influence on the mistuning. The data were analyzed and compared with numerical results. For this, different contact models and boundary conditions were used. The above described characterization of modal behavior of the rotor is the basis for the upcoming aeroelastic investigations and especially for the blade vibration measurements of the rotor, turning with design and off-design speeds.

Application of Parallel Resonant Circuit in Modulator Charging Power Supply

A novel high voltage switching circuit with parallel resonant topology is analyzed.Without adding other resonant components,this circuit makes full use of the transformer leakage inductance and winding capacitance so as to reduce the influence of the transformer parasitic parameter.Using device CD4046 to realize the switching control of MOS,this circuit can realize the constant-current charging to capacitor by operating the above resonance.The operating process of the circuit is analyzed,and the power calculation and some practical waveforms are given.

Decay Process of Quantum Open System at Finite Temperatures

Starting from the formal solution to the Heisenberg equation, we revisit an universal model for a quantum open system with a harmonic oscillator linearly coupled to a boson bath. The analysis of the decay process for a Fock state and a coherent state demonstrate that this method is very useful in dealing with the problems in decay process of the open system. For finite temperatures, the calculations of the reduced density matrix and the mean excitation number for the open system show that an initiaJ coherent state will evolve into a temperature-dependant coherent state after tracing over the bath variables. Also in short-time limit, a temperature-dependant effective Hamiltonian for the open system characterizes the decay process of the open system.