Analysis and proposition of limiting current density measurement protocol

Limiting current density at different temperatures, backpressures, and balance gases can be used to separate molecular diffusion resistance, Knudsen diffusion resistance and local transport resistance of membrane electrode assembly (MEA). However, the measurement of limiting current density has no unified protocol. The diverse choices in the literature, either in the control of current or voltage or in the atmosphere like relative humidity and O₂ concentrations, make it difficult to compare the results and identify the true bottleneck hindering the mass transport. In this work, the current-voltage curves obtained by current scanning/stepping and voltage scanning/stepping methods under dilute O₂ of different concentrations and a wide range of relative humidity were measured and analyzed systematically. It is found that the voltage stepping method is superior to the other three ways of control for the reliable determination of the limiting current density. Aided with simultaneous electrochemical impedance spectroscopy measurement, the limiting current density can be determined with pinpoint accuracy. When the limiting current density is just used to qualitatively evaluate different MEA, the voltage scanning method can be used instead for its high time efficiency. The selection of the atmosphere also plays an important role in suppressing the distortion from excessive water and reducing the spurious contribution from proton conduction resistance. It is found that O₂ concentrations at 0.5 vol% and relative humidity at 90% can give the best estimation of O₂ transport resistance in membrane electrode assembly.     

Compensation of voltage disturbances using PEMFC supported Dynamic Voltage Restorer

The continuity of supply and quality of power are the two main significant aspects of today’s power delivery system. The Dynamic Voltage Restorer (DVR) is a series connected custom power device which improves the quality of power delivered to the consumers. This paper deals with the effectual exploitation of DVR for interconnecting the proton exchange membrane fuel cell (PEMFC) stack to the grid based on optimized proportional integral (PI) and fuzzy logic (FL) Controller. The real coded Genetic algorithm (GA) is used to optimize the PI controller parameters. The PEMFC operated boost converter is used to boost up the fuel cell output voltage to balance the DC side necessities of the voltage source converter (VSC). The proposed DVR provides balanced and unbalanced voltage sag/swell compensation, harmonic reduction as well as an active power injection to the grid. The designed method also protects the sensitive loads from source side power quality disturbances including short term interruption. In addition, the harmonic compensation performance of the proposed work is validated by comparing with the results of the H^∞ controller based DVR under medium level voltage condition. The simulation results from MATLAB/SIMULINK are discussed to prove the effectiveness of the planned method.     

Improved synergetic excitation control for transient stability enhancement and voltage regulation of power systems

This paper proposes decentralized improved synergetic excitation controllers (ISEC) for synchronous generators to enhance transient stability and obtain satisfactory voltage regulation performance of power systems. Each generator is considered as a subsystem, for which an ISEC is designed. According to the control objectives, a manifold, which is a linear combination of the deviation of generator terminal voltage, rotor speed and active power, is chosen for the design of ISEC. Compared with the conventional synergetic excitation controller (CSEC), a parameter adaptation scheme is proposed for updating the controller parameter online in order to improve the transient stability and voltage regulation performance simultaneously under various operating conditions. Case studies are undertaken on a single-machine infinite-bus power system and a two-area four-machine power system, respectively. Simulation results show the ISEC can provide better damping and voltage regulation performance, compared with the CSEC without parameter adaptation scheme and the conventional power system stabilizer.     

磁流变阻尼器抑制转子系统振动试验

针对大型旋转机械通过临界转速时振动过大及运行中故障频发等问题,搭建转子试验台,模拟启停机过程和碰摩、不对中故障。不改变原有支撑形式,安装自主设计的磁流变阻尼器,在不停机的情况下,试验研究阻尼器抑制转子通过临界转速时振动过大及各类故障振动。试验结果表明,阻尼器可以有效抑制转子系统临界转速附近的振动,降幅在60%以上;转子发生碰摩或不对中故障时,阻尼器可以降低其高倍频振动。     

PMU based voltage security assessment of power systems exploiting principal component analysis and decision trees

The need for feature selection and dimension reduction is felt as a fundamental step in security assessment of large power systems in which the number of features representing the state of power grids dramatically increases. These large amounts of attributes are not proper to be used for computational intelligence (CI) techniques as inputs, because it may lead to a time consuming procedure with insufficient results and they are not suitable for on-line purposes and updates.This paper proposes a combined method for an online voltage security assessment in which the dimension of the token data from phasor measurement units (PMUs) is reduced by principal component analysis (PCA). Then, the features with different stability indices are put into several categories and feature selection is done by correlation analysis in each category. These selected features are then given to decision trees (DTs) for classification and security assessment of power systems.The method is applied to 39-bus test system and a part of Iran power grid. It is seen from the results that the DTs with reduced data have simpler splitting rules, better performance in saving time, reasonable DT error and they are more suitable for constant updates.     

Voltage reference architectures for low-supply-voltage low-power applications

This paper presents a voltage reference generator architecture and two different realizations of it that have been fabricated within a standard 0.18 μm CMOS technology. The architecture takes the advantage of utilizing a sampled-data amplifier (SDA) to optimize the power consumption. The circuits achieve output voltages on the order of 190 mV with temperature coefficients of 43 ppm/°C and 52.5 ppm/°C over the temperature range of 0 to 120°C without any trimming with a 0.8 V single supply. The power consumptions of the circuits are less then 500 nW while occupying an area of 0.2 mm² and 0.08 mm², respectively.     

具有初始弯曲转子振动峰值的控制方法

本文从理论上研究了控制初始弯曲引起的转子振动幅值的一种新方法。通过在与转轴初始弯曲方向成较大夹角的方向添加不平衡质量的方法 ,使转子新的不平衡方向与初始弯曲方向夹角接近 180° ,则可以减小转子在临界转速附近的振幅。无论新的不平衡参数大于或小于初始弯曲参数 ,只要新的不平衡参数不至于太大 ,则临界转速附近的振幅响应肯定会得到改善。特别是当新的不平衡参数近似等于初始弯曲参数时 ,无论转速高低 ,振幅都会大幅下降。当重力参数、外阻尼系数和内阻尼系数改变后仍有同样结论。本文援引文献 [2 ]中的实例对这种方法进行了验证 ,并给出了解决类似故障的步骤。本文的方法可用于处理生产实际中转轴初弯引起的振动问题     

无线收、发信机的设计

主要讲述移动通信中收、发信机的工作原理、基本结构和组成.在给出收、发信机原理框图的同时,着重分析了锁相环路的设计.     

关于变压器二次额定电压的计算

推导了变压器二次负载电压的计算公式,建议由此公式代替以往原理书中惯用的近似公式。利用此公式绘制和分析了不同功率因数额定负载时的相量轨迹图。     

Theory of a novel voltage-sustaining layer for power devices

The theory of a novel voltage-sustaining layer for power devices, called a Composite Buffer layer (CB-layer for short) is proposed. The CB-layer can be implemented in several ways, one particular implementation is used here, which consists of alternating n- and p-type regions, that are parallel to the direction of the applied electric field. In the off-state, the fields induced by the depletion charges of both region types compensate each other to allowing the doping in both n-regions and p-regions to be very high without causing a reduction of the breakdown voltage. In the onstate the heavy doping ensures the voltage drop is very low and that the saturation current density high. A simple relationship between the specific on-resistance and R_on and the sustaining voltage V_B can be shown to be R_on=2.53 × 10⁻⁷bV_B^1.23 ωcm², where the breadth b (in μm) of each region is much smaller than the thickness W. The design method of the CB-layer is discussed in some detail. The simulation results are shown to be in perfect agreement with the theory. The structure has application to a wide variety of different power devices. An RMOST structure has been used to demonstrate the benefits of the technique in the paper, for which excellent performance is demonstrated.