The residual ac breakdown voltage of 6.6 kV dry-cured XLPE powercable under wet-accelerated aging tests

The residual ac breakdown voltage decreased to about half of the initial breakdown voltage after 3 months of accelerated aging test under immersion. This decrease was found to be caused mainly by a trace amount of moisture existing in the insulation. However, the breakdown voltage did not further decrease with increasing moisture content. In order to clarify the influence of moisture, the state of water existing in the insulation was investigated using FTIR (Fourier transform infrared spectroscopy) and optical microscopy. From these results, it was concluded that the moisture in the amorphous region and/or at the interface next to the crystal region caused the reduction of the ac breakdown voltage     

High‐frequency voltage oscillation in transformer windings and electrical breakdown properties of interturn insulation immersed in oil at VFT voltage

Switching operations in gas insulated switchgear (GIS) can generate very fast transient overvoltages (VFTO) which propagate in the GIS with little attenuation. When a transformer is directly connected to the GIS through a gas/oil bushing, the VFTO can enter the transformer and excite the voltage oscillation in the windings. In the paper, we firstly show a resonance phenomenon due to the superposition of the traveling waves which was found in the experiments using the coil windings of a 500 kV transformer. The fundamental resonance frequency ranged at about 2 MHz, and its voltage at the interturn (turn‐to‐turn) of the coil could reach 0.25 times of the applied voltage in the worst case. Secondly, the electrical breakdown properties of interturn insulation in oil were studied, applying the unipolar and bipolar voltages with 2 MHz oscillatory frequency. The breakdown voltages at the unipolar and bipolar VFT oscillatory stresses were about 1.25 and 1.40 times higher than that at the standard lighting impulse, respectively. We discuss the experimental breakdown stresses with the ones estimated from the analysis of VFTO in the transformer windings and also show the methods to cope with the VFTO. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 45–52, 2000     

考虑残余应力的大型电力变压器绕组短路强度计算与分析

在电力变压器绕组绕制过程中,绕组截面存在部分残余应力,会对绕组抗短路强度产生不利的影响,现有电力变压器绕组短路强度研究没有对该部分应力予以考虑。推导了圆环形绕组绕制过程中产生残余应力的计算模型,以试验变压器为例对内绕组的辐向短路强度进行了仿真计算,考虑了绕制过程中产生残余应力的影响,并采用国家标准中部分强度考核指标进行了对比校验,结果表明绕制过程中产生的应力同时包含压缩和拉伸分量,残余应力对电力变压器抗短路强度有不利的影响,对环形压缩应力影响较小,对辐向翘曲应力影响较大;所提变压器绕组短路强度计算模型能够考虑残余应力的影响,通过对比试验测量值验证了数值计算模型的准确性。     

三绕组电压互感器实际负荷下误差的推算方法

电压互感器是电力系统中的重要电气设备,它和电力系统的安全经济生产有着密切的关系。电压互感器实际负荷下的误差测试是现场校验工作的重点之一,但是由于不能停电或者无法带实际二次负荷,要得到实际条件下电压互感器的误差值相当困难。本文介绍了一种推算三绕组电压互感器实际负荷下误差的方法,准确可靠,简便易行。具有一定的实际意义。     

动态电压恢复器的时间优化补偿策略研究

动态电压恢复器(DVR)是缓解电压跌落问题最经济、有效的用户电力装置,它的补偿能力指标是持续补偿时间.以补偿电压跌落时DVR装置的最大持续补偿时间为控制目标,通过调整负荷电压超前跌落电网电压的相位角β,提出一种新的时间优化补偿策略.为减轻DVR运行时对负荷的扰动,使补偿开始和结束的暂态过程更平稳,新策略采取了β按恒定步长Δβ缓慢变化的控制方法,并根据DVR装置的T-β曲线,通过判断特定量符号决定β角的补偿方向.此种β角控制方法避免直接求解β的最优解,计算量小.仿真结果证明,新策略能获得比最小能量补偿策略更长的DVR持续补偿时间,有效利用了DVR的直流母线储能.     

动态电压恢复器的时间优化补偿策略研究

动态电压恢复器（DVR）是缓解电压跌落问题最经济、有效的用户电力装置,它的补偿能力指标是持续补偿时间。以补偿电压跌落时DVR装置的最大持续补偿时间为控制目标,通过调整负荷电压超前跌落电网电压的相位角β,提出一种新的时间优化补偿策略。为减轻DVR运行时对负荷的扰动,使补偿开始和结束的暂态过程更平稳,新策略采取了β按恒定步长Δβ缓慢变化的控制方法,并根据DVR装置的T-β曲线,通过判断特定量符号决定β角的补偿方向。此种β角控制方法避免直接求解β的最优解,计算量小。仿真结果证明,新策略能获得比最小能量补偿策略更长的DVR持续补偿时间,有效利用了DVR的直流母线储能。     

Frequency domain modelling of random wound motor windings for insulation stress analysis

The use of conventional low voltage induction motors fed by pulse width modulated (PWM) inverters has begun to present important problems. These waveforms consist of steep-fronted pulses having very short rise times (about 100 ns in modern IGBT bridges) and high frequency repetition rates (up to 20 kHz) whose immediate consequences are additional electrical stresses in an induction motors insulation system. In this paper a frequency domain model for the analysis and characterization of the internal voltage distribution in random wound coils is presented. The model allows voltage prediction in time domain when an inverse fast Fourier transform (FFT) transformation is performed, and requires only a few frequency domain impedance measurements. This methodology will be useful for accurately predicting the voltage distribution in motor windings during the design stage, and reducing the risk of premature failure in motor insulation. Experimental and theoretical results are presented and compared and model effectiveness using different approximations is studied.     

Effects of inorganic fillers on withstanding short‐time breakdown and long‐time electrical aging of epoxy composites

Inorganic fillers were added to epoxy resin for preparing micro and nano filler/epoxy composite, and then the breakdown strengths, depths of erosion caused by partial discharge, and electrical tree experiments of composites were investigated. It was found that the loading of micro fillers usually decreases the short‐time breakdown strength (breakdown under a continuously rising voltage) of the composites. On the contrary, it increases the ability to withstand long‐time electrical aging (erosion or breakdown caused by electrical tree under a discontinuously rising voltage or constant voltage) of the composites. The effect of inorganic fillers in suppressing the electrical tree or erosion channel propagation in the composite is different during the process of withstanding short‐time breakdown and long‐time electrical aging. Defects are introduced during the mixing of fillers and epoxy resin, which play a role to decrease the short‐time breakdown strength and accelerate long‐time electrical aging. It is concluded that micro and nano inorganic fillers can suppress the electrical tree channel propagation and erosion caused by partial discharge, whereas defects play the opposite role in composites. Two opposite effects of inorganic fillers and defects exist simultaneously during the process of electrical aging and breakdown; whereas inorganic fillers play a primary role in long‐time electrical aging, defects play a primary role in short‐time breakdown.     

Dielectric breakdown strength correlation with time-to-failureduring wet aging of XLPE-insulated cables

This paper reviews two sets of failure information, the GMTF (a parameter obtained from time-to-failure results) and GMBD stress (a parameter that is calculated from AC breakdown test results), and suggests a correlation. The test results were obtained from accelerated aging experiments on full-sized medium voltage cables in carefully controlled and monitored water-filled tanks. Results show that the ambient temperature AC breakdown strength for equivalently aged cables is influenced primarily by voltage stress during aging-not by the aging temperature. The AC breakdown strength on equivalently aged cables was also shown to decrease with reductions in the aging voltage at all aging temperatures     

汽轮发电机定子绕组缺线圈运行时的负序过负荷保护分析

一台缺线圈运行的汽轮发电机,实测有不超过6.2 p.u.的负序电流。为防止负序过电流损伤转子铁芯,需要定量评估电枢电流的影响。假设在故障相上增加一个"叠加绕组",采用叠加原理,分析并计算出电枢反应的反转基波磁动势与正序、负序电流之间的定量关系,进一步计算得到等效的负序电流。分析表明因绕组不对称,负序电流和正序电流都会产生反转的基波磁动势。对于这台发电机的负序过负荷保护,可适当降低长期允许负序电流值I2∞至8.5 p.u.,不必降低承受负序电流能力的常数I22t。     

Impacts of material parameters on breakdown voltage and location for power MOSFETs

Journal of Computational Electronics - To improve the electrical performance of power devices, materials used in fabrication need to be analyzed and optimized. By numerical simulations, we reveal...     

一种用于测量快前沿高压脉冲的电阻分压器

分析了电阻分压器中电感和对地电容对分压器响应特性的影响,在此基础上研制了一种用于测量快前沿高压脉冲的同轴型电阻分压器,介绍了分压器的制作过程,对其进行了方波响应标定,并应用于实验测量中。实验结果表明,该电阻分压器能满足测量要求,可用于幅值10kV左右、ns前沿的短脉冲测量。     

利用平面线圈消除电容器寄生电感的影响

电容器是滤波器的重要组成部分,其寄生参数对高频滤波性能有重要影响。本文设计了一种新型的电容器寄生电感的"消除器",以消除电容器的寄生电感的影响,该"消除器"采用平面化的PCB线圈,其上下层绕组的交错排布,有效降低二次寄生电容。本文分析了寄生电感的消除原理及"消除器"的二次寄生电容问题;建立了其3D有限元模型,对"消除器"的参数进行了分析计算,计算与测量结果极好地吻合;将该类"消除器"应用于电容器寄生参数的抵消,实验表明电容器高频性能得到了明显改善。     

Analysis of a polyphase synchronous reluctance motor with two identical stator windings

This paper presents the performance analysis of a cageless two-stator-winding reluctance motor which is capable of developing reluctance torque at good power factors in the absence of inverter control. The machine stator has two separate identical polyphase windings whose pole numbers are the same as that of the cageless rotor. The machine performance is investigated using the traditional circle diagrams, and the d–q rotor reference frame equations derived in space-vector form by applying the concept of winding functions. Core loss and saturation are accounted for in the developed dynamic model. It is shown that if one winding is connected directly to the supply and the other fed with a balanced capacitor, the developed torque is superior to a brushless doubly-fed reluctance machine (BDFRM) of similar size. Other motor characteristics, its line-start behaviour and torque ripples are also presented and discussed. Experimental results corroborate the simulations.     

一起特高压换流站500kV断路器击穿故障分析与处理

针对某特高压换流站发生的一起500kV小组交流滤波器进线罐式断路器击穿事故,通过保护动作情况、现场设备检查、相关试验及后续故障设备解体等对事故原因进行了分析,对事故的处理过程进行了详细描述,同时提出了切实可行的防范措施,以避免同类事故再次发生.     

An application of the residual voltage for diagnoses for detecting irradiated degradation of insulating materials

Electrical cables in nuclear power generating stations must be highly reliable. For further improvement of reliability, the development of nondestructive diagnoses seems desirable for cable maintenance. The authors have been studying residual voltage for developing a diagnosis of electrical cables. The purpose of this paper is to give a foundation for development of diagnostic technology for detecting the irradiated degradation; the relations between γ-irradiation dose and the leakage current, the discharge current, and the residual voltage were studied and attempts made to calculate the amount of polarized charge and characteristic dielectric relaxation times by using data on the residual voltage. The calculated results agreed with the results obtained from the leakage current and the discharge current. These results suggest that the residual voltage in diagnostic technology for detecting the irradiated degradation can be employed.     

Comparison of soft-breakdown triggers for large-area capacitorsunder constant voltage stress

This work quantitatively compares soft breakdown identification methods for constant voltage stress of large-area nMOS capacitors (up to 10 mm²) with 1.8- to 12-nm gate-oxide thickness (with negative gate voltage). We conclude that in the studied range, breakdown is identified more reliably with a current step trigger than through increased current fluctuation. We present a method to quantify the system background noise, and show results of data filtering algorithms that significantly enhance the ratio between the breakdown signal and background noise level     

双线并绕串联结构电感器寄生电容的消除

电感器是滤波器的重要组成部分,其寄生电容对其高频滤波性能有重要影响。本文介绍了一种双线并绕串联结构的电感器,在其中心抽头处接入一个合适电容器可消除寄生电容,该结构较高的耦合系数保证了高频段寄生电容消除的效果。本文简述了滤波器插入损耗的测试原理及电感器的高频模型,分析了电感器寄生电容的消除原理,对比两种绕线结构电感器的耦合系数,通过实验证实了双线并绕串联结构的电感器寄生电容的消除效果更佳。最后将此结构的电感器应用于LC滤波器中,实验表明滤波器的高频滤波性能得到了明显改善。     

200MW发电机定子绕组端部施加直流电压下泄漏电流测量的技术分析

分析了双鸭山电厂200MW发电机组定子绕组端部绝缘缺陷.采用直流耐压试验方法测量发电机定子绕组泄漏电流.测试实测数据表明,该方法对发现发电机定子绕组端部绝缘缺陷是有效的.