Lead-free piezoelectric materials with excellent electromechanical compatibility are essential for industrial applications. However, attaining both a large piezoelectric coefficient (d33) and a high mechanical quality factor (Qm) is generally regarded as challenging because of the inherent trade-off among these properties. In this work, the reduction of internal defects and the redistribution of the second phase in potassium sodium niobate (KNN) based composite ceramics are achieved through a heat treatment technique. This method can achieve a significant improvement of electromechanical properties (d33 = 415 pC/N and Qm = 120), which effectively overcomes the contradiction between piezoelectric properties and mechanical losses. Structural characterizations indicated that the improved electromechanical performance of the annealed KNN composite ceramics could be attributed to the optimized internal defects and the extrinsic local stress field. These findings offer a promising route to enhance the commercial feasibility of lead-free KNN-based piezoelectric ceramics, representing significant progress in the development of high-performance and environmentally friendly piezoelectric materials.

变压器35kV绕组中性点接地方式较多,现有设计没有对不同接地方式下变压器的防雷措施进行专门考虑,导致变压器35 kV侧雷击事故频发.为了对不同接地方式下变压器35kV绕组雷电侵入波特性及防治措施进行研究,采用电磁暂态分析软件ATP-EMTP建立仿真模型,对35kV绕组星形接线的配网变压器和三角形接线的风电场升压变进行了雷电侵入波电磁暂态仿真,分析了导致35kV绕组绝缘损坏的雷电侵入波特点,考察了不同安装方式下避雷器对雷电侵入波防治的有效性.研究表明不同接地方式下变压器35 kV绕组的雷电过电压特性不同,应根据不同接地方式采取针对性防雷措施.

继电保护课程内容知识点多,概念抽象,学生学习时往往难于理解.针对此问题,本文基于Matlab搭建了三段式电流保护的教学仿真模型.该模型能够实现保护的闭环仿真,同时对保护的动作过程进行分析.通过该教学仿真模型,学生能直观理解保护的数据输入、保护算法和保护逻辑功能的实现,有助于学生更好地掌握继电保护基本原理.

直流配电网故障定位是排查故障、保障线路安全和保证供电质量的关键.文中研究了现今直流微网中的故障定位技术,在基于电压源换流器的直流微网中分析了单极故障和极间短路故障时的故障响应特性,提出了一种利用换流器出口侧电容放电过程,提取其中的故障电压电流信息计算线路电感值从而确定出故障位置的新方法.根据电压关系将故障过程分段,利用暂态电流的变化率进行故障选线,再根据故障电路的二阶响应过程,确定出故障位置.在PSCAD/EMTDC中进行了仿真验证,结果表明,定位算法能够准确识别出故障类型以及故障线路并精确定位出故障位置.极间短路故障的定位误差在0.5％以内,单极接地故障的定位误差在3％以内.

随着智能变电站的建设,继电保护原理的实现技术发生了巨大的变化.传统验证性、原理性的实验平台难以适应工程现场继电保护专业技术的应用.因此,迫切需要对现有的教学实验基础条件进行改造和建设.因此,本文阐述四川大学新建继电保护综合实验室的建设思路和方案.实验室以现场电力系统智能变电站建设标准为参考,引进工业化的微机继电保护装置及监控系统,最大限度地模拟实际工程应用,以期达到学生在校实验与工程操作无缝对接的教学目标.

高压直流输电线路及其边界对故障暂态信号高频分量都有明显的衰减作用,这一故障特征被广泛运用于暂态保护,且有研究显示,相较于线路边界,长线路对故障暂态高频分量的衰减作用可能更大,为避免这种情况引起的保护误动,目前常用的故障特征提取方式为利用不同尺度的小波变换获取暂态信号能量比.但基于小波变换的暂态保护存在算法相对复杂,基波选取模式不统一等问题.因此提出一种新的比值算法,针对故障暂态信号中的能量比,用全电流代替低频分量,简化了直流线路故障信息处理过程,由此设计出了新的直流线路故障定位方法,并且利用该判据可直接实现故障极判断.在MATLAB中进行了大量仿真验证,证明该保护算法可以正确反映出故障特征,逻辑简单,并且能够迅速可靠动作.

综述了国内外直流微电网故障定位的研究现状。首先,分析了直流微电网故障定位的基础,指出直流微电网接地方式的选取和故障特征分析对故障定位的重要性。之后,对现有直流微电网故障定位原理进行归类研究,将直流微电网故障定位方法分为注入信号法和故障分析法,指出:各类算法均有优缺点;注入信号法适用于离线故障定位,故障分析法适用于在线故障定位,2种定位方法的准确性均受过渡电阻影响大;现有故障定位算法无法同时对单极接地故障和极间短路故障进行定位,且制约其定位准确性的因素多。最后,指出直流微电网故障定位面临的关键问题,并对未来直流微电网故障定位进行展望。

High-performance air filters were prepared by a simple in situ ionic self-assembly process of amino acid salts.

当直流微网线路发生故障时,为快速切除故障并恢复供电,减少停电时间,需要快速准确的故障定位.以低压双极环形直流微网为研究对象,针对单极接地故障和极间短路故障,提出了一种差分法与伪逆法相结合的新型故障定位方法,可在线确定出不同故障条件下的故障位置,且定位准确性高.根据暂态期间故障回路的KVL矩阵方程,利用差分法和伪逆法,计算故障线路的等效电感,进而计算出故障距离,从而实现故障定位.通过PSCAD/EMTDC仿真验证了该方法的有效性,对两种故障类型及不同故障距离和故障电阻,故障距离估计误差均小于1.43％,故障电阻相对误差小于0.04％.该方法可针对两种不同的故障类型,不受故障距离和故障电阻的影响,能准确地确定出故障点位置并计算出故障电阻,具有较好的实用性.

Due to the access of the distributed generations (DG),the original protection of the distribution network has been changed and can not be adaptable for the network with DG.In order to guarantee the power distribution reliability and make full use of DG,it's necessary to discuss the protection scheme of the distribution networks with DG.The influence of DG on the original protection is summed up in this paper.The improving protection methods with DG are thoroughly analyzed and classified,it can be divided into limiting the penetration capacity of DG,promoted adaptive protection,fault location based on the communication technology,islanding detection and islanding partition.The principles and characteristics of these methods are analyzed,the advantages/disadvantages are illustrated and the developing trends of distribution network protection with DG are pointed out.It can be a reference to the study of distribution network protection with DG.

Objective:To study the diagnostic value of MR diffusion weighted imaging (DWI) and CT and MR perfusion imaging in patients with different degrees of liver cirrhosis.Methods:A total of 60 patients with liver cirrhosis,who were treated in Ziyang Hospital of West China Hospital of Sichuan University from August 2015 to February 2017 were selected.According to the Child-Pugh classification,32 cases of grade A were mild cirrhosis (denoted as group A),16 cases of grade B and 12 cases of grade C were moderate to severe cirrhosis (denoted as group B),30 healthy volunteers were selected as group C in the same period.All the subjects in the three groups were examined by DWI,CT and MR perfusion imaging.The ADC value,hepatic portal perfusion ratio [SSr (CT) and SSr (MR)] were compared among the three groups,and Spearman correlation analysis was used to analyze the correlation between the indexes.Results:There was no significant difference in ADC values among the three groups (P＞0.05),and the ADC values of group A and B were lower than those of group C,but the difference was not statistically significant (P＞0.05).There was no significant difference in ADC values between group A and B (P＞0.05).The levels of SSr (CT) and SSr (MR) in the three groups were compared,the difference was statistically significant (P＜0.05);the levels of SSr (CT) and SSr (MR) in group B were significantly higher than those in group A and C,the difference was statistically significant (P＜0.05).There was no significant difference in the levels of SSr (CT) and SSr (MR) in group A and C (P＞0.05).Spearman correlation analysis showed that SSr (CT) was positively correlated with SSr (MR) in the patients with liver cirrhosis (r=0.687,P=0.000).Conclusion:CT and MR perfusion imaging can reflect the lesion degree of liver cirrhosis,and diagnostic effects of the two are better than DWI imaging,which is worthy of clinical promotion.

With the consideration that the location method for harmonic sources(HSs)based on harmonic state estima-tion(HSE)requires more harmonic meters,a two-stage location method for HSs is proposed in this paper.Firstly,an im-proved independent component analysis(IICA)algorithm considering measurement error is applied to initially select HSs.Then,on the basis of selection results and with the combination of the impact of measurement error and parame-ter error on HSE,total least squares(TLS)algorithm is used to estimate the harmonic injection current on suspicious nodes,thereby realizing the location of HSs accurately.Finally,the validity of the proposed method is verified by simu-lations conducted in an IEEE 14-node system with the evaluation indices of failure rate,mean value of root mean square error,and the minimum number of harmonic meters.

2014年8月21日,IEEE正式通过的IEEE 1668-2014标准,首次给出了1000V以下终端电气设备电压暂降与短时中断穿越（ride-through）性能与兼容性试验标准。基于该标准,在分析敏感设备电压暂降与短时中断穿越能力试验要求与方法的基础上,对标准中尚存在的不足也进行了分析,指出了有待完善和解决的关键问题,并针对这些问题提出了作者的建议,其目的在于更加科学完整地评价终端敏感设备电压暂降与短时中断的抗扰能力。

在SVPWM调制过程中,零矢量的作用会引起实际电压矢量与理想电压矢量之间相角的偏差,从而导致逆变器输出侧存在电压谐波,对于输出电压精度有一定的影响.而在逆变器并联系统中,较小的输出电压相角误差能够引起较大的有功环流,因此要尽量减小零矢量作用效果从而减少环流.首先探讨了零矢量作用对SVPWM调制的影响,然后分析了均分零矢量的法对逆变器并联系统有功环流抑制作用,仿真证明此方法输出谐波更小,对逆变器并联系统的有功环流有一定的抑制效果.

Prony方法具有减少计算量、直接求得信号的幅值、阻尼比、相位和频率,采用最小二乘法意义上的拟合消除噪声影响等优点,将其用于电力系统有着明显的优势和广泛的应用前景.本文结合实例,谈谈Prony方法在电力系统暂态谐波中的应用.

As a green energy,solar photovoltaic technology has a wide prospect and it has been the research hotspot in industrial circles and the academic circles.In this paper,a battery charging circuit controlled by PIC microcontroller for stand-alone photo-voltaic power generation was presented.The shortcomings of the BUCK and BOOST circuits applied in the MPPT circuit were analyzed.The experimental result shows that the pulse charging method is used to charge the battery,which can extend the life of battery and improve system efficiency.

In order to select phase quickly for switching into fault protection,the frequency domain characteristics of transient signal when switching into normal line and fault line are used.FIR digital filter is used for the transient current produced by transmission line under the situation of switching into different faults to obtain the high-frequency and low-frequency current value corresponding to each phase respectively,and then the energy of six high-frequency and low-frequency components of three-phase current are compared to distinguish the phase sequence and the fault types.Lots of simulation experiments with Matlab indicate that this method can distinguish the phase sequence of fault line correctly,which can provide a useful reference for speeding up the transient-based protection.

The principle and identification criterion for switching into fault protection are described,the existing problems are given in theory studies,and new research trends are presented.At last,the developing direction of switching into fault protection is prospected.

In view of the structures and features of transmission lines consisting of overhead lines and power cables,a new accurate fault location scheme based on distribution parameter model of combined transmission lines is presented.The method can discriminate the fault whether happened in overhead lines or power cables,then the fault location can be determined by using the transmission lines parameter and post-fault voltages and currents from the two terminals.The simulation results of EMTP and the data processing of Matlab show that this algorithm is of high accuracy and does not require the synchronized data of two terminals,and it isn't affected by fault resistance,fault type and fault location.The strategy of circuit breaker operation can be decided based on the results of the precise location.The test result shows that it is valid and efficient.

By analyzing a traditional double-terminal method based on distributed parameter model of overhead line,a new double-terminal fault location algorithm based on the comparison of junction voltage dividing hybrid transmission line into parts to solve the discontinuity of parameters is proposed.The algorithm based on the distributed parameter model of hybrid transmission lines,uses the voltage and the current obtained from both hybrid line ends to acquire junction voltage and compare its magnitude to discriminate the fault happened in cable or overhead line,then uses the voltage and the current obtained from both fault line ends calculate the fault location the same as traditional double-terminal method.Theoretical analysis and the simulation of ATP/ MATLAB show that the proposed scheme can accurately measure the hybrid transmission line fault location and be also applied to more complex hybrid transmission lines.