铁心剩磁对电流互感器性能的影响

全面阐述了剩磁对电流互感器的危害.对电流互感器剩磁产生的原因,产生的途径进行了介绍,进而从理论上分析剩磁对测量用、保护用电流互感器性能产生的影响,并通过试验或引用测试结果验证了这种影响,最后提出消除剩磁对电流互感器影响的可行措施.     

剩磁对测量用电流互感器的影响

电流互感器是电力系统中不可或缺的电气设备,其性能直接影响继电保护、电能计量和远程监控能否稳定可靠运行。本文通过阐述电流互感器剩磁产生的原因,从理论上分析了剩磁对测量用电流互感器运行的影响。最后提出了减小或消除电流互感器剩磁的可行措施。     

电流互感器铁心剩磁测量方法研究

剩磁是影响电流互感器传变特性的重要因素,剩磁测量对于电流互感器的应用有着重要的意义。为了有效地测量电流互感器铁心剩磁及相关系数,提出了一种利用交流电压源进行剩磁测量的方法。剩磁测量过程使用交流对电流互感器进行充磁,使其达到深度饱和状态。记录电流互感器感应电压,绘制铁心磁通变化曲线,计算电流互感器剩磁及剩磁系数。试验结果表明测得电流互感器剩磁及剩磁系数与理论分析一致,该方法可以准确地测量电流互感器剩磁。     

保护用电流互感器10%误差曲线现场测试及其二次负载校核

电流互感器饱和是微机保护装置不正确动作的主要原因之一。对电流互感器饱和进行了概述,分析了剩磁引起饱和对微机保护的影响,并着重介绍了保护用电流互感器10%误差曲线现场测试及其二次负载校核方法。     

保护用电流互感器铁芯剩磁衰减规律分析

电流互感器的铁芯具有磁滞效应,运行过程中可能会产生剩磁,进而可能加快电流互感器的饱和,导致保护继电器的误动作。针对该现象,分析了电流互感器剩磁产生的机理及其影响因素;在此基础上,重点研究了电流互感器的剩磁衰减规律。基于分形理论的理论计算公式,可计算短路电流开断后系统无残余电流影响的情况下剩磁的大小;利用PSCAD仿真软件进行试验得到故障后电流互感器一次侧有残余电流情况下磁通的衰减规律,即系统发生短路故障后,电流互感器铁芯内磁通会发生不同程度的衰减,并在局部磁滞回线上运行,既不会在故障切除时刻磁通的基础上运行,也不会衰减至0,为抑制剩磁的产生以及分析电网事故提供了基础理论依据。     

保护用电流互感器10%误差曲线现场测试及其二次负载校核

电流互感器饱和是微机保护装置不正确动作的主要原因之一.对电流互感器饱和进行了概述,分析了剩磁引起饱和对微机保护的影响,并着重介绍了保护用电流互感器10%误差曲线现场测试及其二次负载校核方法.     

电流互感器铁心剩磁影响因素仿真分析

本文通过Matlab中的Simulink仿真软件对不同影响因素下的电流互感器铁心剩磁进行仿真分析,验证电流互感器铁心剩磁在不同影响因素下的变化规律.研究表明,电流互感器铁心剩磁在暂态非周期分量、故障电流的开断时间、一次回路时间常数、故障短路电流大小及二次负载阻抗值的影响下都有明显的变化;影响因素的作用越强,其产生的剩磁就越大.     

剩磁对电流互感器准确级的影响

分析了电流互感器铁心中存在剩磁的原因。阐述了剩磁对电流互感器基本误差的影响程度，提出了减少剩磁影响的措施。     

计量用电流互感器剩磁产生的原因及影响分析

通过理论和实际结合,找到了理论上剩磁对互感器误差影响的原因,和在实验室试验过程中得到大量的一手数据,通过对数据的处理得到电流互感器剩磁产生的原因,并对剩磁的影响进行分析。     

剩磁对保护级电流互感器的影响和PR级的主要参数计算

介绍了剩磁的产生及剩磁对保护级电流互感器的影响.计算了PR级的主要参数,并说明了开气隙铁心的气隙尺寸对误差和剩磁的影响.     

Compensation of the distortion in the secondary current caused by saturation and remanence in a CT

Current-transformer (CT) saturation may cause the maloperation of a protection relay. This is particularly onerous when the remanent flux in the core of the CT adds to the flux change caused by the fault. The CT is forced into deep saturation and the waveshape of the secondary current is severely distorted. An algorithm for compensating the distortion in the secondary current caused by saturation and remanence in a CT is described in this paper. A second-difference function detects when the CT first starts to saturate. At this instant, the negative value of the second-difference function corresponds to the magnetizing current which, in conjunction with the magnetization curve, is used to estimate the core flux. This is then used as an initial value to calculate how the flux changes during the fault. The magnetizing current is estimated by inserting the estimated core flux into the magnetization curve and added to the secondary current; the result, the compensated secondary current, is equal to the secondary referred primary current. Various test results indicate that the proposed algorithm can accurately compensate a severely distorted secondary current and is not affected by remanence. The paper concludes by describing the hardware implementation of the algorithm on a prototype compensation unit based on a digital signal processor.     

Effect of Core Magnetization on Frequency Response Analysis (FRA) of Power Transformers

This paper presents how the frequency response analysis (FRA) measurements on a transformer can be influenced by magnetization condition of the core. Measurements were performed on two transformers at different magnetization levels to show effects of remanent magnetization in the core due to removal of 3-supply, of relaxation demagnetization and of dc flux on the frequency response of winding impedance. The most important observations were that any sudden change of excitation field (applying or removing magnetization current) yields a slowly varying magnetic relaxation, which causes the impedance to change with time due to magnetic viscosity. Results of diagnostic measurements of transformer winding impedance can therefore depend on the level of remanent magnetic flux appearing in the core limbs after switching-off a transformer. These observations lead to a conclusion that, for avoiding the effects of core magnetization, diagnostic FRA measurements on power transformers should not be performed directly after disconnection from network.     

Improved simulation models for current and voltage transformers inrelay studies

Improved models for current transformers (CT), potential transformers, and capacitive voltage transformers (CVT) for a relay software library are presented. The magnetizing characteristic in each of these transformers is based on the nonlinear power curve formulation of J.R. Lucas (1988). The flux-current loops of the transformer core are self-generated rather than predetermined or based on look-up tables. The results presented show that the models developed for the transformers behave as expected, and agree with reported field investigations. Models for the CT and CVT give acceptable results, including those under remanent conditions and ferroresonant conditions. These models are thus necessary components for relay studies carried out with electromagnetic transient programs whether offline or in real time     

A CT saturation detection algorithm

This paper proposes an algorithm for detecting current transformer (CT) saturation by analyzing a secondary current. The current has points of inflection where saturation begins and ends. Then, discontinuity in the first difference of the current arises at the next instants of points of inflection, where the second and third differences of the current may have large values enough to detect saturation. In this paper, the third difference is used because it is more effective than the second difference in terms of saturation detection. However, an anti-aliasing low-pass filter softens the current and, thus, reduces the values of the third difference at those instants. This paper includes the effect of a low-pass filter on the proposed algorithm. The sampling rate of 64 samples/cycle (s/c) is studied by varying a remanent flux and the cutoff frequency of the filter. The test results clearly indicate that the proposed algorithm successfully detects the interval of saturation even with a remanent flux and the softening effect. However, special attention should be paid to the decision of the threshold value when a cutoff frequency is much lower than half the sampling frequency. This paper concludes by implementing the algorithm into a TMS320C6701 digital signal processor and the results are satisfactory.     

Current Transformer Model

This paper presents a current-transformer (CT) model that is useful for low-frequency applications. To describe the iron-core magnetic behavior, a hysteresis model is proposed, which is able to generate minor asymmetric loops and remanent flux. The effects of classic eddy current losses and anomalous losses are represented by linear and nonlinear resistors, respectively. The obtained results are compared with those calculated by the Preisach's model and measured in the laboratory. This model may be applied in power system protection studies, as it is the case of numeric correction of distorted secondary currents in current transformers (CTs)     

电力变压器励磁涌流仿真计算的瞬时功率法

本文提出一种基于变压器铁芯材料的基本磁化曲线Ｂ＝ｆ（Ｈ）、变压器铁芯的几何尺寸、空载损耗数值及瞬时功率的概念，对变压器空载合闸时，在各种不同的合闸角及各种不同剩磁的情况下所产生的励磁涌流进行计算的新方法。通过对计算结果的分析，得出励磁涌流的特征，如峰值、波形及间断角的大小同剩磁和合闸角的关系等。本文所提出的方法有助于对电力变压器设计的正确性进行校验及变压器保护的研究。     

Ferroelectric and dielectric properties of sol-gel and excimer-laser-deposited lead zirconate titanate and barium titanate films

Abstract

Switched remanent polarization was measured as a function of accumulated switching cycles for a variety of ferroelectric films using sinusoidally driven hysteresis loops. Switched remanent polarization and dielectric constant and loss were also obtained as a function of the cycling frequency. PZT films with niobium additives appeared to lose switched remanent polarization with accumulated cycles at a lesser rate than films without niobium. The switched remanent polarization was found to decrease with increasing frequency, which we attribute to the effect of grain size. Also, a decrease of dielectric constant with increasing frequency and an increase of dielectric constant with increased applied voltage are attributed to the effects of domain wall motion contributions to dielectric constant.     

An algorithm for compensating secondary currents of currenttransformers

Current transformer (CT) saturation may cause power system relays to malfunction. The conventional method used to deal with the problem is overdimensioning of the transformer core so that CTs can carry up to 20 times the rated current without exceeding 10% ratio correction. However, this not only reduces the sensitivity of power system relays, but also increases the CT core size. This paper presents a technique of estimating the secondary current corresponding to the CT ratio under CT saturation. The proposed algorithm can improve the sensitivity of relays to low level internal faults, minimize the instability of relays for external faults, and might ultimately assist in reducing the dimension of the required CT core cross-section     

套管型电磁式电流互感器及其应用

电流互感器(CT)是联接高压一次回路与控制保护回路的核心元件。笔者从CT的电磁转换原理探寻CT的结构尺寸及其内在规律,分析了CT误差产生的主要原因是漏磁通和励磁电流,而励磁电流又决定于CT的电气参数、铁心材质及结构尺寸。图解了CT制造时二次绕组均匀分布可将漏磁通降至最低,也只有绕组抽头各级都均匀绕制的CT,其复合误差试验的间接法与直接法才具有等价性。在满足CT使用要求的情况下,适当调整CT的电气参数,如多抽头CT的负载分级、无重合闸要求的暂态保护级CT工作方式由双循环变更为单循环等,可减小CT的误差、降低制造成本。最后,对内置式CT绝缘材料的选择、外置式CT的防潮性进行了说明,特别是对外置式CT现场组装时形成闭合回路的原因、闭合回路对误差的影响以及检测闭合回路存在的方法进行了详尽的解读。