Importance of bandwidth in PD measurement in operating motors andgenerators

It is well known that partial discharge (PD) events produce electromagnetic radiation with frequencies from de to sometimes hundreds of megahertz. In addition, in complex structures such as stator windings, the initial pulses are significantly distorted and attenuated as they propagate through a winding. The detection of the PD pulses needs to account for these effects. In particular, the bandwidth of the detection system will influence the sensitivity of the PD measurement, as well as in what portions of the winding PD can be detected. This paper summarizes over 20 years of research into these effects in stator windings, and discusses the implications for off-line and on-line PD testing, and the calibration of the PD quantities into apparent charge     

Effect of manufacturer, winding age and insulation type on stator winding partial discharge levels

With thousands of machines monitored for as long as 25 years with the same method, on-line partial discharge (PD) testing has become a recognized, proven tool to help maintenance engineers identify which stator windings need off-line testing, inspections and/or repairs. With over 63,000 test results acquired with the same test method, what constitutes a winding with low, moderate or high PD has been identified. This paper presents tables that enable test users to easily identify with some certainty which stators are likely to suffer from groundwall insulation deterioration, with only a single measurement on a machine. The practical importance of these tables is that if one applies PD sensors to a machine and, in the first measurement, one obtains a Qm that exceeds the 90 percentile of the relevant Qm distribution, then one should be concerned enough at the PD level to take action, such as more frequent testing and/or off-line tests and inspections at the next convenient machine shutdown. Within the statistical accuracy possible with several thousands of independent results, it seems that critical PD levels only depend on operating voltage, hydrogen pressure, manufacturer, and the specific type of PD sensor and instrumentation used.     

大型发电机定子局放在线检测中放电量标定研究

发电机局部放电信号强度是判断定子绝缘状态的一个重要参量,目前对发电机定子局部放电信号的强度有多种表示方法,造成了认识上的混乱。本文探讨了大型发电机定子局放量标定中存在的困难及其原因,讨论了采用不同带宽的检测系统对放电量标定带来的影响,对大型发电机定子局部放电信号强度的表示方法提出了建议。     

New practices for partial discharge detection and localization in large rotating machines

This paper presents a new technique for detection and localization of partial discharges (PD) in large rotating machines. In this technique, denoted "InTech", a stator winding with (n) coils is equipped with (n + 1) inductive sensors; as a result, every stator coil is situated between two sensors. The measurement is then focused on the individual stator coils. In this way common difficulties facing partial discharge measurements in rotating machines such as attenuation and distortion of the signals in the stator winding, noise reduction, and the calibration accuracy are reduced considerably. The treatment of the sensor signals, using a special method, results in a measurement of PD in each respective stator coil. In addition, the stator coil from which the PD originates is identified. Results from field measurements on both hydro and turbo machines are presented and discussed.     

Partial discharge, dissipation factor, and corona aspects for high voltage electric generator stator bars and windings

High voltage stator windings in modern electric generators are subject to high electrical, mechanical, and thermal stress. Operation of these machines in both air and high pressure hydrogen results in different considerations. Operation in the air environment does not offer the dielectric advantages of operating in high-pressure hydrogen. The two major design factors that are significant for the design of high voltage stator bars to be operated in air, are corona activity and PD activity. These of course are closely related. With the movement to increasing the power density of generators, which means operation at higher voltages and higher insulation stress levels, it has become more critical to be sure the design constraints are met in the stator bar design to minimize both partial discharge activity and corona activity involving the stator bars. The stator bar insulation system must be designed for minimum partial discharge activity and ideally is corona free at the stator winding operating stress levels, and at the operating conditions (temperature and pressure) the generator is subject to. This paper will present the findings associated with the development work in designing, testing and implementing a PD reduced and corona free design.     

Assessing Test Data

Online partial discharge (PD) testing has been used for more than half a century to provide useful information for diagnosing and monitoring the integrity of stator winding insulation of large generators and motors. Interest in online PD detection and monitoring has as the technology, which was originally used on machines rated higher than 10,000 V, has expanded its application to the smaller, lower-voltage (e.g., 4,160 or 6,900 V) machines. However, questions remain on how to assess stator insulation problems using PD test data. This article documents the most recent developments in research and applicable guides and standards related to online PD monitoring on rotating machines.     

On‐line partial discharge detection of turbine generator using gigahertz‐band spatial phase difference method

Partial discharge (PD) within voids in solid insulating systems is a symptom and/or a direct cause of a deterioration of stator windings of a turbine generator. The bandwidth of the electromagnetic waves emitted from PD is very broad and ranges to the gigahertz. We developed a new system based on the spatial phase difference method for detecting microwaves (GHz) emitted from PD. In this paper we applied this system to two off‐line turbine generators in the overhaul and an on‐line turbine generator which has two built‐in antennas. On‐line PD detection of the turbine generator is very useful for recognizing abnormal and/or deteriorated stator winding insulation without a machine outage. It was found that the detection system was able to detect microwaves emitted from PD in an operating turbine generator. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 38‐43, 2000     

灯泡贯流式水轮发电机组的局放在线监测

针对水轮发电机组结构及汇流母线排走线方式的特点,选择发电机每相高压出线处为系统监测点,利用双安装传感器定向耦合实现干扰脉冲抑制。该系统分别在电站6台发电机主引出线上安装近、远端两路传感器(6只/每台机组),耦合放电信号,经过信号的选通、滤波处理后进入监测系统,利用软件进行时延幅值判别,提取放电信号,并进行二维、三维谱图分析。对比分析了现场测到的放电信号与干扰信号在时域及幅值上的差别,验证了抗干扰方法的有效性。自第一台机组并网发电以来,系统运行稳定、操作方便,所测结果与预试数据基本一致。     

Investigations into the effect of humidity on stator winding partial discharges

Online partial discharge (PD) testing has become a powerful tool to plan the maintenance of stator windings of high voltage motors and generators. Interpretation of PD data is best done by monitoring the trend in PD over time. Unfortunately the trend in PD is sometimes difficult to ascertain, especially if the humidity within the stator is variable over time. This paper shows several examples of how PD is affected by humidity in operating machines. Laboratory experiments were also performed to duplicate this phenomenon. In most practical situations, it appears that the PD activity decreases as the humidity increases.     

Unusual PD pulse phase distributions in operating rotating machines

The position of partial discharges (PD) with respect to power frequency phase position has long been known to provide useful information on the nature of deterioration of electrical insulation. In the past, virtually all such data has been from PD tests performed in the laboratory on static models or equipment. Improvements in the past 15 years in noise elimination technology have made it possible to monitor the PD activity (number, magnitude and phase position) in the HV stator windings of operating motors and generators. Over a two year period, the PD activity was recorded on more than 100 motors, hydrogenerators and turbine generators, and the pulses analyzed with respect to the ac phase position, with a resolution of 3.6°. In the majority of stator windings, the PD pulses occurred in the expected portion of the ac phase, i.e. Between 0 and 90° as well as between 180 and 270°. However a significant number of instances were recorded where the pulses appeared to be shifted ~30°. This is probably due to the phase-to-phase electric stress which occurs in the endwinding portion of a three phase stator winding. In many other machines, the PO phase positions were very different from the expected patterns determined from static tests. In some of these cases, such results were obtained on machines with suspected loose stator windings. It seems reasonable to conclude that the unusual PD patterns may be due to the influence of moving stator coils on the PD patterns. Such an influence is not easy to simulate in static laboratory tests. PD pulse phase analysis is shown to be a useful means of helping to determine the main deterioration mechanisms of stator insulation systems of operating motors and generators     

A Survey of Methods for Detection of Stator-Related Faults in Induction Machines

As evidenced by industrial surveys, stator-related failures account for a large percentage of faults in induction machines. The objective of this paper is to provide a survey of existing techniques for detection of stator-related faults, which include stator winding turn faults, stator core faults, temperature monitoring and thermal protection, and stator winding insulation testing. The root causes of fault inception, available techniques for detection, and recommendations for further research are presented. Although the primary focus is online and sensorless methods that use machine voltages and currents to extract fault signatures, offline techniques such as partial discharge detection are also examined. Condition monitoring, fault diagnostics, insulation testing, interlaminar core faults, partial discharge (PD), temperature monitoring, turn faults.     

发电机局部放电甚高频在线监测现场试验分析

笔者研制了发电机局部放电甚高频在线监测系统,该系统基于双传感器定向耦合的原理进行信号采集,利用数字脉冲逐个识别技术通过软件来实现干扰抑制。结合在四川龚嘴电厂、福建池潭电厂等地的现场实验分析,验证了该系统的测试效果及其抗干扰能力。通过对某电厂发电机连续在线监测及故障预警的分析,证实了相比单一的放电量分析,采用统计参数趋势分析的方法能够更有效地反映发电机放电变化趋势。     

In-service evaluation of motor and generator stator windings usingpartial discharge tests

Partial discharges (corona) are a symptom of most types of deterioration of motor and generator stator windings rated 4 kV and above. Experience indicates that partial discharges occur years before failure. This leaves sufficient time to plan corrective maintenance to avoid the in-service failure of the motor or generator. This paper describes a new type of partial discharge test which can be performed by plant personnel during normal operation of the machine, thus enabling all users of large machines to plan maintenance. The key requirement of the new test is to remove all the interference caused by other sparking and discharging sources in a plant. Such noise has lead to false indications of stator winding deterioration in the past. The new test uses either high-voltage capacitive sensors, high-frequency current transformers, or 1000 MHz electromagnetic couplers to detect the discharge signals. These signals are processed on a pulse-by-pulse basis by a specialized electronic instrument to remove noise and the remaining partial discharges are then categorized according to number, magnitude, and phase position. The test has been implemented on over 100 machines, and the results appear promising     

基于数字局部放电检测的大型电动机主绝缘老化特性的研究

大型电动机在长期的运行过程中,要遭受电,热,机械等多因子应力的联合作用,使主绝缘逐渐老化,为了研究主绝缘老化机理,评估其剩余寿命,需要对电动机定子线棒进行多因子应力综合老化试验,并寻找能反映主绝缘老化程度的特征参量;本文采用先进的数字局部放电检测技术对不同老化阶段的试样线棒进行了综合测试,分析了局部放电各参量与试样线棒老化程度的关系,给出了各种反映绝缘老化特性的指纹图形,测试结果表明,随着试样线棒的进一步老化,局部放电量,放电次数,放电能量呈增长趋势,最大放电脉冲相位产生偏移,其偏斜度由大变小,这为评定大型电动机主绝缘的剩余寿命提供了参考依据。     

Application of wavelet analysis to the determination of partial discharge location in multiple-α transformer windings

The inner insulation system is a critical component of a power transformer. Its degradation may cause the device to fail while in service. If deterioration of the insulation system caused by Partial Discharge (PD) activity can be detected at an early stage, preventive maintenance measures can be taken. Due to the complex structure of power transformers, accurate locating of PD is not an easy task and is one of the main challenges in front of power utilities. Locating PD is more difficult in transformers with multiple-α windings. This problem comes to be vital in open access systems. A method for locating partial discharge within multiple-α windings is proposed, which is based on structural data of a transformer. A 66 kV/25 MVA transformer with fully interleaved winding and connected tap winding is used as test object. Wavelet transform is employed to process the partial discharge signals. Wavelet transform analysis method is a powerful tool for processing transients and non-stationary or time varying signals. Since the wavelet transform provides multi-scale analysis and time–frequency domain localization, it is particularly suitable to process the partial discharge signals. In order to improve the accuracy of the partial discharge location, a new technique for extracting Partial Discharge signals is introduced. Applying wavelet transform to a signal produces a wavelet detail coefficient distribution throughout the time-scale, which depends on the mother wavelet chosen. This technique is based on the capability of the chosen mother wavelet for generating coefficients with maximum values. The wavelet based de-noising method proposed in this paper can be successfully employed to extract PD pulse from the measured signal. It can provide enhanced information and further infer the original site of the PD pulse through capacitive ratio method. The method is described in details and the applications to determine the partial discharge location in multiple-α windings are explored.     

Impact of Slot Discharges and Vibration Sparking on Stator Winding Life in Large Generators

Slot discharge and VS are two stator winding deterioration mechanisms acting on stator winding insulation. Slot discharge can have at least three different causes only one of which is caused by bars vibrating in the slot. Slot discharge will be significant only in bars that are operating at or near rated voltage. Vibration sparking will only occur when two conditions are met simultaneously: a) the slot conductive coating is too conductive, and b) the bars are vibrating in the slot. This can occur throughout the stator.The manifestations of the two mechanisms can be very similar. The major difference between slot discharge caused by loose coils and VS is that VS can occur on bars located anywhere within the winding. Vibration sparking is generally the more aggressive of the two mechanisms. Because the root cause and corrective actions are quite different, it is important to distinguish between the two mechanisms. The presence of VS or slot discharge in a stator winding can have significant influence on the projected life of a stator winding. Thus it is important to detect and correctly diagnose either problem in its early stages. If the amount of activity is significant and advanced, repair may be difficult or impossible, particularly in the case of VS.     

基于遗传编程的绝缘内部局部放电缺陷模式识别

采用新的模拟进化技术——遗传编程，进行局部放电模式识别以区分不同的绝缘内部缺陷类型。制作了4种结构的人工缺陷模型以模拟发电机定子中典型的绝缘内部放电，从局部放电试验中获得二维和三维谱图特征，计算局放信号的矩特征值。首先用模糊方法将局部放电信号的矩特征表示为关于对象不确定知识的模糊特征，作为放电数据的预处理。再由遗传编程分类表达式进化生成局部放电缺陷类型判别函数，并采用递增式学习规则以提高最佳特征对局部放电缺陷分类的效果。另外，将Bootstrap统计模拟技术与遗传编程结合，以克服从小样本数据中进行知识获取的“瓶颈”。人工缺陷模型试验数据的测试结果表明，该方法在局部放电缺陷类型识别中得到了良好的识别效果。     

预防气体绝缘开关装置故障的超高频局部放电在线实时监控系统

超高频局部放电在线实时监控系统可预防气体绝缘金属封闭开关设备(gas insulated switchgear,GIS)发生灾难性故障。介绍了超高频局部放电实时监视系统的基本结构、局部放电试验情况,并给出了该系统在韩国应用的2个实例,该系统的数据库储存了实验室条件下的各种超高频局部放电数据和现场实际运行中的局部放电信号,安装该监控系统后,非专业人员也可判断GIS的状态。     

Using correlation coefficients for locating partial discharge in power transformer

Most serious failure of power transformers is due to the insulation breakdown. Partial discharge (PD) that damages insulation by gradual erosion is major source of insulation failure. The effective ability of the wavelet packets analysis as a tool for disk-to-disk partial discharge faults detection and localization in transformer windings is shown in this paper. Techniques for locating a PD source are of the major importance in both the maintenance and repair of a transformer. One of the most well-known methods of PD localization in transformers is based on winding modeling and current of neutral point analysis. Since the impedance between PD location and neutral point of winding depends on the PD location in respect to neutral point, the frequency spectrum of neutral point current varies when the PD location changes. In the other word, the current components of neutral points vary according to the place where PD occurs. So in this paper, detailed model of transformer winding is modeled and the neutral point current is studied for locating PD. The used method is validated by the simulated model of transformer windings. This model produces a very acceptable current when compared to the experimental data. In this paper for locating partial discharge (PD) in transformer windings, a simulated model is developed for the transformer winding and the PD phenomenon mechanism. The impulse current test and wavelet packets transformation are used to locate PD. It is shown that the neutral current measurement of the transformer winding has useful information about PD location.     

高压电机局部放电在线监测方法

局部放电在线监测是诊断高压电机定子绕组绝缘邦联的有效方法之一。文中概述了国外高压电机局部放电在线监测的几种方法：基于汇流环上成对耦合电容器的监测法,基于中线射频监测法,基于电机引出线上耦合器的监测法,基于定槽耦合器的监测法,基于埋城定子槽里的电阻式测元元件导线的监测法等。