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.     

Identification of stator insulation deterioration using on-linepartial discharge testing

This article analyzes why one cannot rely on an absolute partial discharge (PD) magnitude to assess the insulation condition of rotating machines. How to identify stator insulation deterioration effectively and reliably is discussed and demonstrated by case studies. The paper concludes that trending of PD activity is the reliable way to identify severe insulation deterioration in rotating machines     

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     

Stator winding monitoring

Partial discharge tests can determine which motor and generator stator windings are experiencing insulation problems. A deteriorated winding has a PD activity which can be 30 times or more higher than a winding in good condition. This great difference in PD activity enables even nonspecialized maintenance personnel to identify the few motors or generators in a company which need further investigation and/or maintenance. There are several advantages to on-line partial discharge tests. Machines in good condition require less attention. The overall effect is lower maintenance costs. After implementing on-line partial discharge tests, as well as other monitoring such as flux probing or current harmonic analysis and improved temperature sensing, companies can often confidently extend the outage between major machine inspections. This saves on outage costs and reduces the risk of a stator fault due to human error during maintenance. Finally, on older machines, if there has been no increase in partial discharge activity over time, then the life of the stator winding can be confidently extended, saving a considerable capital expenditure     

Performance evaluation of noise reduction method during on-line monitoring of MV switchgear for PD measurements by non-intrusive sensors

Partial Discharge (PD) measurement is a globally accepted method for insulation diagnosis of electrical assets. The consequences of insulation breakdown are well known. The trend is to move from conventional offline testing to online monitoring for insulation life prediction, which results in the inclusion of high frequency noise in the captured signals. Therefore de-noising is of paramount importance in online monitoring to obtain useful information from the signal.In this research, a 20 kV switchgear panel has been subjected to PD faults in the laboratory and measurements have been carried out by using different non-intrusive sensors including a novel sensor, the D-dot sensor and recorded by a high frequency oscilloscope. The measured results show the effective applicability of sensors for switchgear. The Discrete Wavelet Transform (DWT) has been used to de-noise PD signals in this paper. Time domain and frequency domain comparison of original and de-noised PD signals reveals the significance of this technique for online monitoring of Medium Voltage (MV) switchgear. Finally, an adaptive online de-noising concept, based on automatic de-noising is also proposed in this paper.     

Advancements during the past quarter century in on-line monitoring of motor and generator winding insulation

Electrical insulation plays a critical role in the operation of motor and generator rotor and stator windings. Premature failure of the insulation can cost millions of dollars per day. With advancements in electronics, sensors, computers and software, tremendous progress has been made in the past 25 yr which has transformed on-line insulation monitoring from a rarely used and expensive tool, to the point where 50% of large utility generators in North America are now equipped for such monitoring. This review paper outlines the motivation for online monitoring, discusses the transition to today's technology, and describes the variety of methods now in use for rotor winding and stator winding monitoring.     

Partial discharge. VII. Practical techniques for measuring PD inoperating equipment

For pt.VI see ibid., vol.7, no.3, p.9 (1991) Online partial-discharge (PD) tests for monitoring the condition of the solid dielectric insulation in high-voltage equipment are discussed. Sources of external electrical noise are identified, as are attributes of PD and noise that are helpful in drawing distinctions between them. Methods for distinguishing between PD and noise are described. They are based on pattern recognition, time and frequency domain characteristics, or pulse time-of-flight analysis. Several online PD measuring systems for rotating machines, substation equipment, and high-voltage cables are presented     

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.     

Evaluation of the state of the insulation system of a stator winding of high-voltage electric machines based on measurements of statistical characteristics of partial discharges

The statistical characteristics of amplitude and amplitude-phase spectra of PDs in the insulation of model specimens with artificial flaws in a semiconducting coating are investigated. The algorithm for distinguishing the flaws in the stator winding insulation of high-voltage electrical machines based on statistical parameters of the PD spectra is developed and practically implemented.     

An online groundwall and phase-to-phase insulation quality assessment technique for AC-machine stator windings

An online technique for monitoring the condition of the groundwall (GW) and phase-to-phase (PP) insulation for three-phase ac-machine stator windings is proposed in this paper. Insulation-condition indicators such as capacitance, dissipation factor (tan/spl delta/), and ac insulation resistance are calculated online based on the differential leakage-current measurements for each phase. A model for a three-phase ac-machine insulation system is derived and analyzed for both offline and online testing. Guidelines for interpreting the measured indicators for determining the: 1) overall condition and 2) significant degradation in the individual GW and PP insulation for each phase of the stator are given based on the analysis. Experimental results on a 15-hp induction motor under intentional GW and PP insulation degradation conditions show that incipient insulation degradation can be detected and classified with high sensitivity.     

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.     

Voltage endurance testing

The medium-voltage ASD stator insulation systems operate under greater stress than those using the sine-wave power. The repetitive short rise-time pulses generated by most ASDs cause an additional stress. The pulses are often increased by the effect of the cable between the ASD and motor. It is incumbent on the user and ASD manufacturer to account for the magnitude, rate of rise, and repetition rate of these pulses. The medium-voltage machines powered by ASDs operate with some level of PD. The machine manufacturer must ensure that the insulation system will provide adequate life in the PD environment. VE testing is a well-understood and accepted method to qualify insulation systems. It is a reliable means to compare new insulation systems with the documented existing ones. The TS describes several qualification procedures based on the knowledge of the converter waveform. These will assure that all parts of the machine insulation system will present acceptable test life.     

变压器局部放电在线监测超高频Hilbert分形天线研究

论述了Hilbert分形天线基本原理,并根据分形天线电磁场理论,提出了应用于变压器局部放电超高频在线监测的Hilbert分形天线优化设计方法。通过仿真计算,研究了几何参数对Hilbert分形天线性能的影响,设计出用于变压器局部放电监测的3阶Hilbert分形天线。通过气隙放电、沿面放电、电晕放电3种典型变压器绝缘缺陷局部放电实验,采用分形天线和脉冲电流传感器对这3种局部放电进行了测量,并分析了分形天线的局部放电测量信号功率谱。实验结果证明该天线能够有效应用于变压器局部放电超高频在线监测。     

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。     

Recent important changes in IEEE motor and generator winding insulation diagnostic testing standards

IEEE standards and test procedures are widely used by motor and generator vendors and users to commission windings in new machines, as well as evaluate the condition of the winding insulation in operating machines. Until recent revisions, the basic procedures and standards in use were written over 25 years ago. Since the 1970s, motor windings have encountered many changes in their design and manufacture. The result was that the interpretation of results in many of the standards was no longer valid for the more modern motors. Over the past five years, the IEEE Power Engineering Society has conducted a major review and updating of most of these standards. Many important changes in test procedures and interpretation guidelines have resulted. This paper reviews the main insulation standards used for stator and rotor winding diagnostic testing, and discusses the changes that have been made. Standards discussed include: IEEE 43, 56, 95, 286, 522, and 1434. For example, IEEE 43-2000 now requires a minimum insulation resistance of 100 M/spl Omega/ for new stator windings rated 2300 V or more, rather than the "kV+1" that was required in the past. Furthermore, the interpretation for polarization index has changed such that a motor with a polarization index of 1 is no longer automatically classed as bad.     

便携式在线局部放电测量技术经济有效的绝缘诊断方法

9年来，新西兰工业研究公司(IRL NZ)开发并广泛地在电力工业中应用了快速、非侵入、便携式在线局部放电(局放)测量技术^[1-2]。该技术为电力系统中日益增多的超龄老化设备提供了一经济有效的绝缘诊断工具，使工业界实施预防性维护，保护其重要设备。根据其绝缘诊断，合理地决定是否对价值百万的设备进行修理或更换。该技术广泛地用于电机、电缆、特别是户内开关母线及变压器绝缘诊断与监测。介绍和讨论了它的使用原理,经济效益和许多现场运用的结果。该便携式在线局放测量技术的优越性和其非侵入局放传感器用于各种设备的位置示例于这些现场实例中。该技术已由华能时代科技有限公司引进，正在电力系统推广应用。     

Measures and technologies to enhance the insulation condition monitoring of large electrical machines

The utility of condition monitoring of the stator insulation structure of large machines has been enhanced substantially by the use of computer-based analysis of partial discharge signatures. A number of indices are examined through tests in a multistress aging facility to assess their promise in describing insulation degradation in machine insulation. In transitioning such tests to the field environment, an operational test is also described which offers some promise in identifying degradation without prior baseline data for a machine. On the basis that most electrical failures have mechanical precursors, acoustic methods are described for assessing stator bar mechanical integrity. This includes both using the conductors as waveguides and also employing acoustic emission for the location of large discharges.     

An Advanced Stator Winding Insulation Quality Assessment Technique for Inverter-Fed Machines

The lifetime of the stator winding insulation is reduced when operated with pulsewidth-modulation inverters, since insulation degradation is accelerated due to increased thermal and electrical stresses on the motor. This paper focuses on a new approach for monitoring the condition of the stator insulation for failure-prone inverter-fed machines. The main concept of the proposed technique is to apply a dc and/or variable-frequency ac test voltage to the stator insulation using the inverter and to perform standard offline insulation tests whenever the motor is not operating. This allows the stator insulation to be tested more frequently as compared to offline tests performed once every three to six years during periodic maintenance. Two methods for testing the insulation with minimal modifications to the inverter circuit are proposed and verified experimentally. The results show that the proposed technique provides a simple low-cost solution for detecting stator insulation degradation at an early stage for reliable operation of inverter-fed machine systems, since the insulation condition can be monitored frequently. This does not only helps prevent forced outages and safety risks due to insulation breakdown but also helps perform maintenance in an efficient manner, since individual motor maintenance can be prioritized and scheduled based on its present condition.     

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.     

电机绝缘系统在高频冲击下局部放电试验研究

高频冲击局部放电(PD)测试可以有效的检测电机绝缘系统的绝缘状态。根据传感器的不同,电机绝缘系统在高频冲击下PD测试方法主要有两种:高频电流传感器法和超高频天线法。试验表明:在高频冲击电压下,PD主要发生在冲击的上升沿和下降沿,在同一放电电压下,上升沿和下降沿的PD幅值较大;在风力发电机定子绝缘系统鉴别试验中,随着老化试验的进行,线圈的PD起始电压总体呈下降趋势。通过鉴别试验,可以确定绝缘系统的冲击电压绝缘等级及类型。