Condition monitoring of rotating electrical machinery has received intense research interest for more than 30 years. However, electrical machinery has been considered reliable and the application of fast-acting digital electrical protection has rather reduced the attention operators pay to the equipment. The area based upon current literature and the author's experience is reviewed. There are three restrictions: only on-line techniques for rotating machines are dealt with; specific problems of variable speed drives are not dealt with, except in passing; conventional rather than emerging brushless, reluctance and permanent magnet machines of unusual topology are concentrated upon. The art of condition monitoring is minimalist, to take minimum measurements from a machine necessary to extract a diagnosis, so that a condition can be rapidly inferred, giving a clear indication of incipient failure modes. The current state of the art is reviewed in the following ways: survey developments in condition monitoring of machines, mechanically and electrically, over the last 30 years; put that work in context alongside the known failure mechanisms; review those developments which have proved successful and identify areas of research which require attention in the future to advance the subject.

Review of condition monitoring of rotating electrical machines

/pdf/electrical-insulation-for-rotating-machines-5cd6wk7493.pdf

Electrical Insulation for Rotating Machines

Partial discharge (PD) measurement has long been used as a test to evaluate different insulation system designs, and as a quality control test for new equipment. However, in the past 20 years, PD measurement has been widely applied to diagnose the condition of the electrical insulation in operating apparatus such as switchgear, transformers, cables, as well as motor and generator stator windings. Improvements in the capabilities as well the lower cost of sensors, electronics and memory is partly the reason for the increased popularity of PD diagnostics. Another reason has been the development of methods-including the use of ultrawide band detection-to improve the reliability of the PD measurement in the presence of noise. In addition, rapid progress is being made in automated pattern recognition techniques that also helps to suppress noise. This paper reviews the various PD measurement technologies that have been specifically developed to improve PD diagnostic methods, and outlines how they have been implemented for stators, cables, transformers and switchgear. Areas for further research are also presented.

Partial discharge diagnostics and electrical equipment insulation condition assessment

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.

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

Condition monitoring of engineering plant has increased in importance as engineering processes are automated and manpower is reduced. However, electrical machinery receives attention only at infrequent intervals when plant is shut down and the application of protective relays to machines has also reduced operator surveillance. A first edition of Condition Monitoring of Electrical Machines, written by Tavner and Penman, was published in 1987. The economics of industry have now changed, as a result of the privatisation and deregulation of the energy industry, placing emphasis on the importance of reliable operation of plant, throughout the whole life cycle, regardless of first cost. The availability of advanced electronics and software in powerful instrumentation, computers, and digital signal processors (DSP) has simplified our ability to instrument and analyse machinery. As a result condition monitoring is now being applied to a wider range of systems, from fault-tolerant drives of a few hundred watts in the aerospace industry, to machinery of a few hundred megawatts in major capital plant. In this new book the original authors have been joined by Ran, an expert in power electronics and control, and Sedding, an expert in the monitoring of electrical insulation systems. Together the authors have revised and expanded the earlier book, merging their own experience with that of machine analysts to bring it up to date. The book is aimed at professional engineers in the energy, process engineering and manufacturing industries, plus research workers and students.

Condition Monitoring of Rotating Electrical Machines

The theoretical sensitivity of conventional partial discharge detectors is compared with that obtained from ultra wideband (UWB) (up to l GHz) detection systems. The comparison indicates that for relatively lossfree distributed systems, such as SF6 insulated bus, the UWB system is up to two orders of magnitude more sensitive. UWB detection also embodies additional advantages such as facilitating the location of discharge sites and the rejection of external electrical noise. For discharge detection in plastic-insulated cables, true UWB detection is not practical because of frequency-dependent attenuation effects, although certain gains in sensitivity can be achieved with a detector bandwidth of up to 10 MHz.

Fundamental Limitations in the Measurement of Corona and Partial Discharge

The design of concentric-neutral plastic-insulated power cables has been optimized for the transmission and distribution of large quantities of 60-Ηz power at high voltages. Understanding the mechanism of high frequency propagation in these cables is important in order to evaluate the sensitivity of partial discharge measurements [1,2]. Partial discharges in the solid insulation generate frequencies ranging to several hundred MHz [3]. With wide band detection, location of discharge sites in cable through pulse propagation timing techniques is also possible [4,5,6].

Propagation of partial discharge pulses in shielded power cable

The time to electric breakdown, and the electric field necessary to result in breakdown of solid insulation, seem to be best represented by a Weibull probability distribution. This tutorial paper reviews the graphical method of estimating the parameters of the Weibull distribution. It also presents the method of Maximum Likelihood as an objective technique, developed by statisticians for improving the estimates of the Weibull parameters, especially for censored experiments. Procedures for calculating confidence intervals are also given. It is shown in an example that the confidence intervals can be very wide for sample sizes commonly used in dielectric life tests.

Parameter Estimation for the Weibull Distribution

The results of accelerated aging tests on solid electrical insulation are difficult to evaluate objectively, primarily due to the inherently large variability of the test data. This variability is often represented by the Weibull or other extreme-value probability distributions. This paper demonstrates an hypothesis test procedure which permits the objective and unambiguous evaluation of comparative dielectric tests on two different sets of data. The computation techniques are facilitated through the use of a Fortran computer program. A significant difference must be established at low probabilities of failure. Analysis of typical aging tests from the literature indicate that many experiments performed to date may not be statistically significant at utilization levels. The number of tests required to achieve unambiguous significance at low probability levels may render meaningful accelerated aging tests uneconomic.

The Application of Weibull Statistics to Insulation Aging Tests

This paper is concerned with the accelerated aging of solid insulating materials, as a means of estimating their service lives and identifying superior dielectrics. The assumed failure mechanism is electric treeing. A constant-stress accelerated aging experiment on a transparent epoxy at three voltage levels is described. All factors thought to be important in the electric treeing mechanism were rigidly n-controlled, and the inception and growth intervals were measured separately. The results indicate that the times to inception and failure are three-parameter Weibull distributed, whereas the growth period may be either Lognormal or three-parameter Weibull distributed. The Inverse Power Law could not be n-confirmed. There was little correlation between the inception and growth intervals. Statistical analysis showed that there is much uncertainty in these results, and this is true for most published accelerated aging tests done to date. The general applicability of accelerated aging tests is questioned.

G. C. Stone

Papers

Fundamental Limitations in the Measurement of Corona and Partial Discharge

Propagation of partial discharge pulses in shielded power cable

Parameter Estimation for the Weibull Distribution

The Application of Weibull Statistics to Insulation Aging Tests

The Statistical Analysis of a High Voltage Endurance Test on an Epoxy