On-line Partial Discharge Testing Research Articles

Advances in Interpreting On-Line Partial Discharge Test Results on Stator Windings

On-line partial discharge (PD) testing has been used for decades to help maintenance personnel detect rotating machine stator winding insulation problems at an early stage and thus optimally plan any required maintenance. Specifically, the test can find loose, overheated, and contaminated windings, well before these problems lead to failure. The test has also found use in determining the effectiveness of any repair work.The widespread application of on-line PD testing has enabled the accumulation over the past 20 years of a database with, currently, approximately 700,000 test results. This proposed contribution presents the basics of on-line data collection, including the implementation of noise rejection algorithms and possible methods in improvements in interpreting test results. One case study describing application of on-line partial discharge testing on a hydro generator will be presented.

Online Partial Discharge Condition Monitoring of Complete Networks: Challenges and Solutions Explained through Case Studies for the Pulp and Paper Industry

Unplanned outages within the paper industry lead to significant costs to operators. To reduce unplanned outages and losses, condition monitoring (CM) techniques can be implemented to determine asset health and help target maintenance to where it is required most. The challenges faced by the paper industry to implement condition-based maintenance (CBM) on their medium-voltage (MV) and high-voltage (HV) networks and solutions for online partial discharge (OLPD) monitoring are due to the range and number of assets present and the geographical spread coupled with problems of retrofitting the technology on pre-existing plants. The implementation of OLPD testing or monitoring can vary based on several factors: network topology, connected assets, switchboard type, and the owner's budget and desired local engineer involvement. This article discusses different approaches to implementing an OLPD test program, ranging from simple handheld survey units to complete network monitoring. Its focus is on case studies of the deployment of OLPD testing and monitoring solutions at several U.S. paper sites. The case studies present diagnostic testing of unshielded MV cable at one paper mill up to full permanent OLPD monitoring systems at two others.

Noise Rejection and Detection of Partial Discharges Under Repetitive Impulse Supply Voltage

Partial discharge (PD) detection is a fundamental tool for the design and diagnosis of insulation systems, particularly organic materials that must operate throughout their life without PD. While the methods for PD measurements under sinusoidal supply voltage are well-established, the same does not hold under repetitive-pulse voltage waveforms, such as those provided by inverters. Electromagnetic noise generated by electronic switch commutation in inverters can overlap with PD pulses, making it difficult to measure, separate from noise, and identify PD. This is particularly more cumbersome when fast rise-time switches are employed. To address these issues, this paper investigates methods to detect PD pulses on enameled wires under repetitive impulsive voltages with different rise times of 60, 500, and 1000 ns, proposing techniques which allow separation of PD from noise even when they overlap in time and frequency domains. This paper employs a time-domain approach using standard deviation curve acquired from a set of measurements to discriminate PD from repetitive commutation disturbance. These techniques can be implemented to achieve unsupervised noise rejection, which would be an important goal for both offline and online PD testing. The proposed approach is validated successfully through tests carried out on a low-voltage motor.

Remote detecting and locating partial discharge in bushings by using wideband RF antenna array

It is bushing that performs an essential function in power transformer. Since the existing on-line partial discharge (PD) test methods for bushings require the current from the test tap, it is difficult to be applied to in-service bushings, especially to some old bushings without voltage taps. In order to detect PDs without changing grounding circuit, a novel non-contacting on-line approach to detecting and locating PDs in bushings using time delay of arrival (TDOA) of radio frequency (RF) antenna array signals is proposed in this paper. Based on a real 126 kV oil-impregnated paper (OIP) condenser bushing, four typical PD defect models were made, which were suspended discharge on bushing's top cap, poor connection in bushing tap, surface discharge on oil-side porcelain insulator and suspended discharge near the oil-side grading ring. The detection and location of PDs induced by these defects were carried out and different features were obtained. For the defects of suspended discharge on bushing's top cap and poor connection in bushing tap, this approach gives good location accuracy and the error is less than 20 cm. For the defects inside the bushing housing, the RF signals generated from PDs still can be detected and located, but the calculated location results are all focused around the bottom of the air-side porcelain insulator. In order to reveal the test results, the propagation of electromagnetic (EM) waves in the bushing housing was simulated, which indicated that the coaxial gap between the condenser core and the fixing flange allows various modes of EM waves to propagate with a attenuation from 40 to 60 dB. In the on-site tour-inspection tests, a real PD fault in an 110kV cable terminal bushing was located in a substation, which verified the availability and effectiveness of this approach.

Continuous Remote Online Partial Discharge Monitoring of HV Ex/ATEX Motors in the Oil and Gas Industry

This paper presents results from the continued application of a new technique for the remote online partial discharge (OLPD) testing and monitoring of in-service high-voltage (HV) explosive atmosphere (Ex)/atmosphere-explosive (ATEX) motors operating in hazardous gas zones. The technique employs high-current wideband high-frequency current transformer sensors located remotely from the motor under test, at the switchboard end of the HV feeder cable. Significant cost and operational benefits can be gained from this remote monitoring technique as the stator winding condition of these motors can be monitored without having to enter the hazardous gas (Ex/ATEX) zone. The work described in this paper is a continuation of that carried out by the same authors, as reported at the 2012 IEEE Petroleum and Chemical Industry Technical Conference, and includes the development of proposed condition guidelines for assessing the condition of the stator winding insulation systems of large populations of aged HV motors, based on an OLPD “league-table” database. Two case studies are presented wherein the HV stator insulation condition and reliability of in-service motors, operating in hazardous gas zones, and HV generators were assessed using both online and offline partial discharge (PD) testing and monitoring techniques. The case studies emphasize the importance of carrying out extended continuous OLPD monitoring of in-service rotating HV machines to detect underlying trends in PD activity over time. This paper concludes with how condition monitoring data can support reliability-centered maintenance and condition-based management regimes.

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.

세라믹 커플러를 이용한 고전압회전기 고정자권선의 On-line 및 off-line 부분방전 특성 시험 및 분석

Partial discharge(PD) test can be performed either when the rotating machine is not operating(off-line) or during normal machine operation(on-line). This paper presents an on-line and off-line PD test on a large hydro-generator and induction motor using the same PD acquisition system(PDAS) and ceramic coupler(CC) sensor. PD signal characteristics of CC sensor proved similar with that of epoxy mica coupler(EMC) sensor as a results of PD test for simulated defect winding and frequency response test. A comparison of on-line and off-line PD test for PD characteristic parameters-phase resolved PD(PRPD), maximum PD value(Qmax) and PD occurrence energy(POE)-indicated that on-line PD test could reliably and effectively diagnose insulation conditions which were verified by off-line PD test.

Objective methods to interpret partial-discharge data on rotating-machine stator windings

Partial-discharge (PD) measurements have long been used to assess the condition of the electrical insulation in motors and generators rated 3.3 kV and above. There are many ways to measure PDs during normal service of the motor or generator. Unfortunately most of the measurement methods mix stator PD with electrical-interference signals from poor electrical connections, power tool operation, corona from transmission lines, etc. The result can be false indications of stator-winding problems, reducing confidence in PD measurements. Another issue with online PD testing is interpretation, i.e., identifying which machines are in good condition and which machines need maintenance. In the past decade, a database of over 60 000 test results has been assembled. In hundreds of machines, the condition of the insulation determined by a visual inspection has been compared to the PD levels. The result is a table that provides an objective means of determining the stator-insulation condition relative to other similar machines. An analysis of the results also shows that there are significant differences in PD activity between manufacturers. This paper gives a review of the methods that can reduce the risk of false indications, thus making the measurement less subjective.

Comparison of Off-line and On-line Measurement of Partial Discharges for Hydrogenerator Stator Windings using Acoustic Emission Sensors

The establishment of Condition Based Maintenance for hydrogenerators is very important for monitoring the conditions of stator windings insulation. On-line partial discharge (PD) testing has been used to provide useful information for monitoring the integrity and diagnosing problems in hydrogenerator stator winding insulation. The on-line insulation diagnosis technology can assess thermal, electric, ambient, and mechanical stresses received by the hydrogenerator stator windings. We have already indicated in a previous study that on-line PD measurements using acoustic emission sensor (AE) techniques can be useful for assessing the conditions of stator winding insulation. In this paper, the relationship between AE signals and maximum discharge magnitude was investigated in a non-destructive test for hydrogenerators using off-line measurements. AE signals were investigated by the use of multiple AE sensors, which are produced from an internal discharge for two hydrogenerator stator windings in 11kV and 13.2kV classes, in addition to the load and temperature, and humidity measurements taken on-line in twenty-month and three-month long studies respectively.

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.

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.

On-line Partial Discharge Measurement of Hydrogenerator Stator Windings using Acoustic Emission Detection Techniques

The on-line partial discharge testing has been used to provide useful information to diagnose and monitor the integrity of stator winding insulation of hydrogenerators. We are aiming for the establishment of a method of on-line diagnosis of insulation deterioration on hydrogenerator stator winding by acoustic emission (AE) detection method. We have developed a new AE measuring system that is controlled by output signal of hydrogenerator. In this paper, it was clarified that the insulation deterioration of hydrogenerator stator winding depended remarkably on the temperature of stator winding during a starting operation period.

Using improved self-organizing map for partial discharge diagnosis of large turbogenerators

Partial discharge (PD) classification is a powerful way to predict insulation problems of the windings of rotating machinery. While online PD tests have been carried out for over 40 years, effective diagnostic methods are still under development. In this paper, a practical diagnostic method is proposed based on an improved self-organizing map (SOM) with improved clustering indications. Three feature extraction methods are employed for the SOM implementation, including Weibull analysis, statistical operators, and fractal parameters. Experimental PD data of industrial model bars are used to validate the efficiency of using SOM for PD classification. The method is applied to investigate the turbine generator analyzer (TGA) data obtained from a power plant of British Nuclear Fuels Ltd. Diagnostic results are included to demonstrate that the relationship between the new PD measurement and historical data can be visualized and more confidential diagnostic information can be provided, especially when small-size database, new class of data, and doubtful measurements are involved in a practical environment.

Importance of bandwidth in PD measurement in operating motors and generators

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.

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.

New noise rejection techniques on pulse-by-pulse basis for on-line partial discharge measurement of turbine generators

On-line partial discharge tests for turbine generators are useful for recognizing abnormal or deteriorated stator winding insulation without a machine outage. One problem related to such a test is additional installation of sensors. We propose that wires of a resistance temperature detector (RTD) embedded in a stator slot are applied as a partial discharge sensor in the form of an RF coupler. The other problem is electrical noise that is a pulse similar to partial discharge in frequency characteristic and has higher amplitude than partial discharge. The sensors in two steam turbine generators showed enough sensitivity and broad frequency bandwidth to facilitate the elimination of noise from the partial discharge measurement. Two new techniques of noise rejection on a pulse-by-pulse basis are investigated with the multiple sensors. One is founded on the correlation between pulse height in two frequency bands. The other is based on the correlation between pulse height from the two partial discharge sensors. It was found that each noise rejection technique was successful in a trial testing.

Practical on-line partial discharge tests for turbine generators and motors

Several utilities have found partial discharge testing to be very useful for identifying motors or generators with deteriorated stator winding insulation. Such partial discharge tests are sensitive to problems associated with overheated insulation, loose stator bars, and pollution, all of which can cause in-service failures with long associated outages and high repair/rewind costs. Unfortunately, the partial discharge test is not widely applied since present test methods require considerable skill to perform, and/or require machine outages. These problems are due to the stator winding partial discharges being masked by high levels of internal and external electrical noise. The authors describe how partial discharge tests have now been developed which eliminate the effect of noise, thus allowing plant personnel to perform the test without any machine outage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Partial discharge. VII. Practical techniques for measuring PD in operating 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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>