| 1. |
- Edvall, Martin, et al.
(author)
-
Experimental study of current damages in bearings
- 2007
-
In: Proceedings of the Third International Conference on Advances in Processing, Testing and Application of Dielectric Materials. - 0324-9441. ; , s. 141-146
-
Conference paper (other academic/artistic)
|
|
| 2. |
- Jarvid, Markus, 1985, et al.
(author)
-
Evaluation of the Performance of Several Object Types for Electrical Treeing Experiments
- 2013
-
In: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 20:5, s. 1712-1719
-
Journal article (peer-reviewed)abstract
- A comparison of the efficiency of various types of object for studying the development of electrical trees in polymers is presented. Two types of newly developed wire-plane objects are investigated and compared with two versions of the traditionally used needle-needle objects. The two needle-needle objects are prepared according to ASTM standard and act as references, whereas the alternative objects rely on use of an ultra-thin tungsten wire (10 and 20 µm) in the test objects for providing the highly divergent electric stress. In one type of those object the wire extends from a semiconducting tab embedded in the tested material, while in the other type, the embedded wire is extended and connected externally by means of a copper tape. The comparison is made using cross linked polyethylene (XLPE) for the testing purposes. The wire type objects provide some promising advantages, including parallel formation of several electrical trees and an exposure of a larger material volume. Further advantages include the considerable simplification of both the manufacturing and the measuring procedures. Among the newly developed test objects, the one without the semiconducting tab is considered most advantageous as it allows for more accurate treeing initiation measurements at a lower voltage level. On the other hand the disadvantage is a formation of kinks on the wire that results from polyethylene shrink during sample preparation, which may yield some difficulties in correctly estimating the local field strength locally as well as straining the surrounding material. However the multitude of electric trees formed in each sample allows to easily discard the trees growing at such imperfections in following data analyses. Although finding a statistical method that makes good use of all the data poses a challenge, a suggested approach is presented.
|
|
| 3. |
- Johansson, Anette, 1977, et al.
(author)
-
A Versatile System for Electrical Treeing Tests under AC and DC Stress Using Wire Electrodes
- 2011
-
In: 8th International Conference on Insulated Power Cables, Jicable’11, 19 – 23 June 2011, Versailles - France.
-
Conference paper (other academic/artistic)abstract
- An alternative method to evaluate electrical treeing in solid dielectric materials is presented. Here the test sample utilises a wire electrode for creating the necessary high divergent electric field. This introduces a different preparation method and several of the disadvantages with the traditional needle sample can be avoided. Together with the test object a versatile test setup for both AC and DC measurements is illustrated. This alternative method has proven successful and an evaluation thereof as well as a comparative study between the two electrode systems is included.
|
|
| 4. |
|
|
| 5. |
- Bergman, Becky, 1970, et al.
(author)
-
Reflections on an Integrated Content and Language Project-Based Design of a Technical Communication Course for Electrical Engineering Students.
- 2013
-
In: Journal of Academic Writing. - 2225-8973. ; 3:1, s. 1-14
-
Journal article (peer-reviewed)abstract
- Effective ways of teaching technical communication skills to engineering students have been much discussed. This article reflects on one setting, a first year course in Technical Communication at a university in Sweden, where electrical engineering teachers, language and communication teachers and student counsellors work in close, team-based cooperation using a project model which requires the students to analyse, implement and communicate technical problems. The paper discusses the change in this course - from an EAP course primarily prioritizing language training which ran parallel with a project course - to one unified ICL course. The progression is described through the changes in the organization of the course, and the article focuses on one learning activity: interdisciplinary tutorials on project reports. Through a pilot study where these sessions were video recorded and mapped, we conclude that the presence of different roles became an asset for the range of what the students see as relevant for their project report. In particular, the technical report genre was critically analysed, including problematic areas such as textual sequencing and display of technical problems; data visualisation and commentary; and referencing.
|
|
| 6. |
- Bernelo, Marie, 1958, et al.
(author)
-
May an Increased Focus on Students’ Personal Development Contribute to Increased Motivation, Better Academic Performance and Teamwork in Engineering Programs?
- 2011
-
In: Proceedings of 7th International CDIO Conference, Copenhagen, Denmark.
-
Conference paper (peer-reviewed)abstract
- Our hypothesis is that an increased focus on engineering students’ personal development in the curricula will increase their motivation, academic performance and teamwork. With this starting point we have developed the EDIT model for personal development aimed at the engineering students in the 5-year EE, CE and SE programs at Chalmers University of Technology. The EDIT model comprises the topics and the process and timing of the delivery in the curricula at the bachelor level. It is based on behavior-scientific theories and on 40 years of experience in guiding engineering students at Chalmers. The fundamental concept for the model is that introspective knowledge gives extrovert ability. It comprises four topics: Motivation and learning, Teamwork, Leadership, Career and professional life, and a complementary reflection package. Based on motivational theory and pedagogical literature, we discuss why we have selected these topics and how they should be implemented in the curricula and syllabi to facilitate the development of the students. We argue that these topics should be placed in a context and at a time that makes them meaningful to the students. We give practical examples from the project test implementation and discuss practical issues that are likely to hinder the long-term success. In conclusion, we find that there is some evidence from our experiments that motivation and teamwork is improved. The possible effect on academic performance is so far very hard to assess.
|
|
| 7. |
- Blennow, Jörgen, 1966, et al.
(author)
-
Active High Voltage Insulation
- 2002
-
In: Journal of Electrostatics. ; 55:2, s. 159-172
-
Journal article (peer-reviewed)
|
|
| 8. |
- Blennow, Jörgen, 1966
(author)
-
Active High Voltage Insulation - A New Hybrid Insulation Concept with Dynamic and Active Features
- 2000
-
Doctoral thesis (other academic/artistic)abstract
- In this thesis, the concept of a new environmental-friendly and high-performance high voltage insulation is presented. Basically, it is an electric field steering technique where the dynamic process of charge accumulation, actively and advantageously, re-distributes the electric field within an insulation system. The concept is theoretically and experimentally demonstrated on a one-dimensional plane-parallel insulating structure consisting of an air-gap bounded by two dielectrically covered electrodes. The vital process of charge deposition and relaxation was experimentally studied in a highly lightning impulse (LI) voltage stressed air-gap. The temporal and spatial development of dielectric barrier discharges, studied through current measurements correlated with sequential high speed imaging, revealed fundamental properties of the highly stressed system. In general, an intense discharge was found at the front of the LI followed by a pulse train of significantly smaller discharges, but of opposite polarity, at the tail of the LI. During both a slowly rising dc-voltage and during a sequence of LI- voltages; voltage levels remarkably above the level which, according to the capacitive field distribution gives critical field strength in the air-gap, were reached. The improvements ranged from about 50% for a 27 mm dc-stressed gap, up to 350% for a 3 mm LI-stressed gap. Actual maximum withstand levels were not reached. It can be concluded that a significant feature is that the system adapts its insulation level with respect to what the applied voltage level requires by an accumulation of the appropriate quantity of charge. If the demand of charge exceeds the supply, additional free charge is automatically created through ionization. Although, ionization is not a pre-requisite behind the concept. Under ideal conditions, such a system will in equilibrium exhibit a zero air-gap field, e.g., the electric breakdown strength is only determined by the coatings. Thereby, the breakdown strength could be considerably improved in comparison with a conventional uncovered air-gap.
|
|
| 9. |
|
|
| 10. |
- Blennow, Jörgen, 1966, et al.
(author)
-
Comparative study of dielectric spectroscopy measurements and anayses of oil and paper samples from a transformer undergoing repair
- 2005
-
In: In Proc. of 19th Nordic Insulation Symposium, Trondheim, Norway, June 13-15.
-
Conference paper (other academic/artistic)abstract
- New methods, based on characterisation of dielectric response measurements, for assessing moisture content of pressboard and paper are gaining ground. However, there is a need for calibrating results with results from other techniques. This communication presents results from a study of a 19.5/6.8/6.8 kV - 40 MVA power transformer, which was opened on-site during maintenance work, thus allowing for collecting paper samples. Dielectric spectroscopy measurements were conducted before and after its opening. Results were compared with measurements on oil and paper samples. These showed that the moisture levels in the interior of the solid insulation were unchanged. Further, it was found that FDS measurements gave more accurate estimates of moisture levels than estimates based on oil analyses and equilibrium curves.
|
|
| 11. |
|
|
| 12. |
|
|
| 13. |
|
|
| 14. |
|
|
| 15. |
|
|
| 16. |
- Blennow, Jörgen, 1966, et al.
(author)
-
Field experiences with measurements of dielectric response in frequency domain for power transformer diagnostics
- 2006
-
In: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 21:2, s. 681-688
-
Journal article (peer-reviewed)abstract
- Some new methods, based on characterization of the dielectric response of transformer insulation in time and frequency domains, have been pointed out by CIGRE as suitable to assess moisture content in pressboard and paper. Measuring devices available today on the market are quite robust in field conditions; however there is a risk that undesired internal and external factors might influence the measurements. These influences can lead to mistaken interpretation of insulation wetness. Operators should therefore be aware of the risks appearing at different measuring conditions and of the precautions that can be taken to minimize their effects. This work presents a systematic investigation on the influence of some factors on the results of dielectric response measurements in field conditions. The errors that can be committed during the measurements if certain precautions are not taken, with different equipment connected in parallel, are evaluated. The influence of rain and electromagnetic disturbances are also analyzed proposing solutions to attenuate their effects. Finally, a discussion on the influence of temperature distribution follows. The work is concentrated on frequency response measurements, although some results from the use of polarization and depolarization current technique are also discussed. © 2006 IEEE.
|
|
| 17. |
|
|
| 18. |
|
|
| 19. |
- Blennow, Jörgen, 1966, et al.
(author)
-
Sequential Comparative Study of Dielectric Response and Analyses of Oil and Paper from a Power Transformer Undergoing Repair
- 2005
-
In: In Proc. 14th International Symposium on High Voltage Engineering.
-
Conference paper (peer-reviewed)abstract
- Methods, based on characterisation of dielectric response, for assessing moisture content in pressboard and paper are gaining ground. However, there is a need for calibrating obtained results with results from other techniques. This communication presents results from a study of a 50/10 kV - 40 MVA power transformer, which was sent to a workshop for repair, thus allowing for collecting samples of the solid insulation. Dielectric spectroscopy measurements in frequency domain were conducted at several occasions before and after the repair. Moisture estimation based on dielectric response measurements as well as on titration of oil samples yielded similar results to those obtained by direct titration of paper samples, as long as appropriate temperature values were used in all the analyses mentioned. The temperature value during measurements should be used for the analysis of the dielectric response, while in-service temperature of oil should be used for the evaluation based on Karl Fisher titration.
|
|
| 20. |
- Chen, Xiangrong, 1982, et al.
(author)
-
Electrical Treeing Behavior of DC and Thermally Aged Polyethylenes Utilizing Wire-Plane Electrode Geometries
- 2014
-
In: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 21:1, s. 42-52
-
Journal article (peer-reviewed)abstract
- This paper presents electrical treeing behavior in low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) after exposure to thermal and DC electro-thermal ageing. Both the ageing and the treeing tests were performed by means of two different types of test objects with wire-plane electrode geometry. One type of the tested objects contained only wire electrode of 10 µm diameter, whereas in the other type the wire electrode was attached through a semiconducting tab. The ageing was performed at 80°C with and without 10 kV DC voltage of both polarities connected to the wire and lasted up to 800 hours. The AC electrical treeing tests were applied afterwards for detecting changes of material properties after the ageing. The results showed that the electrical tree inception voltage consistently decreased with increasing time of thermal exposure, whereas the applied DC electric stress had a negligible effect on the observed behavior. Similar effects were found in both the tested materials (LDPE and XLPE) though the object type also influenced the results. For the objects with semiconducting tab, a higher level of the scale parameter was registered because of shielding effect of the tab on the electric field strength at the wire electrode. It also yielded less number of trees growing in parallel at the electrode. The dominant effect of thermal stress on the ageing of LDPE was elucidated by using various analytical techniques, like differential scanning calorimetry, infrared spectroscopy, inductively coupled plasma optical emission spectrometry and oxidation induction time, and it is believed to mainly affect antioxidant content in the test objects.
|
|
| 21. |
|
|
| 22. |
|
|
| 23. |
- Gadd, Gustaf, et al.
(author)
-
A Nonlinear Relation between Body Mass Index and Long-Term Poststroke Functional Outcome-The Importance of Insulin Resistance, Inflammation, and Insulin-like Growth Factor-Binding Protein-1.
- 2024
-
In: International journal of molecular sciences. - 1661-6596 .- 1422-0067. ; 25:9
-
Journal article (peer-reviewed)abstract
- Both high serum insulin-like growth factor-binding protein-1 (s-IGFBP-1) and insulin resistance (IR) are associated with poor functional outcome poststroke, whereas overweight body mass index (BMI; 25-30) is related to fewer deaths and favorable functional outcome in a phenomenon labeled "the obesity paradox". Furthermore, IGFBP-1 is inversely related to BMI, in contrast to the linear relation between IR and BMI. Here, we investigated s-IGFBP-1 and IR concerning BMI and 7-year poststroke functional outcome. We included 451 stroke patients from the Sahlgrenska Study on Ischemic Stroke (SAHLSIS) with baseline measurements of s-IGFBP1, homeostasis model assessment of IR (HOMA-IR), BMI (categories: normal-weight (8.5-25), overweight (25-30), and obesity (>30)), and high-sensitivity C-reactive protein (hs-CRP) as a measure of general inflammation. Associations with poor functional outcome (modified Rankin scale [mRS] score: 3-6) after 7 years were evaluated using multivariable binary logistic regression, with overweight as reference due to the nonlinear relationship. Both normal-weight (odds-ratio [OR] 2.32, 95% confidence interval [CI] 1.30-4.14) and obese (OR 2.25, 95% CI 1.08-4.71) patients had an increased risk of poor functional outcome, driven by deaths only in the normal-weight. In normal-weight, s-IGFBP-1 modestly attenuated (8.3%) this association. In the obese, the association was instead attenuated by HOMA-IR (22.4%) and hs-CRP (10.4%). Thus, a nonlinear relation between BMI and poor 7-year functional outcome was differently attenuated in the normal-weight and the obese.
|
|
| 24. |
|
|
| 25. |
|
|
| 26. |
|
|
| 27. |
- Hammarqvist, Maria, 1975, et al.
(author)
-
Improvement of frequency response for a zero-flux current measuring system
- 2014
-
In: CPEM Digest (Conference on Precision Electromagnetic Measurements). - 0589-1485. - 9781479952052 ; , s. 618-619
-
Conference paper (peer-reviewed)abstract
- Magnetic zero-flux current transformers are widely utilized to measure AC and DC electrical currents. To improve the AC characteristics between 10 Hz and 100 kHz, a method is proposed to evaluate adjustments in the circuitry; increase of the amplification in the AC feed-back circuit and introduction of an external coaxial read-out shunt resistor. The method is generally applicable and includes a coaxial current path within the entire reference system and traceability to national standards. The AC working range is extended from about 1 kHz to about 3 kHz. Outside the nominal AC working range, the scale factor variation is reduced from 45 % to 3.5% and the phase angle error offset changes from ca 5.8 μs to below 0.3 μs.
|
|
| 28. |
- Hammarqvist, Maria, 1975, et al.
(author)
-
Investigation of the Frequency Dependency of Phase Angle Error in a Zero-flux Current Measuring System
- 2013
-
In: ISH 2013; 18th International Symposium on High Voltage Engineering; August 25-30, 2013; Seoul, Korea.
-
Conference paper (peer-reviewed)abstract
- Abstract: An important technique for traceable measurement of electrical current utilizes magnetic zero-flux current transformers. The technique is well established, but much remains to be understood regarding its practical upper frequency limit. Typically, zero-flux systems consist of a measuring head mounted around the primary current carrier and an electronic unit creating a balancing current in the compensation winding. In this paper the phenomena responsible for the frequency dependency of the phase angle error in zero-flux systems in the span 5 Hz – 10 kHz are described using two-channel power measuring systems as reference systems. The aim was to find out which part of the zero-flux systems that contributes most to the phase angle error. It was found that the error increases linearly with frequency between 22 Hz and 2500 Hz. At higher frequencies, the error passes a maximum in transition between the normal operation and the work regime as an ordinary transformer. It is shown that the electronic unit introduces the most significant contribution. A comparison is made between two reference systems using one zero-flux system as a transfer standard. The coincidence between the two reference systems is good.
|
|
| 29. |
- Hammarström, Thomas, 1974, et al.
(author)
-
CHANGES IN PD PROPERTIES WITH DECREASING VOLTAGE RISE
- 2011
-
In: The Proceedings of 17th International Symposium on High Voltage Engineering. - 9783800733644 ; , s. D-013
-
Conference paper (peer-reviewed)abstract
- The increased use of power electronic components in power systems requiresunderstanding of how rapidly rising voltages affect insulation systems. One importantaspect of this challenge is to measure partial discharges, PDs, which are considered asbeing a sign of insulation weakness and can affect its life considerably. This paperpresents a continuation of our earlier investigations on the difference in PD behaviourwhen voltages characterized by different rise times are applied. The results indicate thatthe effect on the insulation system is indeed dependent on the voltage wave shape.Applying square-like voltages to a cavity with dielectric insulated electrodes significantlyaffects the discharge amplitude, its rise time, the inception voltage and the distributionshape. It is further examined how the dimensions of the cavity affect the PDcharacteristics and the investigation shows that PD amplitude increases while its durationdecreases for shorter voltage rise times, being indications of a possible change in thedischarge mechanism. This in turn can lead to faster deterioration and reduction ofinsulation service life. To illustrate the degradation process, microscopic images showhow shorter rise times affect differently the cavity surface deterioration, which isconsistent with the other observations.
|
|
| 30. |
- Hammarström, Thomas, 1974, et al.
(author)
-
Evidence for Changing PD Properties at Short Voltage Rise Times
- 2011
-
In: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 18:5, s. 1686-1692
-
Journal article (peer-reviewed)abstract
- The increased use of power electronic components in power systems makes it important to understand how rapidly rising voltages affect insulation systems. One vital aspect of this challenge is to measure partial discharges, PDs, which are considered as being a sign of weakness and can affect the life of insulation considerably. In this paper we report on continuation of our earlier investigations on the different behavior of PDs when voltages characterized by different rise times are applied. Significant differences in PD characteristics are found, which indicate that the effect on the insulation system is dependent on the voltage wave shape. Applying square-like voltages to a cavity with dielectric insulated electrodes significantly affects the discharge amplitude, its rise time, the inception voltage and the distribution shape. The investigation shows that PD amplitude increases while PD rise time decreases for shorter voltage rise times, being indications of a possible change in the discharge mechanism. This in turn can lead to faster deterioration and reduction of service life and may therefore need to be considered when designing insulation systems exposed to fast transients.
|
|
| 31. |
- Hammarström, Thomas, 1974, et al.
(author)
-
PARTIAL DISCHARGES IN A STATOR UNDER SYNTHEZISED VOLTAGE WAVEFORMS
- 2013
-
In: Proceedings of the 18th International Symposium on High Voltage Engineering 2013.
-
Conference paper (peer-reviewed)abstract
- This paper introduces a novel way of processing results of measurements of partial discharges (PDs) under the action of pulse width modulated waveforms (PWM) that opens for a flexible analysis of data since voltage shapes appearing in-service can be utilized. Several waveforms gradually smoothened towards a sinusoidal voltage shape are exploited to characterize PD properties in the stator of a three phase motor.
|
|
| 32. |
- Hammarström, Thomas, 1974, et al.
(author)
-
Partial Discharges in Motor Wires at PWM Voltages of Different Smoothness
- 2014
-
In: Proceedings of 2014 International Symposium on Electrical Insulating Materials (ISEIM 2014), June 1-5, 2014, Niigata City, Japan. - 9784886860866 ; , s. 184-187
-
Conference paper (peer-reviewed)abstract
- Author’s recent work has been focused on exploring possibilities for measuring partial discharges (PDs) under the action of pulse width modulated waveforms (PWM), which resulted in a development of an electrical measurement method adopted for such voltages. This solution allows a more flexible PD analysis since voltage shapes appearing in service can be utilized. In this paper, results are presented where several waveforms are applied to quantify the PD properties for motor wire test objects, starting from a non-filtered PWM waveform and continuing with gradually smoothened ones towards an AC voltage shape. The results suggest that non-smoothed PWM voltage introduces considerably more PDs and with larger magnitudes, which most probably influences the lifetime of the insulation system in all tested cases. Additionally, above a certain level of filtering, the use of lower carrier frequencies implies higher PD exposure, which suggests that the filters used should be evaluated together with the carrier frequency to ensure a longer lifetime of the insulation. It was also observed that the changes in duty cycle reduce the PD density at higher frequencies. This suggests that only applying square voltage waveforms with 50 % duty cycle for PD testing may fail to capture the actual stress inflicted.
|
|
| 33. |
- Hammarström, Thomas, 1974, et al.
(author)
-
PD Properties when Varying the Smoothness of Synthesized Waveforms
- 2013
-
In: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 20:6, s. 2035-2041
-
Journal article (peer-reviewed)abstract
- The increased use of power electronic components in power systems makes it important to understand how rapidly risingvoltages affect insulation systems. One vital aspect of this challenge is the measurement of partial discharges, PDs, which are considered as being a sign of weakness and can affect the life of insulation considerably. In this paper an approach is presented to measure PDs in a dielectrically insulated cavity when exposed to pulse width modulated (PWM) voltage shapes with different degree of smoothness. This is a continuation of our earlier investigations on the different behavior of PDs where voltages characterized by different rise times were applied. The present investigation shows that the PD amplitude decreases significantly already at a moderate level of PWM voltage smoothness to a magnitude that is about the same as for sinusoidal voltage shape. For the phase resolved PD (PRPD) pattern to become similar to the normal AC pattern it is required that the remains from PWM steps are lower than the extinction voltage. This work elucidates how PDs are affected by synthesized waveforms and limits for a sufficient smoothing level are found, which is of importance when designing insulation systems exposed to fast transients.
|
|
| 34. |
- Hammarström, Thomas, 1974, et al.
(author)
-
Resonant PD signal decoupling circuit for rapidly changing voltages
- 2011
-
In: 22nd Nordic Insulation Symposium (NORDIS 11), June 13-15, Tampere, Finland. - 9789521525629 ; , s. 149-152
-
Conference paper (other academic/artistic)abstract
- A method for electrical detection of partial discharges(PDs) when the voltage is rapidly changing has recentlybeen presented. The method is based on moderatelysharp frequency filters in the PD decoupler, highresolutiondigitizers and time-domain stochasticfiltering. A limitation with high-resolution digitizers istheir lower sample rate. To enable use of fasterdigitizers with less resolution, the filters in the PDdecoupler must be optimized for voltage slew rate andPD magnitude.In this paper, the design of a versatile PD coupler filteris discussed. Although entirely passive, the filtersuppression can be made to change two orders ofmagnitude in half a decade of frequency. Thus, a lowresolutionfast digitizer can be employed with which nsvariations in the PD duration can be studied. The filtercan be optimized by changing a few parameters only.Examples are presented which illustrates the advantagesof this modified PD decoupler on actual measured PDs
|
|
| 35. |
- Johansson, Anette, 1977, et al.
(author)
-
Wire-Plane Electrode System for Electrical Tree Initiation Exposed to AC and DC Voltage Stress
- 2011
-
In: The Proceedings of 17th International Symposium on High Voltage Engineering, s. C-025. - 9783800733644
-
Conference paper (other academic/artistic)abstract
- Electrical treeing in XLPE exposed to DC (of both polarities) and 50 Hz AC voltages has been analysed in a wire-plane electrode geometry using real-time optical detection and microscopic observations. It has been found that treeing mechanisms differ between AC and DC stresses. For AC voltage, the tree initiation and growth is easily observable in real time during the testing. In the case of DC stress a complete breakdown of the XLPE occurs before electrical trees can be discerned with the use of CCD camera. However, in the microscopic observations of the specimens, very fine tree structures can be distinguished. The effect of voltage ramping rate was also analysed. For AC stress, the faster ramping results in a higher tree inception voltage. The opposite trend is true for DC stress, where the faster ramp results in a lower breakdown voltage level. When comparing the effect on polarity of the DC breakdown voltage, negative polarity results in higher voltage levels than positive polarity.
|
|
| 36. |
|
|
| 37. |
- Joshi, Abhishek, 1985, et al.
(author)
-
Electrical Characterization of Bearing Lubricants
- 2014
-
In: 2014 IEEE Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2014; Des Moines Marriott Downtown700 Grand AvenueDes Moines; United States; 19 October 2014 through 22 October 2014. - 0084-9162. - 9781479975259 ; , s. 586-589
-
Conference paper (peer-reviewed)abstract
- In this paper, commercial bearing lubricants have been characterized with respect to their electrical properties as function of lubricant type, additives and usage. In a wider perspective, the research aims at studying, characterizing and modeling bearing currents and the associated premature failure of bearings within electrical generators of wind turbines. In parallel to the work presented here, the phenomenon is also investigated through experiments on bearings under running conditions in test rigs under different conditions. As a complement to these investigations, this paper presents the results from characterization of electrical properties of lubricants such as relative permittivity and conductivity, by means of frequency domain spectroscopy, and breakdown voltage. The relative permittivity at 40 Hz was found to be 2.1 for a lubricant oil without additive, 2.2 for a lubricant with additive for wear protection, 3.8 for fresh grease and 4.7 for a used one. The conductivity at 50 °C for lubricant oil without additives was 2.9 pS/m, with additives 114 pS/m and for grease 4200 pS/m. The breakdown voltage for a 100 μm gap for oil without additives was 24.4 kV/mm and for oil with additives 30.8 kV/mm.
|
|
| 38. |
- Joshi, Abhishek, 1985, et al.
(author)
-
Investigation of the Static Breakdown Voltage of the Lubricating Film in a Mechanical Ball Bearing
- 2013
-
In: Proceedings of the 23rd Nordic Insulation Symposium (NORD-IS 13), June 9-12, 2013, Trondheim, Norway. - 9788232102747 ; , s. 94-97
-
Conference paper (peer-reviewed)abstract
- In this paper, the current pulse (discharge) activity in a small bearing is investigated when current-limited DC voltages of different levels are applied to the shaft under varying rotational speeds and mechanical loads. The measurements show that the inception voltage of discharges in the bearing is higher than the extinction voltage. However, when the discharge activity has been intense the bearing exhibits difficulties to recover its insulating properties when the voltage is decreased to a low voltage level and thus the discharge activity continues. The measurements also show that at low voltages the bearing acts as an insulator but if the voltage is increased the bearing starts to act as a conductor. This transition occurs during a narrow voltage interval. The time interval between two consecutive current pulses decreases significantly with increasing shaft voltage.
|
|
| 39. |
- Joshi, Abhishek, 1985, et al.
(author)
-
Investigations of Particle-Initiated Insulation Breakdowns in Bearings
- 2019
-
In: Journal of Renewable Energy. - : Hindawi Limited. - 2314-4386 .- 2314-4394. ; 2019
-
Journal article (peer-reviewed)abstract
- Early failures in bearings of wind turbine drivetrains have increased after introduction of power electronic switches, which leads to shaft voltages and bearing currents. In presence of voltage, a rupture of bearing insulation could occur due to several plausible electro-physical mechanisms viz., asperities, electric breakdowns, particles, etc. The flow of high amperage current through the bearing during a breakdown mechanism could lead to early failures. Our aim is to understand the electrical behaviour of a bearing and elaborate by an equivalent electric circuit model, emphasizing on particle-initiated breakdowns. In presence of a shaft voltage, the particles form a path of low resistance through the bearing and results in flow of shaft or bearing currents, which could cause pre-mature failure of the bearing. Particles such as Arizona Test Dust (ATD), carbon black, aluminium powder and fine iron powder were mixed in lubricant at particle concentrations ranging between 7.5 mg/L and 150 mg/L. The breakdown characteristics of electrical insulation of the bearing during a given test is quantified as time of conduction, which is expressed as a percentage of the time the bearing is in the conducting state during a test. An investigation of time of conduction for different lubricant samples was conducted along with studying the effects of start and stops of the rotating shaft. The electrical conductive nature of the particle played no role in breakdown of bearing voltage. At a fixed concentration of 150 mg/L, the insulation breakdown events were highest in lubricant with ATD, followed by iron powder, aluminium power and carbon black particles. The time of conduction increases up to 24 times for the same test lubricant, as the particle concentration was increased from 7.5 mg/L to 150 mg/L. The current activity reduced to almost half in the test after stopping the shaft rotation. The resistance of bearing during insulation breakdown events is highest for aluminium powder, followed by fine iron powder, carbon black and ATD.
|
|
| 40. |
|
|
| 41. |
|
|
| 42. |
|
|
| 43. |
|
|
| 44. |
- Lindell, Elisabeth, 1980, et al.
(author)
-
Influence of rise time on partial discharge extinction voltage at semi-square voltage waveforms
- 2010
-
In: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 17:1, s. 141-148
-
Journal article (peer-reviewed)abstract
- This work presents measurements of the partial discharge (PD) extinction voltage in three different types of test objects, using semi-square voltages with 2 mu s and 100 mu s rise time. A needle creating corona discharges, a twisted pair specimen commonly used for testing motor insulation and a paper/oil test object modelling the turn-to-turn insulation of a transformer winding were investigated, presenting extinction voltages between approximately 1 kV and 8 kV. For the twisted pair specimen the PD extinction voltage was significantly lower at the shorter rise time, whereas for the other test objects, just a small or no difference could be detected. The conclusion is that depending on what range of rise times and what insulation system that are studied, there may be an influence of the rise time of the applied voltage on the PD extinction voltage.
|
|
| 45. |
|
|
| 46. |
- Lindell, Elisabeth, 1980, et al.
(author)
-
Partial discharges measured at semi-square voltages
- 2006
-
In: 2006 Annual report conference on electrical insulation and dielectric phenomenon, October 15-18, 2006, Kansas City, USA. - 1424405475 ; , s. 631-634
-
Conference paper (other academic/artistic)
|
|
| 47. |
|
|
| 48. |
|
|
| 49. |
- Mantsch, Adrian Razvan, 1983, et al.
(author)
-
The Effect of DC Electro-thermal Ageing on Electrical Treeing in Polyethylene
- 2013
-
In: 23rd NORDIC INSULATION SYMPOSIUM. ; , s. 29-32
-
Conference paper (peer-reviewed)abstract
- Electrical treeing is among the main mechanisms responsible for electric breakdown in polymeric high voltage cables. In this paper, electrical treeing in electro-thermally aged low density polyethylene (LDPE) was evaluated by means of a real-time microscopic detection system. A wire – plane electrode test object with semiconducting tab was used in the study. The ageing was performed at a constant temperature of 80°C and an applied DC voltage of 10 kV of both polarities and lasted from 100 up to 800 hours. For testing the resistances to electrical treeing, tests were performed by applying 50 Hz AC voltage at a ramping speed of 0.5 kV/s. The obtained experimental results showed that mainly the thermal component of the ageing had an influence on electrical treeing parameters, namely on the initiation voltage as well as the shape and occurrence frequency of the trees. With an increase of the ageing time, the tree inception voltage gradually decreased.
|
|
| 50. |
- Romano, Pietro, et al.
(author)
-
Partial Discharge Measurements under DC Voltages Containing Harmonics Produced by Power Electronic Devices
- 2018
-
In: Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP. - 0084-9162. ; 2018-October, s. 558-561
-
Conference paper (peer-reviewed)abstract
- Partial Discharge (PD) monitoring is one of the main diagnostic instrument to evaluate the reliability of modern electrical transmission and distribution systems. The widespread use of High voltage dc (HVDC) connections increase the demand of voltage source converters (VSCs) which, as a consequence, has brought about new challenges in the field of partial discharge measurements. In fact, the output voltage waveforms generated by AC/DC modular multilevel converters (MMCs) are affected by a significant harmonic content which influences the Partial Discharges (PDs) activity. For this reason, previous research mainly investigated the PD phenomenon on the AC side of the converter. The aim of the present work was to evaluate the PDs behavior applying a voltage waveform containing ripple at the DC side of the converter. At first, a pure DC voltage has been used to stress the tested specimen and then harmonic content has been added by chancing the number of modules of the simulated AC/DC converter. For the different generated waveforms, measurements of PDs have been carried out and a comparison of the different PDs pattern have been made. The effect on the PDs behavior has been also evaluated for different switching frequencies.
|
|
