| 1. |
- Joshi, Abhishek, 1985, et al.
(författare)
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Investigations of Particle-Initiated Insulation Breakdowns in Bearings
- 2019
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Ingår i: Journal of Renewable Energy. - : Hindawi Limited. - 2314-4386 .- 2314-4394. ; 2019
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Tidskriftsartikel (refereegranskat)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.
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| 2. |
- Romano, Pietro, et al.
(författare)
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Partial Discharge Measurements under DC Voltages Containing Harmonics Produced by Power Electronic Devices
- 2018
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Ingår i: Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP. - 0084-9162. ; 2018-October, s. 558-561
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Konferensbidrag (refereegranskat)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.
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| 3. |
- Romano, P., et al.
(författare)
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The Effect of the Harmonic Content Generated by AC/DC Modular Multilevel Converters on HVDC Cable Systems
- 2019
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Ingår i: Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP. - 0084-9162. ; , s. 666-669
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Konferensbidrag (refereegranskat)abstract
- With the increasing penetration of renewable and decentralized energy sources into the power grid, an extended use of DC voltages is expected on both distribution and transmission levels. Generation of DC voltages by means of voltage source converters is associated with a wide spectrum of harmonic distortions at converter terminals, both on the ac and on the dc sides. This can lead to partial discharges in power cables, which deteriorate insulation material thus weakening its performance and reducing cable life-time. In the previously published paper, the effect of harmonic distortion on appearance of partial discharges in cable insulation was evaluated. Here, the study related to the PD behavior has been made focusing on the influence of multilevel converter parameters such as the number of submodules and the switching frequency chosen to generate different voltage stresses. In addition, effects of using a function generator for producing desired waveform instead of the dSPACE board as in the previous work, are highlighted. In particular, it is shown implementation of the former made it possible utilizing the Pry-Cam synchronization and thus the realization of the PD pattern.
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| 4. |
- Stanne, Tara M, 1979, et al.
(författare)
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Low Circulating Acute Brain-Derived Neurotrophic Factor Levels Are Associated With Poor Long-Term Functional Outcome After Ischemic Stroke.
- 2016
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Ingår i: Stroke; a journal of cerebral circulation. - 1524-4628 .- 0039-2499. ; 47:7, s. 1943-1945
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Tidskriftsartikel (refereegranskat)abstract
- Brain-derived neurotrophic factor (BDNF) plays important roles in brain plasticity and repair, and it influences stroke outcomes in animal models. Circulating BDNF concentrations are lowered in patients with traumatic brain injury, and low BDNF predicts poor recovery after this injury. We sought to investigate whether circulating concentrations of BDNF are altered in the acute phase of ischemic stroke and whether they are associated with short- or long-term functional outcome.
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| 5. |
- Xu, Xiangdong, 1984, et al.
(författare)
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Loss Current Studies of Partial Discharge Activity
- 2015
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 22:1, s. 472-481
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Tidskriftsartikel (refereegranskat)abstract
- Partial discharge measurements have important applications in evaluating insulation systems as well as for dielectric materials studies. This work investigates a measurement technique to study partial discharge activity by measuring the PD loss current. The method is similar to dielectric response measurement and measures the current through a test object with and without PD. By comparing the dielectric current and the discharge current, all conduction processes, both fast and slow, can be explored. This is in contrast to traditional pulse PD detections, where only the fast PD transients have been measured. The method is demonstrated through measurements of PDs appearing in twisted pair and needle-plate test objects under sinusoidal and square wave voltage excitations. Comparisons are made to results of simultaneously employed traditional pulse PD detection technique.
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| 6. |
- Xu, Xiangdong, 1984, et al.
(författare)
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Spontaneous Partial Discharge Oscillations Studied by Excess Current
- 2015
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Ingår i: Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP. - 0084-9162. - 9781467374972 ; , s. 55-59
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Konferensbidrag (refereegranskat)abstract
- This work discusses appearance of spontaneous partial discharge (PD) oscillations in specimen containing dielectrically isolated gas cavity as well as a measurement technique that allows to study such PD activity by quantifying the resulting excess current, which includes both fast PD current pulses and relatively slower charge movements. This is achieved through a comparison of currents measured with and without PDs by simultaneously performed conventional pulse PD detection and dielectric response (DR) measurement from which the excess current wave shape is deduced. The obtained results reveal appearance of an additional “slow” current contribution and this paper discusses how this current is generated and why it may influence the PD activity.
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| 7. |
- Xu, Xiangdong, 1984, et al.
(författare)
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Study of Partial Discharge Activity by Excess Current
- 2015
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 22:5, s. 3028-3036
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of partial discharges (PD) play an important role in study of dielectric materials. This work applies a measurement system based on dielectric response technique to study an excess current associated with PD activity, which appears during and after PD activity and shows some prominent properties. The value of such a measurement is demonstrated through studies of two types of test objects, twisted pairs and cavity specimens. A simultaneous use of pulse PD detection in the study shows a correlation between changes in PD amplitudes and non-PD excess current levels. It is concluded that the used methodology can serve as an important complement to the traditionally used pulse PD detection.
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| 8. |
- Åberg, Daniel, 1973, et al.
(författare)
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Increased Cerebrospinal Fluid Level of Insulin-like Growth Factor-II in Male Patients with Alzheimer's Disease
- 2015
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Ingår i: Journal of Alzheimers Disease. - : IOS Press. - 1387-2877 .- 1875-8908. ; 48:3, s. 637-646
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Tidskriftsartikel (refereegranskat)abstract
- Background: Insulin-like growth factor-II (IGF-II) is important for brain development. Although IGF-II is abundant also in adult life, little is known of the role of IGF-II in Alzheimer's disease (AD). Objective and methods: This was a cross-sectional study of 60 consecutive patients under primary evaluation of cognitive impairment and 20 healthy controls. The patients had AD dementia or mild cognitive impairment (MCI) diagnosed with AD dementia upon follow-up (n = 32), stable MCI (SMCI, n = 13), or other dementias (n = 15). IGF-II, IGF-binding protein-1 (IGFBP-1), and IGFBP-2 were analyzed in serum and cerebrospinal fluid (CSF). Results: Levels of IGF-II, IGFBP-1, and IGFBP-2 were similar in all groups in the total study population. Gender-specific analyses showed that in men (n = 40), CSF IGF-II level was higher in AD compared to SMCI and controls (p < 0.01 and p < 0.05, respectively). Furthermore, CSF IGFBP-2 levelwas increased inADmen versus SMCI men (p < 0.01) and tended to be increased versus control men (p = 0.09). There were no between-group differences in women (n = 40). In the total study population (n = 80) as well as in men (n = 40), CSF levels of IGF-II and IGFBP-2 correlated positively with CSF levels of the AD biomarkers total-tau and phosphorylated tau protein. Conclusion: In men, but not women, in the early stages of AD, CSF IGF-II level was elevated, and CSF IGFBP-2 level tended to be increased, compared to healthy controls.
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| 9. |
- Åberg, N David, 1970, et al.
(författare)
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Serum erythropoietin and outcome after ischaemic stroke: a prospective study.
- 2016
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Ingår i: BMJ open. - : BMJ. - 2044-6055. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- Erythropoietin (EPO), which is inversely associated with blood haemoglobin (Hb), exerts neuroprotective effects in experimental ischaemic stroke (IS). However, clinical treatment trials have so far been negative. Here, in patients with IS, we analysed whether serum EPO is associated with (1) initial stroke severity, (2) recovery and (3) functional outcome.
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