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Mechanisms of Electrical Ageing of Oilimpregnated Paper due to Partial Discharges
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. (High voltage)ORCID iD: 0000-0003-1766-8077
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, partial discharge (PD) phenomenon in oil-impregnated paper (OIP) is investigated under accelerated electrical stress. The thesis is mainly focused on the characteristic of PD activity and the influence it has on the insulation properties of OIP. PD source was created by introducing an air filled cavity embedded between layers of OIP. PD activity is investigated from the initiation up to final puncture breakdown of the OIP. The time-evolution of number, maximum magnitude and average magnitude of PD is investigated for cavities with different diameter and height. It was found that time to breakdown is shorter if the cavity diameter is larger and cavities with higher depth produce larger PDs. Comparison between PD activity in three cases, i.e. unaged OIP, thermally aged OIP and OIP samples with higher moisture content is performed. In general, it is found that for all cases the number and the maximum magnitude of PD follows a similar trend versus ageing time. During the very beginning of the experiment large discharges occur and they disappear after a short ageing time. Number and maximum magnitude of PD increase with time until reaching a peak value. Finally both parameters decrease with time and puncture breakdown occurs in the sample. Even though PD activity in thermally aged OIP is higher compared to the unaged OIP samples, the time to breakdown for new and thermally aged OIP samples is similar while it is shorter for OIP samples with higher moisture content.

In this thesis, partial discharge (PD) phenomenon in oil-impregnated paper (OIP) is investigated under accelerated electrical stress. The thesis is mainly focused on the characteristic of PD activity and the influence it has on the insulation properties of OIP. PD source was created by introducing an air filled cavity embedded between layers of OIP. PD activity is investigated from the initiation up to final puncture breakdown of the OIP. The time-evolution of number, maximum magnitude and average magnitude of PD is investigated for cavities with different diameter and height. It was found that time to breakdown is shorter if the cavity diameter is larger and cavities with higher depth produce larger PDs. Comparison between PD activity in three cases, i.e. unaged OIP, thermally aged OIP and OIP samples with higher moisture content is performed. In general, it is found that for all cases the number and the maximum magnitude of PD follows a similar trend versus ageing time. During the very beginning of the experiment large discharges occur and they disappear after a short ageing time. Number and maximum magnitude of PD increase with time until reaching a peak value. Finally both parameters decrease with time and puncture breakdown occurs in the sample. Even though PD activity in thermally aged OIP is higher compared to the unaged OIP samples, the time to breakdown for new and thermally aged OIP samples is similar while it is shorter for OIP samples with higher moisture content.

Breakdown strength of OIP samples is measured before and after ageing with PDs. It is found that the breakdown strength of OIP samples decreases by around 40% after the sample is exposed to accelerated electrical ageing. Furthermore a thermal model was developed to investigate the possible transition of breakdown mechanism from erosion to thermal breakdown in OIP dielectrics. It was found that PD activity can lower the thermal breakdown voltage of OIP up to four times.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xiv, 112 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:006
Keyword [en]
partial discharges, oil-impregnated paper, oil, transformer insulation, dielectric frequency response, breakdown strength, ion mobility, thermal ageing, electrical ageing, moisture in paper, life time, thermal breakdown.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-159670ISBN: 978-91-7595-443-1 (print)OAI: oai:DiVA.org:kth-159670DiVA: diva2:786741
Public defence
2015-02-27, H1, Teknikringen 33, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Note

QC 20150206

Available from: 2015-02-06 Created: 2015-02-06 Last updated: 2015-10-08Bibliographically approved
List of papers
1. Partial Discharges in a Cavity Embedded in Oil-Impregnated Paper: Effect of Electrical and Thermal Aging
Open this publication in new window or tab >>Partial Discharges in a Cavity Embedded in Oil-Impregnated Paper: Effect of Electrical and Thermal Aging
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2015 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 2, 1071-1079 p., 7076808Article in journal (Refereed) Published
Abstract [en]

In this paper, the rapid aging due to Partial Discharge (PD) activity was investigated in unaged and thermally aged oil-impregnated paper with a disk-shaped cavity between the sheets of paper. The PD inception voltage and the voltage for instantaneous breakdown were measured, and the time to breakdown was measured by applying voltages less than the instantaneous breakdown voltage. A comparison between time to breakdown of unaged and thermally aged paper shows no significant difference between thermally aged and unaged paper at sustaining PD activity. The time to breakdown as a function of applied electric field was plotted and compared for both cases showing points scattered around a line in log-log scale. Changes of the PD parameters, such as PD magnitude and PD repetition rate were analyzed from the beginning of PD activity up to the moment of breakdown. The results show that the number and magnitude of PD increase at the beginning of aging until they reach to a maximum value, then both quantities decrease slowly over time until the final puncture breakdown occurs. The results emphasize the importance of PD monitoring on real equipment with oil-impregnated paper as insulation system, such as power transformers, since a focus on the number and magnitude of PD at just the present time may mislead the interpretation. Dielectric spectroscopy measurements performed on unaged and thermally aged paper showed an increasing trend of epsilon '' after the sheets of paper were exposed to thermal aging for longer time. Dielectric spectroscopy measurements performed on samples before and after exposure to PD activity showed a big change of epsilon '', This change can be attributed to byproducts and ions produced by PD activity.

Keyword
Partial discharges, power transformers, thermal aging, oil-impregnated paper, life time, dielectric spectroscopy
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-159673 (URN)10.1109/TDEI.2015.7076808 (DOI)000352793300051 ()2-s2.0-84959365431 (Scopus ID)
Note

QC 20150508. Updated from manuscript to article in journal.

Available from: 2015-02-06 Created: 2015-02-06 Last updated: 2017-12-04Bibliographically approved
2. Dielectric Frequency Response of Oil-impregnated paper: the Effect of Partial Discharges Compared to other Influences
Open this publication in new window or tab >>Dielectric Frequency Response of Oil-impregnated paper: the Effect of Partial Discharges Compared to other Influences
(English)Manuscript (preprint) (Other academic)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-159674 (URN)
Note

QS 2015

Available from: 2015-02-06 Created: 2015-02-06 Last updated: 2015-02-06Bibliographically approved
3. Change in Partial Discharge Activity as Related to Degradation Level in Oil-Impregnated Paper Insulation: Effect of High Voltage Impulses
Open this publication in new window or tab >>Change in Partial Discharge Activity as Related to Degradation Level in Oil-Impregnated Paper Insulation: Effect of High Voltage Impulses
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2014 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, no 3, 1243-1250 p.Article in journal (Refereed) Published
Abstract [en]

High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oil-impregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper.

Keyword
Partial discharge, high voltage impulses, oil-impregnated paper, dielectric properties
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-142024 (URN)10.1109/TDEI.2014.6832271 (DOI)000337847200039 ()2-s2.0-84903201251 (Scopus ID)
Note

QC 20140806. Updated from accepted to published.

Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2017-12-05Bibliographically approved
4. The effect of PD by-products on the dielectric frequency response of oil impregnated paper insulation comprising of a small cavity
Open this publication in new window or tab >>The effect of PD by-products on the dielectric frequency response of oil impregnated paper insulation comprising of a small cavity
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(English)Manuscript (preprint) (Other academic)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-159675 (URN)
Note

QS 2015

Available from: 2015-02-06 Created: 2015-02-06 Last updated: 2015-02-06Bibliographically approved
5. Aging of oil impregnated paper due to Pd activity
Open this publication in new window or tab >>Aging of oil impregnated paper due to Pd activity
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2013 (English)Conference paper, Published paper (Other (popular science, discussion, etc.))
Abstract [en]

Air filled cavity may appear inside solid insulation either due to bad manufacturing or as a result of insulation ageing. A cavity is a weak point in insulation and if the electric stress is high enough; partial discharge can be initiated inside the cavity. In this paper changing of PD parameters (number of PD, maximum magnitude of PD and average magnitude of PD) was investigated over time on oil impregnated paper with an artificial cavity in between the sheets of paper. PD parameters were recorded from the inception of PD until final puncture breakdown occurs in the sample. Experiment was performed on sample with different thickness. Specific pattern on changing of PD number and maximum magnitude of PD was observed for most of the experiments. The pattern includes three stages, big discharges appear at inception of PD and over a very short time they disappear. At stage two, the number and magnitude of PD increase rapidly from small values until they reach to a maximum value. At stage three, the number and magnitude of PD decreases and reach to a constant value. The PD parameter keep constant until the time that final breakdown occurs. The dielectric spectroscopy performed on the sample before and after exposure to PD shows that PD activity causes a permanent shift on ε" which can be explained by PD by-products.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-136428 (URN)
Conference
18th International Symposium on High Voltage Engineering, August 25th-30th 2013, Seoul, South Korea
Note

QC 20131219

Available from: 2013-12-05 Created: 2013-12-05 Last updated: 2015-02-06Bibliographically approved
6. Effect of Partial Discharges on Thermal Breakdown of Oil Impregnated Paper
Open this publication in new window or tab >>Effect of Partial Discharges on Thermal Breakdown of Oil Impregnated Paper
2015 (English)In: IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING, ISSN 1931-4973, E-ISSN 1931-4981, Vol. 10, S14-S18 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we investigate the effect of partial discharges (PDs) on the thermal breakdown voltage of oil-impregnated paper (OIP). A disk-shaped cavity in a stack of paper sheets formed the PD source. The paper sample was exposed to PDs for 18 h. Dielectric spectroscopy measurements were performed on the samples before and after exposure to PD activity. The dielectric spectroscopy results showed a big change in the real and imaginary part of the permittivity. These results were used to calculate the activation energy and heat losses inside the insulation. In order to investigate the possibility of thermal breakdown in the system, the OIP system was simulated by using the FEM software COMSOL Multiphysics. The heat produced by PD pulses and dielectric losses was considered in the simulation. The results show that for the OIP insulation, PD activity can reduce the thermal breakdown voltage by a factor up to 4 times.

Place, publisher, year, edition, pages
John Wiley & Sons, 2015
Keyword
oil-impregnated paper, partial discharges, thermal breakdown
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-159681 (URN)10.1002/tee.22159 (DOI)000369951400003 ()2-s2.0-84944519439 (Scopus ID)
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Note

Updated from manuscript to article in journal.

QC 20160202. QC 20160318

Available from: 2015-02-06 Created: 2015-02-06 Last updated: 2017-12-04Bibliographically approved

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Ghaffarian Niasar, Mohamad

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