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Change in Partial Discharge Activity as Related to Degradation Level in Oil-Impregnated Paper Insulation: Effect of High Voltage Impulses
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. (Electrical Systems)
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. (Electrical Systems)ORCID iD: 0000-0003-1766-8077
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. (Electrical Systems)
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. (Electrical Systems)
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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.

Place, publisher, year, edition, pages
2014. Vol. 21, no 3, 1243-1250 p.
Keyword [en]
Partial discharge, high voltage impulses, oil-impregnated paper, dielectric properties
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-142024DOI: 10.1109/TDEI.2014.6832271ISI: 000337847200039Scopus ID: 2-s2.0-84903201251OAI: oai:DiVA.org:kth-142024DiVA: diva2:699535
Note

QC 20140806. Updated from accepted to published.

Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2017-12-05Bibliographically approved
In thesis
1. The Effect of HV Impulses on Partial Discharge Activity and on the Dielectric Response in Oil-impregnated Paper Insulation
Open this publication in new window or tab >>The Effect of HV Impulses on Partial Discharge Activity and on the Dielectric Response in Oil-impregnated Paper Insulation
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This work investigates how HV impulses affect the behavior of partial discharge (PD) activity and the low voltage dielectric response of oil-impregnated paper insulation. It also investigates how the change in the PD activity is related to the degradation level of oil-impregnated paper insulation. In order to accomplish these objectives, the ageing were done under three different electrical stress conditions, i.e. HV impulses following an early stage 50 Hz AC PD activity, a prolonged PD activity at a 50 Hz AC stress alone, and a combination of HV impulses and a prolonged PD activity at a 50 Hz AC stress. In order to predict the level of deterioration caused by each ageing stress condition, the dielectric spectroscopy (DS) measurements in a frequency range of 1.0 mHz to 1.0 kHz were performed before and after subjecting a test object to each of the ageing stress conditions.

     The investigations were mainly done on the test samples consisting of a cavity deliberately introduced between the layers of oil-impregnated paper. Additionally, the investigation about the effect of HV impulses alone on the DS results was done on aged oil-impregnated paper transformer bushing.

     The PD experimental results presented in this thesis indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity will neither cause a significant change in phase resolved partial discharge (PRPD) patterns nor damage oil-impregnated paper insulation to a level that can be noticed with visual observations. On the other hand, a prolonged PD activity at a 50 Hz AC stress can cause the change in PRPD patterns by decreasing the total PD charge and the number of PD pulses, but cannot quickly damage the oil-impregnated paper insulation as it would do when it is combined with HV impulses. In addition to that, the results show that the combination of both, HV impulses and a prolonged PD activity at a 50 Hz AC stress can cause a high drop in the PD parameters (total PD charge and number of PD pulses). 

     The DS results show that HV impulses below the impulse breakdown stress following an early stage 50 Hz AC PD activity will not cause a significant increase in the real part of the complex capacitance and in the dissipation factor as they will do when they are combined with a prolonged PD activity at a 50 Hz AC stress. Further, the dielectric spectroscopy results obtained every three hours during the ageing of oil-impregnated paper insulation by a prolonged PD activity at an AC stress show that the dissipation factor will increase, but the PD parameters (total PD charge and the repetition rate) will decrease with time of PD application. For a case of the aged oil-impregnated paper transformer bushing, HV impulses of amplitudes up to 200 kV did not result in the change in the dissipation factor curve before removing insulating oil from the bushing. However, after removing about 2.5 liters of insulating oil from the bushing, HV impulses resulted in the change in the dissipation factor curve. The magnitudes of the dissipation factor curves appeared to be much higher in the middle frequencies region, i.e. the frequencies between 10 mHz and 100 Hz. After refilling the bushing with the same insulating oil, the loss peak shifted towards the higher frequencies.

     To understand how the ageing by-products initiated by PDs in the small cavity can modify the geometry of oil-impregnated paper insulation; the model of oil-impregnated paper insulation, comprising of a small cavity, was implemented in Finite Element Method (FEM) software (COMSOL Multiphysics 4.2a). The comparison between the simulation and experimental results show that PD by-products will result in two zones, i.e. aged and unaged zones, and the aged zone will grow with time of PD application; thereby increasing the dissipation factor. On the other hand, in order to interpret  the change in the dissipation factors for the dielectrics in aged oil-impregnated paper transformer bushing after had been exposed to HV impulses, a model of a part of the condenser body (oil-paper insulation) was also implemented in the FEM software (COMSOL Multiphysics 4.2a). To model a condition of low insulating oil level in the bushing, a part of oil subdomains was replaced with the air dielectric properties. A comparison between the simulation and experimental dissipation factor curves indicate that HV impulses will produce the by-products (ions), which will increase the conductivity of air when the bushing has low insulating oil level. On refilling the bushing with the same insulating oil, the insulating oil will take these ions and the reactions between the aged insulating oil by-products (such as acids) and the ions, may produce more ions, thereby increasing further the conductivity of the insulating oil.

 

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. x, 62 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2014:006
Keyword
Partial discharge, high voltage impulses, oil-impregnated paper, dielectric spectroscopy, Finite Element Method.
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-142027 (URN)978-91-7595-024-2 (ISBN)
Public defence
2014-03-26, Sal H1, Teknikringen 33, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140303

Available from: 2014-03-03 Created: 2014-02-27 Last updated: 2014-03-03Bibliographically approved
2. Mechanisms of Electrical Ageing of Oilimpregnated Paper due to Partial Discharges
Open this publication in new window or tab >>Mechanisms of Electrical Ageing of Oilimpregnated Paper due to Partial Discharges
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
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:nbn:se:kth:diva-159670 (URN)978-91-7595-443-1 (ISBN)
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

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Ghaffarian Niasar, Mohamad

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