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The Effect of HV Impulses on Partial Discharge Activity and on the Dielectric Response in Oil-impregnated Paper Insulation
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. (Electrical Systems)
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 [en]
Partial discharge, high voltage impulses, oil-impregnated paper, dielectric spectroscopy, Finite Element Method.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-142027ISBN: 978-91-7595-024-2 (print)OAI: oai:DiVA.org:kth-142027DiVA: diva2:699546
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
List of papers
1. Comparison of phase resolved partial discharge patterns in small test samples, bushing specimen and aged transformer bushing
Open this publication in new window or tab >>Comparison of phase resolved partial discharge patterns in small test samples, bushing specimen and aged transformer bushing
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2012 (English)In: 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE , 2012, 88-91 p.Conference paper, Published paper (Refereed)
Abstract [en]

The transformer bushings are one of the key devices in electrical power networks. With time, the electrical insulations in the transformer bushings become aged. One effect of a long time ageing would be due to Partial Discharge (PD) defects. In this work, the phase resolved partial discharge (PRPD) pattern of the defects naturally present in an aged 36 kV Oil Impregnated Paper (OIP) bushing was compared with the PRPD patterns of defects deliberately introduced in simple test samples and in a bushing specimen. The results presented in this paper show that it might be possible to predict the type of the defects in aged transformer bushings through comparing measured PRPD patterns with PRPD patterns from defects deliberately introduced in simple test samples and in bushing specimens.

Place, publisher, year, edition, pages
IEEE, 2012
Series
Conference on Electrical Insulation and Dielectric Phenomena. Annual Report, ISSN 0084-9162
Keyword
Electrical power networks, Oil impregnated papers, Phase resolved partial discharge patterns, PRPD patterns, Test samples, Transformer bushings
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-104837 (URN)10.1109/CEIDP.2012.6378729 (DOI)000316899800018 ()2-s2.0-84872092506 (Scopus ID)978-1-4673-1252-3 (ISBN)
Conference
2012 IEEE Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2012; Montreal, QC; Canada; 14 October 2012 through 17 October 2012
Note

QC 20121115

Available from: 2012-11-15 Created: 2012-11-13 Last updated: 2015-02-06Bibliographically approved
2. Effect of high voltage impulses on partial discharge activity in a cavity embedded in paper insulation
Open this publication in new window or tab >>Effect of high voltage impulses on partial discharge activity in a cavity embedded in paper insulation
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2012 (English)In: 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE , 2012, 133-136 p.Conference paper, Published paper (Refereed)
Abstract [en]

High voltage impulses in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular insulation at the instant of interaction with a high voltage impulse would determine the level of the degradation of the electrical insulation caused by the high voltage impulse. In this work, the effect of high voltage impulses on the partial discharge (PD) parameters was investigated. The investigation was done in test samples consisting of a cavity deliberately introduced between oil-impregnated papers used in power transformer bushings. The measurement results presented in this paper show that high voltage impulses can have a large impact on the behavior of the PD parameters if the test sample is aged by partial discharges, but a smaller impact when the test sample is unaged.

Place, publisher, year, edition, pages
IEEE, 2012
Series
Conference on Electrical Insulation and Dielectric Phenomena. Annual Report, ISSN 0084-9162
Keyword
Electrical power system, High voltage, Lightning strikes, Measurement results, Oil-impregnated paper, Paper insulation, Partial discharge activity, PD parameters, Switching operations, Test samples, Transformer bushings
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-105037 (URN)10.1109/CEIDP.2012.6378739 (DOI)000316899800028 ()2-s2.0-84872091683 (Scopus ID)978-1-4673-1252-3 (ISBN)
Conference
2012 IEEE Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2012; Montreal, QC; Canada; 14 October 2012 through 17 October 2012
Note

QC 20121115

Available from: 2012-11-15 Created: 2012-11-15 Last updated: 2015-02-06Bibliographically approved
3. Changes in phase resolved partial discharge patterns in a cavity embedded in paper insulation due to AC voltages andhigh voltage impulses
Open this publication in new window or tab >>Changes in phase resolved partial discharge patterns in a cavity embedded in paper insulation due to AC voltages andhigh voltage impulses
2012 (English)In: proceedings 10th NORDAC 2012, 2012, 93-103 p.Conference paper, Published paper (Refereed)
Abstract [en]

Electrical insulation materials such as pressboards, oils and paper are important components inelectrical power equipment. The breakdown of insulation materials will cause severe failure of the equipment. Therefore, it is important to have condition monitoring methods in insulation materials of power equipment. A phase resolved partial discharge (PRPD) measurement is one of them.

In this work, the effect of high voltage impulses on PRPD patterns was investigated in test samples consisted of a cavity deliberately introduced between oil-impregnated paper used in power transformer bushings. In the first study, the test sample was highly stressed by 50 high voltage impulses. In the second study, the test sample was aged at elevated AC voltage during almost 5 hours, but with four high voltage impulses injected at selected times during the experiment. In the third study the test sample was aged by a PD activity at elevated AC voltage up to about 18 hours. The PRPD patterns were periodically recorded by an Insulation Condition Monitoring (ICM) system. In order to investigate how different electrical stresses affect the dielectric properties of the paper insulation, the fourth study about the dielectric spectroscopy (DS) measurements were performed. The results show that high voltage impulses can have a large impact on the behavior of PRPD patterns if the test sample is aged by partial discharges, but a smaller effect when the test sample is unaged.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-105038 (URN)
Conference
proceedings 10th NORDAC 2012, Espoom Fubdabdm 10-11 Septenber 2012
Note

QC 20121115

Available from: 2012-11-15 Created: 2012-11-15 Last updated: 2014-03-03Bibliographically approved
4. 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
Show others...
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
5. The effect of PD by-products on the dielectric frequency response of oil-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-paper insulation comprising of a small cavity
Show others...
2015 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 5, 2923-2930 p.Article in journal (Other academic) Published
Abstract [en]

This work investigates the effect of alternating current partial discharge (AC PD) byproducts on the dielectric frequency response (DFR) of oil-paper insulation in the frequency range of 1.0 mHz to 1.0 kHz. The investigations were done by utilizing experimental results in combination with a Finite Element Method (FEM) model. The results presented in this paper show that AC PD by-products in the oil-paper insulation can result in a dissipation factor curve whose magnitudes increase with increasing volume of the aged zone. The change in a dissipation factor curve due to PD byproducts was observed to occur mainly in the middle and low frequency regions. The results indicate that the dissipation factor curves in the middle and low frequency regions depend on the level of PD by-products in oil and paper, respectively.

Place, publisher, year, edition, pages
IEEE Press, 2015
Keyword
Cavity, dielectric frequency response (DFR), partial discharge, oil-paper insulation, Finite Element Methods (FEM), dissipation factor
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-142025 (URN)000364993800066 ()2-s2.0-84947982758 (Scopus ID)
Note

QC 20151214

Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2017-04-28Bibliographically approved
6. Effect of High Voltage Impulses on Surface Discharge at the Oil-Paper Interface
Open this publication in new window or tab >>Effect of High Voltage Impulses on Surface Discharge at the Oil-Paper Interface
Show others...
2013 (English)In: 23rd Nordic Insulation Symposium (NORD-IS 13), 2013, 108-111 p.Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

High voltage transients in electrical power systems are mainly caused by lightning strikes and switching operations. The high voltage transients above the basic insulation level (BIL) would degrade the electrical insulation of the equipment. In this work, the effect of high voltage impulses on the trend of the surface discharge at the oil-paper interface was investigated. The partial discharge patterns were recorded by the Insulation Condition Monitoring (ICM) system. The HV impulses were generated by a standard 1.2/50 μs HV impulse generator. The measurement results presented in this paper seem to show that surface discharge would cause complete breakdown (continuous flashovers) at the oil-paper interface much faster when the test sample is first aged by HV impulses compared to when it is unaged.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-142026 (URN)9788232102747 (ISBN)
Conference
Nordic Insulation Symposium (NORD-IS)
Note

QC 20140228

Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2017-03-03Bibliographically approved

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